lic qb with answer

8/7/2019 Lic Qb With Answer

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SBM COLLEGE OF ENGINEERING AND TECHNOLOGY, DINDIGUL 624005

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

Branch : ECE Subject : Linear Integrated Circuits

Year : II Subject code: EC1254

Sem : IV Faculty : Mr. S.Senthil KumaranQUESTION BANK

UNIT-I

TWO MARK QUESTIONS:

1. Mention the advantages of integrated circuits.

*Miniaturization and hence increased equipment density.

*Cost reduction due to batch processing.

*Increased system reliability due to the elimination of soldered joints.

*Improved functional performance.

*Matched devices.

*Increased operating speeds.

*Reduction in power consumption.

2. Write down the various processes used to fabricate ICs using silicon planar technology.

*Silicon wafer preparation.

* Epitaxial growth

*Oxidation.

*Photolithography.

*Diffusion.

*Ion implantation.

*Isolation.

*Metallization.

*Assembly processing and packaging.

3. What is the purpose of oxidation?

*SiO2 is an extremely hard protective coating and is unaffected by almost all reagents.

*By selective etching of SiO2, diffusion of impurities through carefully defined windows can be

accomplished to fabricate various components.

4. Why aluminium is preferred for metallization?

*It is a good conductor.

*it is easy to deposit aluminium films using vacuum deposition.


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*It makes good mechanical bonds with silicon.

*It forms a low resistance contact.

5. What are the popular IC packages available?

Metal can package.

Dual-in-line package.

Ceramic flat package.

6. Define an operational amplifier.

An operational amplifier is a direct-coupled, high gain amplifier consisting of one or more

differential amplifier. By properly selecting the external components, it can be used to perform a

variety of mathematical operations.

7. Mention the characteristics of an ideal op-amp.

* Open loop voltage gain is infinity.

*Input impedance is infinity.

*Output impedance is zero.

*Bandwidth is infinity.

*Zero offset.

8. What happens when the common terminal of V+ and V- sources is not grounded?

If the common point of the two supplies is not grounded, twice the supply voltage will get

applied and it may damage the op-amp.

9. Define input offset voltage.

A small voltage applied to the input terminals to make the output voltage as zero when the two

input terminals are grounded is called input offset voltage.

10. Define input offset current. State the reasons for the offset currents at the input of the

op-amp.

The difference between the bias currents at the input terminals of the op-amp is called as input

offset current. The input terminals conduct a small value of dc current to bias the input

transistors. Since the input transistors cannot be made identical, there exists a difference in bias

currents.

11. Define CMRR of an op-amp.

The relative sensitivity of an op-amp to a difference signal as compared to a common mode

signal is called the common mode rejection ratio. It is expressed in decibels.

CMRR= Ad/Ac

12. What are the applications of current sources?


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Transistor current sources are widely used in analog ICs both as biasing elements and as load

devices for amplifier stages.

13. Justify the reasons for using current sources in integrated circuits.

* Superior insensitivity of circuit performance to power supply variations and temperature.

* More economical than resistors in terms of die area required providing bias currents of small

value.

* When used as load element, the high incremental resistances of current source results in high

voltage gain at low supply voltages.

14. What is the advantage of widlar current source over constant current source?

Using constant current source output current of small magnitude (micro amp range) is not

attainable due to the limitations in chip area. Widlar current source is useful for obtaining small

output currents. Sensitivity of widlar current source is less compared to constant current source.

15. Mention the advantages of Wilson current source.

*provides high output resistance.

*offers low sensitivity to transistor base currents.

16. Define sensitivity.

Sensitivity is defined as the percentage or fractional change in output current per percentage or

fractional change in power-supply voltage.

17. What are the limitations in a temperature compensated zener-reference source?

A power supply voltage of at least 7 to 10 V is required to place the diode in the breakdown

region and that substantial noise is introduced in the circuit by the avalanching diode.

18. What do you mean by a band-gap referenced biasing circuit?

The biasing sources referenced to VBE have a negative temperature co-efficient and VT has a

positive temperature co-efficient. Band gap reference circuit is one in which the output current is

referenced to a composite voltage that is a weighted sum of VBE and VT so that by proper

weighting, zero temperature co-efficient can be achieved.

19. In practical op-amps, what is the effect of high frequency on its performance?

The open-loop gain of op-amp decreases at higher frequencies due to the presence of parasitic

capacitance. The closed-loop gain increases at higher frequencies and leads to instability.

20. What is the need for frequency compensation in practical op-amps?

Frequency compensation is needed when large bandwidth and lower closed loop gain is desired.

Compensating networks are used to control the phase shift and hence to improve the stability.

