Low Distortion and High Thermal Stability in Transistor Audio

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					         Low Distortion and High Thermal Stability in
             Transistor Audio Power Amplifiers

    Summary-Two paramount objectives in the design of a high-                                                                 sary and suficient is the following: There shallbe no
quality transistor audio power amplifier are attaining good linearity                                                         nonlinearity in the open-loop transfer characteristic at
and ensuring good thermal stability. Conflicts frequently arise in
realizing these objectives in that steps taken to achieve one tend to
                                                                                                                              any signal level below clipping which causes the incre-
worsen the other. This paper examines these two subjects to deter-                                                            mental gain, ;.e., the slope of the transfer characteristic,
mine the essential conditions which must be fulfilled in order for the                                                        to approach zero. Expressed differently, if and y are
objectives     be met. As a result of this examination the requisite                                                          the open-loop input and output, respectively, and      B is
conditions are found and several useful circuit techniques are de-                                                            the closed-loop feedback ratio, one requires that
veloped. These techniques provide excellent thermal stability and
at the same time permit attaining good linearity with reasonable
amounts of negative feedback. Experimental results are given which
confirm the validity of the principles and the value of the techniques.                                                                                       B
                                                                     If (1) is satisfied, then the closed-loop incremental gain
                                                                    will beconstant at 1 / B andthe closed-looptransfer

A      CHIEVING LOW distortion in anaudio power characteristic will belinear.Theimplications
          amplifier requires that the over-all transfer char-
          acteristic exhibit no appreciable nonlinearity in
 the portion of the characteristic in which normal pro-
                                                                    condition are as follows: 1) class B operation is ruled
                                                                                                                             of this

                                                                              class A B operation is acceptable and with eased
                                                                    bias tolerances, and3) nonlinear techniques may be con-
 gram material spends an appreciable amount              of time. sidered for use in the achievement of thermal stability.
 For music and speech this regionis that around the                 This last point will now be considered in detail.
 origin of the transfer characteristic, and for this reason
 low-level or crossover distortion is of major concern in
 the design of a high-quality audio power amplifier.                    As is well known, the thermal stability of a transistor
    The presence or absence of crossover distortion in a amplifier is governed by the thermal-electrical positive
 transistor power amplifier depends on the type            of bias feedback process whereby heating causes increased            col-
 applied the
        and             effectiveness of distortion-reducing lector current which in turn causes still more heating.
 feedback. Because class A operation is too inefficient t o For this process to reach a stable equilibrium the “loop
 be acceptable, push-pull class B or A B operation must             gain” for this process must be made less than unity by:
 beemployed.Sinceitmaybeassumedthatover-all                         1) selectingsuitablevalues         of the various parameters
 negativefeedback          ill beincorporated      in anyhigh-      aroundtheloop,                       ,
                                                                                           e.g., I c ~ othermal resistance 0, and
 quality audio power amplifier, the choice between these power supply voltage Vcc, and/or 2) introducing nega-
 twomodes of operation depends on the way the bias                  tive feedback to modify one or more           of the processes
 arrangement influences the        nature     of the open-loop      comprising the loop. Since the useful choices for Icno, 6,
 transfer characteristic and thereby the effectiveness           of and VCC usually very limited, and since adequate
 the feedback in producing linear closed-loop response.             stability can seldom be attained by this means alone,
    A suficient conditionfor goodclosed-looplinearity               the feedback approach is essential in achieving thermal
 would be to require that the open-loop transfer char-              stability.Negativefeedbackcanbeappliedeitheras
 acteristicbe as linear as possible.On thisbasispure                direct electrical feedback or as thermal-electrical feed-
 class B operationwouldbeexcludedsinceobjectional                   back. The latter method consists         of utilizing tempera-
 crossover nonlinearities appear even with high-inlped-             ture-sensitive bias elements to reduce the bias current
 ance drive. On this same basis classB operation would as temperature increases. limitation of this method is                                            A&

be acceptable if, but only if, the bias were set at pre-            that the temperature         influencing thebiaselements      is
cisely the correct value. This would be a very stringent            the ambient or heat sink temperature rather than the
requirement. Fortunately this condition is not a neces- transistor junction temperature, the parameter which
sary one. A less stringent condition which is both neces- actually determines the electrical characteristics of the
                                                                    transistor. Appreciable time lags occur between changes
                                                                    in thesetemperatures so thatthermal-electricalfeed-
    Manuscript received April     1964.
    R. A. Greiner is with the Department of Electrical Engineering, back is ineffective in stopping rapid thermal runaway
Lniversity of Vv?sconsin, Madison, IITis.                           processes. This type of feedback is so slow t h a t i t is
    D. G. Daugherty is with the Department of Electrical Engineer-
ing, University of Kansas, I,aa.rence, Kan.                         perhaps better not regarded as feedback but rather as

