Dept. for Speech, Music and Hearing

Quarterly Progress andStatus Report

The useful complementarytwo-stage amplifier

Liljencrants, J.

journal: STL-QPSRvolume: 7number: 1year: 1966pages: 023-036

http://www.speech.kth.se/qpsr

111. INSTRTJl*lENTATION FOR ANALYSIS AND SYNTHESIS

J . L i l jencrant s

I n p r i n c i p l e t h e ampl i f i e r c i r c u i t t o be discussed here

is almost trivially simple, but i t allows t h e c i r c u i t des igner t o

develop many v a r i a t i o n s of p r a c t i c a l i n t e r e s t . The development of

semiconductor technology has given us complementary t r a n s i s t o r s wi th

exoe l len t d a t a t o a very low p r i c e . I n many cases t h i s involves

t h a t t h e cos t of paasive elements f o r i n t e r s t a g e connection and

b i m i n g g e t s out of propor t ion. For direct-coupled complementary

amp l i f i e r s modern transisvtors allow a minimum of t h e number o f such

oomponents which a l s o f a c i l i t a t e s t h e design. On t h e o the r hand

miorooiroui t blocks w i l l t ake over i n many a reas . This i s though

st i l l i n t h e f u t u r e f o r many app l i ca t i ons , e s p e c i a l l y i n analog

airouitry when economy IE of apy importance.

Some of Che fol lowing c i r c u i t s have appeared e a r l i e r i n

t h e l i t e r & t u r e but do not seem t o have received due a t t e n t i o n .

I n t h e moment of wr i t ing , however, a paper r e l a t e d t o t h i s w a s pub-

l i s h e d (2), which may i n d i c a t e a spreading use of t h i s type of c i r -

ou i t ry . A t t h e Speech Transmission Laboratory t h i s ampl i f i e r was

used t h e first t ime about t h r e e years ago i n t h e design of an a c t i v e

f i l t e r . Since then a number o f v a r i a t i o n s of t h e ba s i c c i r c u i t have

oome forward i n t h e l abora tory t o such a degree t h a t they now dominate

our analog c i r c u i t r y . Our general experience wi th these c i r c u i t s i s

q u i t e good and t h e l abo ra to ry work has moved i n t h e d e s i r a b l e d i rec -

t i o n from c i r c u i t problems t o system problems.

P r i n c i p l e

The simple bas ic c i r c u i t i n Fig. 111-A-1 shows t h e two

complementary , t r a n s i s t o r s which both a r e used i n t h e common-emitter

"English vers ion of paper presented a t HVK 66, Gothenburg, Sweden, 1 March 14-1 6, 1966. I

mode. If t h e two supply vo l tages a r e of equal magnitude t h e usefu l

output vo l t age range w i l l be symmetrically l oca t ed around zero.

Fig. 111-A-1 .

This i s v e r y a t t r a c t i v e when i t comes t o t h e app l i ca t i on of negat ive

feedback f o r s t a b i l i z i n g ga in and b i a s po in t s . Feedback i s gene ra l l y

appl ied as i n Fig. 111-8-2.

Fig. 111-A-2.

I f a d i r e c t cur ren t can pass through Z but not Z2 we 1

have u n i t y feedback a t low frequencies and t h e problem of b i a s i n g

i s e s s e n t i a l l y solved. A condi t ion f o r s t a b i l i t y a t h igher tempera-

t u r e s i s however t h a t t h e leakage cu r r en t s of t h e t r a n s i s t o r s a r e

s u f f i c i e n t l y small . I f t h e f i r s t s t age i s cut o f f by t h e input s igna l

i t s c o l l e c t o r leakage cur ren t w i l l @ti l l e n t e r t h e base of t h e second

s t age , where it i s amplif ied, and corresponding i s v a l i d f o r t h e

second s tage . The r e s u l t i n g drop over t h e load R should i n t h a t case

not be s i g n i f i c a n t . I n general t he se demands a r e f u l f i l l e d by e.g.

s i l i c o n p lanar t r a n s i s t o r s . I f t h i s i s not t h e case f u r t h e r elements

have t o be introduced. A s exemplified i n Fig. 111-A-3 leakage

cu r r en t s a r e dra ined through a c o l l e c t o r r e s i s t o r at t h e f i r s t

s t age . The diode, o r poss ib ly a Zener diode, wi l l g ive r i s e t o

a h igher DC vo l tage over t h i s r e s i s t o r without in t roducing i n t e r -

na l feedback i n t h e second s tage .