21. Mention the frequency compensation methods.

*Dominant-pole compensation


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*Pole-zero compensation.

22. What are the merits and demerits of Dominant-pole compensation?

*noise immunity of the system is improved.

*Open-loop bandwidth is reduced.

23. Define slew rate.

The slew rate is defined as the maximum rate of change of output voltage caused by a step input

voltage. An ideal slew rate is infinite which means that op-amps output voltage should change

instantaneously in response to input step voltage.

24. Why IC 741 is not used for high frequency applications?

IC741 has a low slew rate because of the predominance of capacitance present in the circuit at

higher frequencies. As frequency increases the output gets distorted due to limited slew rate.

25. What causes slew rate?

There is a capacitor with-in or outside of an op-amp to prevent oscillation. It is this capacitor

which prevents the output voltage from responding immediately to a fast changing input.

16 MARK QUESTIONS:

UNIT II

TWO MARK QUESTIONS:

1. Mention some of the linear applications of op amps:

Adder, subtractor, voltage to- current converter, current to- voltage converters, instrumentation

amplifier, analog computation, power amplifier, etc are some of the linear op-amp circuits.

2. Mention some of the non linear applications of op-amps:-

Rectifier, peak detector, clipper, clamper, sample and hold circuit, log amplifier, anti log

amplifier, multiplier are some of the non linear op-amp circuits.

3. What are the areas of application of non-linear op- amp circuits?

. Industrial instrumentation

. Communication

. Signal processing

4. What is the need for an instrumentation amplifier?

In a number of industrial and consumer applications, the measurement of physical quantities is

usually done with the help of transducers. The output of transducer has to be amplified So that it

can drive the indicator or display system. This function is performed by an instrumentation

amplifier.


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5. List the features of instrumentation amplifier:

. High gain accuracy

. High CMRR

. High gain stability with low temperature co-efficient

. Low dc offset

. Low output impedance

6. What are the applications of V-I converter?

. Low voltage dc and ac voltmeter

. L E D

. Zener diode tester

7. What do you mean by a precision diode?

The major limitation of ordinary diode is that it cannot rectify voltages below the cut in voltage

of the diode. A circuit designed by placing a diode in the feedback loop of an op amp is called

the precision diode and it is capable of rectifying input signals of the order of millivolt.

8. Write down the applications of precision diode.

. Half - wave rectifier

. Full - Wave rectifier

. Peak value detector

. Clipper

. Clamper

9. List the applications of Log amplifiers:

. Analog computation may require functions such as ln x, log x, sin hx etc. These functions can

be performed by log amplifiers

. Log amplifier can perform direct dB display on digital voltmeter and spectrum analyzer

. Log amplifier can be used to compress the dynamic range of a signal

10. What are the limitations of the basic differentiator circuit?

. At high frequency, a differentiator may become unstable and break into oscillations

. The input impedance decreases with increase in frequency, thereby making the circuit sensitive

to high frequency noise.

11. Write down the condition for good differentiation:-

For good differentiation, the time period of the input signal must be greater than or equal to

Rf C1

T > R f C1

Where, Rf is the feedback resistance Cf is the input capacitance


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12. What is a comparator?

A comparator is a circuit which compares a signal voltage applied at one input of an op-amp

with a known reference voltage at the other input. It is an open loop

op - amp with output + Vsat .

13. What are the applications of comparator?

yZero crossing detectors

y Window detector

y Time marker generator

y Phase detector

14. What is a Schmitt trigger?

Schmitt trigger is a regenerative comparator. It converts sinusoidal input into a square wave

output. The output of Schmitt trigger swings between upper and lower threshold voltages, which

are the reference voltages of the input waveform.

15. What is a multivibrator?

Multivibrators are a group of regenerative circuits that are used extensively in timing

applications. It is a wave shaping circuit which gives symmetric or asymmetric square output. It

has two states stable or quasi- stable depending on the type of multivibrator.

16. What do you mean by monostable multivibrator?

Monostable multivibrator is one which generates a single pulse of specified duration in response

to each external trigger signal. It has only one stable state. Application of a trigger causes a

change to the quasi-stable state. An external trigger signal generated due to charging and

discharging of the capacitor produces the transition to the original stable state.

17. What is an astable multivibrator?

Astable multivibrator is a free running oscillator having two quasi-stable states. Thus, there is an

oscillation between these two states and no external signals are required to produce the change in

state.

18. What is a bistable multivibrator?

Bistable multivibrator is one that maintains a given output voltage level unless an external triggeris applied. Application of an external trigger signal causes a change of state, and this output level

is maintained indefinitely until a second trigger is applied. Thus, it requires two external triggers

before it returns to its initial state

19. What are the requirements for producing sustained oscillations in feedback circuits?

For sustained oscillations,

y The total phase shift around the loop must be zero at the desired frequency of oscillation, fo.