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                                                                                                                             Daugherty and Greiner: Distortion and Thermal Xtability   in Transistor Amplifiers   27

empirical compensation changes ambient
           bias               for        in                      capacitivebypassing of theseresistances in class A B
temperature. rapid
                The      runaway            processes must       amplifiers is not useful because the emitter signal cur-
sensed and inhibited by electrical means. Direct electri-        rentshaveinherentlynonzeroaverageswhichwould
cal feedback is thus the primary method for ensuring             cause the capacitors to charge and bias the amplifier
thermal stability.                                               into cutoff. Accordingly either small emitter resistances
    T o determine how stability may actually be achieved,        mustbeemployedorsomenoncapacitivebypassing
i t is necessary to consider the precise mechanisms by           arrangement must be found.
which thermal instability can occur. A pair transistorsof            Suchanarrangementisthat             of Fig. 2, wherethe
comprising a push-pull class A B amplifier can be con-           emitter resistor is bypassed a t large currents by a series
nected in essentially two d-c configurations, series and         pair of silicon diodes. Since normal silicon diodes require
parallel. parellel
          The         arrangement   applies trans-
                                            to                   approximately 0.6-volt forward bias before appreciable
former-coupledcircuitswhereastheseriesconnection                 conduction occurs, by suitable selection of RE and the
prevails in virtually all output-transformerless circuits.       bias current the quiescent operating point may be set         In
The latter group is of concern here, and a series con-           the center of the moderate sloperegion of the trans-
figuration will therefore be used as the basis of analysis.      conductance characteristic, as shown in Fig. 3. In this
    Consider the circuit of Fig. 1 and currents il, iz, and      case diodebypassing            when emitter
                                                                                      commences the
i ~ . notedabove,thermalrunawayinvolvesanin-
     As                                                          current is driven to twice the quiescent value. This ar-
crease in collector current. For an increase in il to occur      rangement providesgood bias stability by allowing large
there must be an equal increase                                  quiescent RE while at the same time permitting signal
                                         in ( i ~ + i 2 ) . Since the
output stage is presumably included within a negative            operation at highcurrentswithhighgainandsmall
feedback loop, an appreciable increase i~ is precluded
                                               in                losses. An apparentdisadvantage of thisapproach is
in t h a t such a n increasewouldconstitutedistortion            that it is nonlinear. T h e open-loop transfer character-
which would be reduced by the feedback. Thus Qz must             istic of a class AB power amplifier using a pair of tran-
provide the path for an increase in the collector current        sistors connected as in Fig. 2 would be as shown in Fig.
of Q1 and vice versa. For examining the collector cur-           4.This transfer characteristic has a slope which is much
rent behavior of Q1 and Qz under class A B operation,            greater than zero everywhere. Therefore, the necessary
we may    consider separately near-quiescent
                              the                  and           condition for low closed-loop distortion is satisfied and
large-signalphases of operation.Underano-signalor                t h e open-loop nonlinearity is not in fact a disadvantage.
small-signalconditionclass         A operationprevails,and       Thus byjudicious application of nonlinear elements itis
there is an appreciable collector current flowing due to         possible to obtain excellent bias stabilization without
the applied bias. Accordingly, quiescent runaway is a            causing appreciable amounts of distortion.
distinct possibility if the quiescent current is not care-           Consider now the large-signal phases of operation of
fully controlled.Hence requirement thermal
                        one             for                      the class A B system of Fig. 1, i.e., consider a time when
stability is adequate bias stabilization.                        one transistor is conducting and the other is nominally
                                                                 OFF. If the current Ioff the OFF transistor is actually
                                                                 zero, then there is no dissipation in that transistor dur-
                                                                 ing its OFF time, and the dissipation in the               tran-
                                                                 sistor is exactly predicted      from simpletheory.
                                                                 However, if I o f f # O , excess current flows in both tran-
                                                                 sistors, and there is an excess dissipation per transistor
                                                                 given bylicc(lorr,~+I,tr,2)/2,where V C C is definedin
                                                                 Fig. 1. To prevent thisexcess dissipation from becoming
                                                                 excessive dissipation, (Ioff,~+Iof~,2) small.
                                                                                                           must be kept
                                                                 Thus the second condition for thermal stability is effec-
                                                                 tive minimiziation of Ioff.
          Fig.      transformerless amplifier circuit for            A technique for minimizing I o f t in germanium tran-
                   analyzing thermal stability.
                                                                 sistors (Ioff  isnegligible in silicon) is showninFig.         5
                                                                 where a silicon diode has been added in series with the
   An effective technique for stabilizing transistor emit-       emitter of the germanium transistor. For this arrange-
ter and collector current      is to apply negative current      ment, IoorrgIcBo if the base circuit conditions are such
feedback by introducing resistance in the emitter cir-           t h a t V B<0.6 volt during the OFF time. Note that this
cuit of the transistor. This is of course extensively used circuitdoesnotprovideanybiasstabilization.How-
in low-power class A circuits. However, two major ob-            ever, by combining this technique with that          of Fig. 2,
jections to the use of large emitter resistances may be          as shown in Fig. 6, an arrangement providing both bias
raised in thecaseofpoweramplifiers:               1) the voltage stabilization and Ioff     minimization results. In this cir-
and power gains the output stage are severely reduced            cuit, as in t h a t of Fig. 2, RE and the quiescent current
and 2) power losses in the emitter resistances can be-           IQ should be chosen sucht h a t IQRE 0.6 volt. While RE
come prohibitively large. It must be further noted that should be large forgood bias stability, some upper limit

