PRINCIPLES OFELECTRONICS

INTRODUCTIONSAFETYCHAPTER 1- MEASURING INSTRUMENTS AND TESTING METHODS

Meters The Cathode Ray Oscilloscope Simple Component Testing

CHAPTER 2- ECG SEMICONDUCTORS – REPLACEMENT PROCEDURES

2.1 Universal Replacements2.2 Replacement Techniques 2.2.1 Forming Pins 2.2.2 Mounting 2.2.3 Soldering2.3 Mosfet Handling Precautions2.4 CMOS Handling Precautions2.5 Selecting a Bi-Polar Transistor for an unlisted type2.6 SCR,Triac,Rectifier and Bridge Replacement2.7 Personalized Service for unlisted Type2.8 Testing Solid – State Devices2.9 Testing Bi-Polar Transistor2.10 Testing Field Effect Transistor 2.11 Testing Diodes2.12 Testing SCRs and Triacs (Thyristors)2.13 Symbol, Terms and Definitions

CHAPTER 3 – SEMICONDUCTORS3.1Semiconductors Materials3.2 Atomic Structure3.3 Diode Characteristics3.4 Diode Applications3.5 Zener Diodes3.6 Light Emitting Diode3.7 Point Contact Diode3.8 Transistors3.9 Transistors Circuit Configurations3.10 Transistors Data

CHAPTER 4 – POWER SUPPLY CIRCUITS4.1 Basic Principles of DC Power Supplies4.2 The Linear Stabilized Power Unit4.3 Switching Mode Power Supplies4.4 Power Supply Protection Circuits4.5 Testing Power Supply Circuits4.6 Fault Finding Techniques and Typical Fault Conditions

CHAPTER 5 – SINGLE STAGE TRANSISTOR AMPLIFIER 5.1 Basic Principles5.2 Resistor Faults

5.3 Capacitor Faults5.4 Transistor Faults

CHAPTER 6 – THE FIELD EFFECT TRANSISTOR6.1 Operation of the JFET6.2 Operation of the MOSFET6.3 Operation of the VMOSFET6.4 Operation of the IGFET6.5 Common Drain Amplifier

CHAPTER 7 – OPERATIONAL AMPLIFIER SYSTEMS7.1 Sign Changer7.2 Scale Changer7.3 Phase Shifter 7.4 Summing Amplifier7.5 Noninverting Summoning7.6 Transconductance Amplifier

CHAPTER 8 – AMPLIFIER CIRCUITS8.1 Types and Classes of Amplifiers8.2 Negative Feedback8.3 Testing Amplifiers: Basic Measurements 8.3.1 Measurements of Gain 8.3.2 Measurement of Frequency Response and Bandwidth 8.3.3 Measurement of Input Impedance 8.3.4 Measurement of Output Resistance 8.3.5 Measurement of Power Output, Efficiency and Sensitivity for an Audio Amplifier8.4 Transient Testing of Amplifiers8.5 Distortion Measurements 8.5.1 Amplitude Distortion 8.5.2 Frequency Distortion 8.5.3 Phase Distortion 8.5.4 Cross-Over Distortion 8.5.5 Intermodulation Distortion8.6 Faults in Amplifiers

CHAPTER 9 – OSCILLATOR AND TIME BASE CIRCUITS9.1 Principles of Oscillators9.2 Measurement of Frequency9.3 Frequency Stability9.4 Harmonic Distortion9.5 Square and Pulse Waveforms9.6 Sawtooth and Ramp Circuits9.7 Negative Resistance Oscillators9.8 Fault Finding on Oscillators

CHAPTER 10 – PULSE AND WAVEFORM SHAPING CIRCUITS10.1 Introduction10.2 Linear Passive Circuits – the Integration and Differentiation

10.3 Diode Waveform Shaper10.4 Active Pulse Shaping Circuits10.5 The Schmitt Trigger Circuit10.6 The Monostable10.7 Fault Finding in Pulse and Waveform Shaping Circuits

CHAPTER 11 – CIRCUITS USING ANALOGUE AND DIGITAL INTEGRATED CIRCUITS11.1 Introduction to Integrated Circuits11.2 Analogue ICs11.3 Digital ICs11.4 Servicing Instruments Continuing ICs

ForewordThis text was produced specifically for use in the short course “Principles of Electronics”. It provides specific and general information for persons wishing to obtain an introductory practical knowledge of electronics. The step-by-step design takes the reader through the use of electronic tools, the workings of electronic devices, stan-dard electronic circuits and the practical use of electronics.

This is not a text manual but a course guide and handbook. The information in this manual should be followed by additional reading and practice by the participant after, or even during, the programme.

This manual was compiled from information contained in several published sources and experiments developed by the author.

The following sources were used: ECG Manual Electronic Fault Dagnosis Practical Electronics Electrical Engineers Hand Book Lucas Nulle Training Guide Tele-A-Train

Should further reading be required MIC/NSDP will be pleased to supply names of books and publishers where such specifics are available.

MIC/NSDP makes no claims to the material contained herein.

THIS BOOK IS NOT FOR SALE

Compiled by:Tagore RamlalMarch 1997Introduction by Garth Joseph

INTRODUCTIONElectronics has been defined as “that field of science and engineering which deals with electron devices and their utilization”. Electronic devices are “devices in which electrical conduction is principally electrons moving through a vacuum, gas or semi-conductor”.

Generally Electronics can be defined as “the science and technology of the passage of charged particles in a gas, in a vacuum, or in a semi-conductor”. Electrons and mobile holes are the most important of charge carriers in electronic devices but ions also play part.

Brief HistoryBefore electronic engineering came into existence, electrical engineering already flourished. Electrical engineer-ing deals with devices that depend solely on the motions of electrons in metals, for example, generators, motors, light bulbs and telephones. The principal benefactors of these devices are the wire telephone or telegraph com-panies and the power industries.

The introduction of the semiconductor transistor in the late 1940’s caused dramatic change in the electronics industry. Apparent unlimited miniaturization has resulted. Advantages of semiconductors include smaller and lightweight equipment, no heater requirement or heater loss (as required for tubes), more rugged construction, etc. Tubes are still used in a few isolated areas.

Progress is seen in radio, television, computer, automobile and space age development. Most manufactured products used today have been made better or more economical through the use of electronics. Automation usu-ally means electronic control of machine operation. In 1958 at Texas Instruments the monolithic was conceived whereby an entire circuit out of germanium or silicon would be built. The bulk semiconductor was used to form a resistor. A capacitor was built by using an oxide layer (for the dielectric) on silicon. These concepts led to the solid circuit or integrated circuit being announced in 1959 at the IRE convention.

Types of DevicesMiniature semiconductor systems are encased in containers simply to provide a means of handling, protecting and connecting the. Reliability, speed and production yields have improved, while cost, power consumption, and size have been reduced drastically.

Microelectronics describe high-density Integrated Circuits (ICs) or chips, and refers to the technologies in-volved in their design, fabrication and use. The prefix “micro” alludes to the tremendous size reductions that have been taken. Micro-electronic devices are used in printed circuits as dual-in-line packaged (DIP) monolithic ICs or as thick – or thin-film hybrids. Functional devices include Gunn diodes, IMPATT diodes, charge injection devices (CID’s), charge-coupled devices (CCD’s), bipolar function transistors and MOSFET integrated circuits.

The most remarkable aspect of the modern technology is the ability to pack a tremendous number of functions into a physically small area. A nonmenclature has developed in the digital area to describe the degree of packing density. Small-scale integration (SSI), medium-scale integration (MSI) and large-scale integration (LSI). Very-large-scale integration (VLSI) refers to digital chips with 10,000 or more devices.

LSI chips with several thousand devices or memory elements, plus associated input and output circulatory for such items as digital watches, pocket calculators and microprocessors are common. The impact of LSI on calculators and computers is obvious. These devices have caused great changes in the industrial process control, automotive electronics and other fields where data acquisition, computation or controls are necessary. An exten-

sive vocabulary associated with microelectronics has developed.

SummaryAs electronics moves rapidly towards very extensive use of integrated-circuits arrays, the fundamental oper-ations with linear and non-linear active elements become increasingly important. Hence, primary emphasis continues to be placed on feedback, gain elements, modulation, frequency conversation, oscillation, and logic.

Electronic equipment is made up of three basic circuits, rectifiers, amplifiers and oscillators. There are also a few additional types of special circuits which are basically variations of the basic circuits. Finally there are only six commonly used types of electronic circuits, these are resistors, capacitors, inductors, transformers switches and valves (vacuum tubes diodes transistors).

SAFETYA technician will install, maintain, and repair electrical and electronic equipment in which dangerously high voltages are present. This work is often done in confined spaces. Among the hazards of this work are electrical shock, electrical fires, harmful gases which are sometimes generated by faulty electrical and electronic devices, injuries which may be caused by improper use of tools.

Because of these dangers, one should formulate safe and intelligent work habits since these are fully as import-ant as knowledge of electronic equipment. One primary objective should be to learn to recognize and correct dangerous conditions.

Electric Shock

Electric shock may cause burns of varying degree, cessation of breathing and unconsciousness, ventricular fibrillation or cardiac arrest and death. If a 60-hertz alternating current is passed through a person from hand to hand or from hand to foot, the effects when current is gradually increased from zero are as follows: 1. At about 1 milli-ampere (0.001 ampere) the shock will be felt. 2. At about 10 milli-amperes (0.01 ampere) the shock is enough to paralyze muscles and a persons may be unable to release the conductor. 3. At about 100 milli-amperes (0.1 ampere) the shock is usually fatal if it lasts for one second or more.

IT IS IMPORTANT TO REMEMBER THAT FUNDAMENTALLY, CURRENT, RATHER THAN VOLTAGE, IS THE CRITERION OF SHOCK INTENSITY.

It should be clearly understood that resistance of the body will vary. That is, if the skin is dry and unbroken, body resistance will be quite high, on the order of 300,000 to 500,000 ohms. However, if the skin moist or broken, body resistance may drop to as low as 300 ohms. Thus, a potential as low as 30 volts could cause a fatal current flow. Therefore, any circuit with a potential in excess of this value must be considered dangerous.

Care of shock victimsElectric shock is a jarring, shaking sensation resulting from contact with electric circuits or from the effects of lightning. The victim usually experiences the sensation of a sudden blow, and if the voltage is sufficiently high, unconsciousness, severe burns may appear on the skin at the place of contact; muscular spasm can occur, caus-ing a person to clasp the apparatus or wire which caused the shock can kill its victim by stopping the heart or by stopping breathing or both. It may sometimes damage nerve tissue and result in a slow wasting away of muscles that may be become apparent until several weeks or months after the shock is received.

The following procedure is recommended for the rescue and care of shock victims. 1) Remove the victims from electrical contact at once, but DO NOT ENDANGER YOURSELF. This can be done by : (1) throwing the switch, if it is nearby; (2) using a dry stick, rope, leather belt, coat, blanket or any other non-conductor of electricity; or (3) cutting the cables or wires to the apparatus, using a damage control axe while taking care to protect your eyes from the flash when the wires are severed. Even the victim’s shoes, if dry, can be used to pull hi clear. 2) Determine whether the victim is breathing. If so, keep him lying down in a comfortable position. Loosen the clothing about his neck, chest and abdomen so that he can breathe freely. Protect him from exposure to cold, and watch him carefully. 3) Keep him from moving about. After shock, the heart is very weak, and any sudden muscular effort or activity on the part of the patient may result in heart failure. 4) Do not give stimulants or operates. Send for a medical officer at once and do not leave the patient utilize has adequate medical care.

5) If the victim is not breathing, it will be necessary to apply artificial respiration without delay, even though he may appear to be lifeless.

Working on Energized Circuits

Insofar as is practical, a technician should NOT undertake repair work on energized circuits and equipment. However, if it should become necessary, as when making operational adjustments, then one should carefully observe the following safety precautions: 1) Ensure that you have adequate illumination. You must be able to see clearly, if you are to safely and properly perform the job. 2) Ensure that you are insulated from ground by an approved type rubber mat, or layers of dry canvas and/or wood. 3) Where practical, use only one hand, keeping the other either behind you or in your pocket. 4) If the system voltage exceeds 150 volts, rubber gloves should be worn. 5) An assistant should be stationed near the main switch or circuit breaker, so that the equipment may be immediately de-energized in case of an emergency. 6) A man qualified in first aid for electrical shock should be standing by during the entire operation. 7) DO NOT WORK ALONE. 8) Do not work on any type of electrical apparatus when wearing wet clothing, to if he hands are wet. 9) DO NOT wear loose or flapping clothes. 10) The use of thin-soled shoes and shoes with metal plates or hob nails should not be worn. 11) Flammable articles, such as cellulose cap visors should not be worn. 12)All rings, wristwatches, bracelets, and similar metal items should be removed before working on equipment. Also ensure that clothing does not contain exposed metal fasteners such as zippers, snaps, buttons and pins. 13) Do not tamper with interlock switches, that is, do not defeat their purpose by shorting them or block-ing them open. 14) Ensure that equipment is properly grounded before energizing. 15) De-energize equipment before attaching alligator clips to any circuit. 16) Use only approved meters and other indicating devices to check for the presence of voltage. 17) Observe the following procedures when measuring voltages in excess of 300 volts. •Turn off the equipment power. •Short circuit or ground the terminal of all components capable of retaining a charge. •Connect the meter leads to the points to be measured. •Remove any terminal grounds previously connected. •Turn on the power and observe the voltage reading. •Turn off the power. •Short circuit or ground all components capable of retaining a charge. •Disconnect the meter leads. 18) On all circuits where the voltage is in excess of 30 volts and where the deck, bulkheads, or work benches are of metallic constructions, the workers should be insulated from accidental ground by use of ap-proved insulating material.The insulating material should have the following qualities: •It should be dry, without holes, and should not contain conducting materials. •The voltage rating for which it is made should be clearly marked on the material, and the proper material should be used so that adequate protection from the voltage can be supplied. •Dry wood may be used, or as an alternative, several layers of dry canvas, sheets of phenolic insulating material, or suitable rubber mats. •Care should be exercised to ensure that moisture, dust, metal chips, etc. which may collect on

insulating material is removed at once. Small deposits of such materials can become electrical hazards. •All insulating materials on machinery and in the area should be kept of oil, grease, carbon dust, etc., since such deposits destroy insulation.

Safety shorting probe

ALWAYS ASSUME THAT THERE IS VOLTAGE PRESENT when working with circuits having high capaci-tance even when the circuit has been disconnected from its power source. Therefore, capacitors in such circuits should be discharged individually, using an approved type shorting probe. High capacity capacitors may retain their charge for a considerable period of time after having been disconnected from the power source.

When using the safety shorting probe, always be sure to first connect the test clip to a good ground (if necessary, scrape the paint of the grounding metal to make a good contact). Then hold the safety shorting probe by the insulating handle and touch the problem end of the shorting rod to the point to the shorted out. The problem end is fashioned so that it can be hooked over the part or terminal to provide a constant connection by the weight of the handle alone. Always take care not to touch any of the metal parts of the safety shorting problem while touching the probe to the exposed “hot” terminal. It pays to be safe; use the safety shorting probe with care. Some equipment are provided with walk-around shorting such as fixed grounding studs (or permanently at-tached grounding rods). When this is the case, these should be used rather than the safety shorting probe.

Working on De-energized Circuits

When any electronic equipment is to be repaired or overhauled, certain general safety precautions should be observed.They are as follows; Remember that electrical and electronic circuits often have more than one source of power. Take time to study the schematics or wiring diagrams of the entire system to ensure that all sources of power have been disconnected. If pertinent, inform the remote station regarding the circuit on which work will be performed. Use on hand when turning switches on or off. Safety devices such as interlocks, overload relays, and fuses should never be altered or disconnected ex-cept for replacement. In addition, they should never be changed or modified in any way without specific autho-rization. Fuses should be removed and replaced only after the circuit has been de-energized. When a fuse blows, the replacement should be of the same type and have the same current and voltage ratings. A fuse puller should be used to remove and replace cartridge type fuses. All circuit breakers and switches from which power could possibly be supplied should be secured (locked if possible) in the open or off (safety) position and tagged. After the work has been completed, the tag (or tags) should be removed ONLY by the same person who signed it when the work began. Keep clothing, hands, and feet dry if possible.When it is necessary to work in a wet or damp locations, use a dry platform or wooden stool to sit or stand on, and place a rubber mats or other non-conductive material on top of the wood. Use insulated tools and insulated flashlights of the moulded type when required to work on exposed parts.

CHAPTER 1

MEASURING INSTRUMENTS AND TESTING METHODS

1.1 Meters 1.2 The Cathode Ray Oscilloscope 1.3 Simple Component Testing

CHAPTER 1

[1.1] Measuring Instruments and Testing Methods

(1)MetersTo get information about the symptoms of a particular fault, a set of voltage readings at critical points in the circuit must be taken. This information, together with additional information on the circuit performance (i.e. distorted output, overheating component) is usually all that is necessary for correct fault diagnosis. So the only essential piece of test equipment for fault finding is a good, general purpose multi-range meter. This should have a resistance on d.c. ranges of at least 20 kΩ per volt. It is important that the meter has a relatively high resis-tance, otherwise the loading effect of the voltmeter could lead to incorrect conclusions. Also when measuring voltages in circuits that have fairly high resistances, the loading effect must be considered.Take for example the potential divider shown in Fig.1. The voltage across R2 should be 13.3V. If a meter of 20 kΩ on the 10V d.c. range is connected across R2 it will actually indicate nearly 10V. If a higher range of the meter is selected, the meter current is reduced, and a more accurate indication is given. It is always wise to select the highest possible range when measuring voltages in high resistance circuits.

An alternative to the moving coil meter is the small, portable digital multimeter. This displays the measured voltage, current or resistance on a three or more in-line digital display. The more digits used, the greater the accuracy of the reading. The input resistance of these instruments is typically 10 MΩ, which means that the unit takes only a small current from the circuit being measured. It seems that the instrument will replace the moving coil type meter because of the accuracy, readability and high input resistance. However, unless otherwise stated, in all the exercises in the following chapters the measurements have been made using a standard moving coil type meter, mainly because of the ready availability of this instrument.

[1.2] The Cathode Ray OscilloscopeAmong other useful instruments the next important from the point of view of fault finding is the cathode ray oscilloscope (CRO). This is perhaps the most versatile measuring instrument available. With it, it is possible to measure d.c. and a.c., voltage, current, phase-angle, and a whole range of other quantities. The accuracy depends to a great extent upon the care paid to the instruments’ calibration, and in most modern oscilloscopes signals for calibration are built in. The typical input impedance of a CRO is 1 MΩ which has a capacitance of about 20 pF in parallel with it. The input impedance can always be increased by using a special probe unit. A probe is simply a test lead which contains either a passive or an active network at its end or at some point along the lead. The straightforward voltage-divider probe is a basic attenuator with good frequency compensation. The latter is usually adjustable and should be checked before use. A disadvantage of this arrangement is that the signal attenuation is high typically 10:1 or 100:1; this is why the probes are called x10 or x100.The “heart” of an oscilloscope is the cathode ray tube (CRT). This consists of an electron gun, a deflection system and a fluorescent screen. A high velocity, finely focused electron beam is produced by the electron gun. This beam passes between two sets of plates arranged at right angles. Voltages applied to these plates deflect the beam both horizontally and vertically. The beam finally strikes the screen and a fine point of light is produced. This spot of light can be moved to any part of the screen by applying signals to the horizontal and vertical de-flection plates. These signals are produced from the Y-amplifier and the timebase. A signal to be measured is applied to the Y-input of the CRO, is attenuated by the switched attenuator (the Y-amplitude control), then amplified by the Y-amplifier and applied to the vertical plates of the CRT. At the same time the timebase units is triggered to produce a sawtooth signal that, when applied to the horizontal plates, causes the spot to move across the screen at a uniform rate and then fly back and repeat the process. The result is that a bright trace of the input signal appears on the screen.This trace can only be held stationary if the trigger control on the CRO timebase is correctly set. For a single beam CRO there are two possible triggering modes: external or internal. The external position should be se-lected only when a trigger signal is available; this feature can be extremely useful when measuring the time or phase relationship between two signals, as will be seen later. The normal mode for the trigger is to select inter-nal. To hold the trace, switch the trigger select switch to INT and then adjust TRIG. LEVEL (or TRIG. STABIL-ITY) until the trace locks. Suppose we wish to measure the frequency and amplitude of an unknown sine wave signal. The CRO is set up with no input so that first of all the trace is located (some instruments incorporate a beam finder for this pur-pose). The BRILL and FOCUS controls should be set to give a clear fine line on the screen. The signal to be measured is applied to the Y-input as shown in Fig.2 and the Y-amplitude control and the TIME switch set until the signal can be easily measured. In the example the Y-amplitude control is at 2V/cm and the time switch is at 0.1 ms/cm. Therefore the unknown signal has an amplitude of 5V peak and a periodic time of 0.20ms. The frequency is then given by

As stated previously the CRO is a highly versatile instrument but always make sure that it is calibrated correctly and set to a calibrated position.Many modern CROs have double beams which can be used to display two time-related signals. An example is given in Fig.3 where the signals from an astable oscillator are shown. Only one channel can be used to trigger the Timebase so an additional switch is included that allows one to select either Y1 or Y2 for internal trigger. A single beam CRO can also be used for measurements of phase between signals by applying one signal to the external trigger of the timebase while the other is fed to the Y-input.t

[1.3] Simple Component TestingWhen an instrument is being serviced and the checks indicate that a certain component is suspect, it is then necessary to confirm the fault. Often simply replacing the component is a sufficient check, but it is always good practice to test the faulty component to verify the type of fault. This useful for a number of reasons, the most important being the collection of data on component failures. A fault may be caused by defects in component manufacture, a design error, poor production methods, or ageing. Thus, for example, if a large number of com-ponents are failing open circuit, the manufacturer will need to be informed so that future defects can be avoided.Tests to confirm open or short circuit conditions can easily be made using the ohms range of a multi-range me-ter, but while checking for an open circuit it is usually wise to unsolder and lift one end of the component before making the measurements, otherwise other components that are in parallel with the suspect component will give a false indication of the resistance. An alternative method of checking for an open circuit resistor is to “bridge” the suspect component with a known good one, and then recheck the circuit conditions.“Leaky” capacitors can also be tested using an ohmmeter, again by disconnecting one end of the capacitor from the circuit. A good electrolytic should indicate a low resistance initially as the capacitor charges, but the resis-tance should rapidly increase to approach infinity. Open circuit capacitors are best confirmed by placing another capacitor of the same value in parallel and checking circuit operation, or by removing the capacitor and testing it on a simple laboratory set-up as shown in Fig.4 A using a low frequency generator at 1 kHz and two meters. Here Cx = I/2πfV0 with an accuracy of better than ±10% for values from 1000pF to 1µF. An even better method is to use a simple a.c. bridge as shown in Fig.4 B to compare the unknown capacitor against a standard.Tests on diodes, transistors and other semiconductor devices can also be made using the ohms range of a multi-meter. First it is necessary to determine how the internal battery in your multimeter is connected. For example, in one typical instrument the common terminal (marked black) has a positive voltage on the resistance range. If you do not know the connections for the particular meter you are using, the polarity can be determined by connecting the multimeter (on ohms range) to an electronic voltmeter, or by measuring the forward and reverse resistance of a semiconductor diode of known polarity. See Fig.5.Having established the ohmmeter lead polarity, you can discover a great deal about a transistor. First identify the device leads if not known (see Fig.6). Measure the forward and the reverse resistance between pairs of leads until you find two that measure high (over 100 kΩ) in both directions. These must be the collector and emitter (provided the transistor is a good one). The remaining lead is the base. Now measure the resistance from base to one of the other transistor leads; it should be low in one direction (1 kΩ) and high (greater than 100 kΩ) in the other. If the low resistance occurs when the ohmmeter lead with the positive voltage is connected to the transis-tor base, the transistor will be n-p-n type. Of course, it will be the other way around for p-n-p.The above check also tests that both emitter base and collector base junctions in the transistor are good. If either junction shows up high resistance in both directions, it is open circuit; and low resistance in both directions, it is broken down.When testing components, and in particular transistors, FETs and ICs ALWAYS: 1) Check for power supplies near the actual components, and in the case of ICs directly on the appropri-ate pins. 2) Do not use large test probes because they can easily cause shorts. 3) Avoid the use of excessive heat when unsoldering a component and do not unsolder with the unit switched on. 4) Never remove or plug in a device without first switching off the power supply. Components can be damaged easily by the excessive current surges.