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y At fo, the magnitude of the loop gain | A b | should be equal to unity

20. Mention any two audio frequency oscillators:

. RC phase shift oscillator

. Wein bridge oscillator

21. What are the characteristics of a comparator?

. Speed of operation

. Accuracy

. Compatibility of the output

22. What is a filter?

Filter is a frequency selective circuit that passes signal of specified band of frequencies and

attenuates the signals of frequencies outside the band

23. What are the demerits of passive filters?

Passive filters works well for high frequencies. But at audio frequencies, the inductors become

problematic, as they become large, heavy and expensive. For low frequency applications, more

number of turns of wire must be used which in turn adds to the series resistance degrading

inductors performance ie, low Q, resulting in high power dissipation.

24. What are the advantages of active filters?

Active filters used op- amp as the active element and resistors and capacitors as passive

elements.

. By enclosing a capacitor in the feedback loop , inductor less active filters can be obtained

. Op-amp used in non inverting configuration offers high input impedance and low output

impedance, thus improving the load drive capacity.

25. Mention some commonly used active filters:

. Low pass filter

. High pass filter

. Band pass filter

. Band reject filter.

16 MARK QUESTIONS:

1. write notes about,

y Regenerative Comparator (Schmitt Trigger)

y Voltage to current converter

y Precision rectifier

y Square wave generator


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Regenerative Comparator (Schmitt Trigger):

If positive feedback is added to the comparator circuit, gain can be increased greatly.

Consequently, the transfer curve of comparator becomes more close to ideal curve.

Theoretically, if the loop gain is adjusted to unity, then the gain with feedback, A1 becomes

infinite. This results in an abrupt (zero rise time) transition between the extreme values of outputvoltage. In practical circuits, however, it may not be possible to maintain loop gain exactly equal

to unity for a long time because of supply vo1tgemd temperature variations. So a value greater

than unity is chosen. This also gives an output waveform virtually discontinuous at the

comparison voltage. This circuit, however, now exhibits a phenomenon called hysteresis or

backlash.

The circuit is also known as Schmitt Trigger. The input voltage is applied to the (-) input

terminal and feedback voltage to the (+) input terminal. The input voltage Vi, triggers the output

Vo every Lime it exceeds certain voltage levels. These voltage levels are called upper threshold

voltage (VUT) and lower threshold voltage (VLT). The hysteresis width is the difference

between these two threshold voltages.

Voltage to Current Converter (Transconductance Amplifier):

In many applications, one may have to convert a voltage signal to a proportional output current.

For this, there are two types of circuits possib1e:


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V-I Converter with floating load:

Voltage to current converter in which load ZL is floating. Since voltage at node .a. is Vi,

therefore, Vi = iL*R1. That is the input voltage Vi, is converted into an output current of Vi/R1.

The same current flows through the signal source and load and, therefore, signal source should

be capable of providing this load current.

V-I Converter with grounded load:


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A voltage-to-current converter with grounded load shown above in which Vi be the voltage at

node .a.. Writing KVL, we get

i1 + i2 = iL

Since the op-amp is used in non-inverting mode, the gain of the circuit is 1 + R/R =2. The output

voltage is,

Vo = 2 V1 = Vi +Vo . iL*R

iL = Vi/R

Since the input impedance of a non-inverting amplifier is very high, this circuit has the

advantage of drawing very little current from the source. A voltage to current converter is used

for low voltage dc and ac voltmeter, LED and zener diode tester.

Precision rectifier:

The precision rectifier, which is also known as a super diode, is a configuration obtained

with an operational amplifier in order to have a circuit behaving like an ideal diode and rectifier.

It can be useful for high-precision signal processing. The basic circuit implementing such a

feature is shown on the right, where RL can be any load. When the input voltage is negative,

there is a negative voltage on the diode, too, so it works like an open circuit, there is no current

in the load and the output voltage is zero. When the input is positive, it is amplified by the

operational amplifier and it turns the diode on. There is current in the load and, because of the

feedback, the output voltage is equal to the input.

In fact the threshold of the super diode is not actually zero, as it should be for an ideal

one, but it equals the threshold of the normal diode divided by the gain of the operational

amplifier, that is almost zero. This basic configuration has a problem so it is not commonly used:

when the input becomes (even slightly) negative, the output of the operational amplifier can

easily become greater than the voltage supplied to the op-amp, thus causing saturation. Then, if

the input becomes positive again, the op-amp has to get out of the saturation to amplify again.


