INDEX

Amplifi

ers

Base current

drive

Bootstr

ap

Class

B push-pull

Complementary symmetry

Darlington pair

Direct

coupled (small

signal)

Direct

coupled driver

Driver

(single stage)

Grounded base

Grounded collector

(emitt

er follower)

Grounded emitt

er I.F. amplifier

(465kHz)

Push-pull, class

B Push-pull,

single ended

Small

signal

Super-alpha pair

Tone control

Clamps

D.C. clamp to positi

ve voltage

D.C.

clamp to zero voltage

Clippers

Series

diode limiter

Double parallel clipper

(slicer)

Parallel or shunt

diode limiter

Connecti

ons,

transist

or Darli

ngton pair

Dual tone control

Emitt

er follower

Power

supply, stabilis

ed

Pulse shaper,

trigger

Sawtooth generator

(Miller)

PAGE 3 19 7 9 19 5 6 3 2 2 2 4 7 8

2. 3. 5 18 10

20

Signal Generators

Astable multi

vibrator

Bias

oscillator

(tape recorder)

&stable multi

vibrat

or Monostable multi

vibrator

Phase shift

oscill

ator

Sawtooth generator

(Miller)

Tape recorder

bias oscill

ator

Tuned collector

oscillator

Wien bridge oscill

ator

Schmitt

trigger

Super-alpha pair

Tone control

4 6 5 2 8 1 3 17 18 10

TRANSISTOR CONNECTI

ONS

Code lett

er after each type number given

in text refers to appropriate key diagram

below. For examp(e: 2N2926 (I) is type

number 2N2926, connections in key diagram

I below (looking at the underside wire end).

20 e

emitter,

c

coll

ector,

b

base,

s shield, ev

envelope

21 22 21 24 19 10 2 23 22 18

A bSPOT

tac

I e ...

. ,

, _ _,,

.0,.

e.c,,_.ED

b C

D b

E LIME c ..,

sear SOME TYPES

HAVE SW 0 spoT

b t c b

c b e

c s e

\ci1/4t

,4 .4

0.‘ a

F 6

II I

'V

-s

TR5ci TL 471).

' TR8 a

NKT403

1134

TRANSISTOR CIRCUITS

Present

ed free with

the April

1968 issue .of pktACTI

0'34

IQ,34

24

CI 50pF

0-1

INPUT VRI

50LO

TREBLE

I2—DUAL TONE CONTROL

RI 4.71‘11

C3 0. 047pF

With any form of tone control the signal

level will

be reduced at pre-determined

frequencies.

If bass boost is required, the

high frequencies are cut.

Then the audio

spectrum is amplified to bring them up to

normal, whil

e the bass frequencies will be

higher

than the treble. Similarly for

treble

boost the bass

frequencies are cut.

The

reactance of a capacit

or (X

() is inversely

proporti

onal to frequency.

When the

10

R4 56k fl frequency is such that X,. is equal to the

associated resist

or, he frequency response

will be 3dB down. It will continue to go

down at 6dB per octa

ve as frequency

increases for a parall

el capacit

or, and as

frequency decr

eases for a series capacit

or. The C and R form a frequency sensiti

ve

potentiometer.

Transistors:

almost any small signal

type such as 0071(G), NKT2I4(A)

I3—TUNED COLLECTOR OSCILLATOR

Current

changes

through the transist

or are

reflected back through the transformer to

the base. A continuous

oscill

ation is

produced and is tuned to the required

frequency by the inductance of T1 primary

and the value of C3.

R4 acts

as the

load across

the output.

Frequency of

oscillation is 1/(2n x LC) where L is the

inductance of collector winding, and C is

the parallel tuning capacit

ance.

Transistor:

0084(F),

NKT2II(A)

I4—TAPE RECORDER BIAS

OSCILLATOR

This circuit

is based on similar

principles

to those in Fig.

13. Provides

I mA bias

current

at 42kHz for

a tape record head but

is unsuita

ble for

erase. Frequency

of

oscillation is much higher than the highest

audible recorded frequency.

Oscill

ator

frequency should be about five times this

recorded frequency.

Output voltage is

about

32V peak into a 250mH record head.

Transformer details:

Ferr

oxcube pot core

assembly LAI.

Output

winding I 2, 150 turns 34 s.w.g.