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28                                                                                                                                                   IEEE TRANSACTIONS O N AUDIO                                          M arch,-April

                                                                                                                                                                            Fig. 6---A stabilization-minimization technique combining
          Fig. 2-A                                                                                             nonlinear bias stabilization technique.                        the features of the techniques shown in Figs. 2 and 5.

                                                                                                                                                                       must be observed. If RE is too large the slope         of the
                                                                                                                                                                       open-loop transfer characteristic becomes too small to
                                                                                                                                                                       sarisfy (1). This occurs both because      of the degenera-
                                                                                                                                                                       tion introduced by R E and because the necessarily small
                                                                                                                                                                       quiescentcurrent ( I Q = ~ . ~ / R E ) areduction
                                                                                                                                                                                                            causes                 in
                                                                                                                                                                             for small-signal operation. The appropriate upper
                                                                                                                                                                       limit on RE can be determined from analysis of the cir-
                                                                                                                                                                       cuit in which these techniques are      to be utilized. This
                                                                       0                                                                                               analysisshouldinclude       a feedbackstabilityanalysis.
                                                                                       0                                                                               The amplifier
                                                                                                                                                                  such be mustdesign
                                                                                                                                                                              over-all                                 the that
                  Fig. 3-The                                                                                                    transconductance characteristic         smallestslope of the open-loop transfercharacteristic
                                                                                                                             for the circuit of Fig. 2.
                                                                                                                                                                        will satisfy ( l ) ,and at the same time all values of slope
                                                                                                                                                                        between this and the maximum must imply acceptable
                                                                                                                                                   stability.           feedback
                                                                                                                                                                           I t should be noted that     in theseseveraltechniques
                                                                                                                                                                        silicondiodeshavebeenemployed             in spite of rather
                                                                                                                                                                        than because       of theirtemperaturedependence.        Ac-
                                                                                                                                                                        cording-ly the diodes should be isolated thermally frotn
                                                                                                                                                                        thepower             and     be
                                                                                                                                                                                 transistors should provided  with
                                                                                                                                                                        suitable heat dissipators of their own.

                                                                                                                                                                                         EXPERIXEXTAL htPLIFIERS
                                                                                                                                                                           The validity of theprinciplesgivenaboveandthe
                                                                                                                                                                        value of the techniques described are confirmed by the
                                                                                                                                                                        results for two experimental amplifiers. Fig. 7 shows the
                                                                                                                                                                        output configuration employed in these amplifiers. This
Fig. 4-.A typicalopen-looptransfercharacteristicfor          Push-pull                                                                                                  circuit is basicallS7 the L ~ quasi-complementary ampli-
                                                                                                                                                                                                        ~ I
        amplifier using two transistors connected as in Fig.. 2.
                                                                                                                                                                        fier modified to utilize the stabilization techniques de-
                                                                                                                                                                        scribed above. For the compound transistor composed
                                                                                                                                                                        of Q 3 and Q 6 the stabilization-minimization technique of
                                                                                                                                                                        Fig. 5 is employed. resistance
                                                                                                                                                                                                For                R: thecondition
                                                                                                                                                                        R 5 RL 8 ohms is observed in order to limit the varia-
                                                                                                                                                                        tion in open-loop   transfercharacteristicslopeto         no
                                                                                                                                                                        more than a factor of two. Since six-ohm resistors were
                                                                                                                                                                        a t h a n d R is ohms andIQ 100 ma. I n the uppercorn-
                                                                                                                                                7r                      pound transistor Q2 Q 4 a modification of Fig. 5 is ap-
                                                                                                                                                                        plied t o Q4, and the compound as a whole is stabilized by
                                                                                                                                                                        use of the technique of Fig. 2. Diode D Sminimizes I o f f

       Fig. off-current
       effective             technique
                    minimization                                                                                                                                             H. C. Lin, complementary amplifier,”
                                                                                                                                                                                      “Quasi          transistor                    Elec-
                 for use
                              transistors.                                                                                                                              tronics, vol. 29, pp. 173-175; September, 1956.