The complete circuit of most electronic instruments can be broken down into a series of functional blocks; for example in a general purpose sine wave generator these would be power supply, variable sine wave oscillator, buffer amplifier, and output attenuator. By treating the instrument in blocks, rather than as a whole, it is pos-sible to narrow down the search for a faulty component first of all to one block, then by measurements within the block to locate the actual faulty component. The methods used to decide which block is faulty are: a) Input to output (or beginning to end). b) Output to input. c) Random. d) Half-split.All of these have their particular advantages and uses. The RANDOM METHOD, which implies a totally non-systematic approach, is rarely used. A method based on the reliability of components can also be used when there is a wealth of service knowledge and experience concerning a particular instrument. For exam-ple a service engineer might make the reasonable assumption that, because a particular electrolytic capacitor has been at fault in 60% of the instruments recently returned, it is a strong possibility that the next faulty instrument also has a faulty electrolytic capacitor. He would naturally check this first, and in most cases save valuable service time. It must be stressed, however, that this method depends upon the availability of a large amount of data on the reliability of the various components within an instrument. Most service engineers would use a logical systematic approach to system fault location.

The INPUT TO OUTPUT and OUTPUT TO INPUT methods are examples of this systematic approach. The method is fairly obvious. A suitable input signal (if required) is injected into the input block and then mea-surements are made sequentially at the output of each block in turn, working either from the input towards the output or from the output back to the input, until the faulty block is located. This logical method is the one most service engineers use on equipment containing a limited number of blocks. The HALF-SPLIT method is very powerful in locating faults in instruments made up of a large number of blocks in series. Take for example a superhet radio receiver shown in Fig.7. Since there are eight blocks it is possible to divide the circuit in half, test that half, decide which half is working correctly, then split the non-functioning section into half again to locate the fault. An example is the best way of really understand-ing the method. Assume that a fault exists in a demodulator of the receiver, the sequence of tests would be as follows: Split in half, inject signal into the input of (1) (the aerial circuit), and check output at (4) (IF). Output correct. Therefore the fault is somewhere in blocks (5) to (8). Split blocks (5) to (8) in half by checking output to (6). Input signal can be left at (1). No output. Leaving signal at (1), check output from (5). Output should be correct, indicating that the faulty block is (6), the demodulator.

You can try this method for yourself by assuming that the fault is in block (3) for example, and you will find that the number of checks necessary to locate the fault is still three. On average, four tests would be required by using the input to output technique. The half-split method is most useful when the number of components or blocks in series is very large, for example where several series plug and socket connections are used, or for heater chains in valve equipment. There are, however, several assumptions made for the half-split: (a) that all components are equally reliable; (b) that it is possible and practical to make measurements at the desired point; and (c) that all checks are similar and take the same amount of time. These assumptions will not always be valid and it is up to the service engineer to then decide the best method of approach.The half-split method can also be easily complicated by: An odd number of series units. Divergence: an output from one block feeding two or more units. Convergence: two or more inputs being necessary for the correct operation of one unit. Feedback: which may be used to modify the characteristics of the unit or in fact be a sustaining network for an oscillator.

When using any of the methods as described, try and use the method, or a combination of them, that will locate the faulty block in a system in the shortest possible time.

CHAPTER 2

ECG SEMICONDUCTORS – REPLACEMENT PROCEDURES

2.1 Universal Replacements2.2 Replacement Techniques 2.2.1 Forming Pins 2.2.2 Mounting 2.2.3 Soldering2.3 Mosfet Handling Precautions2.4 CMOS Handling Precautions2.5 Selecting a Bi-Polar Transistor for an unlisted type2.6 SCR, TRIAC, Rectifier and Bridge Replacement2.7 Personalized Service for Unlisted Types2.8 Testing Solid-State Devices2.9 Testing Bi-Polar Transistors2.10 Testing Field Effect Transistors2.11 Testing Diodes2.12 Testing SCR’s and Triacs (Thyristors)2.13 Symbol, Terms and Definitions

ECG Semiconductors Replacement Procedures

[2.1] Universal Replacements 1. ECG Semiconductors can be used with confidence because they are specially selected prime parts whose specifications generally exceed those of the original part or application. A maximum number of replacement requirements can be satisfied with a minimum inventory of ECG Semiconductor devices. For example, ECG125 diode can be used to replace more than 625 JEDEC (1N) types (1N867, 1N4001, 1N4003, 1N4011, etc.) and ECG123A transistor can be used to replace more than 200 (2N) types (2N708, 2N708A, 2N2096A, 2N3115, etc.) plus thousands of other standard industry transistor types.

2. The frequency of equipment repair can be reduced by upgrading with ECG Semiconductors. For example, if a 100 volt, 15 amp rectifier is frequently replaced in an important control unit, replace it with a higher voltage and/or higher current part, as explained in rectifier replacement section. Or, if the device is overheating, addi-tional heat sinking may be required.

3. ECG Replacement Semiconductors are available through an ‘ international network of electronic distributors. These distributors provide local availability of ECG replacement semiconductors to help keep equipment down-time at a minimum.

[2.2] Replacement Techniques [2.2.1] Forming Pins When replacing the original device with an ECG unit, certain mechanical and electrical requirements must be observed. Compare the lead or terminal arrangement of the original part with the ECG replacement. If neces-sary, bend the leads to the proper basing arrangement and insulate them to prevent possible shorts. For socketed devices, cut the leads on the replacement to proper length. Check the outline dimensions of the replacement if mounting space is a problem. Replacement in untuned stages can generally be made with a minimum of effort.

1. To avoid pulling when bending a plastic device pin, always restrain the pin firmly by holding with a pair of needle-nosed pliers or tweezers located at least 1/8—inch away from the device body. Internal connections of the pins to their junctions may be disrupted if this procedure is not observed.

2. If pins must be spread apart, bend the pin only in the narrow portion of its length, again restraining the pin during bending as per Step 1.

3. Maintain a bend radius of at least 1/16-inch.

4. Do not repeatedly bend pins. Insulate pins if necessary to avoid short circuits.

5. Mount the device before soldering to leads.

[2.2.2]Mounting

(See Illustration)

1. Use the proper mounting hardware, such as insulating bushings, insulating washers, etc., called for by the specific application.

2. To promote efficient heat transfer, insulating washers, when required, should be thin (typically three mils for a mica washer). Use a thin, even layer of ECG424 Heat Sink compound on both sides of the washer.

3. Exercise care that the tool used to tighten the transistor retaining nut or bolt does not contact the device body.

4. Unless otherwise specified do not exceed six inch-pounds torque (an average male can exert about 50—inch pounds torque with a screwdriver).

5. Avoid exerting pull of the device pins while connecting them to their circuit connections. Provide a means of strain relief if these connections tend to place pulling forces on the pins.

6. To extend transistor life in high-power dissipation applications where a large thermal interface is not provided by the original equipment, the use of a heat sink is recommended when space permits. The ECG line includes a variety of transistor heat sinks, insulator kits, and heat sink compound for this purpose.

Soldering [2.2.3]

1. Solid-state circuitry is generally miniaturized and is most often fabricated on a printed-circuit board. Any repairs to this type of circuitry require a well-tinned, pencil-type soldering iron. Care must be taken in soldering, both to prevent damage to the printed circuit board and to make sure that the solid-state device itself is not over-heated. A good quality 60/40 (60% tin, 40% lead) solder helps to make joints quickly with a minimum of heat. When solid-state devices must be removed or installed, some means must be provided to conduct heat away from their junctions. Long-nose pliers or hemostats may be used as a heat sink.

2. Leakage between the heating element and the soldering-iron tip can cause the tip to be above ground poten-tial. This leakage voltage may cause transistor damage if the chassis has a return to ground. To be on the safe side when soldering or unsoldering solid-state devices (especially non-gate protected MOSFET’S), it is recom-mended that a flexible grounding strap be connected from the metal neck of the soldering iron to a good ground.

3. The same considerations about line leakage also apply to the use of oscilloscopes and signal generators. Even though the equipment may have a power transformer, the “line-filter” capacitors which are usually connect-ed between the transformer primary and chassis ground permit an ac flow between chassis ground and earth ground. Therefore, if you connect the ground lead of such test equipment to a sensitive point in transistorized equipment having an earth ground, damage may result.

4. After soldering the connections using good solid-state practice, complete the replacement job with a check of the bias, following the procedures provided by the equipment manufacturer. [2.3] Mosfet Handling Precautions

In handling non-gate protected MOSFETS, such as the ECG220 and ECG221, the following precautions should be observed:

Prior to assembly into a circuit, all leads should be kept shorted together with the metal spring. When devices are handled, the hand being used should be at ground potential. Tips of soldering irons should be grounded. Devices should never be inserted into or removed from circuit with power on. Gate-protected devices, such as the ECG222, incorporate special back-to-back diodes that are diffused directly into the M.O.S. pellet and are electrically connected between each insulated gate and the FET’s source. These diodes effectively bypass any voltage transients which exceed approximately ±10 volts and protect the gates against damage in all normal handling and usage.

[2.4] CMOS Handling Precautions

The input protection networks incorporated in all CMOS devices are effective in a wide variety of device han-dling situations. To be totally safe, however, it is desirable to restate the general conditions for eliminating all possibilities of device damage.

CMOS devices may be damaged if exposed to high static charges. The handling procedure shown below should be followed in order to assure against damaging the devices: 1. The leads of the devices should be shorted out with some type of conductive material except when being test-ed or when actually in the circuit. This will prevent build up of static charges. 2. All tools, jigs and fixtures, soldering-irons and any type of handling device should be grounded. 3. Transient voltages can damage CMOS too. Therefore, units should not be plugged into, or removed from, circuits while power is still on. Signals should not be applied if the power to the device is off. 4. If a lead is not used, it must be either grounded or connected to the device power supply. Which one will depend on the logic circuit involved. Table I indicates general handling procedures recommended to prevent damage from static electrical charges.

Total protection results when personnel and materials are all at the same or ground potential.

[2.5] Selecting a Bi-Polar Transistor For An Unlisted Type If the transistor to be replaced is unmarked or not listed in the ECG Replacement Guide, the following proce-dure can be used to make an accurate replacement selection.Step-by-step determine each of the following parameters: 1. Polarity NPN or PNP2. Type of material v silicon or germanium 3.Operating frequency range 4. Maximum voltage, collector to emitter 5.Maximum voltage, collector to base 6. Maximum collector current 7. Maximum power dissipation 8. Current gain 9. Case packaging 10. Lead configuration

Step 1 Is it an NPN or PNP device? Your first source of information would be the schematic drawings. If the arrow on the emitter of the transistor symbol is pointing toward the base, you know it is a PNP device, or if it is pointing away from the base, it is an NPN.

Now let‘s say for argument sake that the schematic has been drawn incorrectly or suppose you have no sche-matic, your next clue would be to determine the polarity of the voltage between the emitter and collector. If the collector voltage is positive with respect to the emitter voltage, then it is a NPN device. If the collector voltage is negative with respect to the emitter voltage then it is a PNP device. Therefore, if the VCE (collector to emitter voltage) is positive, it’s NPN, or if the VCE is negative, it’s PNP. An easy way to remember the polar-ity of the collector voltage for each type is:

Step 2Next you must decide whether the device is silicon or germanium. This is most effectively done by the use of a schematic drawing. If you find that the D.C. bias voltage level between the base and emitter is 0.2 volts or less, it is probably a germanium device. Now, if you get a bias voltage reading of 0.4 volts or more, it is probably a

silicon device. There will, of course, be some cases where there will be either no bias or reverse bias voltage present. This would be the case in oscillator and sync clipper circuits. Another way of determining the type of material of a transistor is to look at the complexity of the circuit with respect to the number of components in a single stage. Germanium circuits are much more complex due to ger-manium’s unstable nature with respect to temperature changes and its high leakage currents, therefore, several components are required per stage to make it stable. Voltage dividers are used to compensate the bias voltage and an emitter limiting resistor is always used. A silicon device, however, is very stable with respect to gain when temperature changes and silicon has very low leakage cur- rents. Therefore, a single silicon amplifier stage may only consist of one resistor for base bias and one load resistor. Some circuits using silicon still have more compensating components than they require but as design engineers have become more aware of the stability of silicon, circuits have become simpler.

Step 3 After you have determined the polarity and type of material of a device, you next determine the operating fre-quency range of the circuit in which it is used. This is done mainly by identifying the type of circuit and wheth-er it is working in the audio range, the kilohertz range or the megahertz range.

Step 4 Next you look at the Schematic or the circuit and determine the maximum collector to emitter voltage present in the circuit. In most cases, it is best to use the supply voltage figure as your reference. This figure will then be used to select a replacement device, which has a collector to emitter breakdown voltage at least slightly higher than the supply voltage. Preferably the higher the collector to emitter breakdown voltage, the better.

Step 5 Next the collector to base maximum voltage must be determined. If you have determined the collector to emitter maximum voltage requirements, then you can use this figure for the collector to base maximum voltage require-ments.

Step 6 The sixth step is to determine the maximum collector current. To do this you would consider the DC condition with the device fully on, which of course would give you the highest current required of the device.

Step 7 Now that you have determined the maximum voltage and collector current requirements, you can use them in determining the maximum power requirements. However, the type of circuit where the device is used is the major factor with respect to power dissipation. Here we have some general wattage ranges for different circuit types. 1. Input stages, AF or RF, 50-200 milliwatts 2. IF stages and driver stages, 200 MW - 1 watt 3. Higher power output stages 1 watt and up

Step 8 Next you determine the gain expected from the circuit. This is determined primarily by its application in a cir-cuit. Some typical gain categories are:

RF, Mixers, IF and AF 80 - 150 RF and AF Drivers 25 - 30 RF and AF Output 4 - 40

High Gain Preamps and Sync Separators 150 - 500

Step 9 Next you determine the case packaging and over-all dimensions. (Case type and size need only be considered where an exact mechanical fit is required. Otherwise, if you can fit the device into place even though it is not the exact case, it will probably do. the job as well as the original. Some high frequency circuits may require exact replacements but even these circuits will require some alignment touch-up, especially in the case of UHF circuits.)

Step 10 Lastly you note the lead configuration. (Lead configuration generally is not a prime consideration for replace-ment transistors although it may be desirable for ease of insertion and appearance.)

With the above 10 parameters determined, the application selector guides in the transistor section can be used to find an accurate replacement device.

In an emergency situation where a replacement is listed in the ECG Semiconductor Guide but you do not have the recommended type in stock, this procedure could be used to determine alternate ECG replacements with higher ratings or different case or lead configurations.

[2.6] SCR, Triac, Rectifier and Bridge Replacement

A substitute rectifier, SCR, triac or bridge can be used as long as it is: 1. Equal to or greater in current rating 2. Equal to or greater in voltage rating 3. In a similar case 4. Has (for SCR’s and triacs) the same or lower IGT rating 5. Has equal or faster switching speed

The important factor in point 3 is that for proper heat transfer, substitutes for stud type devices should have the same stud size this, so you can simply take out the defective device and install the new device without having to drill a new size hole in the heat sink. In the case of axial lead devices, however, you may substitute if the case is equal to or smaller than the original device.

Not all ECG devices can be substituted in the above manner. The above-mentioned five substitution factors apply to SCR’S, triacs, rectifiers, and bridges only. Voltage regulation zener substitutions are somewhat less flexible, since each zener is manufactured for, and used as, a specific voltage regulator you cannot substitute a different zener voltage.

[2.7] Personalized Service For Unlisted Types

Upon the user’s request, the ECG Semiconductor Field Engineering Department will try to cross reference any type number or part number not in this ECG Semiconductor Replacement Guide. Sometimes this is possible, sometimes not. However, the chance of success is greatly increased as the amount of information to work with goes up. Therefore, when requesting a new cross reference, please give as much of the information requested in Table II as possible.

______________________________________________________________________________Table II

1. Type Number 2. Part Number 3. Number on Device 4. Type of Device (i.e., Diode, Rectifier, PNP or NPN Transistor-Silicon or Germanium, SCR, Triac, Integrated Circuit, etc.)5. If IC, Specify Digital, Linear, or Other Description 6. Case Style (i.e., TO-5, DIP, 3/8” Stud, etc.) 7. Number of Leads 8. Application of Device (i.e., TV, Motor Control, Battery Charger, Arc Welder, Tape Deck, CB, Scanner, Auto Radio, etc.)9. Manufacturer’s Name 10. Trade Name 11. Chassis Number 12. Model Number 13. Description of Device Function ______________________________________________________________________________Inquiries for such cross reference information should be sent to: ECG Semiconductor, Field Engineering Philips ECG 1025 Westminster Drive Williamsport, Pennsylvania 17701

[2.8] Testing Solid-State Devices PrecautionsOhmmeters - Ohmmeters must be used with great care in transistor circuits. It must be remembered that the ohmmeter has an internal voltage source. Also, some instruments are capable of delivering high currents. Meter test probes with sharp points facilitate checks on printed- circuit boards. They minimize the danger of accidentally bridging adjacent conductors. Also, the needle points easily pierce resin, varnish, or surface corro-sion on the conductors. False readings are often the result of not making good connections on the printed-circuit board.

ln-circuit measurements are often misleading because of the shunt paths provided by transistor junctions that be-come forward biased by the ohmmeter’s supply. An example is shown in Fig.A. With the ohmmeter connected in this fashion, the internal battery places a forward bias on the emitter junction of the transistor. This effectively places RE in shunt with R2.

In addition to shunt paths provided by transistor junctions, the physically small electrolytic capacitors employed as coupling and bypass capacitors also provide low-resistance shunt paths if the ohmmeter’s internal supply should polarize them incorrectly. These components can be permanently damaged by even a low voltage of incorrect polarity. Damage can also be caused if the polarity is correct, but the working voltage of the capacitor is exceeded by the ohmmeter’s supply. Some capacitors are rated at only 3 volts, while the internal supply of many ohmmeters is 7.5 volts and may run as high as 30 volts. (Most electronic VOM’s employ a 1.5-volt supply and are therefore always safe as far as voltage breakdown is concerned.)

As a general rule, ohmmeter measurements in the transistor circuit should be made by disconnecting one lead of the component to be checked. This removes any possibility of a shunt path. However, the reading in Fig.A could be made if the ohmmeter leads are reversed, in which case the emitter junction becomes reverse biased. This requires a knowledge of the polarity of the ohmmeter voltage at the meter’s test leads.

Ohmmeter connections that reverse-bias transistor junctions sometimes run the risk of exceeding the breakdown potential of the junction. This is particularly true in the case of the emitter junction, which breaks down at lower reverse voltages. If the V(BR)BO of the transistor is not known and if a battery voltage of several volts or more is used in the ohmmeter, it is best to disconnect the transistor.

Ohmmeter readings that intentionally or accidentally forward bias a transistor junction also run the risk of caus-ing excessive current flow through the transistor. The forward-biased junction is practically a short, so that the total current flowing is determined mainly by the ohmmeter’s voltage supply and its internal resistance. Many ohmmeters, including electronic VOM’s, supply a short-circuit current of 100 mA when used on the Rx1 scale. This current can damage many transistors. To prevent any danger of damage, only use those resistance ranges where the short circuit current is below 1 mA. For most service-type instruments, use of the Rxl00 and Rxl0k ranges is safe. Do not read forward-bias currents on the in scale.

