Overcurrent protecting circuit for push-pull transistor amplifier

Sikorra, D. J.

English

NASA Technical Reports Server

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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASHINGTON, D.C. 20546 
November 19, 1970 
u s ~ / s c i e n t i f i c  & Technical Information Division 
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Sept. 1, 71970 D. J. SIKORRA 3,526,M5 
APPARATUS FOR OVERCURRENT FROTECTYON OF A PUSH-PULL AMPLIFIER 
Fi led  Dec. 19, 3966 
FIG. I 
FIG. 2 
ATTORNEY 
United States Patent Bffi 3,526,845 ce Patented ScpL I* 1970 
3,526,845 
APPARAlkUS FOR OVERCURRENT PROTECTION 
OF A PUSH-PULL AMPLIFIER 
Daniel 3. Sikorra, Champlin, Minn., assignor, by znesne 
assignments, t@ the United States of America as repre- 
sented by the Administrator of the National( Aeronao- 
tics and Space Administration 
Filed Dee. 19, 1966, Ser. No. 602,828 
Int. @I. H03f 3/18,3/30,3/50 
U.S. 330-11 3 Claims 
and a collector. The emitter of transistor $2 is connecti.:I 
to a terminal 14 which is adapted to be connected to n 
source of direct voltage of a first polarity. A resistor 16 
is connected between a junction poii~t 18 and the collector 
Ci of transislor 12. Junction point 18 also functions as an input means to one-half, a side, or a part of the p:ish-pull 
amplifier. A series of diodes 20, 22, and 24 arc connected 
between junction point 18 and a junction point 26, so that 
the easy direction of a current flov~s from junction point 
18 lo junction point 26. Junction point 26 functions as 
an input means to the other half. side, or part or the 
ABSTRACT OF THE DISCLOSURE push-pull amplifier. A resistor 28 'is coknec<ed between junction point 26 and a terminal 30. Terminal 30 is 
This disclosure relates to electronic circuitry which may adapted to be connected to a second source of direct 
be used to provide overcurrent protection to- a push-pull 15 
amplifier. A pair of con~plementary amplifying transistors 
arranged in push-pull amplifier configuration are provided 
with voltage biasing circuits coupled to the input of each 
transistor and degeneration resistance means connected 
between each transistor and a common node output 20 
terminal. Voltage breakdown means are coupled between 
the comnlon node and each input terminal of the ampli- 
fying transistors in a direction such that excess current 
in the degeneration resistance means associated with either 
amplifying transistor causes the voltage breakdown 25 
rneans associated with the other amplifying transistor to 
condcct and apply a voltage to turn off said other tran- 
sistor whereby short circuit protection is provided the 
amplifier by the voltage breakdown means in conjunction 
with the biasing means. 30 
The invention described herein was made in the per- 
formance of work under a NASA contract and is subject 
to the provisions of Section 305 of the National Aeronau- 2.j 
-. 
tics and Space Act of 1958, Public Law 85-568 (72 Stat. 
435: 42 U.S.C. 2457). 
voltage of a second polarity. Another diode, aon-linear 
impedance means, or non-linear voltage breakdown means 
34 has its cathode connected to junction point 18 and its 
anode connected to a junction point 35. Still another 
diode, non-linear impedance means, or non-linear voltage 
breakdown means 36 has its anode connected to junction 
point 26 and its cathode connected to junction point 35. 
An NPN transistor or three ternlinal amplifying t~ansistor 
38 having a base, emitter, and collector f o ~ n ~ s  a portion 
40 of the push-pull amplifier in the embodiment shown. 
Transistor 38 has its collector connected to terminal 14, 
its base connected to junction point 18 and its emitter 
connected to junction point 35 through an impedance 
means or dcgene~ation resistor 42. A PNP transistor or 
three terminal amplifying transistor 44 havinz a base, 
emitter, and collector forms another portion 46 of the 
push-pull amplifier. Transistor 44 has its collector con- 
nected to terminal 30, its base connected to junction point 
26, and its emitter connected to junction poin 35 through 
an impedance means, or degeneration resistor 48. A load 
means or resistor 50 is connected between junction point 
35 and a common potential or ground 52. 
In FIG. 2. a PNP transistor 100 havine a base. emitter. 
BACICGROUND AND SUMMARY and a collector and an NPN transistor 1i2  having a base: 
emitter, and collector are interconnecled such that col- 
In many push-pull amplifiers if the load is shorted in- 40 lector of transistor 102 is connected to the emitter of 
advertently, the amplifier will be destroyed. The present transistor 100 and the base of transistor 102 is connected 
