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fimilili8FAnew high-efficiency audio amplifier circuitthat can reducethe size,weight and cosfof luture hi-fi amplifiers. Here's.an inside tookat the theory and circuitry

SOME TIME AGO, THE AUDIO INDUSTRY WASstartled by the announcement that InfinitySystems, Inc. (best known for their loud-speaker systems) had come up with amore efficient way to build audio poweramplifiers. They called the technique"Class D" amplification. Briefly, the sys-tem involves the use of high-frequency(in excess of 200 kHz) pulses that are flrstmodulated by the audio signal to be am-plified and then decoded by an integratingcircuit that restores the audio envelopeor waveshape. Since the duty cycle of eachhigh-frequency pulse is relatively short,conduction of the output transistors issuch that heat dissipation is a fraction ofthat encountered with more conventionalClass-B circuits and overall efficiency (atleast when the arnplifier delivers close toits maximum power output) is high. Thusfar, the product has not reached the con-sumer market but Infinity claims that allproduction problems have been licked andthat the Class D, or "switching" arnplifierwill soon be a commercial reality.

In the meanwhile, other companieshave been working on improving the effi-ciency of audio ampiifiers. This work isso widesplead, in fact, that the HitachiCompany of Japan (whose approach tobetter amplifier efficiency is the subject ofthis article) has had to change the nameof their invention from Class E (whichthey had first proposed to use) all the wayto Class G, the designation they currentlyplan to assign to the new and innovativecircuitry we will describe here.

Glass-B elficiencyWhen a Class-B audio amplifier delivers

its maximum rated power, its efficiency(power delivered to the load divided bypower used by the amplifier is quite high-707o or more. Studies show, however,that under music listening conditions, an

LEN FELDMAN*CONTRIBUTING HI-FI EDI TOR

audio amplifier is calied upon to deliverfull or nearly ful1 output for only a verysmall fraction of the time it is operating.Figure 1 represents the results of studies

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LEVEL OF AVERAGE ITUSIC PROGRAI.4

FIG, 1-AVERAGE MUSIC PROGRAM LEVELexpressed in lerms of percentage of totaltime at which that Ievel exists.

of a variety of musical selections. It showsthat while music may reach peaks of +14dB (referred to a 0-dB average powerlevel), for much of the time, actual powerlevels are even well below the 0-dB aver-age level. In fact, for nearly 50% of ttretime (Fig. 1, left vertical scale), powerlevels are some 30-dB below the 0-dBpoint, while a { 10-dB level is reached foronly 0.7% (Fig. 1, right hand verticaiscale) of the time. Even allowing forhighly compressed music (in which dy-namic range is restricted and music istherefore more uniformly "loud") andassuming only a 10-dB crest-factor (aver-age power is 10Vo of peak power), we cansee from the curve of Fig. 2 (efficiencyof a Class-B circuit versus the ratio ofactual output to designed maximum out-put) that for most of the time that anamplifier cf this type is reproducingmusic, it is operating at approximately207o efficiency.

The Class-G ideaHitachi's invention is designed to en-

able amplifiers to operate more efficientlyover more of their operating range, basedupon the way in which they are calledupon to actually amplify musical signals.The simple diagram of Fig. 3 illustratesthe Class-G idea. The input voltage, Vi",is the signal to be amplified and it is ap-

0 20 40 60 80 100PERCENTAGE OF MAXIMUM OUIPUI (%)

FlG.2-CLASS-B EFFICIENCY at various per-centages of maximum output,

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FlG. 3-BASIC CLASS-G amplifier.

plied to the base of transistors Ql and Q2.A load resistor, Rr, is connected to theemitter of Ql. Supply voltage V' is appliedthrough diode Dl to the collector Ql andthe emitter of Q2. The collector of Q2 isconnected to a second supply voltage V""that is higher than supply voltage Vl.

Operation of the circuit is as follows: Ifinput signal voltage V'" is lower than Vl,Q2 is reverse biased between its base andits emitter and is therefore cut off. Currentflowing through load R', is supplied fromV: thiough diode D1. Under these condi-tions, the instantaneous efficiency of thecircuit is given as: Efficiency (%) :YnlVl. If the signai voltage increases to avalue beyond that of Vl (but less thanV""), transistor Q2 becomes forwardbiased and is tumed on. Current flowingthrough load Rr is now supplied from thesecond, higher supply-voltage V"" throughQ2. If we neglect saturation voltage be-tween collector and emitter of Q2 (assum-ing it is sufficiently low), the instanta-neous efficiency of the circuit is given as:Efficiency (%): V'"/V"".

Figure 4 represents the two effciency

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FlG. 4-INCREASED EFFICIENCY resullstrom two-level power-supply system inClass-G amplilier.

levels of the system and the vertical linerepresents the point at which the supplyvoltage transition takes place. Thus, in thelower ranges of input signal voltage, theefficiency of this circuit is improved con-siderably and the amount of heat gener-ated in the output transistor is reducedcompared with conventional Class-B am-plifiers. Referring once more to Fig. 3, itshould be noted that diode Di alsoserves to prevent current flowing from thehigher supply-voltage source (V"") fromflowing back into the first power sourcev1.