Summing up, ohmmeter readings require some judgment before they are made. You need to know three things about the ohmmeter before making measurements: the polarity of the voltage at the leads, the voltage of the internal battery, and the short-circuit current. Also, unless shunt paths can be definitely eliminated by proper polarization, one lead of the component to be checked must be disconnected.

Identifying Leeds on Unmarked Transistors - Occasionally, identifying marks may be obliterated on the tran-sistor case. The leads may then be identified with the few ohmmeter checks shown in Fig. B. In Step 1, ohmme-ter checks are made between each pair of leads in both the forward and reverse directions. Low readings (below 500 ohms) will be found when the ohmmeter places a forward bias across emitter and collector junctions. The highest forward reading is obtained when the meter is placed across the emitter and collector leads. This check establishes the base lead as the one that is not involved in the high forward-resistance reading.

Step 2 identifies the transistor type. An ohmmeter check is made between the base and one other lead. If a low-resistance reading is obtained when the negative side of the ohmmeter is connected to the base, the transis-tor is a PNP type. A low resistance reading when the base is positive indicates an NPN unit.

(2.9) Testing Bipolar Transistors

A transistor that is operated within its ratings with respect to voltage, power dissipation, and temperature is nor-mally expected to have an almost unlimited life. Failures in transistorized circuits are more often the result of damage or malfunctioning of some other component. This is particularly true when miniature transformers and electrolytic capacitors are employed. Despite the reliability of the transistor itself, failures occur due to shorts or opens in the bias circuitry, temporary overloads, physical damage, or even mishaps while servicing.

A great number of transistor testers and analyzers are available. Some only check leakage and current gain, while others are capable of measuring all of the transistor parameters. From a servicing viewpoint, a few simple tests are enough to reveal a great majority of troubles. These tests, to be described, reveal shorts, opens, exces-sive leakage, and provide a rough check of current gain. Fortunately, little equipment is required. Some of the tests require only an ohmmeter. The more elaborate checks can be made with just a few additional components.

Testing the Junctions - The transistor contains two p-n junctions or diodes. Most of the characteristics of the transistor are tied in with the behavior of the junctions, while the rest of the device simply serves as connective material. Damage to the transistor, therefore, almost always shows up as a malfunctioning of one of the rectify-ing junctions. The fault may be an open or shorted junction, or excessive reverse current (leakage).

A rough but useful check of the condition of the junctions may be made with an ohmmeter. First, the forward resistance of each junction is measured, as shown in Fig. C. In this figure, the connections for a PNP transis-tor are shown. The negative terminal of the ohmmeter is connected to the base. The forward resistance of both junctions is checked by touching the emitter and then the collector terminal in turn with the positive lead. A high reading indicates an open junction. A normal unit should show a reading below 500 ohms. Observe the precau-tions given earlier for using the ohmmeter. The forward resistance of the junctions of an NPN unit is checked with the same setup shown in Fig. C, but with the leads to the ohmmeter reversed.

To check for shorts or excessive leakage, reverse the ohmmeter connections and switch to a higher resistance scale, as shown in Fig. D. Now the ohmmeter places a reverse bias on each junction in turn, and leakage current is registered on the meter. A low resistance reading indicates a shorted or leaky junction. Low and medium-pow-er germanium transistors should show a resistance reading of at least 500 kilohms.

Typical readings taken with an ohmmeter on the Rxl0k scale are 700 kilohms to 1.5 megohms. Silicon transis-tors give much higher resistance readings. Power transistors have larger junctions and therefore greater leakage currents. Reverse-bias resistance readings should be 50 kilohms or greater for power transistors.

Reverse-resistance checks on NPN transistors are made by reversing the ohmmeter leads so they are opposite to that shown in Fig. D. Note that the actual numerical reading in ohms is meaningless, because the ohmmeter can only measure linear resistances. The specific ohms-reading changes from meter to meter and is not the same for different settings of the range switch. The minimum and maximum values given here apply in the majority of cases. To increase the accuracy of the ohmmeter check, the readings should be compared with those made on a known good transistor of the same type.

Current Gain - Transistor action may be checked with an ohmmeter by means of the setup shown in Fig. E. The meter registers ICEO before the 500-kilohm resistor is touched to the base. Connecting the resistor allows a small base bias to be applied, and the meter shows an increase in current (decrease in resistance reading).

[2.10] Testing Field-Effect Transistors

Testing field-effect devices is somewhat more complicated than testing bipolars and must take into account the following: 1. Is the device a JFET or MOSFET? 2. Is the FET an n-channel or p-channel type? 3. If the device is a MOSFET, is it an enhancement or depletior type?

Do not attempt to remove from the circuit or handle a FET unless certain that the device is a JFET or an insulat-ed-gate-protected MOSFET. This verification is essential, because an uninsulated-gate-protected MOSFET may be damaged unless proper handling precautions are taken. When handling or inserting or removing this type of MOSFET, the following should be observed: 1. Prior to assembly into a circuit, all leads should be kept shorted together by either the use of metal shorting Springs attached to the device by the vendor, or by the use of a conductive foam. Note - polystyrene “snow” should not be used because it can acquire high static charges.

2. When devices are removed by hand from their carriers, the hand being used should be at ground potential.

3. Tips of soldering irons should be grounded.

4. Devices should never be inserted into or removed from circuits with the power on.

Testing the JFET — The forward resistance of a JFET can be checked with a low-voltage ohmmeter, preferably on the Rx100 scale. Connect the positive lead to gate and the negative lead to the drain or source, if an n-chan-nel JFET. Reverse the leads if a p-channel type.

To test the reverse resistance of an n-channel JFET, connect the negative lead of the ohmmeter to the gate and the positive lead to the drain or source. The device should show almost infinite resistance. Lower readings indi-cate either leakage or a short. Reverse the leads to test a p-channel device.

Testing the MOSFET - The forward resistance and reverse resistance can be checked with a low-voltage ohm-meter on the highest “R” scale. The insulated-gate MOSFET has an extreme high input resistance. Hence, we should obtain almost infinite resistance readings for both forward and reverse resistance testing, between gate and drain or source. Lower readings indicate a breakdown in the insulation between gate and drain or source.

[2.11] Testing Diodes

Diodes and Rectifiers - Because diodes and rectifiers are non-amplifying devices, simple tests for shorts, opens, or excessive leakage are useful methods to determine if they are functioning properly. The following tests are not applicable to the special case of focus diodes and high-voltage triplers, however. The forward resis-tance of a diode or rectifier is checked by connecting the positive and negative leads of an ohmmeter, preferably set to the Rx100 scale, to their respective positive (anode) and negative (cathode) terminals. A reading of about 500 to 600 ohms is normal for silicon types, about 200 to 300 ohms for germanium types, and for larger-type rectifiers (germanium or silicon) the resistance is somewhat lower than their respective diode types. Because high-voltage types may have several diodes in series, higher resistance readings can be expected.

As a quick go/no-go test, the ohmmeter procedure just described is a good technique.

To check for shorts or excessive leakage, switch to a higher resistance scale and reverse the ohmmeter leads. A

low resistance reading indicates a short or leaky device. Germanium diodes should show a resistance reading from about 100 kilohms to 1 megohm. Silicon diodes show higher resistance readings and can go up to 1000 megohms. However, some diodes may show lower resistances but function satisfactorily in some circuits. Recti-fiers, because they generally have larger junctions, have higher leakage currents. ‘

Zener Diodes - To quickly determine if Zeners have opens, shorts, or leakage, connect an ohmmeter in the for-ward direction in the same manner as described for standard diodes.

However, these tests, although helpful, do not provide the primary information needed for a Zener diode, name-ly, is the device regulating at its rated value? A regulation test is accomplished with a metered adjustable power supply that preferably indicates voltage and current.

Connect the output of the power supply through a limiting resistor in series with the Zener diode to be tested and slowly increase the output voltage until the specified current is flowing through the Zener (see Fig. F). Now connect a voltmeter across the Zener to monitor the Zener voltage. Fluctuate the current on either side of the specified Zener current; if the Zener is Operating properly, the voltage should remain constant.

[2.12] Testing SCRs and TRIACs (Thyristors)

The functional testing of SCRs and TRlACs usually requires test equipment capable of supplying the specified gate current (IGT) and minimum hold current of the Thyristor. These parameters are given in chart form in the SCR and TRIAC sections of this book.

Testing with an ohmmeter is not recommended for high current Thyristors and should only be used for relative indications in low current Thyristors. The IGT and IHold parameters of the Thyristor may exceed the source current capability of the ohmmeter causing false readings and therefore, may not always indicate the true func-tion of the device.

However, a simple ohmmeter test on low power Thyristors may provide an approximate evaluation of their gate-firing capabilities by connecting an ohmmeter as shown in Fig. G. The negative lead is connected to cath-ode and the positive lead is connected to anode.

Using the Rx1 scale, short the gate to the anode. A reading of approximately 15 to 50 ohms is normal. Note: When the gate- to-anode short is removed, the same reading should still show on the meter until the leads are removed from cathode or the anode. Now, reconnecting the meter leads to cathode and anode should show no reading until the gate is again shorted to the anode.

Testing of Gate turn off SCRs requires special test equipment. Field testing of these SCRs is difficult and typi-cally provides erroneous results.

Diodes, Rectifiers, Thyristors

Ct - Total Capacitance - The Total Small-Signal Capacitance Between The Diode Terminals.

di/dt - Rate Of Change Of Current Versus Time.

dv/dt - Rate Of Change Of Voltage Versus Time.

IF - Forward Junction Current - The Value Of DC Current That Flows Through A Semiconductor Diode 0r Rectifier Diode In The Forward Direction.

IFRM - Peak Forward Current Repetitive Peak - The Peak Value Of The Forward Current Including All Repetitive Transient Currents.

IFSM - Forward Surge Peak DC Current - Maximum (Peak) Surge Forward Current Having A Specified Waveform And A Short Specified Time Interval.

IGT MIN - Gate Trigger Current - Minimum Gate DC Current Required To Trigger The Device Under The Conditions Specified.

IGO MAX - Peak Gate Turn-Off Current - Maximum Negative Gate Current Required To Switch on.

IH - Holding Current - Anode Current Necessary To Maintain On-State.

IO - Average Rectifier DC Forward Current - The Value Of The Forward Current Averaged Over A 180° Conduction Angle At 60 Hz.

IR - DC Reverse Current - Value Of DC Current That Flows Through The Code In The Reverse Direction. (Leakage Current.)

IRM - Maximum Reverse DC Current - The Respective Value Of Current That Flows Through The Junction In The Reverse Direction.

Itrms - Continuous On-State Current.

Itsm - Surge (Non-Repetitive) Peak On-State Current - A Surge Current Of Short Time Duration.

LS - Series Inductance - The Inductance Between The Terminals On The Diode.

PRV - Peak Reverse Voltage - Maximum Repetitive Peak Reverse Blocking Voltage That May Be Applied To The Anode-Cathode Of The Device.

RS - Series Resistance - The Total Small Signal Resistance Between The Diode Terminals

TA - Ambient Temperature - The Air Temperature Measured Below A Device, In An

Environment Of Substantially Uniform Temperature, Cooled Only By Natural Air Convection And Not Materially Affected By Reflective And Radiant Surfaces.

TC - Case Temperature - The Temperature Measured At A Specified Location On The Case Of A Device.

TJ - Semiconductor Junction Temperature.

TQ - Turn Off Time.

Trr - Reverse Recovery Time - The Time Required For The Current Or Voltage To Recover To A Specified Value After Instantaneous. Switching From A Stated Forward Current Condition To A Stated Reverse Voltage Or Current Condition.

VB - DC Breakdown Voltage - Value Of Voltage Measured At The Point Which Breakdown Occurs With The Diode Reverse Biased.

V(BR)R¬ - Static Reverse Breakdown Voltage - The Value Of Negative Anode-To-Cathode Voltage At Which The Differential Resistance Breakdown Between The Anode And Cathode Terminals Changes From A High Value To A Substantially Lower Value.

VDRM - Repetitive Peak Off-State Voltage - Maximum Instantaneous Value Of The Off- State Voltage That Occurs Across The Devices. Including All Repetitive Transient Voltages. But Excluding All Non- Repetitive Transient Voltages.

VF - Forward Voltage - The Voltage Drop In A Semiconductor Diode Resulting From The Respective Forward Current.

VFM - Maximum Forward Voltage - The Voltage Drop In A Semiconductor Diode Resulting From The Respective Forward Current.

VGFM - Maximum Forward Gate Voltage - Maximum DC Forward Gate Voltage Permitted To Produce A Specified Forward Gate Current.

VGO MAX - Peak Gate Turn-Off Voltage - Maximum Reverse Gate Voltage Required To Switch off.

VGRM - Maximum Reverse Gate Voltage - Maximum Peak Reverse Voltage Allowable Between The Gate Terminal And The Cathode Terminal When The Junction Between The Gate Region And The Adjacent Cathode Region Is Reverse Biased.

VZ - Zener Regulator Reference Voltage - Value Of DC voltage Across The Diode When It Is Biased To Operate In Its Breakdown Region.

ΔVZ/ΔT - Change In Zener Voltage To Change In Temperature.

Transistors BVCBO - Collector To Base Breakdown Voltage - Voltage Measured Between Collector And Base With Emitter Open.

BVCEO - Collector To Emitter Breakdown Voltage - Voltage Measured Between Collector And Emitter With Base Open.

BVCER - Collector To Emitter Breakdown Voltage - Voltage Measured Between A Collector And Emitter When The Base Terminal Is Returned To The Emitter Terminal Through Specified Resistance.

BVCES - Collector To Emitter Breakdown Voltage - Voltage Measured Between Collector And Emitter With The Base Terminated Through A Short Circuit To The Emitter.

BVCEV - Collector To Emitter Breakdown Voltage - Voltage Measured Between Collector And Emitter When A Specified Voltage (V) ls Applied Between The Base And Emitter.

BVCEX - Collector To Emitter Breakdown Voltage - Voltage Measured Between Collector And .Emitter When The Base Is Terminated Through A Specified Load (X) To The Emitter.

BVDSS - Drain To Source Breakdown Voltage - Voltage Measured Between The Drain And Source Terminals With The Gate Short-Circuited To The Source Terminal.

BVEBO - Emitter To Base Breakdown Voltage - Reverse Voltage Measured Between Emitter And Base With The Collector Terminal Open.

BVGSS - Gate To Source Breakdown Voltage - The Breakdown Voltage Between The Gate And Source Terminals With The Drain Terminal Short-Circuited To The Source Terminal.

CISS - Input Capacitance - The Capacitance Between The Terminals (Gate And Source) With The Drain Short-Circuited To The Source.

CRSS - Reverse Transfer Capacitance - The Capacitance Between The Drain And Gate Terminals.

fT - Gain Bandwidth Product - Frequency At Which Small-Signal Gain Becomes Unity.

gfs - Forward Transfer Conductance - Common Source Forward Transconductance.

GPE - Power Gain Emitter Output.

hFE - DC Current Gain - The Ratio Of Collector Current To Base Current At A Specified Collector-Emitter Voltage.

IB - DC Base Current - Value Of DC Current Into The Base Terminal.

IC - DC Collector Current - Value Of DC Current Into The Collector Terminal.

IDSS - Zero Bias Drain Current - Amount Of Current Which Flows In The Drain When The Gate Is Connected To The Source.

N.F. - Noise Figure.

PD - Average Power Dissipation.

PIN - Signal Input Power To Device.

POUT - Signal Output Power.

RDSS - Drain-Source On-State Resistance.

VCC - DC Supply Voltage Applied To The Collector Terminal.

Special Purpose Devices BVCER - Breakdown Voltage Between Collector And Emitter With A Specified Resistor Between Base And Emitter.

BVGKF - Gate To Cathode Forward Breakdown Voltage.

BVGKR - Gate To Cathode Reverse Breakdown Voltage.

hFE - DC Current Gain - Ratio Of DC Output Current To The DC Input Current.

IBO + (MAX) - Maximum Forward Breakover Current.

IBO - (Max) - Maximum Reverse Breakover Current.

IE - Value Of The DC Current Into The Emitter.

IEO - Emitter Current With One Base Open. IG - DC Gate Current - The DC Current Flowing Through The Gate As A Result Of Applied Gate Voltage.

IT PK - Total Peak Current.

IV - Valley Current - The Valley Current Is The Emitter Current At The Second Lowest Current Point.

η - Intrinsic Stand Off Ratio.

RBBO - Base 1 To Base 2 Resistance With Open Emitter.

VAK - Anode To Cathode Voltage - The Maximum Value Of Voltage Applied Between Anode And Cathode Without Failure.

V(BO)+ - Forward Breakover Voltage.

V(BO)- - Reverse Breakover Voltage.

Δ¬VF - Forward Breakback Voltage.

ΔVR - Reverse Breakback Voltage.

VGT - Gate Trigger Voltage - The Gate Voltage Required To Produce The Gate Trigger Current.

Opto Electronic Devices

ID - Dark Current - The Current Which Flows In A Photodetector When There Is No Incident Radiation On The Detector.

IFT - Input Trigger Current - Emitter Current Necessary To Trigger The Coupled Device.

IL - Light Current - The Current That Flows Through A Photo Sensitive Device When It Is Exposed To Illumination.

PT - Total Device Power Dissipation.

Response - The Time It Takes The Device To React To An Incoming Time Signal. Time

Rise Time - The Time Duration During Which The Leading Edge Of A Pulse ls Increasing From 10% To 90% Of Its Maximum Amplitude.

VISO - DC Isolation Surge Voltage - The Dielectric Withstanding Voltage Between The Input And Output.

λP - Wavelength At Peak Emission - The Wavelength At Which The Power Output From A Light-Emitting Diode Is Maximum.

θHI - Half-Intensity Beam Angle - The Angle Within Which The Radiant Intensity Is Not Less Than Half Of The Maximum Intensity.

CHAPTER 3SEMICONDUCTOR DEVICES

3.1 Semiconductor Materials

3.2 Atomic Structure

3.3 Diode characteristics

3.4 Diode Applications

3.5 Zener Diodes

3.6 Light Emitting Diode

3.7 Point Contact Diode

3.8 Transistors

3.9 Transistors Circuit Configurations

3.10 Transistors Data

CHAPTER 3

SEMICONDUCTOR DEVICES

[3.1] SEMICONDUCTOR MATERIALS Semiconductor materials lie between conductors and insulators in their ability to conduct electric current. A one-centimeter cube of a good conductor, silver, for example, has a resistance of 10-6 ohms. A one-centi-meter cube of mica, a good insulator, has a resistance of 1012 ohms. In contrast, a one-centimeter cube of pure silicon, the most widely used semiconductor material, has a resistance of 50 to 60 kilohms.

Semiconductor materials, (germanium and silicon, for example) are crystals. Conduction in semiconductor crystals differs from conduction occurring in either insulators or conductors. This characteristic is exploited by solid-state devices. To understand semiconductor conduction, it is necessary to review the atomic structure of crystals.

[3.2] ATOMIC STRUCTURE All matter is composed of atoms. Atoms are complex dynamic structures containing a nucleus with a net pos-itive charge surrounded by orbiting electrons that exactly cancel this charge. But semiconductor action can be explained without recourse to the atom’s actual physical structure, which is not yet entirely known. Instead, we use a simplified model called the Bohr model (Figures 1 and 2). In the Bohr model, electrons with nearly the same energy are grouped into shells. Each shell is represented by a single circular orbit containing one or more electrons. The maximum number of electrons in any shell is fixed by physical considerations. The outermost shell is called the valence shell. The electrons in this shell are most easily lost or gained. Therefore this shell participates most often in atomic interactions. When this shell is filled, the atom is stable.

Crystals are composed of atoms that join to form symmetrical and repetitive patterns. These atoms complete their valence shells by sharing valence electrons with their neighbors. Silicon requires four valence electrons and combines with four neighboring atoms to achieve stability. Another less widely used material, germanium, also requires four valence electrons and bonds in the same way. In the presence of external energy (light or heat), semiconductor material is not completely Stable. The external energy frees some valence electrons, leav-ing unfilled niches called holes.

A voltage applied across a slab of relatively pure semiconductor material will produce a small electric current. This current flows in two modes, free electron and hole flow. Free electron flow is similar to conduction in metal. Electrons move through the semiconductor in erratic paths, colliding with atoms and other electrons. In the aggregate they move steadily toward the positive voltage. Conduction by holes is also a result of electron movement, though it is the valence electrons that move. These electrons jump from atom to atom, filling exist-ing holes. These electrons also move toward the positive voltage, but the holes appear to move in the opposite direction. Therefore it is convenient to think of electrons as negative carriers and holes as positive carriers. In the pure state, free electrons and holes are evenly divided and relatively few.

[3.3] Diode Characteristics Diodes pass current more readily in one direction (forward bias) than in the other (reverse bias). The circuit symbol defining this action and the circuit actions of a diode are shown in Figures 8, 9, and 10. Forward-biased diodes may carry large currents and dissipate power. If the maximum allowable junction temperature is exceed-ed, the diode will be damaged. This temperature may be held down by connecting the diode leads or case to a large, finned mass of metal called a heat sink. Some commercial silicon diodes are shown in Figure 11. Small diodes (glass cases) are mounted on heat sinks by their leads. Large diodes (stud mounted) are thermally Con-nected to heat sinks through their metal cases. Figure 12 displays the voltage-versus-current characteristic of a typical silicon diode. (Note that the region below the horizontal axis is expanded.) The forward-bias character-istic is a nonlinear curve, but it can be approximated by a straight line meeting the horizontal axis at 0.7 volts. The slope of the straight line represents the forward or on resistance of the diode. This value is supplied by the manufacturer. The reverse-bias curve is initially horizontal, showing that a small constant current (microamps) flows when the diode is reverse biased. As the reverse bias is increased, the curve begins to bend sharply toward the vertical. This is the breakdown region. Rectifying diodes cannot operate here, but special diodes, called zen-ers, can. For most applications, the diode can be considered as a switch that closes when the voltage across it is zero in the forward-bias direction and opens when the voltage across it is less than zero (reverse bias).