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This change takes some time, and this greatly reduces the frequency response of the circuit.

Square wave generator:

In the basic square wave generator circuit, a gradually-charging capacitor was used to help set

the timing or frequency of the circuit. However, since it was only charging through a resistor, it

necessarily charged on a logarithmic curve, rather than as a linear ramp.

In the circuit uses a separate integrator to generate a ramp voltage from the generated square

wave. As a result, we can get both waveforms from a single circuit. The phase relationship

shown between the two output waveforms is correct - remember that the integrator inverts as

well as integrating, so it will produce a negative-going ramp for a positive input voltage, and

vice-versa. Because we are now using an op amp integrator to get the triangle wave, we no

longer have a logarithmic response anywhere in the circuit. Therefore, the equation for the

operating frequency is simplified to:

fout = (1/4RtC)(R2/R1)

The squarewave amplitude is still the limit of voltage transistion, which we are assuming here to

be 10 volts. The triangle wave's amplitude is set by the ratio of R1/R2.


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UNIT III

TWO MARK QUESTIONS:

1. Mention some areas where PLL is widely used:

*Radar synchronization

*satellite communication systems

*air borne navigational systems

*FM communication systems

*Computers.

2. List the basic building blocks of PLL:

*Phase detector/comparator

*Low pass filter

*Error amplifier

*Voltage controlled oscillator

3. What are the three stages through which PLL operates?

*Free running

*Capture

*Locked/ tracking

4. Define lock-in range of a PLL:

The range of frequencies over which the PLL can maintain lock with the incoming signal is

called the lock-in range or tracking range. It is expressed as a percentage of the VCO free

running frequency.

5. Define capture range of PLL:

The range of frequencies over which the PLL can acquire lock with an input signal is called the

capture range. It is expressed as a percentage of the VCO free running frequency.

6. Define Pull-in time.

The total time taken by the PLL to establish lok is called pull-in time. It depends on the initial

phase and frequency difference between the two signals as well as on the overall loop gain and

loop filter characteristics.

7. For perfect lock, what should be the phase relation between the incoming signal and

VCO output signal?

The VCO output should be 90 degrees out of phase with respect to the input signal.

8. Give the classification of phase detector:

*Analog phase detector

*Digital phase detector


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9. What is a switch type phase detector?

An electronic switch is opened and closed by signal coming from VCO and the input signal is

chopped at a repetition rate determined by the VCO frequency. This type of phase detector is

called a half wave detector since the phase information for only one half of the input signal is

detected and averaged.

10. What are the problems associated with switch type phase detector?*The output voltage Ve is proportional to the input signal amplitude. This is undesirable because

it makes phase detector gain and loop gain dependent on the input signal amplitude.

11. What is a voltage controlled oscillator?

Voltage controlled oscillator is a free running multivibrator operating at a set frequency called

the free running frequency. This frequency can be shifted to either side by applying a dc control

voltage and the frequency deviation is proportional to the dc control voltage.

12. On what parameters does the free running frequency of VCO depend on?

*External timing resistor, RT

*External timing capacitor, CT

*The dc control voltage Vc.

13. Give the expression for the VCO free running frequency.

fo = 0.25 / RT CT

14. Define Voltage to Frequency conversion factor.

Voltage to Frequency conversion factor is defined as,

.Y_ _ _Io___ _9c= 8fo / Vcc

ZKHUH___ _9c is the modulation voltage required to produce the frequency shift _Io

15. What is the purpose of having a low pass filter in PLL?

*It removes the high frequency components and noise.

*Controls the dynamic characteristics of the PLL such as capture range, lock-in range, band-

width and transient response.

*The charge on the filter capacitor gives a short- time memory to the PLL.

16. Discuss the effect of having large capture range.

The PLL cannot acquire a signal outside the capture range, but once captured, it will hold on till

the frequency goes beyond the lock-in range. Thus, to increase the ability of lock range, large

capture range is required. But, a large capture range will make the PLL more susceptible to noise

and undesirable signal.

17. Mention some typical applications of PLL:

Frequency multiplication/division


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Frequency translation

AM detection

FM demodulation

FSK demodulation.

18. What is a compander IC? Give some examples.

The term companding means compressing and expanding. In a communication system, the audio

signal is compressed in the transmitter and expanded in the receiver.

Examples: LM 2704- LM 2707; NE 570/571.

19. What are the merits of companding?

*The compression process reduces the dynamic range of the signal before it is transmitted.

*Companding preserves the signal to noise ratio of the original signal and avoids non linear

distortion of the signal when the input amplitude is large.

*It also reduces buzz, bias and low level audio tones caused by mild interference.

