TRANSISTOR AND DIODE STUDIES. Prof. H. J. Zimmermann Prof. S. J. Mason C. R. Hurtig Prof. R. B. Adler Dr. W. D. Jackson R. E.

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1 XI. TANSISTO AND DIODE STUDIES Prof. H. J. Zimmrmann Prof. S. J. Mason C.. Hurti Prof.. B. Adlr Dr. W. D. Jackson. E. Nlson A. DESIGN OF TANSFOMEESS TANSISTO AUDIO AMPIFIES Considrabl ffort by many oranizations has bn put into th dvlopmnt of hihfidlity, hih-fficincy audio amplifirs that us powr transistors. Ths amplifirs may b dividd into two classs: thos that us transformrs, and thos that mploy complmntary symmtry. lativly littl, if any, fdback is mployd in th transformr-coupld amplifirs. This is du, in part, to th limitations that th transformrs plac on th frquncy rspons. Th us of complmntary symmtry liminats this limitation but, unfortunatly, crats two nw limitations. First, it is difficult to obtain commrcially availabl complmntary powr transistors that ar rasonably matchd for ain of hih-currnt lvls. Scond, a srious problm of bias stability xists. W hav rcntly studid th dsin of sinl-ndd, push-pull, transformrlss output stas that us th sam typ of transistor and hav built amplifirs that incorporat as much as 36-db ovr-all nativ fdback. This dsin tchniqu liminats th problm of matchd ain at hih-currnt lvls, but dos not compltly liminat th stability problm. Bfor dscribin this output sta, a fw nral points of intrst in transformrlss amplifir dsin will b statd. It is commonly known that th roundd bas connction is not usful in transformrlss dsin bcaus th rsultin powr ain is lss than unity. Th advantas of th roundd collctor ovr th roundd mittr from th standpoint of distortion ar also wll known (1). Th advanta of th roundd collctor from th standpoint of maximum phas shift at hih frquncis is not as wll known. It will b discussd hr. An quivalnt circuit for a roundd-mittr sta is shown in Fi. XI-1. With th assumption that th collctor impdanc z is infinit, th volta ain is in a (1-a)( + rb) + r Middlbrook' s (2) approximation for th frquncy dpndnc of th currnt ain a is a (1 - m Ts) 1 + Ts (2) This rsarch was supportd in part by Purchas Ordr DD-B222 with incoln aboratory, which is supportd by th Dpartmnt of th Army, th Dpartmnt of th Navy, and th Dpartmnt of th Air Forc undr Contract AF19(122)-458 with M.I. T.

2 TANSISTO AND DIODE STUDIES) in r i I [ Fi. XI-1. Equivalnt circuit of a roundd-mittr amplifir. whr m = 0.204, T = 1. 04/2Trf c, and fc is th frquncy at which th currnt ain is 3 db blow its low-frquncy valu. This approximation is vry accurat for frquncis as hih as fc' and vn rasonably ood for frquncis as hih as 4 fc. Thus ao[l - m Ts] o in (1 - a ) + r + Ts[ (I + am)+ r] o o (3) whr = + rb. Th sinificant point in Eq. 3 is that th maximum phas shift is Introducin nativ fdback by th simpl mans of incrasin th ratio r / dos not affct th maximum phas shift. Th quivalnt circuit of a roundd-collctor sta is shown in Fi. XI-2. Th volta ain of this sta is 0= in + r + K(1-a) (4) Substitutin for th frquncy dpndnc of a, and lttin = + r, w obtain 1 + Ts in -(5) ( a) 1 +mm) +Ts + Ts ( Ts (6) I + Ts 1 +- (1+m) t Thus, th volta ain of a roundd-collctor connction has th intrstin proprty of a lad-la ntwork. Th maximum phas shift is 90 or lss. Th frquncy at

3 TANSISTO AND DIODE STUDIES) I, - -I -- Fi. XI-2. Equivalnt circuit of a roundd-collctor amplifir. which th maximum phas shift occurs is ivn by f f c max / - (+m) Th valu of th maximum phas shift is (7) Th valu of th maximum phas shift is - (1+m) i max tan max tan (m] 2- (l+m ( 1/2 ) As th ratio / approachs unity (th approximat condition for an availabl powr ain of unity) th maximum phas shift approachs a valu of a fw drs. For practical valus of / btwn 5 and 10, th maximum phas shift varis from 450 to 580. Th circuit diaram of a sinl-ndd push-pull output sta tothr with a complmntary drivr sta is shown in Fi. XI-3. Class B opration is usd in both th output and drivr stas. For nativ-oin wavforms at th input, both T 1 and T 2 conduct; for positiv-oin wavforms, T 3 and T 4 conduct. It should b notd that T 3 and T 4 ar both roundd-mittr connctions. Howvr, w shall show that th mthod of connction that includs a hih dr of nativ fdback causs th ain and th frquncy rspons of two roundd-mittr stas to closly approximat thos of th