The thermal efficiency of the systemwill, of conrse, depend upon the choice ofthe two power-supply operating voltages.

0" o.z 0.4 0.6 0,8 l.o

ACTUAL OUTPUT/DESIGN |V]AXIMUM OUTPUT

FtG. 5-EFF|ClEhiCY OF CLASS-G amplifierdepends upon Vr/V. ratio.

Figure 5 compares the efficiency level atvarious outputs (expressed as a fractionof maximum design output) for two dif-ferent Class-G designs having differentVl/V"" ratios as compared with conven-tional Class-B operation. Regardless ofwhether V1 is half or two thirds as greatas V"", we see that efficiency is far greaterthan that of Class-B operation, particu-larly at lower output levels where, as wehave $een earlier, the amplifier is likelyto operate most of the time when repro-ducing actual music programs.

Along with improvement in efficiencycomes reduced internal heat dissipation ofthe output devices used in the Class-G ap-proach. Figure 6 illustrates this point for

ACTUAL O UTPUT/DESIGN OUTPUT

FlG. 6-HEAT DISSIPATION for variousVry'V"" ralios.

various Vi/V"" ratios. Internal dissipa-tion is plotted as a fraction of maximumpower output on the vertical axis, whilethe ratio of output power to designedmaximum power is shown along the hori-zontal axis of the graph. As expected. in-ternai dissipation is lower at all operatingconditions for Class-G compared withconventional Class-B operation.

DListortionCloser examination of Fig. 3 points up

certain problems that exist in the basicconcept of Class G. In the simple form

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simplest form of Class-G circuit.

of the circuit shown, Q2 is not turned onuntil input signal voltage exceeds the col-lector voltage of Ql by an amount equalto, the base-emitter voltage Vr" of Q2,Thus, when the value of input signal volt-age Vi" is in the range between Vl andV1 * Vt", Q1 is already saturated (be-tween collector and emitter) before con-duction of Q2 begins. This results in adistolted output waveform signal as

shown in Fig. 7.To prevent this form of distortion dur-

ing the changeover from one power sup-ply level to the other, the circuit must bemodified so that saturation of Q1 does notoccur until Q2 is turned on. This is ac-complished by adding another diode, D2,as shown in the simplified schematic ofFig. 8. Now, when V1 is less than theinput signal, the voltage between the col-lector and the emitter of Q1 is lower thanthe saturation level by an amount equal tcthe threshold value of D2 and thus Qlremains unsaturated. Diode D2 may bea Zenet diode or even a resistor since itis only required to maintain a voltagedifierence equal to the Vr. voltage of Q2.Still another diode, D3, is added to the:

basic circuit as shown in Fig. 7. Since a.

reverse bias is applied between the baseand emitter of Q2 when the signal voltageis lower than supply voltage V', the base-emitter junction of Q2 must be abie tostand a reverse voltage higher than Vl.Since the maximum inverse voltage of thebase-emitler circuit of mosi transistors isgeneraily low, diode D3 is provided toprevent the flow of reverse currentthrough the base-emitter junction of Q2,thus protecting this junction against thereverse voltage.

Push-Pull operationFigure 9 shows the required configula-

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not turned onceeds'thejcol-amount $quiil: Vr" of: Ql.ut'signal-volt-wees Vi andatuiated (be-

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iistortion dur-e po\\'er sup-rcuit rhust bef QI.does noir. Tfis is ac-er diode. D2schenratii oi'less than. the,rfen thetoiii lower ihanount equal li.and thus{ Q i

D? may brist0r sinie iiin p voh-,igrrltage of,Q?.rdded tct th{g. :7. Since :een the bast:ignal voltagrVr, the base-t be able tcrer than Vl.oltage of thetra!sistors isprovided to)rse currentction of Q2.

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ilon for a push-pull output circuit. DiodesD2 and D3 shown in Fig. 8 have beeoomirtecl for the sake of clarity, Two val-ues of positive and negative supply volt-ages are required and transistors Q3 and

Q4 operate at opposite polarity voltagescompared witlt Ql and Q2 to form thefamitiar complementary configuration(NPN nd PNP pairs are used). Fig. l0-ashows the input signal waveforn, to'gether with the voltage levels Vcr and Vc,(for the fir'st half of the cycle) and Vc,and Vcn for the opposite half of the signalwaveform. Fig. l0-b shows the currentwaveforms resulting from tbe four sup-ply-voltages (two voltages for each polar-ity). Fig, l0-c represents that portion ofthe output waveform Powered bY thelower-lcvcl supply voltage Vc', whileFig, l0-cl shows the contribution of outputwaveform powered by the higher supplyvoltage Vcg. Finally, Fig' 10-e shows aconrparison of power losses (or dissipa-tion) in Ql and Q2 (for the half cycleshown) as compared with the power lossthat would take place in a conventionalClass-B configuration'

Practical Class-G circuatThe first product which Hitachi intends

to introduce that will incorporate tleClass-G principle is their Model SR-903AN{/FM stereo receiver, pictured in Fig.1l By way of illustrating the improvement

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FlG. 11-HITACHI SR-903 stereo receiveruses Class-G oulpul circuit.