20. List the applications of OTA:

OTA can be used in

Programmable gain voltage amplifier

sample and hold circuits

Voltage controlled state variable filter

Current controlled relaxation oscillator.

16 MARK QUESTIONS:

1.Explain the working of PLL. Explain its usage in FM demodulation.y The phase-locked loop (PLL) is an important building block of linear systems. The high

cost of realizing PLL in discrete form limited its use earlier.

y PLLs are available as inexpensive monolithic ICs. Used in satellite communication

systems, air borne navigational systems, FM communication systems, computers etc.

y The basic block of PLL consists of

Phase detector/comparator

A low pass filter

An error amplifier

A Voltage Controlled Oscillator (VCO).


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y

y The VCO is a free running multivibrator and operates at a set frequency fo called free

running frequency. This frequency is determined by an external timing capacitor and an

external resistor.

y It can also be shifted to either side by applying a dc control voltage v to an appropriate

terminal of the IC. The frequency deviation is directly proportional to the dc control

voltage and hence it is called a Voltage Controlled Oscillator..

y If an input signal v of frequency f, is applied to the PLL, the phase detector compares the

phase and frequency of the incoming signal to that of the output Vo of the VCO.

y If the two signals differ in frequency and/or phase, an error voltage Ve is generated. The

phase detector is basically a multiplier and produces the sum (f+f0) and difference (f-fo)

components at its output. The high frequency component (f+f0) is removed by the low

pass filter and the difference.

y Input frequency component is amplified and then applied as control voltage v to VCO.

The signal v shifts the VCO frequency in a direction to reduce the frequency difference

between f and

y f0. Once this action starts, we say that the signal is in the capture range. The VCO

continues to change frequency till its output frequency is exactly the same as the input

signal frequency.

y The circuit is then said to be locked. Once locked, the output frequency f, of VCO is

identical to f, except for a finite phase difference. This phase difference generates a

corrective control voltage v to shift the VCO frequency from f, to f, and thereby maintain

the lock. Once locked, PLL tracks the frequency changes of the input signal.

y Thus, a PLL goes through three stages (i) free running, (ii) capture and (iii) locked or

tracking.


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y

As capture starts, a small sine wave appears. This is due to the difference frequency

between the

y VCO and the input signal. The dc component of the beat drives the VCO towards the

lock. Each successive cycle causes the VCO frequency to move closer to the input signal

frequency. The difference in frequency becomes smaller and a larger dc component is

passed by the filter, shifting the VCO frequency further. The process continues until the

VCO locks on to the signal and the difference frequency is dc.

y The low pass filter controls the capture range. If VCO frequency is far away, the beat

frequency will be too high to pass through the filter and the YLL will not respond. We

say that the signal is out of the capture band. However, once locked, the filter no longer

restricts the PLL. The VCO can track the signal well beyond the capture band. Thus

tracking range is always larger than the capture range.

Some of the important definitions in relation to PLL are:

Lock-in Range: Once the PLL is locked, it can track frequency changes in the incoming signals.

The range of frequencies over which the P11 can maintain lock with the incoming signal is

called the lock4n range or tracking range. The lock range is usually expressed as a percentage of

f0, the VCO frequency.

Capture Range: The range of frequencies over which the PLL can acquire lock with an inputsignal is called the capture range. This parameter is also expressed as percentage of f0. Pull-in

time: The total time taken by the PLL to establish lock is called pull- in time. This depends on

the initial phase and frequency difference between the two signals as well as on the overall loop

gain and loop filter characteristics.

FM demodulation:

If PLL is locked to a FM signal, the VCO tracks the instantaneous frequency of the input signal


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The filtered error voltage which controls the VCO and maintains lock with the input signal is the

demodulated FM output. The VCO transfer characteristics determine the linearity of the

demodulated output. Since, VCO used in IC PLL is highly linear, it is possible to realize highly

linear FM demodulators.

2. Explain the operation of Multiplier and explain how multiplier working as divider:

There are a number of applications of analog multiplier such as frequency doubling, frequency

shifting, phase angle detection, real power computation, multiplying two signals, dividing and

squaring of signals. Two signal inputs (Vx and Vy) are provided. The output is the product of the

two inputs divided by a reference voltage, Vref. Vo = VxVy/Vref

If both inputs are positive, the IC is said to be a one quadrant multiplier. A two quadrant

multiplier will function properly if one input is held positive and the other is allowed to swing

both positive and negative. If both inputs may be either positive or negative, the IC is called a

four quadrant multiplier. The log-antilog method relies on the mathematical relationship that the

sum of the logarithm of two numbers equals the logarithm of the product of those numbers.