[3.4]DIODE APPLICATIONS

RectificationThe diode can be used to convert ac to dc. This action, called rectification, exploits the fact that the diode conducts most readily in one direction only. Figure 13 shows a half-wave rectifier, and Figures 14 and 15 show that the diode conducts only on the positive half-cycles and transforms the ac wave into a unidirectional pul-sating dc. For most applications, the output of Figure 15 must be smoothed (filtered). A relatively large capac-itor placed across the load can accomplish this (Figure 16). On each positive half-cycle, the capacitor will be charged to the peak value of the voltage across the transformer secondary. When the supply voltage decreases, the diode will turn off, and the capacitor will discharge into the load. If the circuit values are correctly chosen, the much smoother output of Figure 17 will be obtained. Full-wave rectification provides a smoother output than does half-wave rectification. Figures 18 and 19 show that the center-tapped, full-wave rectifier is essentially two half-wave rectifiers working into the same load. The upper diode, D1 conducts on the positive half-cycle, and D2 conducts on the negative half-cycle. Both send current through the load in the same direction, producing the output shown in Figure 20. The pulsations in this wave occur at twice the frequency of the half-wave rectifier. A capacitor placed across the load (Figure 21), is charged twice as often, giving the smoother output shown in Figure 20b.

Figure 22a shows a full-wave bridge rectifier. On the positive half-cycle, electron current flows from B- through D2, the load, D3, and back to A+. On the negative half-cycle, electron current flows from A+ through D1 the load, D4, and back to B-. In both cases the current flows through the load in the same direction. The bridge rectifier uses the full output of the transformer and does not require a center-tapped transformer. However, four diodes are required, and the total volt-drop across the diodes is twice that of the full-wave rectifier of Figure 18. Figure 22b is another representation of Figure 22a.

[3.5] Zener DiodesThe zener diode is used whenever a constant voltage is required. Figure 23 shows its circuit symbols, and Figure 24 displays atypical characteristic. This shows the voltage across the diode versus the current through it. When operated beyond the zener breakdown point, this characteristic becomes an almost vertical line. This means that in this region the voltage across the zener is almost constant and independent of current. A load placed across the zener (Figure 25) shares this constant voltage. R1 in Figure 25 limits the zener current and restricts its operation to the zener region.

[3.6] Light-emitting diode (LED) (a) Action. A LED, shown in Fig. 26 with its symbol, is a junction diode made from the semiconductor gallium arsenide phosphide. When forward biased it conducts and emits red, yellow, or green light depending on its composition. No light emission occurs on reverse bias which, if it exceeds 5V, may damage the LED.

When a p-n junction diode is forward biased, electrons move across the junction from the n-type side to the p-type side where they recombine with holes near the junction. The same occurs with holes crossing from the p-type side. Every recombination results in the release of a certain amount of energy, causing in most semicon-ductors, a temperature rise. In gallium arsenide phosphide some of the energy is emitted as light which escapes from the LED because the junction is very close to the surface of the material.

(b) External resistor. Unless a LED is of the ‘constant-current’ type it must have an external resistor R connect-ed in series to limit the forward current which, typically may be 10 mA (0.01 A). The voltage drop VF is greater across a conducting LED than across an ordinary diode and is about 2V, therefore R can be calculated from:

R= (supply voltage-2.0)V/(0.01 A)

For example, on a 5.0V supply, R = 3.0/0.01 = 300Ω(c) Uses. LEDs are used as indicator lamps, especially in digital electronic circuits to show whether outputs are ’high’ or ’low’. One way of using a LED to test for a ’high’ output (9V in this case) is shown in Fig. 27 a and for a ’low’ output (0V) in Fig. 27 b. In the first case the output acts as the ’source’ of the LED current, in the second case the output has to be able to accept or ’sink’ the current

The advantages of LEDs are small size, reliability, long life and high operating speed. [3.7] Point-contact diode (a) Construction. The construction of a germanium point-contact diode is shown in Fig. 28. The tip of a

gold wire presses on a pellet of n-type germanium. During manufacture a brief current is passed through the diode and produces a tiny p-type region in the pellet around the tip, so forming a p-n junction of very small area.(b) Use. Point-contact diodes are used as signal diodes to detect radio signals (a process similar to rectification in which radio frequency a.c. is converted to dc) because of their very low capacitance. When reverse biased, the depletion layer in a junction diode acts as an insulator sandwiched between two conducting ‘plates’ (the p- and n-regions). It therefore behaves as a capacitor and the larger its junction area and the thinner the depletion layer, the greater is its capacitance.A capacitor ‘passes’ a.c. and the higher the frequency and the greater the capacitance, the less opposition does it offer (XC= 1/(2πfC)). At radio frequencies therefore a normal junction diode would not be a very efficient detector (rectifier) because of the comparatively large junction area; its opposition in the reverse direction would not be large enough. A point-contact diode on the other hand is more suited to high frequency signal detection because of its tiny junction area. Germanium is used for signal diodes because it has a lower ‘turn-on’ voltage than silicon (about 0.1V compared with 0.6 V) and so lower signal voltages start it conducting in the forward direction. For the OA91 point-contact diode IF (av) = 20 mA, VF ≈ 1 V and VRRM = 100V.

[3.8] TRANSISTORS

The transistor is a three-terminal, solid-state device that provides current, voltage, and power amplification. Typical body shapes are shown in Figure 29. The three transistor terminals attach to the emitter, base, and collector, in an NPN transistor, the base is an extremely thin, P-type slab sandwiched between two N-type slabs (Figure 30). In a PNP, the base is N-type, and the emitter and collector, are P-type (Figure 31). Figure 32 shows the circuit symbols of the transistor. The arrow on the emitter indicates the direction of the positive current flow (opposite to the electron flow) when the base-emitter circuit is forward biased. Transistor amplification is illustrated in Figure 33. Figure 33 a shows an NPN transistor with the emitter grounded, the base connected either to the ground or to a positive voltage, and the emitter positive. Figure 33 b shows the potential distribution from emitter to collector. This transistor may be viewed as two joined diodes (Figure 34). Diode 1 comprises the emitter and base and diode 2 comprises the base and the collector. With its base grounded, diode l is forward biased, but not completely, as a residual voltage opposes the current flow. The depletion region of this diode lies between c and d in the diagram. The upward sloping line between emitter and base shows that internally the emitter is slightly more positive than the base. (Upward motion means moving from positive to negative.) Free electrons are the carriers of current in the emitter. When the base is made more positive, the potential hill is lowered, and electrons enter the base. Only electrons above the hill have sufficient energy to enter the base and create current. As the hill is raised or lowered by a signal applied to it, fewer or greater number of electrons can surmount the hill. Therefore the base current mimics the base signal. Diode 2 is strongly forward biased from base to collector (reverse biased from collector to base). This is shown by the long fall from e and d. Electrons that are able to leave the emitter and enter the collector will gain energy by falling through this voltage change. Because the base is always very thin, 98 percent or more of the electrons entering the base will pass through it, enter the collector, and descend the base collector slope. As a result, small current changes controlled by a weak signal at the base will generate large current changes out of the collector. The transistor therefore functions as a current amplifier. The PNP transistor acts in exactly the same way except that all voltages and currents are reversed.

[3.9] Transistor-Circuit Configurations

The three possible circuit configurations of transistor amplifiers are shown in Figure 35. Each configuration has its own function. The common base (CB) provides voltage and power gain but no current gain. it has a low input impedance and a high output impedance. Consequently, it is often used for matching a low impedance source to a device requiring a high input impedance. The common emitter (CE) provides current, voltage, and power gain and it is widely used as an amplifier. The common collector (CC) provides current and power gain. It has a high input impedance and a low output impedance and is widely used to match devices. For all configu-rations, R1 limits the emitter-base current, and the output is taken across R2.

[3.10] Transistor data Transistors are identified by one of several codes. In the American system they start with 2N followed by a number, e.g. 2N3053. In the Continental system the first letter gives the semiconductor material (A = germani-um, B = silicon) and the second letter gives the use (C = audio frequency amplifier, F = radio frequency amplifi-er). For example, the BC108 is a silicon a.f. amplifier. Some manufacturers have their own code. While one type of transistor may replace another in many circuits, it is often useful to study the published data when making a choice. Table 1 lists the main ratings, called parameters, for five popular n-p-n silicon tran-sistors. All are general purpose types, suitable for use in amplifying or switching circuits. The BCl08 is a low current, high gain device, the others are medium current, medium gain types.

(a) Current, voltage and power ratings. The symbols used have the following meanings.

IC max is the maximum collector current.VCEO max is the maximum collector-emitter p.d. when the base is open-circuited.VEBO max is the maximum emitter-base p.d. when the collector is open-circuited.PT max is the maximum power rating at 25°C and equals VCE x IC approximately.

(b) d.c. current gain hFE. Owing to manufacturing spreads, hFE is not the same for all transistors of the same type. Usually minimum and maximum values are quoted but sometimes only the former. Also, since hFE de-creases at high and low collector currents, the IC at which it is measured is stated.

In general, when selecting a transistor type we have to ensure that its minimum hFE gives the current gain re-quired by the circuit.

(c) Transition frequency fT. This is the frequency at which hFE = 1 and is important in high-frequency circuits.

(d) Outlines. These are given in Fig. 36 for the transistors in Table 1.

CHAPTER 4

POWER SUPPLY CIRCUITS

4.1 Basic Principles of DC Power Supplies

4.2 The Linear Stabilized Power Unit

4.3 Switching Mode Power Supplies

4.4 Power Supply Protection Circuits

4.5 Testing Power Supply Circuits

4.6 Fault Finding Techniques and Typical Fault Conditions

CHAPTER 4

Power Supply Circuits 4.1 Basic Principles of D.C. Power Supplies Practically all electronic instruments require a source of d.c. power before they will operate. Sometimes the source is a battery, but more usually the power is obtained from a unit that converts the normal single phase a.c. mains supply (240 V at 50 Hz) to some different value of d.c. voltage. The function of the power supply is to provide the necessary d.c. voltage and current, with low levels of a.c. ripple (mains hum) and with good stability and regulation. In other words it must provide a stable d.c. output voltage, irrespective of changes in the mains input voltage and of changes in the load current. A further important requirement of a modem unit is that it should be able to limit the available output current in the event of an overload (current limiting) and also limit the maximum output voltage. Damage to sensitive components, such as ICs, in the instrument can easily occur if excessive voltages appear on the power supply lines. There are various methods of achieving a stable d.c. voltage from the a.c. mains, but only two methods are com-monly used. These are (i) Using a linear stabilizer (ii) Using a switching mode stabilizerBoth have their advantages and disadvantages as will be seen. The switching mode power supply unit (SMPU) is a relatively new innovation and finds its main use in high-power applications (100 W upwards).

4.2 The Linear Stabilized Power Unit

The block diagram of a conventional power unit is shown in Fig. l. The TRANSFORMER serves two main pur-poses: it isolates the equipment d.c. power lines from the mains supply, and it changes the level of the a.c. mains voltage to some lower or higher value. The ratio of the secondary voltage to primary voltage is determined by the number of turns on each winding.

The RECTIFIER unit converts the a.c. voltage from the transformer secondary winding into pulses .of unidi-rectional current. Three types of rectifier circuit are used for single phase: the half-wave, the full-wave, and the bridge. These, together with their output waveforms, are shown in Fig. 2. The half-wave rectifier, although being a simple circuit, has the main disadvantage of low efficiency. The diode conducts only on one half of the cycle, so the efficiency cannot be greater than 50 per cent. The full-wave rectifier uses two diodes, each conducting on alternate half cycles to give much higher efficiency. However, to achieve this, a transformer with a centre tapped secondary winding is necessary. This means that twice the num-ber of turns is required on the secondary winding. This circuit was common when valve rectifiers were in use, since it was cheaper to wind extra turns on the transformer than to use more valves. The bridge rectifier, now the circuit of choice, uses four diodes to achieve rectification over the whole cycle, and no centre tap is required. The four diodes can now be supplied in one encapsulated unit, which is more convenient and somewhat cheaper than wiring in four separate diodes. However should one part of the encapsulated bridge circuit fail, the whole unit then has to be replaced.Following the rectifier is the FILTER which serves to smooth out the pulses received from the rectifier. The circuit can have either a capacitive or an inductive input as shown in Fig. 3. The inductive filter, or choice input

filter, is more commonly used when the power unit has to supply a large load current. On low power equipment a capacitive input filter is more typical. The input capacitor, called the

the diode becomes reverse biased, and the capacitor now discharges through the load resistor. The voltage across the load now more nearly represents a d.c. level, but superimposed upon it is an alternating wave- form, called the ripple. The value of the ripple amplitude depends upon the size of the capacitor and the load resis-tance. To achieve low values of ripple a high-value electrolytic capacitor, typically 500 μF or more has to be used. A number of points should be noted concerning the reservoir capacitor. (a) Since it is an electrolytic, it is polarized and must be connected correctly in the circuit. (b) Its d.c. working voltage must be greater than the peak of the transformer secondary voltage. (c) It must be physically large since it has to absorb large pulses of current when it is charging, The peak values of which may be several amperes. If too small a capacitor is fitted, it may overheat and possibly explode! Check the ripple current rating. The other components of the circuit form a low pass filter, which reduce still further the output ripple voltage. Typical values for the iron-cored inductor are 1 to 5 Henries and for C2 , 500 μF. In some circuits these com-ponents may be omitted, especially when an efficient regulator is used. The inductor also is often replaced by a wire-wound resistor of low value, say 22Ω, in which case there will be a voltage drop across this component resulting in a lower output voltage. The last block is the REGULATOR, which is used to keep the output voltage constant irrespective of changes in the mains input voltage and of changes in the load current. These two functions are called line stabilization and load regulation respectively. All linear regulators comprise (a) a control unit (b) a reference element (usually a zener diode) (c) an error amplifier as shown in Fig 5. In operation, the circuit compares a portion of the d.c. output voltage with the reference voltage. Any difference between the two levels is amplified by the error amplifier, and the output fed to the control unit. The stability and regulation of the output voltage depends upon the stability of the reference element and the gain of the error amplifier. High gain Op-Amps in IC form are now commonly used as the

error amplifier to give power supplies of excellent performance. The main advantage of the linear regulator is that the output is continuously controlled to give good stabilization against mains input changes and good regulation against load current changes.

A typical specification for an output voltage of +15 V @100 mA load current is Line stability 10 000:1 (a 10 V change in mains supply giving a 1 mV change in d.c. output)Output ripple 0.1 mV pk-pk at full loadD.C. output impedance 005 ohmsTemperature coefficient 200 μV per degree C Load regulation 0.033% from zero to full load (i.e. an output change of 5mV)

The limitation in the linear regulator circuit is that good performance is achieved at the expense of inefficiency. Power is dissipated and lost in the series control transistor and this power loss increases with load current. A large heat sink is required to ensure that the junction temperature of the series transistors is kept within its rated value. For power units supplying above about 100W, the switching mode regulator becomes a preferred alterna-tive.

4.3 Switching Mode Power Supplies (SMPU) There are two main variations of this type. In one, a fast switching transistor is used as the control element in the regulator (Fig. 6). This transistor is switched on and off at a frequency above audio (usually 20 kHz). The d.c. output voltage, after being

smoothed by a low pass filter, is controlled by varying the mark-to-space ratio of the switching signal. Such techniques are known as secondary switching. The error signal, generated by comparing the d.c. output with a reference level, is used to control the duty cycle of a free-running oscillator. The advantage of this type of circuit is that the series transistor heat dissipation is greatly reduced, hence greater regulator efficiency.

Another form of the SMPU is shown in Fig. 7 and uses a principle called primary switching. The mains supply itself, after rectification and smoothing, is switched at high frequency by high voltage switching transistors. With this method the transformer following the switching transistors can be much smaller than the bulky 50 Hz transformer required in conventional supplies. Regulation is achieved by again varying the switching duty cycle of the transistors. Naturally RF suppression circuits must be

included to reduce the switching spikes that would otherwise be fed back into the mains supply. This SMPU offers considerable advantages in terms of efficiency, reduction in heat loss, and reduction in overall volume. However it does not possess the regulation performance that can be achieved in the linear circuit. Switching mode supplies are now commonly used where large currents at low voltage are required, as in equipment using many digital ICs.

4.4 Power Supply Protection CircuitsSome form of protection must be incorporated in even the simplest power supply. A common form is the stan-dard fuse which serves to disconnect the unit from the mains supply when an overload or short occurs. A pow-er unit may have fuses in the line and neutral mains wires, and also a fuse in the dc. unstabilized line. Fuses usually do not blow soon enough to protect the series transistor in the regulator if the output is shorted, and so some form of current limiting device is used. A simple circuit for achieving this is shown in Fig. 8 where the load current flows through a low value current monitoring resistor. If the load current increases beyond a pre-de-termined value, the voltage developed across this resistor turns on Tr2 which in turn tends to turn off Tr1, the series transistor. Over-voltage protections can be provided by a circuit which senses the d.c. output voltage, and compares it with a reference level as in Fig. 9. If the d.c. output voltage rises above VZ a signal is generated which triggers the thyristor and this short circuits the output, either blowing a d.c. line fuse or operating the current limit. Such circuits are called “crowbars”. Naturally the fault must be cleared before the circuit can be reset.

4.5 Testing Power Supply Circuits

The main parameters which ought to be measured either in a test department or by the service technician after he has repaired a power unit are the following: (a) D.C. output voltage (b) Available d.c. output current (c) Output ripple voltage at full load (d) Stabilization against mains supply changes (e) Regulation from zero to full load These can all be measured using a standard test set-up as shown in Fig. 10.

The d.c. output voltage should be measured, and if necessary adjusted, when the unit is fully loaded. However it is sometimes advisable to measure the output on a low load and then gradually increase the load current to maximum. There should, of course, be little change in the output voltage. The peak-to-peak ripple amplitude can be checked best by measuring at the output with an oscilloscope. A sen-sitive a.c. range must be selected because the ripple should be quite low, typically less than 20 mV. Measurement of stabilization and regulation requires that any small change in d.c. output be carefully noted, and

therefore a digital voltmeter is often necessary. For stabilization measurement, the unit should be fully loaded and the change in d.c. output voltage noted for say a ±10% change in the a.c. input. The mains input can be var-ied using an adjustable auto-transformer as shown. Then if, for example, the d.c. output changed by 50 mV from 10 V, i.e. an output change of 0.5%, then the line stabilization would be 40:1. Load regulation is measured, keeping the 3-6- input constant, by noting the change in output when the load is varied from zero to full load.

Load regulation = (Change in d.c.output )/(D.C.output on no load) x 100% For example suppose the output changed by 20 mV from 10 V. The load regulation is

((20 ×10-3)/10) ×100%=0.2%

To obtain fuller information on a power supply’s performance it is often necessary to plot the load regulation curve. This is a plot of output voltage against load current. A typical result for a unit with current limiting is shown in Fig. 11.

4.6 Fault Finding Techniques and Typical Fault Conditions

When a faulty power unit is returned for repair, the fault has to be isolated to some particular portion of the unit. The fault may lie in the transformer, the rectifier, the filter section, or the regulator, and measurement with a voltmeter will be necessary to locate the fault.However its probably best to start diagnosis with a few rather obvious but often overlooked checks. First measure the d.c. output voltage. If this is zero, the next check should be on the mains input. Is the mains supply reaching the transformer primary? If it isn’t, there is the possibility of a faulty plug (relative-ly simple to repair!), open circuit mains wires, or a blown fuse. If the fuse is suspected, always test its continuity with an ohmmeter, never rely on just a visual inspection. It is also worth noting that both the live and neutral wires may have a fuse in circuit, so make sure both are checked. If the fuse is blown it has done so because of some fault condition and the fault must be cleared before a new fuse is fitted. Resistance checks (with the mains unplugged!) must be used to locate such a fault. Use an ohm-meter to measure the resistance of the transformer primary, the secondary, the rectifiers, and so on. The winding resistance depends, of course, on the size of the transformer. The primary resistance, for a medium size trans-former, should be low, typically about 50 Ω. The secondary, usually supplying a lower voltage, may have a resistance of only a few ohms. Detecting shorted turns on a winding can therefore be quite difficult. Wherever possible compare the measured resistance with any available data on the type of transformer being used. Anoth-er useful check is to run the transformer off load and test for overheating. When using an ohmmeter take care to use the correct polarity for resistance checks where diodes, electrolytic capacitors and transistors are present. It is all too easy to get misleading results. For example, in Fig.12, if the meter is used to measure the resistance of the unstabilized line, the positive prod (connected inside the meter to the positive plate of a battery) should be placed on the positive line and the negative prod to earth. If the meter is reversed

there will be a low resistance path through the rectifiers and a leakage path through the capacitor. Returning to the faulty power unit, suppose however that the fuse is intact, and that the mains is reaching the primary. The next step is to measure the secondary a.c. voltage, the unstabilized d.c. voltage then the d.c. volt-age in the regulator and so on, until the fault is located. Table 1 lists some typical faults together with the associated symptoms. The faults are only a sample of those which may occur. Locating a faulty component from a given set of symptoms will come with practice and the following exercises are designed for that purpose.

FAULT SYMPTOMS

Mains transformer, open circuit D.C. output zero. Secondary a.c. zero, high primary or secondary resistance primary or secondary.______________________________________________________________________________Mains transformer shorted turns Two possibilities: (a) mains fuses blown or on primary or secondary (b) low d.c, output and transformer over heating because of excessive current being drawn.______________________________________________________________________________Mains transformer, windings shorting Fuses blown. Low resistance between to frame or screen windings and earth. ______________________________________________________________________________ One diode in bridge open circuit Circuit behaves as a half-wave rectifier. Lower d.c. output with poor regulation. Increased ripple at 50 Hz not 100 Hz as should be the case. ______________________________________________________________________________One diode in bridge short circuit Mains fuse blown, since secondary winding will be practically shorted every other half cycle. A resistance check across each arm of bridge is required, measuring the resistance of each diode in the forward and reverse direction______________________________________________________________________________ Reservoir capacitor open circuit Low d.c. output with very high values of a.c. ripple on output.______________________________________________________________________________Reservoir capacitor short circuit Fuses blown. D.C. resistance of unstabilized line low in both directions.______________________________________________________________________________Error amplifier in regulator open High d.c. output that is unregulated. No circuit control signal for the series element.______________________________________________________________________________Series transistor open circuit base Zero d.c. output. The unstabilized d.c. will emitter be slightly higher than normal since no current is being drawn. ______________________________________________________________________________

Reference zener short circuit Low d.c. output. Possibility of series transistor overheating.