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FlG. 14-DISTORTION resulting from storagetime delay in Class-G circuil.

FlG, 1s-DISTORTION resulting from storagelime delay is reduced by the addition of Lito circuit.

as opposed to 110 watts-per-channel forthe Class-B unit against which it was com-pared.

Hitachi plans to introduce many moreunits employing Class-G circuitry. Typicalof these will be the stereo power amplifier,model HMA-8300, pictured in Fig. 12. Acircuit of the power output stages of thisunit is shown in the schematic diagram ofFig. 13. In addition to the cilcuit elementsalready described, we see one more refine-ment that should be mentioned. Note, thatin series with supply voltages Vcr and Veare small inductors Ll and L3. Becausetransistors have finite turn-on and turn-offtimes, a form of distortion can be intro-duced to tle output waveform as illus-trated in the waveforms of Fig. 14. Thisform of dstortion is due to an effect knownas storage time-delay and is quite inde-pendent of the base-emitter Vt" voltagediscussed earlier. To counteract this effect(and to further reduce output waveformdistortion in Class-G circuits), a coil isadded in series with diode D6 shown inthe improved circuit of Fig. 15.

The effect of storage delay would be

FlG. 16-THD VS. POWER OUTPUT at 20-kHzbefore and after addition of L1 (Fig. 15) incircuit. Dislorlion characleristic at 1-kHz i9also shown,

particularly noticeable when reproducinghigh frequencies, since the time requiredfor amplification of a single cycle of a 20kHz signal is only 50 microseconds. Thecurves of distortion versus output mea-sured at 1 kHz and 2A kHz for a typicalCIass-G circuit shows clearly that without

(continued on page 87)

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T,!ME

Flc._12-HITACHt iMA-g300 power amptifierwill be introdueed in the neaifuture an'd willconlain Class-G oulput circuitry,

in efficiency attained because of this newoutput.circuit, Dr. Centaro Miyazaki ofHitachi Consumer products Researchcenter was kind enough to supply Dre withsome agvance comparisons between this/: watls^-qelchannel receiver (from 20_Hz 1o ,20-kHz, 8_ohm toads, 0.3% maxi_mum THD) and a typical Class-B unithavtng- the.same FTC power rating. The)5 90 will weigh in at 28.7 lbs as against40.8 .lbs for the Class-B unit. Undermuslc power" rneasurenlent conditions(abandoned by the industry since the ad_

vent of the FTC power rule, but nevedt;_tess indicative of the short-term poweroutput capability of an amplifierj, theSR-903 will deliver 160 watts-per_channel

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49

FlG. 13-OUTPUT CIRCUIT of lhe Hitachi HMA-S300 power amptitier.

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10.0 20.0 50.0 100 200PO$/ER OUTPUT {WATTS}

ELASS-G AMPLIFIERS(continued from page 49)

the addition of the extra coils (Ll'andL3) in the circuit of Fig. 13, harmonicdistortion would rise more rapidly for a20 k}{z signal while remaining at a muchlower level for lower frequency signals.With the addition of this final refinement,total harrnonic distortion is maintained atvery low levels for both md- and high-frequencies, as shown by the solid-line 20-kl{z curve of Fig. 16.

FTC and Glass-GThe Class-G circuit seems to be partic-

ularly attractive at this time in light of thepre-conditioning tests now required by theFTC in connection with determining thepower output ratings of audio amplifiers.As many readers know, the FTC requiresthat ampliflers be able to sustain onethird of their rated continuous po\Mer out-put for one hour. In Class-B circuits, thispower level results in almost the greatestinternal heat dissipation for the outputdevices and, in many cases, this has forceddesigners to increase heat-sink dimen-sions (and cost to consumers) withoutreally providing audible benefit to users.On the other hand, Class-G operation canresult in very nearly the most efficientoperation at this one-third rated point(and at lower levels more often encoun-tered in musical reproduction) with ap-propriate economies in weight, power-supply demands and, most important ofa1l, retail prices that the consumer has topay for high-quality audio amplifica-tion. R-E

*The author is indebted to Dr. Gentaro Mi-yazaki of Hitachi for allowing us to publishthe firsi definitive descripiion of Hitachi'sinnovative new amplifier circuit. Severalclaims of a pending U.S. (as well as foreign)patent have already been allowed by thePateni Office, and it should be noted thatthe actual inventor of the Class-G circuit isMr. Tohru Sampei, of Hitachi. At one pointin its development, classes of amplifiersseemed to be developing so rapidly that thecompany had thought to call the new circuitthe "S"-system in honor of its inventor, butthey settled for Class-G instead, after re-searching the matter thoroughly here andabroad.

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