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                                                                                                                                  and Distortion
                                                                                                                                   Greiner:   and Stability
                                                                                                                                               Thermal        in Transistor
                                                                                                                                                                         Amplifiers   29

                                                                         The excellence of the thermal stability is evidenced
                                                                      by the following data. With the amplifier operating un-
                                                                      der maximum dissipation conditions, the power supply
                                                                      current, and the output transistor heat-sink tempera-
                                                                      ture were recorded a t frequent intervals during a 41-
                                                                      minute test period. The heat-sink temperature varied
                                                                      from an initial value of 30°C to its equilibrium value of
                                                                      62OC. During interval         was detectable
                                                                                                there no
                                                                      change in powersupplycurrent.Fromtheconstancy
                                                                      of this current i t is clear that the dissipation was also
                                                                      constant and that the thermal stability was very good.
                                                                         T h e secondamplifierbasedonFig.          7 employs V g g
                                                                         2 VCC= volts, CBs= 1000 uf, R1= 100 ohms, and
                                                                      R 500 ohms. This combination produces 150 watts of
                                                                      sine-wave power in an 8-ohm load and exhibits distor-
                                                                      tion and stability characteristics similar to those given
                                                                      above for the 50-watt unit. As a check on the effective-
                                                                      ness of the diode-resistor networks, this 150-watt am-
                                                                      plifier was “simplified” by omitting several of the diodes
                                                                      and adjusting the bias accordingly. This modified cir-
                                                                      cuit exhibited poor thermal stability and failed destruc-
Fig, 7-The basic output configuration used in the two experimental tively at a power level of about 50 watts. From these
   amplifiers described in the text. All three P-N-P transistors are
   germanium, both N-P-N transistors are silicon, and all diodes resultsit is seen thatthediode-resistornetworks                 of
   are silicon.                                                       Figs. 2 and 6 do promote thermal stability and at the
                                                                      same time permit achieving low distortion performance
                                                                      with reasonable amounts of negative feedback.
of Q4 while R6 and D prevent Q 4 from turning off too                    T h e use of nonlinear techniques is often complicated
abruptly and introducing crossover distortion. By sym- and expensive and involves a time-consuming selection
           s 7                 e
metry, R R so t h a t R has a value of 6 ohms.                        andmatching of components.Theapplication              of the
   T h e use of diodes Dl,Dz,and D to provide the base techniques
                                          3                                      described however,
                                                                                         here,                      is completely
bias voltage is a standard practice. The merits of this straightforward. The utilization of the silicon diodes is
practice are twofold. One obvious merit is the good bias based on the fact that for appreciable current                     flow a
compensation provided for variations            in ambient tem-       foward drop of about 0.6 volt is required. Since exactly
perature. A second and more important feature               of the 0.6 volt is not required, no matching         of V-I character-
practice is that the diodes exhibit a dynamic resistance              istics and/or temperature dependences is involved. The
which is much smaller than the static resistance              V / I . diodes are never reverse-biased, and therefore reverse
Thismeansthattheinstantaneousvariation                     of bias characteristicsareunimportant.Thusanyhighcon-
voltage with signal is much less than would be observed ductance silicon diode of appropriate current rating can
if a linear resistance were        used in place of the diodes.       be used.
The constancy of this bias voltage not only improves the
symmetry of the transfer characteristic but, more im-                                          CONCLUSION
portant, promotes thermal stability by providing ade-                    Low-distortion amplification requires adequate nega-
quate turn off bias for Qz- Q 4 when Q3 Q 6 are driven tive feedback and an open-loop transfer characteristic
ON.                                                                   which has reasonable slope a t all levels other than clip-
   In the first amplifier based on Fig. 7, VBB 83 volts, ping. Thermal stability requires adequate bias stabiliza-
  VO= 75 volts, CBS=0, and (Rl+R2) 2000 ohms. tionandeffectiveoff-currentminimization.Thesere-
This amplifier delivers 50 watts sine-wave power to an                quirements can be met simultaneously by application        of
8-ohm load and exhibits open-loop harmonic distortion                 the techniques of Figs. 2 and 6, as exemplified in Fig. 7,
whichvariesbetween           2 and 4 percentasthesignal               and including the output stage in an over-all negative
level is varied from zero to full power. When a voltage               feedback loop. By these means        it ispossible to build
amplifier is added and 20 t o 26 d b of feedback is applied, verystable,low-distortiontransistorpoweramplifiers
the distortion drops by an order           of magnitude, as ex- using either silicon orgermaniumtransistorsorany
pected, and a stable, low distortion amplifier results.               combination thereof.

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