CHAPTER 5

SINGLE STAGE TRANSISTOR AMPLIFIER

5.1 Basic Principles

5.2 Resistor Faults

5.3 Capacitor Faults

5.4 Transistor Faults

5.1 Basic Principles

This section is concerned with the effects of individual component failures in a single-stage common emitter amplifier. The circuit shown in Fig. 1 usually has eight components. Remembering that resistors can fail high or open circuit; capacitors either open or short circuit; and that the transistor can fail open or short circuit between any of its connections; it can be seen that a total of at least twelve faults is possible. For each of these faults a unique set of conditions will exist. Before considering any fault conditions, the Operation Of the circuit must be understood. In a class A amplifier a mean current flows through the transistor, and the input signal causes this current to either increase or de-crease. This change in collector current then develops a voltage signal across the collector load resistor R3. The operating point of the collector voltage, that is the d.c. voltage between collector and the 0 V rail, should be a value that allows equal positive and negative swings of the output signal. As a rough approximation, VC should be half the supply voltage. The whole purpose of the bias components (R1, R2, R4) is to fix this operating point, and to keep it stable. Stability is very important since a number of factors cause the operating point to change. For silicon transistors the most important is the change in current gain hFE. This can have a value from 50 to 500 for the same type of transistor and; obviously without some form of stabilizing circuit, the operating point would change drastically in the circuit every time the transistor was changed. The bias circuit achieves stabili-zation by fixing the value of the base voltage V3 and by keeping it constant irrespective of changes in the base current. To do this the values of R1 and R2 must be chosen so that the current flowing through them is much greater than the transistor base current. These resistors form a potential divider

and if we neglect base current, the d.c. base voltage can be calculated from

and the emitter voltage VE is given by

where VBE is the forward bias voltage between base and emitter, typically 0.7 V for a silicon transistor. Then

and as we are neglecting base current, so the d.c. voltage at the collector VC is given by

Now, since VB is fixed, the d.c. current through the transistor will be fixed, and this gives the operating point VC. In operation, the circuit is an example of series negative feedback. Imagine that the collector current increases, thus causing the Operating point to fall. The emitter current also increases, raising the emitter voltage VE. However, since VB is fixed by the potential divider, any increase of VE must reduce the voltage between base and emitter of the transistor, and this in turn causes a decrease of collector current. This tends to counteract the original rise to stabilize the operating point. Having set the correct bias with the resistors, the ac. input and output signals must be coupled to and from the circuit without disturbing the d.c. levels. To do this capacitors C1 and C2 are used. Both these should be fairly high-value electrolytics, say 10 µF, to enable the circuit to amplify low frequencies. Capacitor C3, the decou-pling capacitor, ensures that no a.c. signals appear at the emitter which would reduce the gain of the circuit. Since the internal resistance of the emitter base junction is quite low, C3 must be high in value. A typical value is 100 µF. For the circuit shown in Fig. l the calculations of the d.c. bias voltages would be as follows:

This is assuming that the current gain is high and that the base current can be neglected. This is nearly always the case. Therefore

Putting these in table form we have, for the calculated values:

TP | 1 | 2 | 3 |V | 2.4 | 5.3 | 1.7 |

In fact when the circuit is built, the actual voltages measured with a 20 kΩ/V meter will be slightly different. This is to be expected since the bias resistors used have a tolerance of 10%. The actual readings were TP | 1 | 2 | 3 |Meter | 2.3 | 5.5 | 1.7 | reading

This shows the close agreement between the Calculated and the measured values. When fault finding on any circuit always try to make a rough calculation of the voltage you would expect. This can be an invaluable guide as to which parts of the circuit are functioning correctly. Now let us consider the effect of component failures, taking each in turn.

5.2 Resistor Faults

R1 OPEN CIRCUIT (Fig. 2)

TP | 1 | 2 | 3 | No output |MR | 0 | +12 | 0 | signal |

When R1 goes open circuit, the current flowing in R2 and the base is zero. It follows that the transistor is cut off so both the emitter and base voltages are zero. Since no collector current is flowing the voltage dropped across the collector load R3 is zero and the collector voltage itself is the same as the supply voltage VCC.

R2 OPEN CIRCUIT (Fig. 3)

TP | 1 | 2 | 3 | Grossly distorted output; negative going signals MR | 3.2 | 2.6 | 2.5 | clipped Without R2 in circuit the current that was flowing through R2 now tries to flow into the base of the transistor. But the value of base current will be limited by the transistor current gain, so less current flows through R1. This means that the base voltage must rise. The base current in fact rises to a value that completely “turns on”, or saturates the transistor, so that the collector voltage is only about 0.1 above the emitter voltage.

R3 OPEN CIRCUIT (Fig. 4)TP | 1 | 2 | 3 | No output |MR | 0.75 | 0.1 | 0.1 | signal |

Without R3 in circuit the collector current is zero, so any current flowing in the emitter must now be supplied from the base. The base/emitter junction acts like a forward biased diode placing R4 in parallel with R2. Since R4 is a low value resistor (560Ω) the emitter voltage falls to a very low value. The base voltage, as expected, is about 650 mV greater than the emitter voltage. It might be reasonable to assume that the voltage reading at the collector would be zero since the resistance is open circuit. However, when the meter is connected it presents a high resistance path from the collector to 0V, and the base/collector junction acts like a forward biased diode passing a small current through the meter.

R4 OPEN CIRCUIT (Fig. 5)TP | 1 | 2 | 3 | No output |MR | 2.3 | 12 | 2 | signal |

With an Open circuit between emitter and 0V, no currents flow through the transistor. The collector voltage therefore rises to VCC. The voltage at the base is fixed by the potential divider R1 and R2 and since the base current is small in comparison to the current through R2 this voltage hardly changes at all. As with the previous example when the meter is connected between emitter and 0V, a small emitter current flows so the voltage indicated at the emitter is slightly higher than normal.

5.3 Capacitor Faults

C1 or C2 OPEN CIRCUIT (Fig. 6)TP | 1 | 2 | 3 | No output |MR | 2.3 | 5.5 | 1.7 | signal |

With this type of fault the bias conditions of the circuit are unchanged. The fault can only be an open circuit coupling capacitor. A check with an oscilloscope is necessary to determine which one is actually faulty.

C3 OPEN CIRCUIT (Fig. 7)TP | 1 | 2 | 3 | Low GainMR | 2.3 | 5.5 | 1.7 |

Again the bias conditions are unchanged. The symptom that identifies this fault is the fact that the amplifier voltage gain has fallen. With C3 open circuit, a.c. signals will appear across R4 introducing negative feedback. The voltage gain will fall to a value given by R3 + R4 , i.e. approximately 4.

C3 SHORT CIRCUIT (Fig. 8)TP | 1 | 2 | 3 | No output |MR | 0.7 | 0.15 | 0 | signal |

The emitter resistor R4 is shorted out, so the emitter voltage reads 0V. The transistor heavily forward biased sat-urates and therefore attempts to pass a large current. However, the transistor current is limited to a value given by VCC ÷R3 which prevents the transistor from being damaged. The base voltage must be 0.7 V higher than the emitter.

5.4 Transistor Faults

COLLECTOR/BASE JUNCTION OPEN CIRCUIT (Fig. 9)TP | 1 | 2 | 3 | No output MR | 0.75 | 12 | 0.1 |

Since the collector is open there can be no collector current flowing, so the voltage at TP2 rises to +12 V. The base/emitter junction now acts as a forward biased diode in a similar way as for the fault of R3 open circuit.

COLLECTOR/BASE JUNCTION SHORT CIRCUIT (Fig. 10)TP | 1 | 2 | 3 | MR | 3 | 3 | 2.3 |

As with any short circuit a clue to the fault is given by the fact that the voltages on the base and collector are equal. With this fault the circuit effectively reduces to R3 in series with the base/emitter diode and R4.The resistance of this path is much lower than R1 and R2, so the effect of the latter resistors can be neglected. The current flowing in R4 is given by

The voltage at the emitter will then be I x R4 = 2.3V. The voltages at TPl and 2 will be 0.7V higher than this, sufficient to forward bias the base/emitter diode.

EMITTER/BASE JUNCTION OPEN CIRCUIT (Fig. 11)TP | 1 | 2 | 3 | No output MR | 2.3 | 12 | 0 | signal

With this fault there can be no current flowing in the transistor. The voltage drops across R3 and R4 are zero, so the collector voltage rises to VCC and the emitter voltage is 0V. The voltage on the base is determined by the potential divider R1 and R2 and therefore remains at 2.3V. There is no difference in the symptoms if the base or the emitter connection to the junction is open circuit.

EMITTER/BASE JUNCTION SHORT CIRCUIT (Fig. 12)TP | 1 | 2 | 3 | No output MR | 0.13 | 12 | 0.13 |

The voltages at TP1 and TP3 will be equal and at a low value since R4, a low resistance, is placed directly in parallel with R2. With a shorted base/emitter junction all transistor action ceases, so the collector voltage rises to VCC.

COLLECTOR/EMITTER SHORT CIRCUIT (Fig. 13)TP | 1 | 2 | 3 | No output |MR | 2.3 | 2.5 | 2.5 | signal |

The voltage at the emitter is equal to that on the collector, indicating a short. The value of the voltage will be de-termined by R3 and R4 which now form a potential divider. The base voltage remains unchanged at 2.3V since the emitter voltage has risen, thus cutting off the base/emitter diode.

CHAPTER 6

THE FIELD EFFECT TRANSISTOR

6.1 Operation of the JFET

6.2 Operation of the MOSFET

6.3 Operation of the VMOSFET

6.4 Operation of the IGFET

6.5 Common Drain Amplifier

THE FIELD EFFECT TRANSISTOR

The advent of solid-state technology has brought forth a large variety of solid-state devices. One of these de-vices is the field effect transistor (FET). Although the impact of the FET was not felt on the electronics industry until about a decade after the development of the transistor, the FET is actually a very “old” device. The first patent granted for the FET dates back to 1928.*The major difference between the ordinary transistor and FET is the very high input impedance of the FET. The ordinary transistor is usually called the bipolar junction Transistor (BJT) to distinguish it from the FET. The FET is manufactured in two different ways: the junction field effect (JFET) and the insulated gate effect (IG-FET). The IGFET is sometimes also referred to as the metal oxide semiconductor field effect (MOSFET). The difference between the JFET and the IGFET is that the IGFET has a higher input impedance than the JFET.

6.1 Operation of the Junction FET (JFET)

A pictorial representation of the JFET is shown in Fig. la and the symbol in Fig. lb. The FET is a three-terminal device made of a bar of n-type silicon material with a “ring” of p-type material at the center. The source (S) and drain (D) terminals are situated at each end of the n-type bar and the

*U.S. Patent 1900018 was granted to Lilienfeld for “a device controlling electric current.”

gate (G) terminal is connected to the p-type material. This type of FET is referred to as the n-channel FET. The channel is the bar of n-type material to which the source and drain terminals are connected. p-channel FETs are made as well, with the gate of n-type material. The symbol for a p-channel FET is shown in Fig. 2.

Under normal operating conditions the. FET is biased so that the gate-channel junction is reverse biased. This gives the device the very high input impedance, since the small-.signal resistance between the gate and the channel is the same order of magnitude as a reverse-biased diode.

The input resistance seen by the signal generator looking into the amplifier stage of Fig. 8 is essentially RG in parallel with the resistance of a back-biased diode. Since the diode’s resistance is extremely high, Rm = RG

It is the input resistance that makes the FET valuable as a circuit component. Compared to bi-polar amps, the input resistance of a JFET is much higher, but the gain is generally lower.

6.2 MOSFET

In the JFET, drain current is controlled by the voltage on a reverse-biased diode. There is another type of FET in which drain current is controlled by the voltage on the gate, but no gate diode is used. This device is called the Metal Oxide Semiconductor Field Effect Transistor or MOSET.

Fig. 9 shows how MOSFET are built. Construction starts with formation of a heavily doped substrate materials. The heavy doping is indicated by N +. Next, a layer of very lightly doped (N -) material is deposited onto the substrate. This layer will form the channel. Then, as shown in Fig. 9 C, two “islands” of N + material are doped in to form the source and dram. Next, a very thin layer of silicon dioxide is deposited on top; silicon dioxide is an extremely good insulator. Finally, as shown in Fig 9 E, metal connections are made to the source and drain, and a metal gate is deposited on top of the insulator. For this reason, this device is also known as an Insulated Gate FET, or simply IGFET.

Fabrication of a MOSFET

The channel material is so lightly doped that with 0 V between gate and source practically no current flows from source to drain, as shown in Fig. 10 A However, as shown in Fig. 10 B if a positive voltage is applied to the gate with respect to the source, electrons from the substrate are drawn up into the channel, making it conductive. So current does flow from source to drain through the channel. The conductivity of the channel is thus enhanced by the positive gate voltage, so this is called an enhancement mode device. Notice that no current flows in the gate circuit. The gate acts like one plate of a capacitor, the channel being the other plate. The silicon dioxide layer forms the dielectric.

Controlling current flow in a MOSFET

By increasing and decreasing the gate-to-source voltage, the conductivity of the channel increases and decreas-es accordingly. Therefore, the drain current in the MOSFET is controlled by the gate-to-source voltage, just as in the JFET. We can see two differences here. First, no reverse-biasing voltage is necessary. Second, the input resistance seen looking into the gate is essentially an open circuit, since it is simply a small capacitor. The value or this capacitance is extremely small, usually in the order of 5-10 pF.The symbol for the N-channel MOSFET is shown in Fig.11 A. If all P-type materials are used instead of N-type, a P-channel device can be built with similar characteristics. See Fig. l1 B for the symbol.

N-channel MOSFET.

Some variations exist in the symbols used by different manufacturers. Sometimes the N-channel symbol is drawn as shown in Fig. 12 A. And occasionally, instead of being tied internally to the source, the substrate connection is brought out of the package separately. The symbol for a MOSFET with a separate substrate lead is shown in Fig. 12 B.

Alternate MOSFET symbols.

Fig. 13 shows how a MOSFET can be used as an a-c signal amplifier. Notice that resistors R1 and R2 form a voltage divider, thus applying + 5 VDC between gate and source to bias the device on in the active region. The incoming a-c signal rides on the d-c level, thereby increasing and decreasing the total drain current in accor-dance with signal variations. The gain of the MOSFET is similar to that of the JFET

MOSFET small-signal amplifier.

Since practically no drain current flows with 0 V applied to the gate, the MOSFET makes an excellent switching device. Fig. 14 A shows a MOSFET being driven by a square wave generator. When the input signal, shown in Fig. 14 B, is at 0 V, the drain voltage, shown in Fig. 14C, is equal to VDD, or 15 V. Then when the input is driven sufficiently positive, say to 5V, the transistor turns on. V0 then drops to a low value. Most of the VDD supply appears across RD. How low V0 drops depends on how large RD is and on how low a resistance the transistor becomes. The latter is referred to as RDS(ON), meaning resistance, drain-to source, when on. Both JFET and MOSFET usually have a higher voltage across them when on, as compared to a bipolar transistor. But in low power circuits, MOSFET have an advantage, because no input current is needed to turn them on.

MOSFET switching circuit.

CAUI’ION The insulating layer of silicon dioxide between the gate and channel is extremely thin. For this reason, static electricity, even from your fingers, can damage it permanently. Do not handle MOSFET unneces-sarily. They usually come from the manufacturer wrapped in metal foil, or stuck into conductive foam. Do not remove them from the packaging until you are ready to install them in a circuit Also, never insert or remove a MOSFET when the power is on.

In plants where MOSFET parts are assembled, the assembler sometimes wears a metal bracelet which is grounded by means of a small chain. Otherwise, the entire work surface is conductive and grounded.

6.3 VMOSFET In the late 1970s, a new geometry for MOSFET began to be used, called the Vertical MOSFET, or simply VMOSFET. The operation of the VMOSFET is basically the same as that of the MOSFET, but it has some other excellent characteristics resulting from its different construction. Fig. 15 shows the internal construction of a typical VMOSFET. Note that a very lightly doped P-region sepa-rates the source and drain. Then a V-shaped groove is cut through the P-region, and an insulating layer of silicon dioxide is deposited on the surface. Finally, metal is deposited in the V-groove to form the gate.

VMOSFET construction.

Like the MOSFET, the VMOSFET becomes conductive when a positive potential is applied to the gate with respect to the source pulling electrons into the channel. As the voltage on the gate becomes higher, the channel becomes more conductive and the drain current flows more freely. Because the surface area in the V-groove is small, the capacitance formed by the gate is extremely small. As a result, the high-frequency performance is excellent. The device can be switched on or off in a few nanoseconds making the VMOSFET a very efficient switch. Unlike the bipolar transistor, which tends to conduct more heavily as the temperature increases, the VMOSFET does not. The channel resistance actually tends to increase with an increase in temperature. This increased resis-tance prevents a condition called thermal runaway, which occurs in bipolar transistors and can destroy them. The characteristics that make the VMOSFET very interesting and valuable compared to the ordinary MOSFET are its high current capability, high transconductance and low resistance when on. These characteristics make the VMOSFET an excellent choice for such applications as power amplifiers and switching regulators. For ex-ample, the IRF-100 N-Channel Power MOSFET can handle up to 16-A ID, it has a typical transconductance of 3 mhos, and its conducting resistance RDS(ON) is 0.2Ω. The characteristic curves for the IRF-l00 are shown in Fig.16 along with its package. The symbol for the VMOSFET is the same as that for the MOSFET. Like MOSFET, care must be taken when handling VMOSFET so as not to damage them with static charges. Some types, however, have built in zener di-odes for gate protection. A zener diode is a special diode that acts like an open circuit when reverse biased, until the reverse voltage across it reaches some specific value. Then the diode breaks down (becomes conductive) and prevents the voltage from getting any higher. The zeners are used to protect VMOSFET break down at 15-18 V, before any damage to the gate can occur. The symbol for the protected gate VMOSFBT is shown in Fig.17.

When using protected gate devices, take care not to drive the gate negative with respect to the source. If the gate goes negative with respect to the source, the protective diode becomes forward biased and can conduct heavily if driven from a low-impedance source. One protective measure is to put a resistor, say 1 kΩ, in series with the gate to limit the diode current. This resistor also minimizes a tendency for the circuit to oscillate as a result of the inductance and capacitance of connecting leads.

Protected-gate VMOSFET.

6.4 Insulated Gate FET (IGFET)

The construction of the IGFET (or MOSFET) is similar to that or the JFET, except that a layer of silicon dioxide is placed between the p-type and the n-type materials. This results in increased input resistance and lower input capacitance. If the IGFET is operated in the same way as the JFET, the output curves are of the same type as those of the JFET. When an n channel IGFET is operated so that the gate is more negative than the source, the device is said

to be operating in the depletion mode. However, with the layer of insulation between the channel and the gate, the IGFET can be operated with the gate more positive than the drain without making the input a forward-bi-ased junction. This keeps the input impedance high and allows a larger region of operation. When the IGFET is operated in this manner, the transistor is said to be operating in the enhancement mode. When the transistor is operated in the enhancement mode, the resulting field increases the conductivity of the carriers and allows more drain current to flow. Typical characteristics of an n-channel IGFET are shown in Fig.18a, and the symbol for the device is shown in Fig. 18b. Notice that the value of VDS at which saturation occurs has increased above the pinch-off voltage by the same amount as the applied gate-to-source voltage.

Thus the parameter rd is the change in drain-to-source voltage divided by the corresponding change in drain cur-rent for a constant value of rGS. This parameter is the reciprocal of the slope in the output characteristics. Notice that, if the transistor is operated in the saturation region, rd, is a very large number. Typical values for rd range from 50 kΩ to 1 MΩ. The reminder given here is that the small-signal parameters should be measured at the operating, or Q-point, of the JFET. Typical values for the small-signal and other pertinent parameters for the JFET are given in Table l.

TABLE 1 TYPICAL JFET PARAMETERS ______________________________________________________________Parameter Value IDSS 10 mAgm 5 mA/V at ID ¬= 8 mArd 80 kΩVp 5 V

Example 1: Given a JFET having the characteristics listed in Table l. sketch typical v-i output characteristics for this device.

Solution: (a) The Slope of the output characteristics is examined first. We assume that the characteristics will be plotted for all values of VDS up to 20 V; then calculate the change in i_D that will occur for this range of values.

The above value of ∆iv is quite small when compared to IDSS, so that the lines will be assumed to be horizontal. (b) The first line is new sketched with Vp = 5 V,iD =IDSS = 10 mA , and VGS = 0 V, as shown in Fig.21(c) Since gm is equal to 5 mA V, the next line can be plotted. The drain current will now be approximately 5 mA less than IDSS for V_GS equal to -1.0V. The saturation of the drain current is reached at vDS= 4.0 V. The result-ing curve is added to Fig. 21.(d) A final curve is added for vGS equal to the pinch-off voltage of -5.0 V. The resulting drain current is zero. Additional curves may be sketched in

Biasing the IGFET can be done in the same fashion as the JFET, if the transistor is used in the depletion mode. If the IGFET operates in both the depletion and enhancement modes, it can be made to operate at zero bias, with the gate-to-source voltage set at zero V. This can be accomplished very simply by the circuit shown in Fig. 22 a, the design procedure being the same as in the example discussed previously.If the IGFET is to be biased in the enhancement mode, this can be accomplished by the circuit shown in Fig. 22b. Here the design procedure is the same as that for H-type biasing of a BJT, except that a large resistance, Rg, is placed between the gate and point X, as shown in Fig. 22 b, to raise the input impedance to an acceptable level. This can be done, since no gate current flows.