Secondary winding 3-4, 8 turns 30 s.w.g.

Primary winding: (a) 5-6,25

turns

34 s.w.g.:

(b) 6-7, 125 turns

34 s.w.g.

Transistor: 0072(G),

2G381(H)

R1 181111

FROM f.

AMPLIFIER

BIAS

REJECTOR

Cl 6800pF

TO REC HEAD

250mH

-9V

C3 1200 PF

TR1

R2 4711

II

IS—PHASE SHIFT OSCILLATOR

-16V

R5 10kfl

OUTPUT

D—°

C5 50pF

R6 20011

R7 20.0.

12

Convenient way

of producing a signal

without using transformers.

At low fre-

quencies, where internal phase shift

of the

transist

or is minimal,

an artificial phase shift

is obtai

ned by inserti

ng a CR ladder net-

work between the collector and base. At

least three CR "rungs" are needed to get

180 . Impedance of the ladder will

be

much higher than that of the transist

or. Attenuation over the ladder is 29 using

three equal resista

nces and three equal

capacit

ances.

Frequency 11(2”CR

N. 6)Hz.

Where C - 0.01,,,

F and R

!Oki)

this

gives a frequency of 650Hz but is modified

by transist

or impedances to 800Hz. Output

impedance is high.

High gain transistor is required such as

0075(G) or ACI56(E)

16—WIEN BRIDGE OSCILLATOR

Audio frequency sine wave generator.

Frequency determining components

are

contai

ned in bridge network R6, C3, R8,

and C4.

Regenerati

ve positi

ve feedback

from point

"X" to TR I base occurs

only at

this frequency.

Frequency

l/(2 RC).

Here R - - 4.7k U and C 0.0I

pF. Oscil-

lator is frequency

selective. Any change or

mismatch in component

values

in the series

or parall

el pair of the Wien bridge will

cause the feedback signal to TR I base to

become out

of phase with the bridge input,

preventing oscill

ation.

If resist

ors R6 and

R8 are made into a twin ganged potentio-

meter,

the frequency can be varied.

Transistors:

0071(G),

AC I 55(

E), N KT2 14( A)

13

I7—ASTABLE MULTIVIBRATOR

Free running square wave generat

or. Unbalance between two stages causes TR I

(or TR2) to conduct whil

e TR2 (or

TRI) is

cut off.

Cross-c

oupled capacit

ors Cl and

C2 are alternately charged through base

emitt

er junctions;

discharged through VRI

and VR2. When C2

is discharged through

VR2. TRI

base voltage falls exponentially

to zero, when TRI starts

to conduct.

TR2

is cutoff. When Cl is discharged through

VRI,

TR2 base falls

to zero, when TR2 will

conduct.

The small negati

ve pips

on the

base waveforms are due to rapid initial

recharge of capacit

ors. Waveform at both

collectors approximate to a square wave.

Frequency is adjusted by VRI and VR2.

f 1/(1.4CR) where C is single coupling

capacita

nce (farads)

and R is base bias

resistance (ohms),

and where Cl

C2

and

VRI

VR2. Mark/space ratio is altered

by varyi

ng the value of one capacit

or.

Transistors:

N KT2 1 8(A),

0083(G),

AC 156(E)

18—MONOSTABLE MULTIVIBRATOR

5? 680.11

C1 4700pF

Used for pulse shaping or to provide a

calculable delay period in the output pulse

which is independent of the input trigger

pulse period. May

be employed for

frequency divisi

on. With no trigger pulse

applied in the circuit shown. TR2 would

normally be switc

hed on.

Base bias

is

provided by R3.

TR I is off as a positi

ve

bias potential is applied via

R4 and RS.

C2

is charged to --3V. When trigger pulse is

applied via

the diode shaping circuit

to the

base of TRI,

this transist

or starts

to switc

h on.

The positi

ve going TRI

collector

0

Yb 151 Vb TR2

0 V

t

3 VOLTS

voltage switc

hes off

TR2 This cumulative

effect

on the base volta

ges

is shown at time

t,. With TRI

bott

omed, C2 retai

ns its

original charge with • 3V applied to TR2

base. C2

discharges through R3 and TRI,

bringing TR2 base towards OV, and the

point where TR2 switc

hes on again. The

output pulse period is determined by C2

and R3.