invention was developed in response to this problem. The to the collector of transistor 100. A resistor 104 is con- 
circuitry of the present invention prevents a current great- nected between the emitter of transistor 102 and the col- 
er than a predetermined amount from flowing in the lector of transistor 100. A resistor 106 is connected be- 
amplifier. 43 tween the emitter of transistor 100 and the base of tran- 
Another problem in push-pull amplifiers is that they sistor 100. The base of transistor 100 is also connected 
may oscillate or be driven at a high frequency. If com- to a terminal 110. The collector of transistor 102 is con- 
plementary symmetry transistors are used, the oscillation nected to a terminal 112. The emitter of transistor 102 
and inherent storage effects in amplifiers may cause both is connected to a termillal 114. 
sides of the push-pull amplifier to conduct at the same 50 
time for a brief period. This causes a short circuit be- OPERATION 
tween the two bias supplies providing bias to the push- Assume for the moment that diodes 34 and 35 are not 
pull amplifier and destroys the amplifier. The present in- present. A signai at input means 10 is amplified by tran- 
vention eliminates this problem also by providing that sistor 12 and appears at junctiorl points 18 and 26. If 
overcurrent in one part or side of a push-pull amplifier 63  transistors 35 and 44 are biased class B or class AB, 
turns off the other part or side. then the positive portion of the signal at  janction point 1 8  
DESCRIPTION will be amplified bjr transistor 38 because it is an NPN 
transistor and the negative portion of the signal a t  junc- 
It is an object of this invention to advance the amplifier tion point 26 wili be amplified by transistor 44 becarrss 
art by providing short circuit protection to a push-pull G O  it is :L FNP transistor.. The negative portion of the sign21 
amplifier. at j~rnction point 16 mv.~ill tarn transistor 38 off, apt$  the 
Further objects and advantages ivill become apparent ~ositive portiori BP the signal at jr~nciion point 26 will 
froin the reading of this specification al;d claims in con- i i lrn irnnsistor. 43 OFF. All of this is :veil Ecnow;; to 
junction with the drawings wherein: those skilled in :he art. This means that load means 50 
FIG. 1 shows a preferred embodinlent of a push-pull ti5 is ~lriven by transistor 38 during one-half of the cycle 
amplifier using the teachings of the present invention; and by transistor 44 during the other half of the cycle. 
and This is what is meant by a push-pull amplifier, as 
FIG. 2 shows a substitute circuit to be combined with again is well known to those ski!led in the art. 
a portion of FIG. 1 to provide another embodiment of The diodes 20,  22, and 24 are inserted to  provide bias 
the invention. 70 separation to transistors 38 and 44. That is, two of the 
In FIG. 1, an apparatus input means 10 is connected diodes compensate for the emitter base diode effect of 
to a bas:: of a PNP transistor 12  having an emitter, a base transistors 38 and 44 and the remaining diode biases the 
3,526,845 
3 
,.. I 
4 
ilntrs!sic;.s sligh!ly 0p.' 90 ::c bccoil?a, in this case, I'r is .:.is0 obvio~lr to those sl<il!ed in ille art that diodes 
biased i:1 c l ; ?~ i  AB. i t  will he realized l?y those skilled 20, 22, and 24 are not necessarily the only biasing ar- 
in the Zr-t th.:ll fewer or  mo?a diodes can he used depend- rangement possib!e. Resistors, batteries, or Zener diodes 
iiig oil thc typc or  c l a s  of bias desired. 
-. 
co~lld be used. Hoviever, diodes are preferred at  this 
1 buy far the conve~tional class en~itler follower time. 
lypc amplifier has been explained. Now the circuit with I claim: 
diodes 34 and 36 inserted will be considered. 1. Apparatus providing short circuit protection for a 
Assume that an excessive current is drawn by tran- push-pull amplifier comprising: 
sistor 38. Considering the s e 1 . i ~ ~  circuit formed by re- ( a )  a plurality of complementary symmetry amplify- 
sistor 42, diode 36, diode 24, diode 22, diode 20, and ing transistor means arranded in push-pull amplifier 
emitter base junction of transistor 38, it is seen that all configuration, each amplifying transistor means hav- 
of the voltage drops in this series circuit are fixed by ing three terminals, including a base terminal; 
non-linear charactcrislics except the voltage drop in re- (b)  a plurality of degeneration resistance means, one 
sistor 42. I t  is also seen that the voltage across the of said degeneration resistance means being con- 
emitter base junction of transistor 35 and the voltage 15 nected to a first terminal of a first of said amplify- 
across d i ~ d e  36 approximately balance each other in the ing transistor means and another of said degeneration 
