Chapter A1.2 DC-AC Load Line

Ref:080314HKNEE3110DCandACLoadLine1ChapterA1.2DCandACLoadLine•DCbiasingcircuits•DCandACequivalentcircuit•Q-point(Staticoperationpoint)•DCandACloadline•SaturationCutoffCondition•ComplianceRef:080314HKNEE3110DCandACLoadLine2DCBiasingCircuitsRCRB+VCCicvceibvinvout•TheacoperationofanamplifierdependsontheinitialdcvaluesofIB,IC,andVCE.•ByvaryingIBaroundaninitialdcvalue,ICandVCEaremadetovaryaroundtheirinitialdcvalues.•DCbiasingisastaticoperationsinceitdealswithsettingafixed(steady)levelofcurrent(throughthedevice)withadesiredfixedvoltagedropacrossthedevice.Ref:080314HKNEE3110DCandACLoadLine3PurposeoftheDCbiasingcircuit•Toturnthedevice“ON”•Toplaceitinoperationintheregionofitscharacteristicwherethedeviceoperatesmostlinearly,i.e.tosetuptheinitialdcvaluesofIB,IC,andVCERef:080314HKNEE3110DCandACLoadLine4Voltage-DividerBias•Thevoltage–divider(orpotentiometer)biascircuitisbyfarthemostcommonlyused.•RB1,RB2voltage-dividertosetthevalueofVB,IB•C3toshortcircuitacsignalstoground,whilenoteffecttheDCoperating(orbiasing)ofacircuit(REstabilizestheacsignals)BypassCapacitorRCR1+VCCRER2voutvinC2C1C3Ref:080314HKNEE3110DCandACLoadLine5GraphicalDCBiasAnalysisRCR1+VCCICIERER2cmxyRRVVRRIIIRIVRIVECCCCEECCEECECCCCEC:equationlinestraightofformslope-Point1for0IC(mA)VCEVCE(off)=VCCIC(sat)=VCC/(RC+RE)DCLoadLineRef:080314HKNEE3110DCandACLoadLine6DCLoadLineIC(mA)VCEVCE(off)=VCCIC(sat)=VCC/(RC+RE)DCLoadLine•ThestraightlineisknowastheDCloadline•Itssignificanceisthatregardlessofthebehaviorofthetransistor,thecollectorcurrentICandthecollector-emittervoltageVCEmustalwayslieontheloadline,dependsONLYontheVCC,RCandRE•(i.e.ThedcloadlineisagraphthatrepresentsallthepossiblecombinationsofICandVCEforagivenamplifier.ForeverypossiblevalueofIC,andamplifierwillhaveacorrespondingvalueofVCE.)•Itmustbetrueatthesametimeasthetransistorcharacteristic.SolvetwoconditionusingsimultaneousequationgraphicallyQ-point!!WhatisIC(sat)andVCE(off)?Ref:080314HKNEE3110DCandACLoadLine7Q-Point(StaticOperationPoint)•Whenatransistordoesnothaveanacinput,itwillhavespecificdcvaluesofICandVCE.•Thesevaluescorrespondtoaspecificpointonthedcloadline.ThispointiscalledtheQ-point.•TheletterQcorrespondstotheword(Latent)quiescent,meaningatrest.•AquiescentamplifierisonethathasnoacsignalappliedandthereforehasconstantdcvaluesofICandVCE.Ref:080314HKNEE3110DCandACLoadLine8Q-Point(StaticOperationPoint)•TheintersectionofthedcbiasvalueofIBwiththedcloadlinedeterminestheQ-point.•ItisdesirabletohavetheQ-pointcenteredontheloadline.Why?•WhenacircuitisdesignedtohaveacenteredQ-point,theamplifierissaidtobemidpointbiased.•Midpointbiasingallowsoptimumacoperationoftheamplifier.Ref:080314HKNEE3110DCandACLoadLine9DCBiasing+ACsignal•Whenanacsignalisappliedtothebaseofthetransistor,ICandVCEwillbothvaryaroundtheirQ-pointvalues.•WhentheQ-pointiscentered,ICandVCEcanbothmakethemaximumpossibletransitionsaboveandbelowtheirinitialdcvalues.•WhentheQ-pointisabovethecenterontheloadline,theinputsignalmaycausethetransistortosaturate.Whenthishappens,apartoftheoutputsignalwillbeclippedoff.•WhentheQ-pointisbelowmidpointontheloadline,theinputsignalmaycausethetransistortocutoff.Thiscanalsocauseaportionoftheoutputsignaltobeclipped.Ref:080314HKNEE3110DCandACLoadLine10DCBiasing+ACsignalRef:080314HKNEE3110DCandACLoadLine11DCandACEquivalentCircuitsRCR1+VCCRER2RLvinRCR1+VCCICIERER2R1//R2rCvcerC=RC//RLvinBiasCircuitDCequivalentcircuitACequivalentcircuitRef:080314HKNEE3110DCandACLoadLine12ACLoadLine•Theacloadlineofagivenamplifierwillnotfollowtheplotofthedcloadline.•Thisisduetothedcloadofanamplifierisdifferentfromtheacload.IC(mA)VCEVCE(off)=VCCIC(sat)=VCC/(RC+RE)DCLoadLineICVCEIC(sat)=ICQ+(VCEQ/rC)VCE(off)=VCEQ+ICQrCacloadlineICVCEQ-pointacloadlinedcloadlineRef:080314HKNEE3110DCandACLoadLine13ACLoadLineWhatdoestheacloadlinetellyou?•Theacloadlineisusedtotellyouthemaximumpossibleoutputvoltageswingforagivencommon-emitteramplifier.•Inotherwords,theacloadlinewilltellyouthemaximumpossiblepeak-to-peakoutputvoltage(Vpp)fromagivenamplifier.•ThismaximumVppisreferredtoasthecomplianceoftheamplifier.(ACSaturationCurrentIc(sat),ACCutoffVoltageVCE(off))Ref:080314HKNEE3110DCandACLoadLine14ACSaturationCurrentandACCutoffVoltageR1//R2rCvcerC=RC//RLvinICVCEIC(sat)=ICQ+(VCEQ/rC)VCE(off)=VCEQ+ICQrCacloadlineRef:080314HKNEE3110DCandACLoadLine15AmplifierCompliance•Theacloadlineisusedtotellthemaximumpossibleoutputvoltageswingforagivencommon-emitteramplifier.Inanotherwords,theacloadlinewilltellthemaximumpossiblepeak-to-peakoutputvoltage(VPP)fromagivenamplifier.ThismaximumVPPisreferredtoasthecomplianceoftheamplifier.•ThecomplianceofanamplifierisfoundbydeterminethemaximumpossibleofICandVCEfromtheirrespectivevaluesofICQandVCEQ.Ref:080314HKNEE3110DCandACLoadLine16Usingacloadlinetodeterminecompliance.endofslideRef:080314HKNEE3110DCandACLoadLine17MaximumPossibleComplianceRef:080314HKNEE3110DCandACLoadLine18ComplianceThemaximumpossibletransitionforVCEisequaltothedifferencebetweenVCE(off)andVCEQ.SincethistransitionisequaltoICQrC,themaximumpeakoutputvoltagefromthea