Appendix 1: Transistor parameters

A parameter is a characteristic or property of a transistor which identifies it and distinguishes it from another similar transistor. In choosing a particular transistor to do a specific job, a comparison of parameters is the means by which the most suitable component can be selected. The important parameters of a transistor are the same as those for any amplifier, namely, input and output impedances, gain and feedback ratios. The amplifier characteristics will, of course, be determined by the parameters of the active devices it contains.

Parameters can be expressed in the units of impedance or resistance, in the units of admittance or conductance, or they may have no units and be just plain numerical ratios. The same kind of unit (that is, oluns or siemens) may be used for all four parameters, or the parameters may have mixed units (for example, ohms, siemens and straight ratio). To clarify this, consider expressing the four parameters in ohms; the input and output impedances are already in ohms; the gain and feedback would have to be expressed in the form of a voltage change divided by a current change, because the unit of voltage divided by current is the ohm. Expressing a gain in this form will not necessarily suit the device. A field­effect transistor, for example, has a current change produced by a voltage change. The unit of current divided by voltage is the reciprocal of the ohm, the siemens. Without going further into detail, it is sufficient to say that appropriate para­meters are chosen for a device, depending upon how the device works. Thus the parameters used for the bipolar device are different to those used for the unipolar device.

Parameters expressed in ohms are called z parameters at high frequencies and r parameters at low frequencies. Parameters expressed in siemens are called y parameters at high frequencies and g parameters at low frequencies. The para­meters used for each device will now be given.

The most commonly used parameters for bipolar transistors are the hybrid, or 'h', parameters. The h parameters are input resistance, hi; output conductance, ho ; current gain, hr; and feedback ratio, hr. The input resistance is not extremely

Appendix 1; Transistor parameters 125

high, because for a bipolar transistor current flows at the input. The output resistance, if required, is the reciprocal of ho ; but the output conductance is usually given because it is a more suitable parameter for this particular device. As before, the parameters are defined using small changes so that

small change in input voltage h·=------~--~----~

I small change in input current

small change in output current h = -------=;-----"----

o small change in output voltage

small change in output current hf = 11 h .. sma c ange m mput current

small change in feedback voltage hr = small change in output voltage

The parameters hf and hi are measured with the output terminals open circuited (that is, with no load); ho and hr are measured with output terminals short cir­cuited (that is, with zero resistance load). An examination of the defmitions shows that hi is measured in ohms, ho in siemens, and that hr and hr are pure numbers. Because of the mixed units, these parameters are called hybrid para­meters, the h standing for hybrid.

The h parameters of a transistor can be obtained from the characteristics. For example, if a graph of collector current against base current is plotted (see figure 5.3a) the slope would represent hf for the mode where output current is collector current and input current is base current (that is, the common-emitter mode). For the transistor, a set of h parameters is often given for each mode; for common emitter, common base and common collector. The basic definitions remain the same, but the output and input voltages are those at the output and input elec­trodes for the particular mode. To distinguish between modes a second sufftx is used in which 'e' represents common emitter, 'b' common base, and 'c' common collector.

Thus, hfe which is the ratio between a small change in output current to a small change in input current for the common-emitter mode means, in fact, the ratio between a small change in collector current (the output current in this case) and a small change in base current (the input current in this case). Similarly, hfb is the ratio between small changes of collector current (output) and emitter current (input) for the common-base amplifier. Other symbols used for hfe and hfb are {3 and a, respectively, but these symbols are falling out of use at the present time. The use of capital letters in the sufftx, for example, hFE, hFB, etc. removes the words 'small change' from the definition and the parameters apply to unchanging quantities (that is d.c.). Thus, where hfe means a small change in collector current divided by small change in base current, hFE means the d.c. value of collector current divided by the d.c. value of base current. The para­meters hfe , hfb , etc. are close in value to hFE' hFB' etc. if hfe, hfb are measured

Table A

U

Active device param

eters

Bipolar transistors

Unipolar transistors (F

ET

)