Fig. 111-A-3.

Moreover, when t h e two t r a n s i s t o r s a r e cut o f f by t h e

input s i g n a l t h e ba s i c c i r c u i t o f f e r s no e f f i c i e n t pa th f o r t h e

ex t r ac t i on of t h e base charge of t h e second s tage . I n general t h i s

w i l l l i m i t t h e use fu l frequency range t o t h e order of 100 kc/s.

Also t h i s i s remedied by t h e arrangement of Fig. 111-A-3 which thus

should be used f o r wideband app l ica t ions .

Since t h e f i r s t s t age i s opera t ing a t a very low c o l l e c t o r

cur ren t i t i s a l s o f o r t h i s reason recommendable t o use t he mentioned

type of t r a n s i s t o r t o i n su re a reasonable cur ren t gain.

The base and t h e emi t t e r of t h e first s t age could be re-

garded as t h e i npu t s t o a d i f f e r e n t i a l ampl i f i e r . However, t h e input

impedance of t h e emi t t e r i s only s eve ra l hundred ohms and a more de-

manding app l i ca t i on makes an add i t i ona l bu f f e r necessary. We then

ge t t h e c l a s s i c a l c i r c u i t of Fig. 111-A-4, o f t e n used i n commercial

opera t iona l ampl i f i e r s .

Fig. 111-A-4.

Basic v a r i a t i o n s

To start wi th p r a c t i c a l c i r c u i t s we look at Fig. 111-A-5 showing a degenerate form of feedback network. Here t h e e n t i r e

output vo l tage i s fed t o t h e inpu t emi t t e r g iv ing a ga in ve ry c lo se

t o uni ty . This "augmented e m i t t e r fo l lowern has a performance s i m i -

l a r t o t h a t of t h e Darl ington c i r c u i t i n Fig. 111-A-6. It is easy

t o show t h a t t h e r a t i o of input t o output impedances i s roughly

equal t o t h e product of t h e cur ren t ga ins of t h e t r a n s i s t o r s , wi th

t h e ind ica ted components approximately 20.000. Using d a t a shee t

va lues of t h e parameters an i nves t i ga t i on of t h e vo l tage ga in w i l l

g ive 1 - 4 * 1 0 - ~ f o r t h e Darl ington c i r c u i t and 1 - 2 * l 0 - ~ f o r t h e

augmented e m i t t e r follower. These f i gu re s conform reasonably wi th

p r ac t i c e . It i s q u i t e na tu r a l t h a t t h e augmented emi t t e r fo l lower

should be supe r io r because input and output a r e separated by only one

emit t er-base diode.

Fig. 111-A-6.

Fig. 111-A-8

This way of genera t ing a negat ive r e s i s t a n c e by t h e way

h i n t s at an a l t e r n a t i v e t o t h e C o l p i t t s and Har t l ey c i r c u i t s f o r

o s o i l l a t o r s , o r f o r Q mu l t i p l i c a t i on . A p o t e n t i a l h igh l i gh t i s t hen

t h a t no t a p i s needed i n t h e resonator .

I n app l i ca t i ons demanding a l a r g e r g a i n t h e feedback

c f r c u i t of Fig. 111-8-2 should be used. With t h i s c i r c u i t t h e g a i n

w i l l be t h e i nve r se of t h e feedback f a c t o r , o r

provided t h i s va lue i s much sma l l e r than t h e open loop ga in which

normally i s of t h e o rder of 50 dB.

Fig. 111-8-9.

Fig. 111-8-9 suggests a p rac t i ca l var iable amplif ier

o i r cu i t . A t low frequencies the feedback fac to r i s uni ty giving a

good bias point s t a b i l i t y . The gain a t normal frequencies is ad-

justed with the potentiometer i n the range 0 t o +20 dB. The input

impedance of the amplifier, disregarding the base leak of the f igure,

is as a rule-of-thumb

where the r igh t hand member contains the load (inoluding the co l l ec to r

r e s i s t o r ) , t h e current gains of the t r ans i s to r s , and t h e r e su l t ing

amplif ier gain. Usually Rin i s a t l e a s t several hundred kiloohms.