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Log-amps require the input and reference voltages to be of the same polarity. This restricts log

antilog multipliers to one quadrant operation. A technique that provides four quadrant

multiplications is transconductance multiplier. Some of the multiplier IC chips available areAD533 and AD534. We now discuss two applications of multiplier IC.

Multiplier as divider:

Division, the complement of multiplication, can be accomplished by placing the multiplier

circuit element in the op-amp.s feedback loop. The output voltage from the divider with Input

signals Vz and Vx as dividend and divisor respectively, is given by

Vo = -Vref(Vz/Vx)

The result can be derived as follows. The op-amp.s inverting terminal is at virtual ground.

Therefore,

Iz = Ia and Iz = Vz/R


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The output voltage Va of the multiplier is determined by the multiplication of Vx and Vy

Va = VxVo/Vref

Vo = -Vref (Vz/Vx)

3. Write note about Gilbert cell multiplier:

Monolithic four quadrant multipliers utilize the variable transconductance principle to

achieve errors of fractions of 1% over small signal bandwidths extending well into the MHz

range. Four quadrant operations is achieved by using two transconductance pairs with the basses

driven in Anti phase and the emitters driven by a second VI converter.

The output I-V converter is made up of the op-amp and a third VI converter in its

feedback path. One of the main causes of linearity error is the logarithmic term. This error is, to a

first approximation, compensated for by introducing an equal but opposite nonlinearity term via

the VI converter Q11-Q12 inside the feedback path.

UNIT IV

TWO MARK QUESTIONS:

1. List the broad classification of ADCs.

1. Direct type ADC.


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2. Integrating type ADC.

2. List out the direct type ADCs.

1. Flash (comparator) type converter

2. Counter type converter

3. Tracking or servo converter

4. Successive approximation type converter

3. List out some integrating type converters.

1. Charge balancing ADC

2. Dual slope ADC

4. What is integrating type converter?

An ADC converter that perform conversion in an indirect manner by first changing the analog

I/P signal to a linear function of time or frequency and then to a digital code is known as

integrating type A/D converter.

5. Explain in brief the principle of operation of successive Approximation ADC.

The circuit of successive approximation ADC consists of a successive approximation register

(SAR), to find the required value of each bit by trial & error. With the arrival of START

command, SAR sets the MSB bit to 1. The O/P is converted into an analog signal & it is

compared with I/P signal. This O/P is low or High. This process continues until all bits are

checked.

6. What are the main advantages of integrating type ADCs?

i. The integrating type of ADCs do not need a sample/Hold circuit at the input.

ii. It is possible to transmit frequency even in noisy environment or in an isolated form.

7. Where is the successive approximation type ADCs used?

The Successive approximation ADCs are used in applications such as data

loggers & instrumentation where conversion speed is important.

8. What is the main drawback of a dual-slop ADC?

The dual slope ADC has long conversion time. This is the main drawback of dual slope ADC.

9. State the advantages of dual slope ADC:

It provides excellent noise rejection of ac signals whose periods are integral multiples of the

integration time T.

10. Define conversion time.

It is defined as the total time required converting an analog signal into its digital output. It

depends on the conversion technique used & the propagation delay of circuit components.

The conversion time of a successive approximation type ADC is given by T(n+1)


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Where T---clock period

Tc---conversion time

n----no. of bits

11. Define resolution of a data converter.

The resolution of a converter is the smallest change in voltage which may be produced at the

output or input of the converter.

Resolution (in volts)= VFS/2n-1=1 LSB increment. The resolution of an

ADC is defined as the smallest change in analog input for a one bit change at the output.

12. Define accuracy of converter.

Absolute accuracy:

It is the maximum deviation between the actual converter output & the ideal converter output.

Relative accuracy:

It is the maximum deviation after gain & offset errors have been removed. The accuracy of a

converter is also specified in form of LSB increments or % of full scale voltage.

13. What is settling time?

It represents the time it takes for the output to settle within a specified band LSB of its final

value following a code change at the input (usually a full scale change). It depends upon the

switching time of the logic circuitry due to internal parasitic capacitance & inductances. Settling

time ranges from 100ns. 10s depending on word length & type circuit used.

14. Explain in brief stability of a converter:

The performance of converter changes with temperature age & power supply variation. So all the

relevant parameters such as offset, gain, linearity error & monotonicity must be specified over

the full temperature & power supply ranges to have better stability performances.

15. What is meant by linearity?

The linearity of an ADC/DAC is an important measure of its accuracy & tells us how close the

converter output is to its ideal transfer characteristics. The linearity error is usually expressed as

a fraction of LSB increment or percentage of full-scale voltage. A good converter exhibits a

linearity error of less than LSB.