Hybrid (h

) C

omm

on C

omm

on C

omm

on C

onductance (g) C

omm

on C

omm

on C

omm

on param

eters base

emitter

collector param

eters gate

source drain

small change in

small change in

Input input voltage

input current param

eter sm

all change in

hib

hi.

hic sm

all change in gig

gis gid

input current input voltage

(input impedance)

(input conductance)

small change in

small change in

Output

ou

tpu

t current output current

parameter

small change in

hob hoe

hoc sm

all change in gog

gos god

ou

tpu

t voltage o

utp

ut voltage

(output conductance) (output conductance)

small change in

small change in

Forw

ard output current

output current transfer (gain)

small change in

hfb h

r. hrc

small change in

grg grs

grd param

eter input current

input voltage

(current gain) (transfer or m

utual conductance)

small change in

small change in

Reverse

feedback voltage feedback current

transfer sm

all change in h

rb h

r. h

rc sm

all change in grg

grs g

rd

(internal input voltage

ou

tpu

t voltage feedback) param

eter

Note: C

apital letter suffix means d.c. quantities.

Appendix 1: Transistor parameters 127

at small signal levels. Transistor parameters are summarised and compared in table Al.l.

The most convenient parameters for field-effect transistors are the 'g' para­meters, all of which are measured in the units of conductance (siemens). The four g parameters are very similar to the h parameters for bipolar transistors. The g parameters are

. d small change in input current mput con uctance gi = -----'''-----!....---­

small change in input voltage

small change in output current output conductance go = -----'''--.--.!....----

small change m output voltage

small change in output current transfer conductance gf = II h .. I

sma c ange m mput vo tage

small change in feedback current feedback conductance gr = II h· I sma c ange m output vo tage

Notice that all parameters are measured in terms of current/voltage ratios, and the unit will therefore be the siemens as stated. As with h parameters, a second suffix is used to show the mode; s for common source; g for common gate; and d for common drain. Thus gfs, for example, represents a small change in drain current divided by a small change in gate voltage (for the common-source con­nection, the drain current is the output current and gate voltage is the input voltage). The parameter gfs is obtainable from the characteristics, being the slope of the Id/Vg curve (see figure 5.3b). As with the bipolar transistor, capital letters in the suffix denote the d.c. parameter, gFS, for example, being the ratio of Id to Vg (both d.c. values). See table A.I for an overall summary of parameters.

Appendix 2: Decibels

The gain of an amplifier may be a very large number in itself and if amplifiers are cascaded (connected so that the output of one feeds the input of the next) the overall gain is the product of the individual amplifier gains and will be an even larger number. To make these high gain figures easier to handle we use a logarith­mic unit called the decibel (symbol dB). Use of the decibel also makes gain calculations easier because, since the unit is logarithmic, the process of addition replaces multiplication, as explained below.

We begin by defining the bel (symbol B) which is the logarithm of the ratio of two power levels. Thus for two power levels PI and P2 , the ratio P t/P2 expressed in bels is 10g(P t/P2 ) (the logarithm being to the base 10).

Consider two actual power levels of say, 40 W and 1 mW

The numerical ratio is

that is 40 000

The ratio in bels is given by

40 10-3

log 40000

= 4.602 B (from tables)

The decibel is one-tenth of a bel, that is

1 B = 10 dB

So that in decibels this ratio is 4.602 x 10 which is 46 dB (ignoring the 0.02, which would be done in practice).

In general then for two power levels PI andP2 the ratio in decibels is

1010g(Pt/P2 )

Suppose power level PI is due to a voltage VI volts driving a currentll amperes

Appendix 2: Decibels 129

in a resistor R ohms and power level P2 is due to a voltage V2 volts driving a currentI2 amperes in a resistor of the same value R ohms. Then

V12

PI =II2R or R

and

or

So that

or

or

and

log PI = log (II ) 2 P2 lz

II = 2 log - or 12

and the unit of these ratios is the bel

and

or

PI II log - = 20 log -

P2 lz

VI 20 log-V2

where the unit is the decibel. We see then that the decibel may be used as a unit to express voltage ratios or

current ratios as well as power ratios.