If it is taken as a r u l e i n the system design always

t o put the in te r s t age coupling capacitors a t t he amplifier inputs ,

then the impedance t o ground i n the interconnecting cables w i l l be

very low which i n tu rn w i l l make screening unnecessary i n many cases.

The c i r c u i t can a l so be used i n the operational amplif ier

mode as i n Fig. 111-A-10 where the gain w i l l be

while the input impedance i s Z2.

Fig* 111-8-10,

Before enter ing more exot ic va r i a t ions of t h e c i r c u i t

we f i r s t observe t h a t the augmented emit ter follower w i l l work as

an excel lent current generator i n Fig. 111-8-1 1 . The current through

R is proportional t o the input voltage, and s ince the input current

i s very low the main current has t o pass the output, the impedance

of which w i l l then be very high. This may seem t o be a paradox s ince

the output i s a,n emit ter .

Fig. 111-8-1 2 shows

a phase s p l i t t e r .

Fig. 111-8-1 1 .

t he c i r c u i t correspondingly used

P," Fig. 111-A-1 2.

Specialized c i r c u i t va r i a t ions

I f you take the temperature compensated c i r c u i t of

Fig. 111-A-7 and switch the vol tage t o the co l l ec to r r e s i s t o r a8 i n

Fig. 111-A-13 t h e c i r c u i t can operate as a gate. It i s a normal

buffer when G i s pos i t ive , but when it i s negative a l l th ree semi-

conductors w i l l be cut of f . Since the output impedance is high i n

t h e l a t t e r case several such outputs may be connected together t o

form a multiplexor.

Fig. 111-A-13.

I f you instead load t h e output with a capacitor, pre-

fe rably i n s e r i e s with an overload pro tec t ing r e s i s t o r , t h e r e s u l t

w i l l be a boxcar c i r c u i t control led by G.

The c i r c u i t e a s i l y lends i t s e l f t o the introduct ion of

switched vol tage dividers . This i s exemplified i n Fig. 111-A-1 4

which i s p a r t of a d i g i t a l l y control led amplifier/attenuator. The

amplif ier i s preceded by an a t tenuat ing d iv ider , and the feedback

path contains a second one cont ro l l ing the gain. R and R2 i n a 1

ohain of such u n i t s can be dimensioned f o r a t tenuat ions and gains

e.g. i n the s e r i e s 1 , 2, 4, 8 , ... dB and the overa l l gain i s then

s e t by the d i g i t a l control s igna l s S. Since t h e ac t ive elements

f i l l t h e dual function of working amplif iers and i s o l a t i n g s tages

t h e number of components p e r b i t i s r e l a t i v e l y small.

d second appl ica t ion of a switched vol tage d iv ider i s

seen i n fig. 111-8-15, a transformerless equivalent t o a balanced

r i n g modulator. Via the lower branch R2 - R we get a negative 1

gain, t he value of which i s general ly independent of t h e switoh.

The second s igna l path through R - R and the amplif ier w i l l in- 3 4 I

stead get a pos i t ive gain control led by the switch. It i s thus I

Fig.

f i g . 111-8-1 7 shows an i n t e r e s t i n g v a r i a t i o n of the

augmented follower. Depending of t h e p o l a r i t y of the input s igna l

one of t h e two input t r a n s i s t o r s i s always cut of f . The o ther of

them i s cooperating with the t h i r d t o an i s o l a t i n g buffer. The

output w i l l thus be a ful l wave r e c t i f i e d version of the input . The

transformer can of course be replaced by a phase s p l i t t e r with ap-

propr ia te coupling elements. The remarkable poin ts with t h i s r e c t i -

fier i s i t s high input impedance and a l i n e a r range of about 60 dB.

It i s a l so possible t o introduce nonlinear feedback i n t o the same

o i r c u i t , f o r instance according t o Fig. 111-8-16, giving a l eve l meter

ae a r e s u l t . A somewhat d i f f e ren t diode ladder w i l l make the c i r c u i t

an RMS detector .