16. What is monotonic DAC?

A monotonic DAC is one whose analog output increases for an increase in digital input.

17. What is multiplying DAC?

A digital to analog converter which uses a varying reference voltage VR is called a multiplying

DAC (MDAC). If the reference voltage of a DAC, VR is a sine wave give by

V(t)=Vin_&RV__ IW


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Then, Vo(t)=Vom&RV__ IW______)

18. What is a sample and hold circuit? Where it is used?

A sample and hold circuit is one which samples an input signal and holds on to its last sampled

value until the input is sampled again. This circuit is mainly used in digital interfacing, analog to

digital systems, and pulse code modulation systems.

19. Define sample period and hold period.The time during which the voltage across the capacitor in sample and hold circuit is equal to the

input voltage is called sample period. The time period during which the voltage across the

capacitor is held constant is called hold period.

20. What is meant by delta modulation?

Delta modulation is a technique capable of performing analog signal quantization with smaller

bandwidth requirements. Here, the binary output representing the most recent sampled amplitude

will be determined on the basis of previous sampled amplitude levels.

16 MARK QUESTIONS:

1.Explain the working of R-2R ladder type DAC:R-2R ladder DAC:


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Fig. R-2R ladder DAC & its equivalent circuit

y R-2R ladder type DAC uses only two values of resistors range from 2.5kohm to 10kohm

y The output voltage Vo = (-2R/R)*(-VR/4) = VR/2= VFS/2

y The switch position can be changed to the binary word 001 in 3 bit DAC. The voltages at

the nodes (A, B, C) formed by resistor branches are easily calculated. The output voltage

for other 3-bit binary words can be calculated.INVERTED R-2R LADDER

y In weighted resistor type DAC and R-2R ladder type DAC, current flowing in the

resistors changes as the input data changes.

y More power dissipation causes heating, which in turn, creates non-linearity in DAC. This

is a serious problem and can be avoided completely in Inverted R-2R ladder type DAC..

A 3-bit Inverted R-2R ladder type DAC is shown where the position of MSB and LSB is

interchanged.

y Here each input binary word connects the corresponding switch either to ground or to the

inverting input terminal of the op-amp which is also at virtual ground. Since both the

terminals of switches 4 are at ground potential, current flowing in the resistances is

constant and independent of switch position, i.e. independent of input binary word.

2.Explain the working of successive approximation type ADC:


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y The successive approximation technique uses a very efficient code search strategy to

complete n bit conversion in just n-clock periods.

y An eight bit converter would require eight clock pulses to obtain a digital output.

y The circuit uses a successive approximation register (SAR) to find the required value of

each bit by trial and error.

y The circuit operates as follows. With the arrival of the START command, the SAR sets

the MSB d1 = 1 with all other bits to zero so that the trial code is 10000000.

y The output Vd of the DAC is now compared with analog input Va. if V3 is greater than

the DAC output Vd then 10000000 is less than the correct digital representation.

y The MSB is left at .1. And the next lower significant bit is made .1. And further tested.

y However, if Va is less than the DAC output, then 10000000 is greater than the correct

digital representation. So reset MSB to .0. and go on to the next lower significant bit.


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UNIT V

TWO MARK QUESTIONS:

1. Mention some applications of 555 timer:

*Oscillator

*pulse generator

*ramp and square wave generator

*mono-shot multivibrator

*burglar alarm

*traffic light control.

2. List the applications of 555 timer in monostable mode of operation:

*missing pulse detector

*Linear ramp generator

*Frequency divider

*Pulse width modulation.

3. List the applications of 555 timer in Astable mode of operation:

*FSK generator

*Pulse-position modulator

4. What is a voltage regulator?

A voltage regulator is an electronic circuit that provides a stable dc voltage independent of the

load current, temperature, and ac line voltage variations.

5. Give the classification of voltage regulators:

*Series / Linear regulators

*Switching regulators.

6. What is a linear voltage regulator?

Series or linear regulator uses a power transistor connected in series between the unregulated dc

input and the load and it conducts in the linear region .The output voltage is controlled by the

continuous voltage drop taking place across the series pass transistor.

7. What is a switching regulator?

Switching regulators are those which operate the power transistor as a high frequency on/off

switch, so that the power transistor does not conduct current continuously. This gives improved

efficiency over series regulators.

8. What are the advantages of IC voltage regulators?

*low cost

*high reliability


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*reduction in size

*excellent performance

9. Give some examples of monolithic IC voltage regulators:

78XX series fixed output, positive voltage regulators

79XX series fixed output, negative voltage regulators

723 general purpose regulator.

10. What is the purpose of having input and output capacitors in three terminal IC

regulators?