For power levels PI, P2

voltage levels VI, V2

and for current levels I 1,I 2

P ratio is 10 log ...1. dB

P2

ratio is 20 log VI dB V2

I ratio is 20 log 2. dB

12

It should be noted that the resistance was assumed the same in both cases so that for correct use the decibel must refer to voltage and current levels across or in the same value of resistance. Since normally we use decibels to indicate a change in conditions at the same point in a system this does not usually lead to error.

130 Appendix 2: Decibels

A useful point about the decibel is that because it is a unit based on logaritlnns to the base 10 we obtain the same numerical value of a ratio and its reciprocal. For example consider two voltages Vl and V2 when Vl/V2 = 2. In decibels

10 log Vl = 10 log 2 V2

= 10 x 0.3010

= 3.01 dB (3 dB)

and the reciprocal of the ratio is t so that in decibels this is

V 10 log -.l. = 10 log t

Vl

= 10 x 1.6990

which, since 1.6990 means -1 + O. 6990,

= 10 x (-1 + 0.699)

= -10 + 6.99

= -3.01 dB (-3 dB)

The number is the same, the sign is different. Thus if a voltage level is doubled (from V2 to Vl) we say it is then 3 dB UP (3 dB), if it is halved (from Vl to V2 ) we say it is now 3 dB DOWN (-3 dB).

Another useful aspect of the use of logaritlnnic units is that gains of amplifiers or amplifying stages expressed in decibels may be added when the amplifiers or stages are cascaded. Numerical gains are, of course, multiplied together in such a case. Consider, for example, two stages each of voltage gain 100 which are cas­caded together.

Total gain

or

= 100 x 100

= 10000

20 log 10000 dB

=80dB

Each stage has a gain of 100 or 20 log 100 dB, that is 40 dB. The overall gain is thus 40 dB + 40 dB

= 80 dB as before

Certain laboratory instruments (signal generators, etc.) are calibrated in decibels up and down so that signal output levels may be changed accordingly. It must be remembered, however, that the calibration is only accurate if the effective resist­ance of the circuit at the point where the instrument is connected remains the same.

Appendix 3: Resistor colour code and schematic diagram code as 1852

The resistance of most ftxed resistors (other than wirewound types) is usually indicated by the use of a system of coloured rings grouped together at one end of the component. There are usually either three or four of these rings and they are read as follows: rings one and two, counting from the end at which the rings are grouped, give the first two ftgures of the resistance value; ring three gives the number of noughts following these two ftgures; and ring four (if present) gives the tolerance (that is, a measure of the range within which the actual resistance is permitted to lie). The colour code (table A3.l) is as follows:

Colour

Figure (or number of noughts, for third ring)

Colour

Figure (or number of noughts, for third ring)

Table A3.1

Black Brown Red Orange Yellow Green

o 2 3 4 5

Blue Violet Grey White Silver Gold

6 7 8 9

If a fourth ring is present it will be silver to indicate 10% tolerance or gold to indicate 5% tolerance. If a fourth ring is not present the tolerance may be assumed to be 20%.

Thus, a resistor coloured red-red-red-silver has a nominal resistance 2200 n (that is, 2-2-two noughts) and its actual resistance lies within ±10% of 2200,

132 Appendix 3: Resistor colour code

that is between (2200 - 220) il and (2200 + 220) il. Similarly, a resistor coloured yellow-violet-brown is 470 il, 20% tolerance. A third ring coloured black signifies no nought so that a brown-black-black set of rings would indicate 10 il, that is first two figures 1 and 0, no noughts to follow.

BS 1852 Resistance Code

Throughout the book schematic circuit diagrams have indicated resistance values using standard prefixes. The new British Standard 1852 Code is being adopted by some manufacturers. This code tells more about the resistor but uses fewer characters. Some examples are

0.56 il is written R56 1.0 il is written 1 RO 5.6 il is written 5R6 68 il is written 68R 2.2 kil is written 2K2 10 Mil is written 10M

An additional letter may then be used to give the tolerance

So that

and so on.