6.5 Common Drain Amplifier ( Source Follower)The source follower is analogous to the emitter follower. It offers a very high input impedance, unity gain, and a relatively low output impedance. The major difference between the source follower and the emitter follower is that it need not be biased by a previous stage to exhibit the high input impedance.The JFET is usually the device that is used in making a source follower, since an IGFET, common source am-plifier, already has a very high input impedance. IGFET’s are generally more expensive, and, in many cases, it may be cheaper to use an additional JFET source follower to obtain a higher input impedance.Figure 24 illustrates a circuit that could be used to obtain a practical

source follower. The proper gate-source bias voltage is supplied by the resistor Rs. The biasing procedure is the same as for the common source amplifier.

CHAPTER 7

OPERATIONAL AMPLIFIER SYSTEMS

7.1 Sign Changer

7.2 Scale Changer

7.3 Phase Shifter

7.4 Summing Amplifier

7.5 Noninverting Summing

7.6 Transconductance Amplifier

Chapter 7

OPERATIONAL AMPLIFIER SYSTEMS

Many analog systems (both linear and nonlinear) are constructed with the OP AMP or DIFF AMP as the basic building block. These IC’s augmented by a few external discrete components, either singly or in combination, are used in the following linear systems: analog computers, voltage-to-current and current-to-voltage convert-ers, dc instrumentation amplifiers, voltage followers, and active filters.

BASIC OPERATIONAL AMPLIFIER APPLICATIONS An OP AMP may be used to perform many mathematical operations. This feature accounts for the name oper-ational amplifier. Some of the basic applications are given in this section. Consider the ideal OP AMP of Fig. l. The equivalent circuit of Fig. 1b has a virtual ground (which takes no current), it follows that the voltage gain is given by

Based upon this equation we can readily obtain an analog inverter, a scale changer, a phase shifter, and an adder.

7.1 Sign Changer, or Inverter If Z = Z’ in Fig. 1, then Avf = - 1, and the sign of the input signal has been changed. Hence such a circuit acts as a phase inverter. If two such amplifiers are connected in cascade, the output from the second stage equals the signal input without change of sign. Hence the outputs from the two stages are equal in magnitude but opposite in phase, and such a system is an excellent paraphase amplifier.

7.2 Scale Changer If the ratio Z ’/ Z = k, a real constant, then Avf= - k, and the scale has Bee: multiplied by a factor - k. Usually, in such a case of multiplication by a constant, - l or - k, Z and Z ’ are selected as precision resistors. 7.3 Phase Shifter Assume that Z and Z’ are equal in magnitude but differ in angle. Then the operational amplifier shifts the phase of a sinusoidal input voltage while at the same time preserving its amplitude. Any phase shift from 0 to 3600 (or ± 1800) may be obtained.

7.4 Adder or Summing Amplifier The arrangement of Fig.2 may be used to obtain an output which is a linear combination of a number of input signals. Since a virtual ground exists at the OP AMP input, then

and

and the output is proportional to the sum of the inputs. Many other methods may of course, be used to combine signals. The present method has the advantage that it may be extended to a very large number of inputs requiring only one additional resistor for arch additional input. The result depends in the limiting case of large amplifier gain, only on the resistors involved, and because of the virtual ground, there is a minimum of interaction between input sources. 7.5 Noninverting Summing An adder whose output is a linear combination of the inputs without a change of sign is obtained by using the noninverting amplifier. In Fig. 3 we show such a summer, the output is given by

where the voltage at the noninverting terminal v_+ is found by using superposition. For example, the contribu-tion to , where

R’p2 is the parallel combination of all the resistors tied to the noninverting node with the exception of R’2; that is, R^’p2=R’1 ||R’3 ||R’4 ||…||R’n . For n equal resistors each of value R’2,

and

It is possible to perform analog addition and subtraction simultaneously with a single OP AMP by replacing the resistor R in Fig. 3 by the n input voltages and resistors of Fig. 2. Again superposition is used to find the contri-bution to v_o from any of the input voltages. It should be emphasized that, when one of the voltages v1,v2,…,vn is under consideration, then the positive input terminal is effectively grounded (if the bias current is negligible). Similarly when one of the voltages v’1,v’2,…,v’n is under consideration, then R in Fig. 3 represents the parallel combination of R1,R2,…,Rn. 7.6 Voltage-to-Current Converter (Transconductance Amplifier) Often it is desirable to convert a voltage signal to a proportional output current. This is required, for example, when we drive a deflection coil in a television tube. If the load impedance has neither side grounded (if it is floating), the simple circuit of Fig. 2 with R’ replaced by the load impedance ZL is an excellent voltage-to-cur-rent converter. For a single input v1=vs (t), the current in ZL

Note that i is independent of the load ZL, because of the virtual ground of the operational amplifier input. Since the same current flows through the signal source and the load, it is important that the signal source be capable of providing this load current. On the other hand, the amplifier of Fig. 4a

requires very little current from the signal source due to the very large input resistance seen by the noninverting terminal.If the load ZL is grounded, the circuit of Fig. 4b can be used.

CHAPTER 8

AMPLIFIER CIRCUITS

8.1 Types and Classes of Amplifier

8.2 Negative Feedback

8.3 Testing Amplifiers: Basic Measurements

8.3.1 Measurement of Gain 8.3.2 Measurement of Frequency Response and Bandwidth 8.3.3 Measurement of Input Impedance 8.3.4 Measurement of Output Resistance 8.3.5 Measurement of Power Output, Efficiency and Sensitivity for an Audio Amplifier

8.4 Transient Testing of Amplifiers

8.5 Distortion Measurements

8.5.1 Amplitude Distortion 8.5.2 Frequency Distortion 8.5.3 Phase Distortion 8.5.4 Cross-over Distortion 8.5.5 Intermodulation Distortion

8.6 Faults in Amplifiers

Chapter 8 Amplifier Circuits

8.1 Types and Classes of Amplifier Since there are so many different types of amplifier used in electronics, before going into circuit details it is a good idea to define what is meant by the term amplifier. An amplifier is any device where a small input signal is used to control a larger output power. It follows from this that an amplifier must consist of some active device, such as a valve or transistor; a source of d.c. power; and a load resistor. This is shown in Fig. 1A. Here the input signal is used to control the current that flows through the active device. This current then develops a voltage change across the load resistor, so that the output power isPo=Vo io wattsThe input power Pi=Vi ii watts. POWER GAIN, or power amplification, is given by the ratio of output to input power: Ap=Po/Pi

A more Common Symbol for the amplifier is shown in Fig. 1 B, the signal flow being in the direction of the arrow. Any amplifier increases the power content of its input signal but this is not always the chief consideration. An amplifier may be designed to give primarily voltage gain, current gain or power gain. Thus the first classifica-tion of amplifiers is one which divides them into types designed primarily for power, voltage or current amplifi-cation. We have already seen that power gain Ap=Po/Pi. So it follows that

VOLTAGE gain Av=Vo/Vi CURRENT gain Ai=io/ii It is important to realize that these are all expressions of gain as ratios. In other words if the output

of a voltage amplifier is 2V peak when its input is 100 mV peak, then voltage gain is(2 V)/(100 mV)=(2 V)/(0.1 V)=20

Often the figures involved in working with amplifier gains as ratios can become unwieldy. This is the case when an amplifier exhibits large changes of gain with signal frequency and when these changes have to be recorded graphically. For this reason a logarithmic unit for gain is often used. This is called the Bel. Power gain Ap=log10(Po/Pi ) Bels

A Bel is usually too large a unit for electronic measurements, so tenths of a Bel or decibels (dB) are commonly used. Then

By using decibels for gain units, very large changes in gain ratios can be compressed. The figures are much easier to handle. This can be seen from the following table.

Power gain as ratio | Power gain in dB ________________________|________________________ 10 | 10 dB 100 | 20 dB 1000 | 30 dB 10000 | 40 dB 100000 | 50 dB 1000 000 | 60 dB________________________|________________________

Voltage and current gains can also be expressed in dB, as follows:

This is only strictly true if the amplifier has equal input and output resistance. This is rarely the case, but it is often assumed. The reason why the multiplier 20 is used can be seen from the following:

where Ro = output resistance and Ri = input resistance. If Ro=Ri, then Ap=(Vo/Vi )

2 dB. Therefore

As well as possessing gain, any amplifier also has PHASE SHIFT between its output and input. For example at low frequencies a single stage common emitter transistor amplifier produces an output signal which is an inver-sion of its input. This is shown in Fig. 2, and is explained quite simply by

the fact that, as the input voltage increases, the current flowing into the base increases, which increases the collector current. This current flows through the collector load and therefore the collector voltage falls. At high signal frequencies, the phase shift does not remain exactly at 180°; this is because (a) the current carriers in the transistor take a finite time to reach the collector region and (b) reactive components in the circuit produce addi-tional phase shifts. Phase shifts in an amplifier at high frequencies lead to phase distortion and possibly instabil-ity if a negative feedback loop is used. This will be seen later when negative feedback is discussed. Table 1 Broad classification of Amplifier Circuits

Gain Frequency response Class of operation and typical use

Voltage, Audio and low frequency Class A – Small signal voltage andCurrent Radio frequency (tuned) current amplifiersOr Wideband or videoPower Pulse Class B – Power output amplifiers Direct current Class C – Transmitters and pulse Switches______________________________________________________________________________

A further classification of amplifiers can be made by considering the RANGE of signal frequency over which the amplifier has useful gain. An audio frequency amplifier, for example, should amplify signals over the range from 15 Hz up to perhaps 20 kHz. A graph of amplifier gain against signal frequency is called a frequency response curve. A typical frequency response curve for an audio amplifier is shown in Fig. 3. Note that gain is usually plotted in dB on the vertical axis and frequency on the horizontal axis. Frequency is plotted logarithmi-cally so that a large range can be accommodated. The gain of any amplifier will change because of the reactive components in its coupling and decoupling cir-cuits, stray circuit capacitance and inductance, and because of the frequency limitations of the active devices used. The BANDWIDTH of an amplifier is usually defined as the range of frequencies over which the gain has not fallen by more than 3 dB from its mid-frequency gain. If the response is flat this is equivalent to 50% of max-imum gain for power amplifiers (half power points) and 70.7% of maximum gain for current or voltage am-plifiers. From Fig. 3 it can be seen that the Bandwidth = f2 – f1.Amplifiers can therefore be classified as (a) Audio frequency (AF or LP) (b) Radio frequency (RF). Tuned with narrow bandwidth (c) Wideband or video.(d) D.C. amplifiers. The basic response curves for these types are shown in Fig. 4. For a d.c. amplifier, the active devices must be directly coupled so Special techniques are required to ensure correct biasing. This is discussed later. There is yet a further classification to deal with, namely the CLASS OF OPERATION and the intended use of the amplifier.

There are basically three classes of Operation. Class A The active device (the transistor or valve) is biased so that a mean current flows all the time. This current is either increased or decreased about this mean value by the input signal. This is the most commonly used class, typical examples being small signal amplifiers. Class B The active device is biased just to cut off and is switched into conduction by one half-cycle of the input signal. This class of operation is widely used in push-pull power output amplifiers. Class C The active device is biased beyond the point of cut-off so the input signal must exceed a relatively high value before the device can be made to conduct. This class is used in pulse switching and transmitter circuits.

The preceding discussion of amplifier classification can be rather confusing, but it is important before attempt-ing any repair work on an amplifier that the type of amplifier and its purpose is fully appreciated. The informa-tion in the last few paragraphs is gathered together in Table 1 that sets out the main types of amplifiers in use.

8.2 Negative Feedback No discussion on amplifiers can be complete without mentioning negative feedback. The study of this subject can be very involved, but the basic principles are not too difficult to grasp. An amplifier is said to have negative feedback when a portion of the output signal is fed back to oppose the input signal. This is shown in Fig. 5. Here an amplifier with a gain AO has a portion of its output signal VO, fed back in series with the input in such a way that it opposes the input. The feedback circuit has a fractional gain of β. Therefore the feedback signal is given by

Vf=βVOThe input signal to the circuit is

Vi=Vs+VfBut substituting (l) in (2) we get

Vi=Vs+βVONow the gain of the amplifier is AO = Vo/VS. Thus

VO = AOVSThe gain of the whole circuit, which we shall call AC, is given by

i.e.

The gain of the amplifier without feedback is called the OPEN LOOP GAIN AO. The gain of the amplifier circuit with negative feedback is called the CLOSED LOOP GAIN AC.

The loop referred to is the connection between the output to the input via the feedback network β.The product AOβ is called the LOOP GAIN. It is the gain of the circuit from the amplifier input terminals X—Y to the feedback terminals P—Q. Now if the loop gain is much greater than unity, the gain of the amplifier with negative feedback can be rewritten as follows:

(The l in the denominator can be ignored since it is small compared with AOβ) Therefore

This is an important result since it means that the gain is now dependent only on the characteristics of the feed-back circuit. Thus if the feedback network is a potential divider made by two resistors as shown in Fig. 5 then the amplifier gain is

The gain is therefore given by a ratio of two resistors, and is independent of changes in circuit components, such as current gain changes in transistors. This holds true as long as the loop gain AO β>>1. Negative feedback is widely used for the following reasons:(a) It stabilizes the gain of the circuit, making the gain independent of changes due to components, temperature and power supply lines.

(b) The frequency response is improved, and the bandwidth widened. This can be seen from Fig. 6.

(c) The way in which the feedback signal is derived from the output and applied to the input can be used to modify the input and output impedance of the circuit.

(d) Non-linear distortion and internally generated noise in the amplifier is reduced.

These reasons show that a manufacturer making, say, wideband linear amplifiers, can by using negative feed-back ensure that every amplifier produced has nearly the same characteristics.

A typical example of a two-stage amplifier using negative feedback is shown in Fig. 7. A feedback resistor RF is connected from the output to the emitter of the first stage. The overall gain of the circuit will be given by

This type of feedback is said to be shunt-derived, since the feedback network is in parallel with the output load, and series-applied since the feedback voltage Vf is effectively in series with the input. The output impedance is reduced, while the input impedance is increased. One of the problems associated with negative feedback is concerned with phase shifts round the loop. In a two-stage voltage amplifier the output is phase with the input; this is why in Fig. 7 the feedback is applied to the emitter and not the base. At higher frequencies the reactive components within the amplifier introduce addition-al phase shifts, this in turn changing the phase of the feedback signal. At some frequency the total phase shifts will be such that the feedback signal is adding to input not opposing it. The result is that the circuit will oscil-late. This situation can be avoided by ensuring that the loop gain (AOβ) is less than unity when the total phase shift round the loop is such as to produce positive instead of negative feedback. This is why op-amps such as the 709 have to have a frequency compensation circuit to limit the bandwidth. For this reason also, direct coupling is often used since it eliminates phase shifts due to coupling capacitor Fig. 8 shows

the amplifier of Fig. 7 using direct coupling. An additional feedback path is provided to stabilize the d.c. Operat-ing point via R5.Feedback can, of course, result from fault conditions when for example a decoupling capacitor goes open cir-cuit. In this case the gain will be reduced drastically. Testing amplifiers with negative feedback loops is discussed in the next section.

8.3 Testing Amplifiers: Basic Measurements The various tests that should be made on an amplifier system obviously depend upon the type of circuit under consideration. The basic measurements that should be made are those of gain, frequency response, and band-width. In addition it may be necessary to measure the input and output impedance, the maximum power output, and the efficiency. The latter would apply only to power output stages. All of these measurements can be made, with reasonable accuracy, using the instruments listed below: (a) Stabilized power supply (b) 20 kΩ/V multirange meter (c) Signal generator with sine and square wave output (d) Variable attenuator, calibrated in dB.(e) Oscilloscope. For tests to measure distortion, noise, stability and pulse response more specialized equipment is required, which may include(a) Distortion meter (b) Noise measuring set (c) Spectrum analyser (d) Phase meter (e) Function generator.

8.3.1 Measurement of GainThe layout of the measuring circuit is shown in Fig. 9. Suppose the amplifier’s voltage gain at a frequency of 1 kHz is required. First the signal generator is set to give an output of say 500 mV at 1 kHz, with the attenuator switched to zero dB. This signal, at the amplifier input (point A), is connected to the Y-input of the oscilloscope and the oscilloscope controls are adjusted so that the trace displayed uses a large portion of the screen and has its peaks just on graticule lines. The oscilloscope leads are then moved to the amplifier output (point B) and, leaving the

oscilloscope controls as set, the attenuation is increased until the output is exactly the same height as with the first measurement. The gain of the amplifier is now equal to the setting of the switched attenuator. The advan-tage of this method is that the measurement does not depend upon the accuracy of, the oscilloscope. If the vari-able attenuator has switched ranges down to 0.1 dB, then the result will be obtained to within ±0.1dB.

8.3.2 Measurement of Frequency Response and BandwidthUsing the same set-up as in Fig. 9, the gain of the amplifier can be found at any frequency. The gain, in dB, is then plotted against a frequency on linear/log graph paper. For an audio amplifier 4 cycles of log would be required to cover the frequency range 10 Hz to 100 kHz. The bandwidth can be quickly determined by noting the two frequencies at which the gain falls by 3 dB from the mid-frequency gain.

8.3.3 Measurement of Input Impedance The input circuit of an amplifier can be represented by a resistor in parallel with a low-value capacitor. At low frequencies the input impedance is mostly resistive since the reactance of the capacitor is such a high value. A circuit for measuring input impedance at 1 kHz is shown in Fig. 10. A variable resistor, usually a decade resis-

tance box, is connected between the signal generator and the amplifier input. This

Resistor is set to zero and the amplifier output is connected to the measuring instrument, an oscilloscope or a.c. meter. The controls are set so that a large deflection is indicated. The resistance of the decade box is then in-creased until the indicated output signal falls by exactly a half. Since the resistance box and the amplifier input impedance form a potential divider when the output is halved, the setting of the decade resistance box is equal to the input resistance.

8.3.4 Measurement of Output Resistance The circuit shown in Fig. 11 is used for this measurement. The technique is similar to that of measuring the input impedance. A signal frequency 1 kHz is used and initially RL is disconnected and large deflection obtained on the oscilloscope. The external load RL is then connected and reduced in value until the output falls by exactly a half. The value of RL at which this occurs is equal to the resistance.

8.3.5 Measurement of Power Output, Efficiency, and Sensitivity for an Audio Amplifier For these measurements the loudspeaker should be replaced by a wire-wound load resistor of the same value as the loudspeaker impedance, and the tests should be carried out at a frequency where the loudspeaker impedance would be mostly resistive, typically about 1 kHz. The diagram for the measurement is shown in Fig. 12. The wattage rating of the load resistor should be higher than that of the maximum output power. The input voltage should be adjusted until the output signal indicated by the oscilloscope is a maximum undistorted level. This is when there is no, clipping of the positive and negative excursions of the output signal. Naturally if a distortion meter is available then a more accurate check on distortion levels can be made. Then the maximum output pow-er should be recorded without exceeding the manufacturers’ specified value of harmonic distortion. This may be a value of total harmonic distortion of 0.05% of the output signal.

where VO is the r.m.s. value of the output signal.

The efficiency of the amplifier can be checked by measuring the d.c. power taken by the amplifier from the supply.

The sensitivity of the amplifier is the input voltage required at the input to produce maximum undistorted output power.

8.4 Transient Testing of Amplifiers All the tests previously described are made using an input signal at one frequency. By applying pulses or square waves to an amplifier it is possible to acquire information about the amplifiers frequency response, phase distor-tion, and any tendency to instability. A square wave is made up of a series of pure sine wave components, which area fundamental, having the same periodic time as the square wave, and all odd harmonics. Thus by applying a square wave or pulse to an am-plifier, a large range of signals at different frequencies have to be amplified by the same ratio and without phase shift if the output is to be a perfect replica of the input. For testing low frequency amplifiers, a square wave of 40 Hz or 1 kHz is suitable and the output signal can be observed on an oscilloscope. Departure from squareness in the output signal gives a good indication of the tran-sient distortion that is present in the amplifier. Various conditions are shown in Fig .13. Video and wideband amplifiers can also be usefully tested in this way, but usually a special form of signal called a pulse and bar is used. This, together with various possible outputs, are shown in Fig, 14.

8.5 Distortion Measurements Various types of distortion can effect the shape of the output signal from an amplifier.

8.5.1 Amplitude Distortion The output signal is flattened on one or both of its peaks as shown in Fig. 15. This type of distortion occurs when the amplifier is overdriven by excessively large input signal, or when the bias conditions change, or be-cause of some non-linearity in the characteristics of a transistor or valve.

8.5.2 Frequency DistortionThis results when the amplifier gain changes drastically with frequency within its passband. Suppose an ampli-fier has a frequency response as shown in Fig. 16, which is reasonably flat over the passband, but that the actual response is as shown in Fig. 16B, then the amplifier is said to have frequency distortion. This can take the form of loss of gain at low or high frequencies or increase of gain at low or high frequencies.

8.5.3 Phase DistortionAs the signal frequency is increased so the phase of the output signal relative to the input will change. This type of distortion is troublesome when the input signal is a complex waveform, made up of several sine wave com-ponents all at different frequencies. If these all suffer different phase shifts through the amplifier, the resulting output will not be identical in shape to the input.

8.5.4 Cross-over Distortion Distortion of this type occurs in class B push-pull output stages. In a complementary transistor output stage for example, unless some forward bias is applied, the transistors will not conduct until the input signal to their bases exceeds about 500 mV (this is for a silicon transistor ). See Fig. 17. The purpose of the

bias components is to overcome the distortion by providing a very small amount of forward bias.