A capacitor connected between the input terminal and ground cancels the inductive effects due to

long distribution leads. The output capacitor improves the transient response.

11. Define line regulation.

Line regulation is defined as the percentage change in the output voltage for a change in the

input voltage. It is expressed in millivolt or as a percentage of the output voltage.

12. Define load regulation.

Load regulation is defined as the change in output voltage for a change in load current. It is

expressed in millivolt or as a percentage of the output voltage.

13. What is meant by current limiting?

Current limiting refers to the ability of a regulator to prevent the load current from increasing

above a preset value.

14. Give the drawbacks of linear regulators:

*The input step down transformer is bulky and expensive because of low line frequency.

*Because of low line frequency, large values of filter capacitors are required to decrease the

ripple.

*Efficiency is reduced due to the continuous power dissipation by the transistor as it operates in

the linear region.

15. What is the advantage of switching regulators?

*Greater efficiency is achieved as the power transistor is made to operate as low impedance

switch. Power transmitted across the transistor is in discrete pulses rather than as a steady current

flow.

*By using suitable switching loss reduction technique, the switching frequency can be increased

so as to reduce the size and weight of the inductors and capacitors.

16. What is an opto-coupler IC? Give examples.


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Opto-coupler IC is a combined package of a photo-emitting device and a photo sensing device.

Examples for opto-coupler circuit: LED and a photo diode, LED and photo transistor, LED and

Darlington.

Examples for opto-coupler IC: MCT 2F , MCT 2E .

17. Mention the advantages of opto-couplers:

*Better isolation between the two stages.

*Impedance problem between the stages is eliminated.

*Wide frequency response.

*Easily interfaced with digital circuit.

*Compact and light weight.

*Problems such as noise, transients, contact bounce,.. are eliminated.

18. What is an isolation amplifier?

An isolation amplifier is an amplifier that offers electrical isolation between its input and output

terminals.

19. What is the need for a tuned amplifier?

In radio or TV receivers, it is necessary to select a particular channel among all other available

channels. Hence some sort of frequency selective circuit is needed that will allow us to amplify

the frequency band required and reject all the other unwanted signals and this function is

provided by a tuned amplifier.

20. Give the classification of tuned amplifier:

(i) Small signal tuned amplifier

*Single tuned

*Double tuned

*Stagger tuned

(ii) Large signal tuned amplifier.

16 MARK QUESTIONS:

1. Write about Video Amplifier:


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y A video amplifier has to amplify signals over a wide band of frequencies, say up to

20MHz. For faithful reproduction of the picture, the shape and form of the video

waveform must be preserved during amplification.

y ICMC 1550 is a Video Amplifier. As it uses a cascade amplifier pair, the video amplifier

is called cascade video amplifier. The transistor Q1 is a common emitter amplifier and

transistor Q3 is a common base amplifier and they together from a cascade amplifier. To

properly terminate the co-axial cable carrying the video signal, a 50 ohm resistance is

connected between the pins 1 and 4 of MC 1550.

y Such a small resistance has very negligible effect on the biasing of the transistor Q1. The

load resistance RL is directly inserted in the collector of the transistor Q3.

y Both Q2 and Q3 are operating in their active region, therefore the collector of Q1 sees the

very

y small input resistance of two common-base stages in parallel

3. Explain the working principle of isolation amplifier:


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An isolation amplifier is an amplifier that offers an ohmic or electrical isolation

between its input and output terminals. Isolation amplifiers are often used when there is a very

large common mode voltage difference between the input and output sides of the device. They

can provide voltage difference of several thousands of volts between input and output. The

isolation in the isolation amplifier is achieved by use of transformer.

The ISO100 is an optically-coupled isolation amplifier. High accuracy, linearity, and time

temperature stability are achieved by coupling light from an LED back to the put (negative

feedback) as well as forward to the output. Optical components are carefully matched and the

amplifier is actively laser-trimmed to assure excellent tracking and low offset errors.

The various features of the IC ISO100 are

1. Easy to use, similar to an op-amp

Vout/Iin = Rf, Current Input

Vout/Iin = Rf/Rin, Voltage Input

2. 100% Tested for breakdown : 750 V continuous isolation voltage

3. Ultra-low leakage : 0.3 A, max, at 240 V/60 Hz

4. Wide bandwidth : 60 kHz

5. 18-Pin dip package

The ISO100 is fundamentally a unity gain current amplifier intended to transfer small signals

between electrical circuits separately by high voltages or different references. In most

applications, an output voltage is obtained by passing the output current through the feedback

resistor (Rf). The ISO100 uses single light emitting diode (LED) and a pair of photodiode

detectors coupled together to isolate the output signal from the input.

lic qb with answer

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