series circuit and therefore these two terms may be stricken resistance means being connected to a first terminal 
from consideration. What remains is equal and opposite of the second of said amplifying transistor means, 
voltage drops across the diodes 20, 22, and 24 and the said degeneration resistance means being connected 
resistor 42. Thus it is easily seen that the instantaneous 20 together at  a common node output terminal of said 
voltage across resistor 42 can be no pea te r  than the push-pull amplifier; 
voltage across diodes 20, 22, and 24 or  there will be (c)  circuit biasing means coupled to the input of each 
less instantaneous bias on trar~sistor 38 than required, amplifying transistor means by connection between 
and it will begin lo turn off. This means that if an  excess the base terminal of said first amplifying transistor 
current through resistor 42 causes the voltage across 25 means and the base terminal of said second amplify- 
resistor 42 to equal the sum of ths diode voltages, the ing transistor means wherein the base terminal of 
circuit will begin current limiting. The level of current each transistor means is an  input terminal, in each 
litlliting is deiermined by resistor 42 since the size of amplifying transistor means said third terminal being 
the resistor determines what voltage is developed from operatively connected whereby said amplifying tran- 
the current flowing through it. 30 sistor means provide push-pull amplification; and 
Another feature of this invention is that if transistor (d)  a pair of non-linear impedance voltage-breakdown 
38 tends to dram an excessive current, diode 36 turns means, one of said non-linear impedance means con- 
ON. It is seen that diode 36 impresses a voltage across nected between the base terminal of said first ampli- 
the emitter base junction of transistor 44 which is oppo- fying transistor and the conlrnon node output ter- 
site from that which transistor 4-4 needs to remain in 35 minal and the other of said non-linear impedance 
a conducting state. This means that whenever one tran- means connected between the base terminal of the 
sistor at temp!^ to draw an excessive current, the other second amplifying transistor and said common node 
transistor is turned off. This feature prevents any high output terminal, said non-linear impedance means 
frequency oscillation or  driving voltage from causing both being connected in a direction such that an excess 
transistors to remain conducting a t  the same time which qo of current in one amplifying transistor and its as- 
would cause a short circuit from the voltage impressed sociated degeneration resistance means coupled there- 
at terminal 14  through transistors 38 and 44 to the to  causes the other non-linear impedance means asso- 
voltage impressed at  terminal 30. ciated with the other of said amplifying transistors 
It is obvious to those skilled in the art  that the sym- to  conduct current and apply a voltage t o  turn oft' 
inetry of the circuit requires that the converse explana- 45 said amplifying transistor whereby short cir- 
tion hold for the remainder of the  circuit, if a.n excess cuit protection is provided the push-pull amplifier 
of current is drawn by transistor 44. by the non-linear impedance means in conjunction 
The circuitry of FIG. 2 may be substituted directly with the biasing means. 
for the three terminal element 45 contained within the 2. The apparatus of claim 1 wherein the non-linear 
dashed lines. This is because the circuitry within FIG. 2 50 impedance meanns colllprises diode means. 
performs as a high gain PNP transistor. This circuit is 3. The apparatus of claim 1 wherein the biasing means 
familiar to those skilled in the art. The advantage of  includes diode means. 
this circuit is that an  NPN transistor may be used for 
the power stage and a relatively low power PNP tran- References Cited 
sistor used as a driver. In  the current state of the art  55 
it is easier and cheaper to obtain the high power N P N  UNITED STATES PATENTS 
transistor than to  obtain a high power PNP transistor 3,015,781 111962 Eklov 330-15 
of eaual snecifications. 3,375,455 3/1968 Motta _----------- 330--24 X 
other cbnfigurations wiII be obvious to one skilled in 
;!le art. In fact, it is realized thnt many three terminal 60 NATHAN 'limary 
amplifier configu~ations may replace amplifiers 40 and 
46. Amplifiers 40 and 46 have been represented as a U.S. C1. X.R. 
single transi-tor for the sake of simplicity only. 330-24, 13, 14, 15, 2 6  


Apparatus for overcurrent protection of a push-pull amplifier Patent

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