4 TANSISTO AND DIODE STUDIES) T Ti Fi. XI-3. Circuit diaram of a sinl-ndd push-pull output sta and compl- C 3 mntary drivr sta. 3 2, T Tin T roundd-collctor stas. Th us of complmntary transistors in th drivr sta provids th ncssary phas invrsion. Capacitanc couplin to th load rsistor allows th us of a sinl powr sourc and slihtly improvs th bias stability of th circuit. Th mid-frquncy ain of th two roundd-collctor stas, if w assum that th collctor impdancs ar infinit, is i. in +r 2 + b b ) (1 - a) + ( + rb1)(1 - al)(1 - + r a2) 1 +- (1 - al) (1 - a) (10) If w includ th frquncy dpndnc of al and a 2, th hih frquncy ain bcoms (1 + T 1 s) (1 + T 2 S). in 2 S+ (T + T2) 1 +- (1 - ao) (l+m) s + TIT (l+m) s (11) whr ao = (a)s= 0 = (a 2 )s=. If TT, thn (1 + T 1 s) (1 + T 2 s) 0. ] in 1 + T2s (1-a) + T1TzS _ (12) Th dnominator has two pols. Th location of ths pols is a function of th ratio /. Whn this ratio is small th pols ar locatd on th nativ ral axis; as th

5 TANSISTO AND DIODE STUDIES) ratio incrass th pols mov toward ach othr; and aftr coincidnc bcom complx. Th mid-frquncy ain of th two roundd-mittr stas is in [ + r b 3 (1 - a3) + r ] (1 - ) (13) 1 + [1 - a 3 + a 3 a 4 1 If th manituds of a3 and a4 ar qual, T 4 > T 3, and r and 3 ar nliibl. Thn o 1 + T 4 s + T3T4(1 + m 2 ) s 2. r (14) in T m +- (1-a) (1+m) s + T 3 T 4 (i+m) m s If T 4 is ratr than T 3, w hav o (1 + T 4 s) (1 + T 3 s).. F in T (1-) s + T 3 T s (15) Comparin Eqs. 12 and 13, w s that th hih-frquncy bhaviors ar quit similar. A comparison of Eqs. 10 and 13 for th condition that th currnt ain a is clos to unity indicats that th mid-frquncy ains ar both vry clos to unity. It can also b shown that th input and output impdancs of th two diffrnt circuits ar approximatly qual. Thus th low-frquncy ains will also b qual. Th bias stability of th circuit shown in Fi. XI-3 has not bn studid in rat dtail. Calculations show that th quiscnt currnt has a trm that is dpndnt on saturation currnts of th form I co +I co quiscnt currnt trm 2, whr a' is th approximat currnt ain of th output sta. This factor should indicat a rlativly hih snsitivity of quiscnt currnt to th tmpratur of th collctor junctions of th powr transistors. Howvr, in actual opration, th quiscnt currnt, aftr lon priods of opration at powr lvls as hih as 5 watts, rmains rasonably constant. This may wll rsult from th inhrnt thrmal stability of th powr transistors rathr than from proprly stabilizd circuitry. With th us of commrcially availabl transistors (such as Philco 1041 powr transistors and Gnral Elctric 2N136 and 2N169 drivrs), th maximum phas shift of th volta ain of th output and drivr stas can b of th ordr of Employin

6 TANSISTO AND DIODE STUDIES) ths dsin principls, w hav constructd and tstd 4-sta, 5-watt audio amplifirs which hav provd stabl, with 36-db ovr-all nativ fdback from th spakr to th input sta. Intrstin charactristics of such amplifirs ar: (a) output impdanc ovr th audio ran of th ordr of 0.01 ohms; (b) frquncy rspons that is flat within 1 db from 5 to 50, 000 cps; and (c) squar-wav rspons at 20 kc with lss than 5 pr cnt ovrshoot. C.. Hurti frncs 1.. P. Crow and. D. Mohlr, Dsin of hih fidlity 10 watt transistor amplifir, IE Convntion cord, Part 7, 1956, pp D. Middlbrook, An Introduction to Junction Transistor Thory (John Wily and Sons, Inc., Nw York, 1957), pp B. DIEECTIC MODUATOS Tmpratur tsts of th zro-sinal balanc of th C brid modulator (s th Quartrly Prorss port of July 15, 1957) that mploys junction diods hav indicatd nonrtracabl charactristics. Sinc w now attribut this factor to th prsnc of watr moistur rathr than to a prmannt chan in th paramtr valus, w hav constructd a hrmtically sald box containin th majority of th brid componnts. Th tmpratur tsts will b rpatd in th futur. C.. Hurti

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