An approximation of this period

is 0.7C2R3 seconds.

Transistors:

0071(G),

ACISS(E),

N KT2 1 4 ( A )

15 14

19—BISTABLE MULTIVIBRATOR

20—SCHMITT TRIGGER

ci TR I

3300pF

Commonly used for

counting and generati

ng

square waves

from pulse inputs.

When the supply is applied the circuit

assumes a stable state with one transist

or bott

omed and one cut

off.

When a

positive

pulse

is applied

via

the

differentiator

CI-DI,

the transist

ors reverse

their switched state and the circuit

assumes

16

its second stable conditi

on. D2 and D3

route the pulses for the corr

ect switc

hing

sequence each diode being reverse biased

when its connecting transist

or is cut off.

Outputs

may be taken from either collector.

Transistors:

NKT224(B),

AC113(E)

The Schmitt

or emitt

er coupled bista

ble,

can be considered as a fast

acting switc

h, the

action of which is precipitat

ed by a pre-

arranged d.c. level, the trip volta

ge (see

waveforms).

Often employed as a voltage

level detector.

When the input

is below the triggering level

TR2 is switc

hed on by its base divider net-

w3rk R2 and R4,

with TR I switc

hed off.

If

the input voltage is made to exceed the

emitt

er voltage (neglecti

ng the small base-

INPUT

OUTPUT

TRIP

LEVELS

emitt

er drop at TR I) this transist

or begins

to switc

h on and TR2 switc

hes off.

Since

there are no coupling capacit

ors in this

circuit,

reversi

on to its original conditi

on of

TR2 on can only be achieved by reducing the

amplit

ude of the input

signal below the trip

voltage. The switc

h on and switc

h off

voltages differ in level and this property is

often used to square, or clip, waveshapes.

Transistors:

NKT274(A),

AC I54(E)

17

IS

2I—MILLER SAWTOOTH GENERATOR

Alternatively known as the Miller

integrat

or or linear sweep generat

or, this circuit is

often encountered where sawtooth wave-

forms

are required. Cl, DI,

and RI clamp

the input positi

ve peaks

to OV.

When the

input

goes negative TRI is switc

hed on and

almost all the supply voltage is applied to

TR2 the emitt

er load. Since the base is

almost at zero volts,

C2 charges to this

potential.

As the pulse input

goes

positive,

TRI is cut off and C2 commences to dis-

charge via TR2 and the base bias resist

ors R2 and VR I. The discharge of the Miller

capacit

or C2 is characteris

ed by an initial

small step in the output, after which the

base voltage of TR2 remains consta

nt and

C2 continues discharging linearly on a long

time consta

nt producing a sawtooth output.

33011

Transistors:

2S321(H), 0C203(F)

22—SUPER-ALPHA PAIR

Alpha is the term originally used to indicate

transist

or current gain. Super-al

pha pair

derives

its name from the combined gain of

two transist

ors, these being multi

plied.

Otherwise the characteristi

cs are as in a

grounded emitt

er stage.

Transistor:

npn type-2N2926(1)

23—DARLINGTON PAIR

Has a very low output impedance and high

input

impedance. Input

impedance is

approximately equal to the combined

current gains of the transist

ors multi

plied

by the output impedance. Otherwise the

charact

eristics are as in a grounded collector

(emitt

er follower)

stage.

Transistor: npn type-2N2926(1)

24—BOOTSTRAP AMPLIFIER

A technique used to achieve high input

impedance where base bias

resist

ors are

likely to shunt the input,

is known as

bootstra

pping. Here almost equal and in-

phase volta

ges are applied to either

end of

resistor

R3.

The impedance of this

isolation resist

or can be considered as very

large, as only a small

current

flows.

TR2 is

the emitt

er load of TRI

which acts as an

emitt

er follower.

As is usual with this

type of circuit,

the input

impedance is high,

but the bias divider RI-R2 would reduce

the impedance "seen" by the signal if the

bootstra

pped resist

or R3 was not

included.

Cl couples negative feedback from the

INPUT

emitt

er load to implement this

boot-

strapping.

Transistors:

TRI

AC107(G),

TR2 0071(G)

19

1r I—GROUNDED BASE

R2 C2

10 kft 100yf

Low input

impedance to

emitter;

high

output

•mpedance from collector.