F=±l% G=±2%

J=±5% K =±10%

M =±20%

4R7K means 4.7 il ± 10% 68KK means 68 kil ± 10% 4M7M means 4.7 Mil ± 20% 6K8J means 6.8 kil ± 5%

Appendix 4: Multiple and submultiple units

As indicated in the text, units of resistance, capacitance and inductance are ohms, farads and henrys respectively. These are abbreviated U, F and H. The single unit is not always a convenient size and accordingly multiple units or sub­mUltiple units are used. The abbreviations and symbols in use are

pico P 10-12

nano n 10-9

micro Il 10-6

milli m 10-3

centi c 10-2

kilo k 103

mega M 106

giga G 109

Thus: 1 gigahertz (1 GHz) means 1 x 109 or 1000 million hertz 1 kilohm (l kU) means 1000 ohms 1 microampere (lilA) means 1 x 10-6 amperes; that is, one millionth of one ampere 1 nanofarad (1 nF) means 1 x 10-9 or 1/1 000 000 000 of one farad

and so on.

Self-test questions and answers

Questions

1. The main disadvantage of d.c. signal transmission is

A. Only low power signals may be transmitted B. d.c. power supplies are required C. Electromagnetic propagation cannot be used D. The extent of the transmitted intelligence is limited

2. The synchronising signal in a television transmission is used to

A. Separate sound from vision signals B. Ensure line and frame timebases run at the same frequency C. Ensure camera and receiver timebases run in synchronism D. Ensure the same brightness at the camera and at the receiver

3. A superheterodyne receiver system

A. Has poor selectivity B. Uses a frequency changer C. Has poor sensitivity D. Uses wideband amplifiers

4. For correct working of an npn bipolar transistor the electrodes named should be at the following polarities with respect to the emitter

A. Collector positive, base negative B. Collector negative, base positive C. Collector positive, base positive D. Collector negative, base negative

Self-test questions and answers 135

S. The nature of the impedance of a series tuned circuit below and above resonance, respectively, is

A. Inductive, capacitive B. Capacitive, inductive C. Capacitive, resistive D. Resistive, inductive

6. The nature of the impedance of a parallel tuned circuit below and above resonance, respectively, is

A. Inductive, capacitive B. Inductive, resistive C. Capacitive, inductive D. Resistive, capacitive

7. If the gate-source voltage of a junction-gate n-channel field-effect transistor is positive, that is gate is positive with respect to source

A. The drain current increases B. The drain current is reduced C. The drain current ceases D. The drain current remains at the same value as when the gate-source volt­

age is zero

8. A thyristor may be switched off

A. By reducing the gate voltage B. By reducing the anode voltage C. By increasing the gate voltage D. By increasing the anode voltage

9. If one diode of a bridge rectifier failed

A. The output would fall to zero B. The output would be reduced full wave rectified d.c. C. The output would be half wave rectified d.c. D. The output would be unrectified a.c.

10. A Zener diode

A. Is used in the forward biased mode B. Uses avalanche breakdown when reverse biased C. May be used as a current stabiliser D. May be used as a voltage stabiliser

136 Self-test questions and answers

11. The purpose of a bleeder resistor at the output of a power supply is to

A Stabilise the output voltage B. Prevent the output voltage rising to too high a value on no load C. Reduce output ripple D. Rectify the a.c. input

12. The voltage regulator in a power supply (a.c. to d.c.) is included

A To remove ripple B. To rectify the a.c. C. To stabilise the output voltage D. To stabilise the input voltage

13. A transistor is biased class A when the operating point is

A At zero collector (drain) current B. At cut-off C. Beyond cut-off D. Between zero current and cut-off and in the linear part of the transfer

curve

14. A common-emitter amplifier has the input and output signals at the following electrodes

A Input-base, output-emitter B. Input-base, output-collector C. Input-emitter, output-collector D. Input-emitter, output-base

15. A complementary symmetry circuit uses

A Two pnp transistors B. Two npn transistors C. Two n-type FETs D. One pnp transistor, one npn transistor

16. A phase splitter circuit is used

A To provide feedback in an oscillator B. To provide antiphase signals from a single input C. To improve the gain-frequency response of an amplifier D. As a detector for FM signals