I n a s e r i e s regulated power supply the regulator i s

normally made as a Darlington pa i r . You could just as well use our

oomplementary c i r c u i t as i l l u s t r a t e d i n the lower branch of Pig.

1114-18 t h a t shows the basic p a r t s of a twin power supply. With

t h i s arrangement you can use t h e same kind of power t r a n s i s t o r i n

both suppl ies without a need f o r separate transformer windings and

r e c t i f i e r bridges. The regulated output can by t h e way be made

l a r g e r than t h e reference vol tage by the introduct ion of a feedbaok

vol tage d iv ide r as i n Fig. 111-A-2.

F ina l ly we show two power amplif iers each made as a I

pyramid of th ree basic amplifiers. I n Fig. 111-8-19 t h e f i r s t two

t r a n s i s t o r s give the voltage gain determined by the feedback network,

Fig. III-A-18.

while t h e r e s t i s two power b u f f e r s connected i n pkh -pu l l and

wi th cur ren t ampl i f i ca t ion only. I n t h i s example t h e load Z i s

put i n s i d e t h e feedback network g iv ing an e f f i c i e n t cur ren t feed-

back. The c i r c u i t i s usefu l f o r d r i v i n g CRT d e f l e c t i o n c o i l s ,

osc i l lograph galvanometers, and c e r t a i n servo elements.

Fig. 111-8-1 9.

The l a s t example, Fig. III-A-20, i s a loudspeaker ampli-

f i e r with ordinary vo l tage feedback. Without making any r e a l e f f o r t ,

e.g. by matching t h e output t r a n s i s t o r s , you can e a s i l y ge t a d i s -

t o r t i o n l e s s than 0.1 $. This i s very much due t o t h e i n t e r n a l

Fig. III-A-20.

feedback between t h e power t r a n s i s t o r s and t h e i r d r i v e r s which

makes t h e d r i v i n g I tse l f compensating". The input c i r c u i t s can

n a t u r a l l y be va r i ed t o a high degree. I n t h i s example t h e diode

a t t h e base of t h e f i r s t t r a n s i s t o r works t o make t h e output DC

vo l tage neg l ig ib ly small . The quiescent cur ren t through t h e

power t r a n s i s t o r s regula ted by t h e two small r e s i s t o r s between

t h e i r c o l l e c t o r s i n conjunction wi th t h e diode i n t h e c o l l e c t o r

c i r c u i t of t h e second t r a n s i s t o r . It may seem unfavorable wi th

two supply vo l tages a s compared t o one being twice a s l a r g e , but

t h i s w i l l a s a r u l e not inf luence t h e volume of t h e power supply.

Ins tead t h e usual coupling capac i t o r t o t h e load i s unnecessary.

F i n a l l y a few words of warning may be i n order. It

should be repeated t h a t not a l l t r a n s i s t o r s can be used wi th

t he se simple c i r c u i t s because of leakage cur ren t hazards.

Secondly one should watch out f o r high frequency o s c i l l a t i o n s

apt t o occur i n systems with a high degree of feedback. F ina l l y ,

a l s o t h i s a consequence of high feedback, t h e semiconductors

should be guarded aga ins t d e s t r u c t i v e surge cu r r en t s i n case

of un in ten t iona l sho r t c i r c u i t s . It i s ve ry o f t en s u i t a b l e t o

do t h i s by p u t t i n g a small r e s i s t o r i n s e r i e s wi th t h e emi t t e r

of t h e second s t age .

References :

( I ) Crawford, B e : Itcomplementary two-stage a m p l i f i e r s t t , -. Electro-Technology 73 (1 964), pp. 48-53

( 2 ) OEhmichen, J .P . : ItLes t r a n s i s t o r s cgmpl6mentaires e t l e s a s s o c i a t i o n s NPN-PNPtt, L90nde E l e c t r i q u e - 46 (1 9 6 6 ) ,

P P O 84-98. (3 ) S t a s i o r , A . : "A review o f complementary t r a n s i s t o r s

c i r c u i t s des ign" , E l e c t r o n i c Design - 10 (~962)~ PP* 52-55 *