Cl acts as short circuit to a.c.

while blocking d.c.

There-

;are, the base is grounded to

z.c. and common to both

Input

and output

signal

circuits.

Used for matc

hing

low impedance source to

amplifi

er stages.

Phase shift

zero; a.c. current gain • I ;

a.c. voltage gain high.

Transistor: 0071(G)

2

2—GROUNDED COLLECTOR

RI

CI A

50yF

.,

INPUT

R2 11111

High input impedance;

low

output impedance.

Often

called an emitt

er follower.

Decoupling capacit

or is some-

times

connected

across

supply

lines

to

reduce

power

line

impedance.

Collector

is common to

input and output circuits.

Used to match amplifier to

low impedance load. Phase

shift zero; a.c. current gain

high; a.c. voltage gain • I.

Transistor: 0C35(C)

3—GROUNDED EMI

TTER

RI 18YR CI

50yF

INPUT R2

2.7kfl

-4.5V

C3 50pF

OUTPUT

C2

Medium input

impedance.

Medium output impedance.

Battery positi

ve supply line

is common to both input

and

output circuits.

Decoupling

capacit

or C2 short

circuits

emitt

er to positi

ve supply

line as far as a.c.

is concerned.

Therefore,

emitt

er is

common to both input and

output circuits.

Phase shift

180 ; at.

current gain high;

a.c. volta

ge gain high.

Transistor: 0071(G)

4—BASE CURRENT DRIVE

Low input impedance;

source can be

coupled in series

with base instead of in the

emitter circuit,

where high curr

ent gain is

required and where the nor

mal common

base circuit

is unsuitable. Transformer

can be used to match almost

any impedance

to the base. A.C. signal is superi

mposed on

very small

base bias

current,

determined by

values of RI and R2.

Values of com-

ponents not

given beca

use they depend on

the charact

eristics of the transformer and

signal source impedance.

Transistors:

0071(G),

NKT214(A),

AC 155(E)

C1 50yF

0--

INPUT

5—DRIVER STAGE

Ep -9V

OUTPUT

to push

pull 'toot

45:1+1

TT45

C2 100oF

Common emitt

er stage with a collector

current

of about 3mA. The collector load

is the transformer primary coupled to a

class B push-pull pair. Special packages

contai

ning all three transist

ors (matched)

can be obtai

ned. If the charact

eristics are

dissimilar,

severe distorti

on is likely.

Ideally the two halves of T1 secondary

should be bifilar wound, i.e. both halves

wound at the same time so that the wires

lay side-by-si

de. Commerci

al types are

available (e.g. Repanco TT45).

Transistors:

2G381(H),

OCR! (G),

NKT27I(A),

ACI 54(E)

3

25 & 26—D.C. CLAMPS TO A REFERENCE VOLTAGE

Clamping is the technique of shifti

ng a wave-

form so that

either its positi

ve or negati

ve

peaks are fixed at some refer

ence level.

If a

train of pulses

is passed through a capacit

or the mean d.c.

level is blocked. In the clamp

25—D.C. CLAMP TO POSITIVE

VOLTAGE

OV

OV

20

circuits

the diodes

DI restores

the d.c.

level

so that the peaks are clamped to the

reference voltage. In the examples given

below these peaks

are negative.

26—D.C. CLAMP TO ZERO

VOLTAGE

27—SERIES DIODE LIMITER

Normal output

DI 0A81

RI —8

1/ 6.81E11

+4V BIAS

+4V

+4V

Output with

diode reversed

Clipper or limit

er circuits

are used when a

particular wavefor

m, not necessaril

y sinu-

soidal,

is required which lies

above or below

some refer

ence level.

Since the forward

resistance of a diode is low, and its reverse

resistance high, when the input voltage

raises the 6athode level above the bias

potential, DI does not conduct and the

output remains at the bias

potential.

During the remainder of the cycle, the

diode conducts

and the output volta

ge

almost equals the input.

With the diode

reversed,

conduction only occurs

when the

positive peaks exceed the bias potential.

28—SHUNT DIODE LIMITER

18V 0

—8V + OAB1

+4V BIAS

Normal

output

+4V 0

—8V

R2 101111 44V 0

Output

with

diode reversed

The shunt diode clipper uses the diode to

short-ci

rcuit

the output

above the 4V bias.

