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1、AnalysisofSmallSignalAmplifiersintheMid-FrequencyBandIntroductionIntheotherlesson,asinglesmallsignalequivalentcircuitwasdeveloped,applicabletobothbipolarjunctiontransistorsandfieldeffecttransistors.Thisisthetransconductancemodel,anditisredrawninamoregene
2、ralwayinFig.5.1,whereterminaldesignations1,2and3areused,thenumberscorrespondingtothevariouselectrodes,asshowninthetable5.1.Table5.1TerminalBJTFET1BaseGate2CollectorDrain3EmitterSourceFig.5.1transconductancemodelAspreviouslyexplained,theinputresistanceisv
3、irtuallyanopencircuitfortheFET,anditisequaltofortheBJT.Thetransconductancemodelisasimplifiedversionofanequivalentcircuitknownasthehybrid-model,whichtakesintoaccountequivalentcircuitelementswhichaffectthehighfrequencyperformanceofthetransistor.Inthepresen
4、tlesson,frequencyeffectsatbothhighandlowfrequenciesareignored,butthesimplifiedanalysisisusefuloverarangeoffrequenciestermedthemid-frequencyband,whichisthefrequencyregionofinterestforgeneral-purposeapplications.GeneralDefinitionsIrrespectiveofthemodeofcon
5、nectionofthetransistor,amplifierpropertiescanbedefinedinageneralmanner.Itisessentialtoobservecorrectlythepolaritiesofvoltagesandcurrentsinthesedefinitions,andtheseareshowninFig.5.2.ReferringtoFig.5.2(a),wehaveTerminalvoltagegain:(5.1)Voltagegainreferredt
6、oSource:(5.2)Currentgain:(5.3)Inputresistance:(5.4)Outputresistance:Thisistheinternalresistanceoftheamplifierasseenbytheload.Itcanbefoundbysetting,andapplyingavoltage(inprinciple)totheoutputterminalswiththeloadremoved.WithreferencetoFig.5.2(b),theoutputr
7、esistanceisthen(5.5)Fig.5.2outputresistancecomputationItwillalsobeseenfromthesedefinitionsthat(5.6)andthat(5.7)AmplifierConfigurationsThetransistorequivalentcircuit,Fig.5.1,isseentohavethreeterminals,whiletherathermoregeneralblockdiagram,Fig.5.2,showsthe
8、amplifierwithfourterminals,twoinputsandtwooutputs.Thismeansthatoneofthetransistorterminalsmustbecommontoinputandoutput.Anyoneofthethreetransistorterminalsmaybemadethecommonterminalandthisgivesrisetothreedistinctamplifierco