17. An emitter follower has

A Low input impedance, low output impedance

Self-test questions and answers 137

B. Low input impedance, high output impedance C. High input impedance, low output impedance D. High input impedance, high output impedance

18. An amplifier delivers 100 mV output when the input is 10 mY. If 1 mV is fed back so as to oppose part of the input the new overall gain of the amplifier is approximately

A. 10 B. 11 C. 9 D. 100

19. The fall-off in gain of a transformer-coupled voltage amplifier at low fre-quencies is due to

A. The high reactance of the load B. The low reactance of the load C. The high reactance of the output shunt capacitance D. The low reactance of the output shunt capacitance

20. The fall-off in gain of a resistance-capacitance-coupled amplifier at low frequencies is due to

A. The low reactance of the coupling capacitor B. The high reactance of the coupling capacitor C. The low reactance of the output shunt capacitance D. The high reactance of the output shunt capacitance

21. The fall-off in gain of a resistance-coupled amplifier when the signal frequency is high is due to

A. The high reactance of the coupling capacitor B. The low reactance of the coupling capacitor C. The high reactance of the shunt capacitance across the output D. The low reactance of the shunt capacitance across the output

22. If the load of a resistance-loaded amplifier is increased in value, the HT remaining the same

A. The load line slope is increased B. The load line slope is reduced C. The load line slope remains the same, the load line moving up the h/VA

axes D. The load line slope remains the same, the load line moving down the

lA/VA axes towards zero

138 Self-test questions and answers

23. In the normal operation of an astable multivibrator

A. There is one stable state B. There are two stable states C. There are no stable states D. The output is sinusoidal

24. In a Colpitts oscillator

A. One side of the tuned circuit contains a tapped coil B. One side of the tuned circuit contains two capacitors connected in series C. Feedback is effected via a transformer D. There is no tuned circuit

25. In a phase shift RC oscillator

A. The tuned circuit is transformer coupled B. Phase shift is effected by an RC network C. Feedback is from emitter to base D. The frequency is controlled by varying a coil

26. In a Hartley oscillator

A. One side of the tuned circuit contains a tapped coil B. One side of the tuned circuit contains two capacitors connected in series C. Feedback is effected by a transformer D. There is no tuned circuit

27. A diode detector for AM signals gives

A. Poor selectivity, good sensitivity B. Good selectivity, poor sensitivity C. Good selectivity, good sensitivity D. Poor selectivity, poor sensitivity

28. In ftgure Q.l, if direct voltage readings between base and ground and between emitter and ground were equal, the fault could be

A. R 1 open circuit B. C 3 short circuit C. Transistor internal base-emitter short D. R3 open circuit

Self·test questions and answers 139

figure Q.l

29. In figure Ql, if R3 went open circuit

A. The amplifier gain would fall to zero B. The transistor would overheat C. The output would be distorted D. The transistor bias would increase

30. In figure Ql if capacitor C3 went open circuit

A. The gain of the amplifier would be reduced B. The transistor would cut off C. The transistor would burn out D. The output would fall to zero

Answers

1. C 11. B 21. D 2. C 12. C 22. B 3. B 13. D 23. C 4. C 14. B 24. B 5. B 15. D 25. B 6. A 16. B 26. A 7. A 17. C 27. D 8. B 18. C 28. C 9. C 19. B 29. A

10. D 20. B 30. A

Index

AM detectors 55ff amplifiers

coupling 77 modes 66 see also under type

amplitude limiting 52 amplitude modulation 56 angular velocity 11 anodes (CR T) 108 astable multivibrator 98 audio power amplifiers 86 auto-transformer 33 avalanche breakdown 45

ballast resistor 51 bandwidth 27,79,91 base 57 belt drive 112 bias

classes 75 diode 45 transistor 57, 60

biasing 73 bipolar transistor 57 bistable multivibrator 100 bleeder resistor 118 blocking oscillator 100 bridge rectifier 50

capacitor input filter 116ff cathode ray oscilloscope 107ff cathode ray tube 107ff choke input filter 118 clamping 52 classes of bias 75 clipping 52 collector 57 Colpitts oscillator 93 common base mode 66 common collector mode 66 common drain mode 66