When the input volta

ge is less than the

bias potential the output waveform is

almost

identical

since the diode is not

conducting. When the input

volta

ge

exceeds

the bias

potential the diode

conducts and shunts the load resista

nce so

that the output

volta

ge only slightly

exceeds the bias

potential.

With the

diode reversed, the peaks

are clipped at the

bias voltage.

21

3I—STABILISED TRANSISTOR POWER SUPPLY

29—DOUBLE PARALLEL CLIPPER OR SLICER

In the slicer

the two biased diodes

combine

to trim the input peaks alternately when

the positi

ve and negati

ve half

cycles

exceed

the respective diode bias potentials.

The

diodes act as shunts for the peaks above

. 4V

(bias

voltage).

30—TRIGGER PULSE SHAPER

CI D1

330pF

0A131

K

22

Most

switching sources

produce recta

ngular

waveforms which generall

y have to be

shaped to provide an effective trigger for

other pulse circuits.

In the shaper

shown,

Cl and RI form a differentiator which

converts

the rectangular wave input into

positive and negative going spikes,

the

width of each being dependent

on the time

consta

nt C,R,.

The diode effecti

vely

removes the lower spike so that the

output provides

one positi

ve going narr

ow

pulse for one input cycle. Almost

any

small

signal diode can be used. The

remaining small negati

ve pip is due to the

reverse diode resist

ance.

PSI 2A 240V

•. .•

ax. MAINS

INPUT

Na_

c,04.•

•

220V

•

rj

13V 150 n.A

02

4 • 0A202

03 04

Of the three main types

o recti

ficati

on the

bridge recti

fier shown here (DI,

2, 3, 4)

offers the lowest

impedance source for

d.c. supplies, an importa

nt factor when using

amplifier circuits

to achieve ac. stabilit

y. Ideal

source impedance is zero but is not

practicable. Very high values

of smoothing

capacit

or are used to reduce impedance to

lowest

figure. If a volta

ge stabilis

er is

connected to the output from the bridge

circuit,

any fluctuation of load resista

nce

(R,.) is used to control the volta

ge output

from TRI.

If R1. falls,

the output volta

ge

tends

to drop. A d.c. voltage is picked off

C1 1500

25V

R2 680 ft

RI 1.1kA HS2075

OR ZL7.5

TR1

TR2

e, 05 7.5V

ZENER

R3 VRI

LIN R4 560L1

—8V TO —13V 1

C2 1500yF 25V

DV

by the potential divider R3, VR I, R4 to

control the current

through TR2 and hence

TRI. TR2 base goes more negative, TRI

emitter volta

ge rises thus restoring the

output

voltage. CI reduces

the impedance

across

the bridge and smooths

the d.c.

C2

reduces the impedance at the output

terminals

and reduces

the ripple to less

than

5mV at 10V output.

Maximum current

capacity

150mA. D5 is a Zener diode to

fix TR2 emitt

er voltage at 7.5V.

Transistors:

TRI

N KT304(D),

TR2

NKT217(A)

23

CI SpF

6-465kHz I.F. AMPLIFIER

RI

R7 C4 56pF

CS 18pF

1 20

3 9k11

• 4.1

c LINE

Tuned amplifi

er with transformer

primaries

tuned to the if. High frequency transist

ors essential. Base voltage is determined by

potential divider (R2 and R3)

in con-

junction with a.g.c. volta

ge. Base of TR I is

current

driven by TI : primary is connected

to the r.f. mixer (not shown).

Second

C6 0-25pF

56 4 710.

-7V

TO DETECTOR

stage is similar but with higher current

drain.

Feedback is provided by RI, C4,

R7, and CO.

The transformers shown can

be Repanco type XT26 for TI and T2;

XT27 for

T3.

Transistors:

NKTI42(B),

2G30I(H),

0C45(

G)

7—DIRECT COUPLED SMALL SIGNAL AMPLIFIER

52 27kft

r,

TRI INPUT

-9V

R7 -12Y

4.7YO

C4 5

12LoiF

OUTPUT

C5 rk,

100pF

Grounded emitt

er configurati

on: base bias

is taken from TR2 emitt

er to TR I base.

