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1、Unit3ElectronicAmplifiersTextKeywordsTechnicaltermsExpressionsNotestothetextTextElectronicamplifiersareusedmainlytoincreasethevoltage,current,orpowerofasignal.Alineramplifierprovidessignalamplificationwithlittleornodistortion,sothattheoutputisproportionaltotheinput.Anonlinearamplifierm
2、ayproduceaconsiderablechangeinthewaveformofthesignal.Linearamplifiersareusedforaudioandvideosignals,whereasnonlinearamplifiersfinduseinoscillators,powerelectronics,modulators,mixers,logiccircuits,andotherapplicationswhereanamplitudecutoffisdesired.Todayeitherdiscretetransistorcircuitso
3、rintegratedcircuitsaremostlyused.Nowadays,transistorsarethemostimportantdeviceinelectronicamplifiers.WhentheBEjunctionisforwardbiased,andtheBCjunctionisreversebiased,thebasecurrentiBcancontrolthecollectorcurrentiC,representedbytheformulaiC=βiB.Therearethreefundamentalconfigurationsinte
4、rmsofconnectingtransistorsinacircuit(asshowninFig.3-1):common-emitter,common-collectorandcommon-base.Fig.3-1ThreeBasicConfigurationsofTransistorAmplifiersInthecommon-baseconnection,astheinputcurrentiEisapproximatelyequaltotheoutputcurrentiC,itiscalledthecurrentfollower.Inthisconfigurat
5、ion,thecircuithaslowinputimpedanceandhighoutputimpedance.ThevoltagegainAu>0,thatmeanstheoutputandtheinputisin-phase.TheCBconfigurationcircuitismainlyusedinhighfrequencyandwidebandcircuits.Forthetransistorvoltageamplifier,oftenitisasmallalternatingvoltagethathastobeamplified.Abipolarjun
6、ctiontransistorinthecommon-emittermodecanactasavoltageamplifierifthetransistorhasaproperbiasandasuitableresistor,calledtheload,isconnectedinthecollectorcircuit.Thisconfigurationhasmoremoderateinputandoutputimpedancethanthecommon-basecircuit.Itprovidesbothcurrentgainandvoltagegain,moreo
7、verAu<0.Thethirdtypeofconnectionisthecommon-collectorcircuit.Inthisconfiguration,theinputimpedanceofthetransistorisveryhighandtheoutputimpedanceislow,thevoltagegainisslightlylessthanone,representedbyAu≈1,andthepowergainisusuallylowerthanthatobtainedinacommon-baseoracommon-emittercirc