优化音频桥系杆负载放大器-Optimizing Audio

第第页优化音频桥系杆负载放大器-OptimizingAudioThisaudiobridge-tiedload(BTL)amplifierapplicationnotedescribesauniquearchitecturethatminimizesthedifferentialoutputdistortionandnoise(THD+N).

Low-voltagesystemsdrivingloudspeakersoftenemployabridged-amplifierconfigurationthateffectivelydoublesthevoltageswingatthetransducer(speaker).Inatypicalbridge-amplifiercircuit(Figure1),anAC-coupledinvertingstagewithgaindrivesonesideofthespeaker.Italsodrivesasecondunity-gaininvertingamplifier,whichdrivestheothersideofthespeaker.

Figure1.Thisconventionalbridgedamplifierfeaturestwoamplifiersintandem.

Bridgedamplifiersusuallyincorporateamatchedpairofamplifiers,butthefirstamplifierdominatesoverallperformancebecauseitsoutputnoiseanddistortionarereplicatedinthesecondamplifier.Youcaneliminatethisdrawbackbyplacingthetwoamplifiersinanon-cascadedconfiguration,inwhichoneisinvertingandtheotherinnon-inverting(Figure2).Bothamplifythesameinputsignal,soneitheronereproducesnoise,distortion,orclippingintroducedbytheother.

Figure2.ThiscircuitoptimizestheFigure1configurationbyapportioninggainbetweenthetwoamplifiers.

AsakeyimprovementinFigure2,DCbiasforthenon-invertingcircuitisderivedfromtheinvertingamplifier'ssourceresistor.(BiasisrequiredbecausetheinputisACcoupled).Usingtheotheramplifier'ssourceresistorasabiassourcelowersthecomponentcountandeliminatessignalinjectionintothehigh-impedancebiassource(VCC/2,appliedtothetopamplifier'snon-invertinginput).

AnotheradvantageinFigure2istheeliminationofDCgaininthenon-invertingamplifier.Forthecircuitshown,C2setsthe-3dBpointathalftheinputcutofffrequency,andR1/C1setstheinputhighpasscutofffrequencyat100Hz.

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