The bias

on TR I base is directl

y controlled

by TR2 collector

current,

which automatic-

ally sets

the working point of TR I , an

d hence

TR2 through the potential divider RI, R6.

The bias point of TR2 is set by the con-

trolled current

through TR I collector

load.

Direct

coupling between TR I collector

and

TR2 base is essential for this operati

on.

The input impedance is 1k t for a 50dB

gain. Decoupling of the supply line by R7

and C5 enables this circuit to be used in

front

of that

shown in Fig.

8. R8

indicates

decoupling resist

or value for --9V supply.

Transistors:

0071(G),

NKT2I4(A),

AC 155(E)

5

8—DIRECT COUPLED DRIVER

Same principle as applied in Fig. 7 but

handles larger

signal prior to the push-pull

output stage. Can be coupled to circuit

shown in Fig.

7 if used with 27k0 resist

or and 0.005/4F capacit

or in parallel connected

in series

with the input.

Transformer

details:

Bobbin iin

fin

bore; Primary 1900, turns 42 s.w.g. enam.

L 8.2H, D.C.R.

2170. Secondary two

bifilar windings 950 turns each 40 s.w.g.

enam.

1. — 2.05H,

D.C.R. = 1050 for

each half.

Laminations 15 mil mumetal.

Transistors:

0071(G),

NKT214(A),

AC155(E), 0081 0(G), N KT272(A)

6

INPUT Cl

50pF

T1 b —12V

'kr R2

OUTPUT

to push

pull stags

9—CLASS B PUSH-PULL OUTPUT

The bases are 180 out

of phase (determined

by driver transformer

connecti

ons).

Bias

is provided by R7

and R8.

Crossover

distorti

on can result from the slight non-

linearit

y at very low base currents,

so

transist

ors are given a small bias to partl

y offset this.

Each transist

or conducts

on

alternate half

cycles which are recombined

in the output transformer.

Transformer

details: Bobbin iin = tin bore; Primary

two bifilar

windings 178 turns

each 31 s.w.g.

enam.

L = 190mH,

D.C.R.

3.30 for

each half.

Secondary

29 turns 20 s.w.g.

enam.

L = 5.25mH,

D.C.R. = <

Laminations 15 mil

mumetal.

Power

output

500mW into 30 loudspeaker

when coupled

to circuit in Fig. 8. Heat

sinks must

be

used,

35sq cm 18 s.w.g. aluminium.

Transistor: O

M (G)

INPUT

trom

driver tronsf

m'r

Fetdbict to

R5

100k11

best of driver

.."—

AAA-

--

7

II—COMPLEMENTARY SYMMETRY

10—SINGLE ENDED PUSH-PULL OUTPUT

TRI and TR2 can be considered as two

separat

e class

B operat

ed stages.

Bias

is

provided by RI and R2 for

TR I, and R3 and

R4 for TR2. Since the negative and

positive supply lines are common to ac.,

TRI conducts for one half-cy

cle whil

e TR2

conducts

for alternate half-cy

cles.

The

output

is taken across

the two stages

which

are in parall

el for a.c.

No output trans-

former is needed. Output 200mW into

75iI with 2.7k12

feedback resist

or from

output back to emitt

er of driver.

Driver

transformer (Forti

phone L442), turns ratio

7:1 1; Primary L = SH at I.5mA d.c.,

D.C.R.

7500.

Secondary d.c.r.

• 100 S2 per

winding.

Transistors:

0072(G), NKT2I2(A)

8

Driver trenvfor

mer •

INPUT

— 9II

Cl 1000pF

(Li

INPUT

CI 50)+F RI 101tn

No output transformer is used in this

circuit as the two transist

ors are operated

under similar conditi

ons to that shown in

Fig. 10. No driver transformer is used as

the signal is provided direct

from TRI.

Notice TR3 is an npn type so that it can

operate in opposit

e phase to the con-

ventional stage without

transformers.

The

output is taken from the emitt

er circuits.

Both TR2 and TR3 act

as emitt

er followers.

D.C. stabilisati

on is provided by RI,

2, 6, 7

for all three transist

ors. Heat sinks are

recommended— 35sq cm

18 s.w.g. alu-

minium.

Transistors:

TRI

2G37I(H),

TR2 2G381(H), TR3 2G339(H)

9