common emitter mode 66 common gate mode 66 common source mode 66 complementary symmetry 90 compression bonding 64 coupling, amplifier 77 CR circuits, d.c. 3 crossover distortion 87 crystal oscillator 96 current amplification 58 current feedback 81ff

decibel 80, 128ff damping 28 d.c. restoration 52 deflection (CRO) 110 deflection sensitivity 110 depletion mode 60 depletion region 47 diac 61 diffusion process 64 differentiating circuits 104 diode 42ff

characteristic 47 steering 102

distortion 86, 87 doping 43 drain 59 dynamic impedance 27

eddy currents 31 efficiency, transformer 39 electron gun 107 electromagnetic focusing 109 electrostatic focusing 108 emitter 57 emitter follower 66, 118 enhancement mode 60 exponential curve 3ff extra high tension 111 extrinsic semiconductor 43

Faraday's law 34 feedback 76,81,84 fidelity 75 field effect transistor 46, 57 film circuit 63-4 filter 51, 116 flywheel 111 focusing (eRO) 108ff forward bias, diode 45 frequency 16 frequency response 28,79 full-wave rectifier 49

g parameters 68-9,126 gate 59 gears 112 germanium 42

h parameters 68, 125 half-power points 91 half-wave rectifier 47 Hartley oscillator 93 high-pass circuit 18 hole 43 hole conduction 44, 57 hysteresis 40

IGFET 60 impedance 17

diagrams 18ff input 36,85 matching 37,85 output 37, 85 triangle 19

integrated circuits 63 integrating circuit 104 intrinsic semiconductor 43

JUGFET 58

laminations 31 LC oscillators 92ff LCR circuits, a.c. 20ff limiters 53 linearity 74 load line 47, 70ff low-pass circuit 18 LR circuits, d.c. 3ff

majority carriers 45 matching 37 Meissner oscillator 93 metallising 111

Index 141

microprocessor 64 midband gain 80 minority carriers 45 monolithic circuit 64 monostable multivibrator 102 MOSFET 60 multivibrators 98ff

n-type semiconductor 43 negative feedback 81ff neon oscillator 99 npn transistor 57ff

Ohm's law 1,15 operational amplifier 105, 119 oscillators 92ff

see also under type

p-type semiconductors 44 peak inverse voltage 49 pentavalent material 43 phase shift 9 phase splitter 88 phasors 9 photoelectric devices 62 pinch-off 59 planar transistor 65 pnp transistor 57ff positive feedback 81 ff post-deflection anode 111 power amplifier 86 power dissipation 49 power supplies 115 prime mover 115ff protection, diode 56, 100 pulleys 111 ff push-pull circuits 87

Q-factor 26

RC coupling 78 RC oscillators 96 rectification 48 regulation 51, 118 relaxation oscillators 98 reservoir capacitor 49, 116 resistor colour code 131 resonance 22, 27 resonant frequency 23 restoration, d.c. 53 r.f. choke 55 ripple 55, 117

142 Index

screen ( CRT) screening 31 semicond uctor

107

42ff series resonance 22, 81, 86 silicon 42 single-ended amplifier 85 small-signal amplifier 87 smoothing 51, 116 source 59 source follower 66 stabilisation 118 stabiliser diode 46 steering diodes 102 substrate 64 swinging choke 118

tappings, transformer 32 tetravalent material 43 thermal runaway 76 thyristor 61, 99 time constant 3ff transformers

audio 89 construction 31 coupling 78 efficiency 39 ideal 33 losses 39 toppings 32

transients 2ff transistors

characteristics 58-9

parameters 125 see also under type

triac 61 trivalent material 43 tuned circuits

oscillators 93 see also resonance

tunnel diode 47

veractor diode 47 VDU 107 voltage

feedback 8lff gain 68ff magnification 25 multiplication 120 regulation 117 regulator diode 46 stabiliser 51

waveform shaping 102 Wien bridge oscillator 96

X deflection 109 X plates 109-10

Y deflection 109 Y plates 109-10

Zener breakdown 45 Zener diode 46, 51, 118