8.5.5 Intermodulation Distortion When non-linearity exists in an amplifier circuit, two signals of different frequencies, say 400 Hz and 1 kHz, as well as being amplified will be mixed, and so the output will contain small amplitude signals of the sum and difference frequencies, i.e. at 600 Hz and 1.6 kHz and harmonics of these frequencies. Measurement of distortion levels is usually made using a distortion meter, an instrument which sums the power in all the harmonics and gives the result as a percentage of the output power. This gives the value of the total harmonic distortion resulting from amplitude and non-linear distortion, but does not include frequency, phase or intermodulation distortion. A frequency of 1 kHz is normally used for this measurement. Total harmonic distortion can also be measured by passing the output voltage signal through a filter which at-tenuates the measurement frequency (1 kHz) but passes all harmonics. A good circuit for this is a twin-tee filter as shown in Fig. 18 since this has maximum attenuation at one frequency. The output can be measured using a sensitive r.m.s. millivoltmeter.

Intermodulation distortion can be measured by feeding two signals of 400 Hz and 1 kHz into the amplifier usually with a ratio of about 4:1. Then using a filter at 1 kHz the result of any intermodulation will be indicated using the method detailed previously. A method that can be used to display amplitude distortion, phase shift distortion and harmonic distortion for an audio amplifier is shown in Fig. 19. The signal generator set to 1 kHz is fed to the amplifier input at a suitably low level and to the X-input of the oscilloscope. The output from the amplifier is fed to the Y-input of the oscil-loscope. The oscilloscope

trace will be a straight line at an angle of 45° if the amplifier output is undistorted. Naturally a high-quality os-cilloscope must be used for this test, since any non-linearity in the X and Y oscilloscope amplifiers will also be displayed. Various outputs for different types of distortion are shown in Fig. 19 also.

8.6 Faults in Amplifiers It would not be possible to detail all the possible faults that could occur in all the various types of amplifier cir-cuits. Instead the following is a general guide to assist in fault location. Before considering some typical faults it should be noted that, as well as the d.c. bias levels, the output signal itself is often an invaluable guide to the type of fault. The previous section detailed the types of distortion that could occur. Let’s consider a simple example of how changes in bias components can cause large amounts of amplitude distortion. In Fig. 20A the operating point at the collector is about +5 V to allow equal positive and negative swings at the output. If R1 goes high in value from its nominal 82 kΩ to say 150 kΩ the operating point will now rise to approximately 8 V. The output signal is now distorted on its positive excursion as shown in Fig. 20B. For fault finding on amplifier systems it is best to follow the standard procedure and inject a signal into the input, and by using an a.c. meter or an oscilloscope, check each stage in turn until the

faulty stage is reached. Then measure the d.c. levels at this stage, Table 2 lists some typical faults together with the expected symptoms.

TABLE 2 Typical Faults on Amplifier Systems

Small signal amplifiers FAULT SYMPTOMS Bias component failure open Results in a large change in operating pointcircuit or high value resistors usually tending to cut transistors off. This gives either grossly distorted output or no output at all. ______________________________________________________________________________Short circuit decoupling or Again a large change in operating point usuallycoupling capacitors tending to force transistors to conduct much harder. Grossly distorted output. ______________________________________________________________________________Coupling capacitors No transfer of signal from one stage to next. All open circuit d.c. bias levels normal. No output signal. ______________________________________________________________________________Signal decoupling capacitors Low gain, since series negative feedback is open circuit introduced.______________________________________________________________________________Power line decoupling Increase in “hum” level (100 Hz) at amplifier capacitors open circ- -uit output. The first stage of a preamplifier is normally supplied from a decoupled line.______________________________________________________________________________ Open circuit feedback Excessive gain with instability and possiblyline oscillation. ______________________________________________________________________________Noisy transistor or Poor signal-to-noise ratio.resistor at input (Always check early stages first.) ______________________________________________________________________________Change in coupling and Reduction in bandwidth. decoupling capacitor values Poor low frequency response.to lower value ______________________________________________________________________________

Power amplifiers FAULT SYMPTOMS Bias resistors open circuit For class B amplifiers, the type in common use,or high in value there will be a large amount of crossover distortion. ______________________________________________________________________________Output capacitor short Output fuses blown or transistors overheating. Usecircuit resistance check to find faulty component. ______________________________________________________________________________Bias potentiometer Either (i) increase in cross over distortion orincorrectly set (ii) overheating of output transistors.

CHAPTER 9OSCILLATOR AND TIME BASE CIRCUITS

9.1 Principles of Oscillators

9.2 Measurement of Frequency

9.3 Frequency Stability

9.4 Harmonic Distortion

9.5 Square and Pulse Waveforms

9.6 Sawtooth and Ramp Circuits

9.7 Negative Resistance Oscillators

9.8 Fault Finding on Oscillators

CHAPTER 9Oscillator and Time Base Circuits

9.1 Principles of Oscillators An oscillator is any device or circuit that produces output which varies its amplitude with time. The output may be sinusoidal, square, pulse, triangular, or sawtooth as shown in Fig. 1. These circuits are used in all types of electronic equipment, from radio and tv transmitters and receivers, com-puters, oscilloscopes, signal generators, to digital frequency meters.Oscillators can be constructed using components that exhibit a negative resistance characteristics such as the unijunction transistor and the tunnel diode. The operation of these will be discussed later. However, a large majority of circuits are based round an amplifier with a positive feedback loop. When a portion of an amplifier’s output is fed back in phase with its input, the effective input is increased and so its overall gain.For positive feedback

So, if the loop gain βAO approaches unity

The gain with positive feedback thus approaches infinity. Such high gains will result in oscillations, the frequency of which must be con-trolled by a frequency-determining network. The requirements for a circuit to produce oscillations are (a) Amplification (b) A positive feedback loop (c) Some network to control the frequency (d) A source of power. This is shown in block form in Fig. 2. The network that determines the operating frequency may be part of the feedback circuit or external to it. Typical circuits are combinations of L and C, or R

and C, and we shall consider some of the more basic types first. A good example to start with is the TUNED COLLECTOR oscillator (Fig. 3), a circuit for producing sine wave oscillations at frequencies from a few kilohertz up to 1 MHz. The amplification is provided by the transistor which is connected in common emitter mode. Bias components R1, R2 and R3 cause the transistor to conduct, which then forces the parallel tuned circuit of L1C1 into oscillation. These two components determine the oper-ating frequency, which is given by the familiar formula

In order to maintain oscillations, positive feed-back is provided by the secondary winding of the transformer. This must be connected to give 180° phase shift.

Another method of providing positive feedback in a single stage amplifier, without using a transformer is to use a PHASE SHIFTIING network from the collector to the base. A typical example of a phase shift oscillator is shown in Fig. 4. Each CR network from the collector to the base provides 60° of phase shift so that the resultant feedback is positive. This oscillator is particularly useful for generating sine waves of fixed frequency.

Another circuit which depends upon phase shift is the Wien bridge oscillator shown in schematic form in Fig. 5. This is the standard circuit used in sine wave generators over the range of 1 Hz up to 1 MHz since it can easily be made continuously variable over that range. It has excellent stability and produces an output with low har-monic distortion. An example of this circuit is discussed later.

Square, pulse and sawtooth oscillators usually have an operating frequency that is determined by the charge and discharge times of a capacitor. Such circuits are used as clock pulse generators in digital circuits, time base generators in oscilloscopes, and pulse generators in radar.It would not serve a useful purpose to detail further all the many forms of oscillators that are in current use. Oth-er texts should be consulted for circuits of say the Hartley, Colpitts, Clapp, and so on. What we are concerned with are the important features, or characteristics, of the circuit, and possible fault conditions. Three of the main characteristics are Operating frequency Output amplitude Frequency stability.Depending on the type of circuit however, other important characteristics will be For sine wave oscillators (1)The harmonic distortion (the purity of the sine wave). For square wave and pulse oscillators (1)The, mark-to-space ratio, and width. (2) The rise and fall times. (3) Any overshoot or sag. For sawtooth oscillators (1) The linearity. (2) The flyback time.

9.2 Measurement of Frequency The measurement of frequency and output amplitude can be made using any standard method depending upon the required accuracy. An oscilloscope is perhaps one obvious choice, but even when properly calibrated this only gives an accuracy of about ±3% for both time and amplitude. This may be adequate or a large majority of cases, such as for example the measurement of the frequency of the bias oscillator in an audio tape record-er. When greater accuracy is required the unknown frequency must be measured by comparing it with a stan-dard oscillator of known frequency. Using a CRO to obtain Lissajous figures is one well known method. More commonly the preferred method now is to measure the frequency by a digital frequency meter (Fig. 6). These instruments, fitted with an internal,

highly stable and accurate oscillator, display the unknown frequency on a five digit or more in-line display. The accuracy of such instruments, dependent upon the accuracy of the internal oscillator, can be ±0.01% or better. The instrument counts the number of oscillations of the input frequency that occur within a time period determined by the internal oscillator and the display switch. For example suppose a gating time period of l mil-lisec is selected, and the total count displayed is 1000, then the frequency of the input is 1 MHz.

9.3 Frequency Stability The stability of the output frequency is very important in many applications. Various factors can cause the fre-quency of an oscillator to drift from the preset value. These include (a) Changes in power supply voltage levels. (b) Changes in active component parameter transistor current gains, etc. (c) Changes in load. (d) Variations in components that determine the frequency. The effect of the first three factors can be minimized by using stabilized power units, and a buffer amplifier between the oscillator and load. The biggest cause of instability will come from changes in the components that make up the frequency-determining network. Obviously components with good long-term stability, and very low temperature coefficient, should be used, and quite often these components are housed in a temperature-con-trolled enclosure. To achieve the highest possible stability, the designer must resort to using a piezoelectric-crystal to determine the frequency. Certain substances, typically quartz, when specially cut, will mechanically resonate with an applied voltage. Stabilities of 1 part in 108 are readily attained. Two typical examples of crystal controlled oscillators are shown in Fig. 7A and B. The first circuit is a modified Colpitts circuit with the crys-tal in place of the inductor. The second circuit uses the 710 integrated circuit which is a high-speed differential comparator. Positive feedback is provided from the output to the non-inverting input by the crystal. The d.c. operating point is fixed by R3 and R2, and C1 decouples R2, thus removing negative feedback at the oscillation frequency.

9.4 Harmonic Distortion This results from non-linearity or excessive gain in the amplifier circuit. With most oscillators the gain of the amplifier must be controlled to a value that just maintains the losses in the rest of the circuit. In Fig. 4 for exam-ple, the potentiometer can be adjusted to give more or less negative feedback. If the amplifier gain is too high then the amplitude of the oscillations builds up and the output distorts. Distortion is best measured by a distor-tion-measuring set, but assuming this is not available, another method can be to use a narrow, band stop filter. The oscillator output is passed through this filter, which must have very high attenuation at the oscillator’s fre-quency, but which passes all harmonics. The resulting output from the filter can be measured with a true r.rn.s. reading meter, and this can be used to calculate the percentage harmonic content of the oscillator’s waveform. A twin T-filter is a suitable type.

9.5 Square and Pulse Waveforms Square wave and pulse waveform is shown in Fig. 8, and in some cases the same instrument is used to give either square or pulse out-put. Both waveforms are called rectangular, a square wave being a special case of a rectangular wave with a mark-to-space ratio of one. A typical pulse waveform is shown in Fig. 8, in which the various characteristics are defined. Note that the rise time of the leading edge is measured as

the time taken for the signal level to change from 10% to 90% of its full amplitude. Pulse measurements are made using a wideband oscilloscope, and usually an external synchronizing signal must be provided in order that the trace displays the leading edge of the waveform. With rise time measurements, the rise time of the measuring instrument itself cannot be ignored, and nor must the measuring leads present a relatively high stray capacitance at the oscillator. An attenuating probe must be used to couple the signal into the Y-amplifier of the oscilloscope.The rise time measured on the screen is related to the circuit waveform rise time and the oscilloscope’s rise time by the formula

Wherever the measured value approaches the oscilloscope’s rise time this formula must be applied. Square waves can be generated from relaxation oscillators such as the astable multivibrator or by passing the output of a sine wave oscillator through a squaring circuit such as a Schmitt trigger. This latter method is com-monly used in general purpose laboratory instruments; a typical block diagram is shown in Fig. 9.

9.6 Sawtooth and Ramp Circuits The majority of these circuits are those that produce a waveform which rises steadily with time up to a required amplitude, and then returns rapidly to the point from which the output can again rise. The linear rise is usual-ly called the sweep, and the rapid return the flyback. These circuits find their main use in sweeping the beam across the face of a cathode ray tube, in other words in oscilloscopes, television cameras and receivers, and radar displays. In this case they are usually referred to as timebase circuits. The same type of circuit can be found in digital voltmeters where they are referred to as ramp generators. Circuits like these can be either free running or triggered from an external oscillator, but the basis of all of them is a capacitor which is charged and then rapidly discharged. Apart from the frequency, the important requirement for most circuits is good linearity. This means that the rate of change of the output with time must be uniform. In fact, even in general purpose oscilloscopes, the linearity deviation in the time base should be better than 1%. When a capacitor is charged from zero volts towards a volt-age V via a resistor R, the voltage across the capacitor rises exponentially according to the formula

Thus to obtain reasonable linearity from a simple discharger circuit (Fig. 10), the maximum change in v_c should not be more than 10% of the total voltage.

In this circuit the capacitor charges positively while the input to the transistor switch is zero. When a positive pulse is applied to the transistor switch it rapidly discharges the capacitor. The linearity from a simple cir-cuit such as this is not usually sufficiently high, unless V is very high. Various methods are used to overcome non-linearity, one being to charge the capacitor from a constant current source. A detailed example is dealt with later in this chapter.

9.7 Negative Resistance Oscillators Strictly speaking, negative resistance oscillators and feedback oscillators are identical, since the latter can be regarded analytically as having introduced negative

resistance into the circuit at the operating frequency. This negative resistance makes up the losses in the fre-quency network. However, it is best to classify negative resistance oscillators as those that use devices such as the tunnel diode, tetrode, unijunction transistor, i.e. devices that have an effective negative resistance region in their characteristics. A typical characteristic for a tunnel diode is shown in Fig. 11. The current first rises with forward voltage, then falls with increasing voltage, and finally rises again as the voltage is further increased. Placing a tunnel diode across a resonant circuit as shown in Fig. 12 provides an effective resistanceless tuned circuit which will then oscillate continuously. Very high frequency oscillators, up to as high as thousands of megahertz, are possible. The unijunction transistor (UJT) is made of a bar of n-type material, (sometimes p) with ohmic contacts at each end and a p-type emitter junction formed near the centre (Fig. 13). The resistance of the bar is normally around

10 kΩ, so when connected to a supply, with base 2 positive with respect to base 1, the bar acts as a potential divider, and a p.d. of ղVBB appears between the emitter and B1 where VBB is the voltage between B2 and B1, and ղ is called the intrinsic stand-off ratio (ղ is normally between 0.4 and 0.7).When the emitter voltage is less than ղVBB the emitter junction is reversed. When the applied emitter voltage exceeds ղVBB by about 0.7 V, this voltage being called the peak point, the emitter becomes forward biased and injects holes into the B1 region. Once this happens, the resistance between the emitter and B1 falls to a low val-ue. The action is regenerative.

A typical UJT relaxation oscillator circuit is shown in Fig. 14. Capacitor C charges via R1 and eventually the emitter voltage exceeds the peak point of the UJT. The UJT conducts and discharges C rapidly via R3. As C is discharged the current through the emitter falls, and when it falls below the minimum holding current then the UJT turns off. The capacitor can then charge again to repeat the process. In designing such circuits note that the value of R1 should not be either too low, otherwise the UJT will not be able to switch off, or too high, otherwise the UJT may not receive sufficient emitter current to turn on. Typical values lie between 10 kΩ and 1MΩ. UJT oscillators such as this are commonly used to trigger thyristor and triac circuits.

9.8 Fault Finding on Oscillators Again because the types of circuit in common use are so various the fault finding procedure must be adjusted for the particular circuit and application. As with other parts of an instrument, a good understanding of the pur-pose and operation of the unit is essential. Wherever possible consult the maintenance manual before attempting measurements or adjustments in order to find if any special precautions or test instruments are necessary. A number of oscillator units contain several blocks, such as attenuator, buffer amplifiers, modulators, etc., so a 1ogical approach is essential to locate which block is non-functioning. For example, consider the block diagram of an amplitude modulated RF signal generator (Fig. 15) and suppose a fault exists such that there is (a) An output from Socket A, which is continuous wave with SW1 on position 1, and modulated RF with SW1 on position 2. (b) No output from socket B. The fault can only lie in the AF attenuator block.

TABLE .1 Faults on an Oscillator Circuit FAULT SYMPTOMS ______________________________________________________________________________Positive feedback loop open circuit No output. D.C. bias levels correct.______________________________________________________________________________Negative feedback loop open circuit Output amplitude increased and wave shape distorted. ______________________________________________________________________________Open circuit component on No output on that range only.one switch range ______________________________________________________________________________Bias component open circuit No output all ranges. D.C. levels incorrect.______________________________________________________________________________ Switching transistor in ramp Output from ramp permanently high.generator open circuit ______________________________________________________________________________ Emitter follower with low or short Output from ramp reduced and non-linear.circuit input impedance in ramp generator ______________________________________________________________________________Crystal open circuit in crystal Circuit will probably still oscillate but at a controlled oscillator different frequency with poor stability

For individual oscillator circuits various faults can occur. Table 1 1 lists a few common symptoms.

CHAPTER10PULSE AND WAVEFORM SHAPING CIRCUITS

10.1 Introduction

10.2 Linear Passive Circuits - the Integrator and Differentiator.

10.3 Diode Waveform Shapers

10.4 Active Pulse Shaping Circuits

10.5 The Schmitt Trigger Circuit

10.6 The Monostable

10.7 Fault Finding in Pulse and Waveform Shaping Circuits

Pulse and Waveform Shaping Circuits

10.1 Introduction Much of modern electronics is concerned with shaping and modifying pulses and other signals. The main pulse-forming and waveform-shaping networks can be grouped into (1) Linear passive circuits: those made up of R, C and L elements. (2) Non-linear passive circuits: diode clippers and restorers. (3) Active circuits: those that use transistor switches such as the Schmitt trigger and monostable. Some of the more common examples will be dealt with in this chapter. Such circuits will be found in colour tv receivers, radar sets, and in fact in nearly all electronic equipment.

10.2 Linear Passive Circuits — the Integrator and Differentiator Circuits that contain only R, C or L components are termed linear, since they do not affect the shape of a sine wave input signal. They only produce attenuation and phase shift for a pure sine wave. However, they greatly modify other wave shapes. The INTEGRATOR, often called a low pass filter, is shown in Fig.1 together with output signals for typical in-puts. Since the reactance of the capacitor falls with increasing frequency, this circuit removes the high-frequen-cy components from a pulse wave-form. When a step input is applied the voltage across the capacitor cannot change instantaneously. It rises exponentially according to the formula

Now CR, the product of capacitance in farads and resistance in ohms, is called the TIME CONSTANT of the circuit. In one time constant the voltage across the capacitor changes by about 63%. Note that it takes nearly 4.5 time constants for the voltage across the capacitor to equal V. The effect of an integrator on pulses which have a long width in comparison to the “integrator’s” time constant is to degrade the rise and fall times. If, however, the pulse is short in comparison to the “integrator’s” time con-stant, then the capacitor will not have sufficient time to charge completely, and the output will appear triangular. Circuits such as these are often used to provide short time delays. An example is shown in Fig. 2. The DIFFERENTIATOR, basically a high pass filter, allows high frequencies to pass, but attenuates the low fre-quencies. The circuit together with outputs for various pulse inputs is shown in Fig. 3. When a step waveform is applied, and assuming that C is uncharged, then the voltage across the capacitor cannot change instantaneously. The voltage across a capacitor can only change when it acquires some charge, and this naturally takes time. So the output must rise to the same value as the input. As C charges the voltage across it increases, and the output voltage across R falls exponentially:

The result is that a “spike” equal to the change of state at the input is generated. If an input pulse that is long compared to the differentiator’s time constant is applied, then the output must go negative on the trailing edge. This occurs because the capacitor, already charged by the leading edge of the pulse, cannot change its voltage instantaneously when the trailing edge arrives. The left-hand plate of the capac-itor will be at +V and the right hand plate at 0 V. When the input changes abruptly from +V to 0 the output must now change from 0 to -V. Differentiators are often used to convert pulses of one polarity into “spikes” of the opposite polarity. These spikes can then be used to trigger other circuits.

10.3 Diode Waveform Shapers

Diodes, because of their property of conduction in one direction only, are widely used to remove portions of a signal (clipping) and to clamp a waveform to a reference level (restoring). CLIPPING CIRCUITS are used where only a portion of the input signal is required. A diode, either in series with the signal path or par-allel to it, is used to remove part of the signal that lies either above or below a d.c. reference level. A typical series diode clipper is shown in,

Fig. 4. The diode only conducts when the input signal exceeds the reference bias level VB. Naturally when the diode conducts, its

forward slope resistance forms a potential divider with R to the input signal. The output signal then will be attenuated slightly, i.e.

Therefore, to minimize errors, R is normally made much greater than r_d. In practice this is readily achieved since the diode slope resistance r_d is low, typically less than 100Ω. By reversing the diode the output signal would contain only that portion of the input below VB, and the upper portion would be clipped. Similar effects can be achieved by connecting the diode in parallel with the signal as in Fig.5. Here if the input exceeds VB the diode conducts and limits the output. A circuit can be made using two diodes to clip both the positive and negative portions of a waveform. Such circuits are sometimes called “slicers” and can be used to convert sine waves into semi-square waves. An example of a diode circuit used with a differentiator is shown in Fig. 6. Here the diode is used to remove the positive portion of the waveform so that only a negative “spike” is generated. A CLAMPING CIRCUIT is one which fixes one of the peaks of an alternating waveform to a d.c. reference voltage. Note that a clamper should ideally not affect the wave shape. A typical circuit of a clamper to 0V (d.c. restorer) is shown in Fig. 7. For correct operation the time constant of the CR network should be much greater than the input pulse width. The output can only be clamped correctly when the input consists of a train of pulses as shown.

The capacitor charges when the input waveform goes negative since the diode conducts heavily. After a few cycles of the input waveform the capacitor becomes charged to the peak value of the input, and therefore shifts the mean output level positive. By reversing the diode the positive peaks could be clamped to 0 V. A good example of the use of diodes is in a field-synchronizing-pulse generator used in a mono-chrome tv receiver. An example is shown in Fig. 8 together with the waveforms. The diode D1 is cut off by each negative synch pulse and this allows C2 to discharge through R2. When the synch pulse returns positive, D1 conducts to recharge C1 rapidly. The time constant of C2R2 is such that only a small negative signal is generated at D2 anode by the narrow line synch pulses. However, when the broad field synch puls-es are present, a large negative signal is generated. A diode clipping circuit D2R3R4 removes any line synch information, and the signal is differentiated by C3R5 to produce a series of sharp field pulses at the output.

10.4 Active Pulse Shaping Circuits These are circuits that utilize the switching properties of transistor, tunnel diodes, FETs, etc. to reshape input signals. Under this gen-eral heading we shall be considering the Schmitt trigger circuit and monostables.

A simple TRANSISTOR SWITCH is shown in Fig. 9. The tran-sistor switch is often operated in the saturated mode. This is when the input signal overdrives the transistor and therefore switches it hard ON. Under these conditions the voltage at the collector falls to a low value. This voltage, called the collector emitter saturation voltage VCE(sat), may have a value from 0.1 V to 0.6 V depending upon the type of transistor in circuit. The transistor is OFF when no input signal is present, or when the input is lower than the required base emitter voltage. Under these conditions the collector voltage is high, at V¬CC, and the only current flowing through the collector load is the transistor’s leakage current. With modern silicon transis-tors this is very small, and in most cases can be neglected.When switched to the ON position there is a finite time before the output falls to VCE(sat) this is the transition time for charge carriers to move through the transistor. When switched OFF, however, there is a finite delay time before the collector current ceases, because minority charge carriers are stored in the base region of a saturated transistor. These charges have to be removed before the transistor switches to the OFF position. The waveforms showing transition and storage delays are shown in Fig. 10. Often switching speeds are improved by the use of (a) Speed-up capacitors: capacitors in parallel with drive resistors. (b) An anti-saturation modification. The latter is a circuit that prevents the transistor from saturating (Fig. 1l). The diode D1 is used to hold the collector voltage at a lev-el that just prevents saturation. When an input is applied the collec-tor voltage falls, but when the collector voltage is lower than that of the base, the series diode (often germanium) conducts and diverts the excess input current into the collector. You may find circuits in use with R2 replaced by a silicon diode.

10.5 The Schmitt Trigger Circuit This circuit, shown in Fig. 12, is used for level detection, reshaping pulses with poor edges, and squaring sine wave signals. The Schmitt is basically a snap-action switch that changes state at a specific trip point. Consid-er conditions when the input to Tr1, base is at zero. Tr1 is conducting since it has forward bias provided by the potential divider R2, R3 and R4. The voltage at Tr2 base is approximately

The voltage at Tr1 and Tr2 emitter will be 0.7 V less than VB2 and this positive voltage reverse biases Tr1 , there-by holding it off. The current flowing through Tr2 is determined by

and the collector voltage of Tr2 will be

Usually the circuit is designed so that Tr2 is not saturated, thus allowing faster switching speed. When the input voltage is increased so that it nearly equals the voltage on Tr2 base, then Tr1 starts to conduct, its collector voltage falls, and Tr2 starts to turn off. Because of the positive feedback between the emitters,Tr1 rapidly turns on and Tr2 off. The output voltage rises to +VCC. A special feature of the Schmitt is that the circuit does not switch back as soon as the input signal is reduced just below the threshold level or trip point, but at a much lower level. The circuit possesses hysteresis or backlash, and this is very useful in eliminating

noise superimposed on the input signal.But the reason for hysteresis can be seen by considering that the circuit changes state at the point when the two base voltages are equal. When the threshold level or upper trip point is passed, Tr1 switches on and its collector voltage falls. This means that Tr2 base voltage also falls, so in order for the circuit to switch back to its original

state the input voltage to Tr1 base must be reduced to a value equal to the lower voltage on Tr2 base. The effect of the hysteresis of the circuit is shown in Fig. 13, where it can be seen that the output switches back only when the input is reduced below the lower trip point. A dual Schmitt trigger is available in TTL integrated circuit (SN 7413), and this is often used in interface circuits to improve the noise margin.

10.6 The Monostable The monostable multivibrator, sometimes called a “one shot” is a circuit that is widely used for generating an output pulse of fixed width and ampltude. This output pulse is only produced when the circuit is triggered into operation by a narrow input pulse. The monostable can be made using discrete components or is available in an integrated circuit package (SN 74121). The most common form for producing the circuit using discrete compo-nents is shown in Fig. 14, but it should be noted that there are several variations of this. These include emitter coupled and complementary types. The basic circuit can be seen to consist of a two stage amplifier with resistive coupling from output to input. As the name suggests, the circuit has one fixed stable state. This is with no input trigger pulse, when Tr2 is ON and Tr1 is OFF. Tr2 conducts because it has forward bias provided by R_t. This resistor has a value low enough to provide sufficient base current to just drive Tr2 into saturation. The collector voltage of Tr2 will then be approx-imately 0.1 V, and this ensures that Tr1 is held off in a non-conducting state. The circuit can be switched into a “quasi-stable” condition by applying a positive pulse to Tr1 base. This need only be of short duration, as its purpose is merely to trigger the circuit into operation. The pulse causes Tr1 to conduct and its collector voltage falls. This change in voltage is coupled via CT to Tr2 base. Remember that the voltage across a capacitor cannot change instantaneously, so a change of voltage on one plate appears as an equal change on the opposite plate. The voltage on Tr2 base therefore goes negative and this turns off Tr2 . The collector voltage rises towards VCC and, because of the positive feedback via R3, Tr1 is forced to conduct more. Very rapidly the circuit switches state so that Tr1 is ON and Tr2 is OFF, but this is not a permanently stable condition. The base of Tr2 is negative, while Tr1 collector is at approximately +0.1 V, so the capacitor CT now charges via Rt and Tr1 so that its right-hand plate moves from -VCC towards +VCC. When this voltage reaches nearly +0.6 V, Tr2 begins to conduct again, and the circuit rapidly switches back to its stable state. The output from Tr2 collector is thus a positive pulse of amplitude approximately VCC and with a defined width. The width of the pulse is determined by the time constant Ct Rt, and is approximately

Various modifications can be made to improve the performance of the monostable, and circuits will be found in current use that use catching diodes, protection diodes, speed-up capacitors, etc. Most of these modifications are used to improve the operating speed of the circuit so that an output pulse with fast rise and fall times is achieved. A typical best figure for the rise time is 10ns. The circuit shown in Fig.15 includes catching diodes which operate to limit the output amplitude and improve speed. When Tr2 switches off, its collector load has to charge any capacitive load which is formed by CL and stray circuit capacitance. Initially the rate of rise is fast but, without the diode, the total rise time would be relatively slow. With the diode in circuit, the change in volt-age at the collector is limited and therefore the rise time is improved.

10.7 Fault Finding in Pulse and Waveform Shaping Circuits Some of the faults that occur in pulse shaping and switching circuits are different in form from those in other units, such as amplifiers and power supplies. Quite often the signal is degraded in some way, so that the required wave shape is not produced. This may happen with or without a change in d.c. bias conditions. Locating such faults requires a good understanding of the circuit function. Another example of the special faults that can occur is in a switching circuit, such as a monostable, which produces output pulses or changes state when no input is present. This is called “spurious triggering” and it can be, in some cases, very difficult to locate the cause of the problem. The fault may lie in the circuit itself, but more usually will be caused by interference. This is “noise”, either picked up on the input leads or travelling along the mains wires (mains borne). Such interference is often caused by rotating electrical machines or switching surges from heavy inductive loads situated near the elec-tronic unit. In a very noisy industrial environment special care has to be taken in the design and installation of

electronic units. This involves screens, mains filters, screened leads, and high noise immunity interface circuits. An example of the latter is discussed later. When testing wave-shaping circuits ensure that the bandwidth of the oscilloscope that you use is sufficient for the measurement. It is good practice to use an attenuating probe so that the capacitive loading of the measuring leads are kept to a minimum. With circuits that contain active switches together with linear and non-linear wave-shapers it’s a good idea to follow a standard sequence in fault location:

(a) Measure the power supply lines with a multi- meter. The voltages should be near the correct value and the ripple should be low. (b) Check that the input signal is present. Often the input is generated from a transducer (photocell, thermis tor) or a mechanical switch. Since the input-actuating device is usually positioned well away from the unit, it is much more prone to damage than internal components.(c) Check that input leads and plug and socket connections are good. Ensure that any screen leads are prop erly earthed.(d) If the actuating device is OK, cause the input signal to change state rapidly by operating the input de vice, or apply a suitable input from an oscillator. Then, by following the signal flow from input to output, check each stage until the signal is either degraded or lost. This will locate the faulty stage.(e) The operation of individual transistor switching circuits can be checked without unsoldering the de vice. Remember that the circuit will be causing the transistor to be either ON or OFF. For a transistor that is ON, momentarily short the base to emitter. The transistor should turn OFF and its collector voltage rise to VCC. If the circuit conditions are holding the transistor in the OFF state, check that it can be turned ON by applying a forward bias current. Use a resistor of about 10 kΩ temporarily connected from the supply rail to the base connection. The transistor should turn on and its collector emitter voltage should fall to a low value (typi-cally 100 mV). See Fig. 16.

CHAPTER 11CIRCUITS USING ANALOGUE AND DIGITAL INTEGRATED CIRCUITS

11.1 Introduction to Integrated Circuits

11.2 Analogue ICs

11.3 Digital ICs

11.4 Servicing Instruments Containing ICs

Circuits using Analogue and Digital Integrated Circuits

11.1 Introduction to Integrated Circuits Think of some discrete circuit that you have built, say the monostable and then imagine this circuit made much smaller and completely enclosed in plastic, leaving only the connecting pins showing. What we have then is an integrated circuit, i.e. an encapsulated unit containing all the necessary diodes, transistors and other compo-nents for a particular function. The first integrated circuits were in fact made in this way; but the mass-produced ICs of today are nearly all silicon monolithic types. In a monolithic IC, all the elements are diffused and inter-connected in one piece of silicon. This small piece of silicon is referred to as the chip. The word monolithic comes from the Greek “mono”, meaning single, and “lithos”, meaning stone (in this case the stone is silicon). Other types of IC include the film circuits (both thick and thin films). In these, conducting and resistive tracks are formed on the surface of an insulating inert substrate. To complete the circuit, tiny active components such as diodes and transistors are bonded in position and the unit is then encapsulated. In general film ICs are used where the ratio of passive to active devices is fairly high. The advantages of ICs over discrete circuits are mostly due to the fact that many more active elements can be fitted into a small space. This high component packing density gives low cost, higher reliability, and the possi-bility of producing circuits that could hardly be justified using discretes. From small-scale integration has grown the medium-scale integrated circuits (MSI); then the large-scale integrated circuits (LSI); leading up to very-large-scale integration (VLSI) used on the larger microprocessor and memory ICs. A VLSI circuit may contain more than 100 000 transistors. It follows from the preceding paragraph that the IC must be considered as a functioning block within a system and that, when a fault occurs within the IC, the whole IC must be replaced. In many cases this can make the task of fault finding easier than in systems using discrete components; but it would be a mistake to assume that no knowledge of the internal operation of the IC is required. Having a good understanding of the function of the ICs in a system and the way in which they work is essential. It enables the fault-finder to sort out faults that are definitely not the IC and therefore prevents the unsoldering and replacing of a perfectly good IC when the fault is either an external component or a bad connection. Integrated circuits are usually sub-divided into two main areas: Analogue ICs: those used to amplify, process or operate on input signals that can vary anywhere between defined limits. Digital ICs: the type used in logic and computer systems where the inputs are usually either high (logic 1) or low (logic 0) but not any value between these states. A table can be compiled as in Table 1 to show the distinction between IC types, but it should be noted that not all types are included and also that there may be some overlap.

TABLE 1 ______________________________________________________________________________Linear ICs | Digital ICs |Operational amplifiers | DTL | Logic gates AND/OR, (typical 741) | | NAND/NOR.Audio amplifiers (typical LM380) | TTL | Schmitts. MonostableRF amplifiers | ECL | Bistables. Counters.Wideband amplifiers | | shift registers.Power amplifiers (with heatsink attached) | MOS | Memories. Clocks. Voltage regulators | CMOS | Microprocessors. Demodulators |__________________________________________|___________________________________

11.2 Analogue ICs A true analogue signal is one that, at any instant in time, can be any value within a defined range. For example the output of a microphone is an electrical signal that is analogous to the input sound wave; in other words the microphone output signal varies in time in a similar fashion to the sound wave. A circuit used to amplify this small signal must be an analogue type in order to preserve the waveshape. Analogue ICs are also sometimes re-ferred to as LINEARS since the response required from the circuit has to be reasonably linear in order to avoid distorting the signal as it is being amplified. By far the most commonly used analogue ICs are OPERATIONAL AMPLIFIERS (op-amps). These versatile devices can be used in a wide variety of amplifying, signal processing and waveform-generating applications. An op-amp is basically a low-drift d.c. amplifier with very high open loop differential gain and good common mode rejection. The symbol for an op-amp together with the pin out data for the 741 IC are shown in Fig. 1. There are two input terminals, one called the inverting input (marked —); and the other called the non-inverting input (marked +). The output, assuming zero offset, will be the difference in signal between the two inputs multiplied by the open loop gain:

Here (V1-V_2 ) is the differential input and, ideally, when V1 = V2 the output should be zero. In practice some offset always occurs (caused by the slightly different characteristics of the two input transistors). With both inputs held at zero volts, the resulting output caused by the input offset voltage can easily be trimmed to zero by some offset nulling technique. Most IC op-amps such as the 741 are provided with simple facilities, as shown, to achieve this.

Since the open loop gain Avol is so large (typically 100 dB at d.c. and low frequencies), only a tiny difference in voltage at the input pins is necessary to give a large output. Note 100 dB = 100 000 as a voltage ratio. Take an example with V1 = +150 µV and V2 = +100 µV. Then VO = 100 000 (150 x 10-6 - 100 x 10-6) = +5 V The output will be at approximately +5 V provided that the power supplied to the IC is at least ±7 V. If the in-puts are now changed, giving V1 = +100 µV and V2 = +150 µV, the new value of output will be VO = 100 000 (100 x 10-6 - 150 x 10-6) = -5 V The output is inverted. The examples are illustrated in Fig.2 together with some exercises. In each case calculate the approximate d.c. output. Assume that any error due to input offset is negligible. The following extract from the 741 specification gives an indication of the quality of an IC op-amp.

The high value of open loop gain is obtained only at d.c. and very low frequencies. An internal capacitor, which gives the op-amp stability and prevents oscillations, causes the open loop gain to fall with increasing frequency. Fig.3 shows the open loop frequency response curve, from which it can be seen that the amplifier has useful gain up to nearly 1 MHZ.

There are many other IC op-amps available, some with FET inputs for high input resistance, others with low drift or fast slew rate. Slew rate is the speed at which the output changes when driven by a step input and is specified in volts/µsec under closed loop unity gain conditions. Slew rate limiting has the effect of turning a sine wave input (of a few kilohertz) into a triangular wave output. The 741s is an example of an op-amp with better slew rate performance, 20 V/µsec as compared with 0.5 V/µsec for the standard 741. It is a direct replacement for an 8-pin 741 but will give a full-power bandwidth of 200 kHz. By using suitable external components in association with an IC op-amp, circuits of amplifiers, oscillators, waveform generators, and active filters can be readily designed. Some typical examples are shown in Fig.4. The first circuit shows a non-inverting amplifier. The input signal is a.c. coupled to pin 3 and R2 sets the input resistance of the

circuit. A portion of the output signal is fed back to the inverting input to oppose the input signal. The gain is set by R1 and R3 and is given by

The second example shows a 741 used to produce square waves. When the power is first applied, C_t will be uncharged, so the op-amp output will saturate at its positive level ( Vsat

+). A portion of this output voltage is fed back via R2 and R1 to the non-inverting input. The voltage on the non-in-verting input will be

As Ct charges via Rt the voltage on the inverting pin rises positive. When this voltage just exceeds the level on the non-inverting input, the op-amp switches rapidly to its negative saturated level V-sat. The voltage on the non-inverting input also reverses to become negative. The capacitor Ct now discharges via Rt towards V-sat until the voltage on the inverting terminal is just more negative than the level set up on the non-inverting input. When this occurs, the op-amp output is again forced to switch to V+

sat and the cycle recommences. In this way the circuit produces continuous square waves. Note that both feedback paths control the frequency since the Rt Ct time constant will determine the charge and discharge rate, while the potential divider R2R1 determines the switching points. The frequency is given by the formula:

11.3 Digital ICsIn digital circuits the input data is represented by groups of “highs” or “lows”. Using positive logic convention, a high level is represented by a positive voltage and a low level by a voltage at or near zero. Thus the inputs and outputs of digital ICs are signals switched between two well defined states. With TTL, logic 0 is typically 200 mV (not greater than 400 mV) and logic 1 is typically 3.3 V (not less than 2.4 V). The particular advantages of using information in digital rather than analogue form are (1) A signal is indicated as either high or low so there is less ambiguity and much less chance of error. (2) Digital information can be easily transmitted, stored, and processed without degradation. (3) Data can be reshaped after transmission. (4) Noise and interference has much less effect. (5) Many two-state devices exist. The last point shows that digital logic is based on two-state devices, i.e. the device is either ON or OFF and gives either a HIGH or a LOW output. Digital ICs can be grouped into combinational and sequential types. With COMBINATIONAL LOGIC, various input conditions must be met simultaneously to give an output. Therefore combinational logic is made up of gates such as the AND, OR, NAND, NOR, NOT and exclusive OR. In SEQUENTIAL LOGIC, the digital elements possess a memory and the resulting output from a sequential logic IC will depend upon the input and the previous state of the circuit. The basic building block of all sequen-tial circuits is the bistable. These can be connected together, often inside an IC, to make counters, shift registers, and memories. It is not proposed to explain here the theories of digital logic or in great detail how the internal circuit of a digital IC operates (except where required in the exercises). This chapter is intended only as a brief introduction to the subject. However some knowledge of the various digital IC logic families is essential. The earlier types such as RTL (resistor transistor logic) and DTL (diode transistor logic) are now obsolete. However there is much equipment using DTL still in use. The main important logic families are as follows.

(1) Transistor Transistor Logic (TTL) A widely used logic type that is available with many functions (look up a manufacturer‘s catalogue), TTL combines fast speed with moderate power consumption and reasonable levels of noise immunity. It was devel-oped as a successor to DTL and is continually being updated. Later versions use Schottky type transistors which improve the switching speed.

TTL types Standard 74 series Schottky 74S Low power Schottky 74LS Advanced low power Schottky 74ALS High speed 74H being Lower power 74W phased out

(2) Complementary MOS Logic (CMOS) These are made from combinations of p and n channel enhancement mode MOSFETs. This type of construction results in a number of particular advantages when compared to TTL. The unique features are (a) A very low power consumption (about 10 nW/gate under d.c. conditions). (b) A wide operating supply voltage range (+3 V to +15 V). (c) A very high fan-out (at least 50). (d) Excellent noise immunity (45% of the supply voltage). Therefore CMOS logic is often chosen for low-cost low power consumption systems, especially those for use in an electrically noisy environment and where speed of operation is not the prime consideration. The devices are not as fast as TTL. The type numbers are the 4000 series.

(3) Emitter Coupled Logic (ECL) This is more rarely used and does not have the same extensive range as TTL and CMOS. It has a fast operating speed, typically 2 nsec, but a relatively high power consumption. Type numbers are the 10 000 series. 11.4 Servicing Instruments Containing ICs Since the IC itself constitutes a functional block, with a minimum of external components being used, it is obvious that a failure of one part inside the IC will lead to a complete loss of performance and the IC will then have to be replaced. One internal component failure renders the whole IC useless. Naturally ICs are designed to give very high reliability but failures will and do occur. Some of these failures will be caused by the natural environmental stresses acting on the IC, weakening perhaps an internal connecting lead and finally causing an open circuit. Alternatively “spikes” on the supply rails, or large current surges at switch-on, can cause failure at semiconductor- junctions. It is the designer’s job to ensure that the power supply is well regulated and filtered. TTL logic, for example, has a maximum supply voltage rating of 7 V; no fault must be allowed to occur in the power supply which would put an over-voltage on to the ICs. This would cause many of them to over-heat and burn out. When testing equipment containing ICs take care not to short pins by using large test probes, avoid the use of excessive heat when unsoldering a component, and never remove or plug in an IC to its socket without first switching off the power supply. This is when excessive surge currents can occur, and it is possible for a com-plete batch of ICs to be destroyed one after another by an unskilled operator. For fault finding on IC units follow a logical procedure: (a) Check power supply at the IC pins. Is it within its rated value? Is the ripple level low? If the answer is yes, proceed. (b) Make sure that the required input is present at the IC pin indicated on the diagram. (c) Check for a suitable output. (d) Check visually and with a meter for any open or short circuits in the copper track to the IC. Several aids for servicing ICs are available, such as IC inserters, test clips (reduce risk of accidental shorting), logic probes, etc. Use them wherever possible. If an IC has to be removed by unsoldering, always use a desoldering tool to remove the solder from each IC pin in turn until the IC can be lifted out. It’s worth taking time over this job to avoid damaging the copper track of an expensive printed circuit board.