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Unit3ElectricalTechniqueUnit3ElectricalTechniquePassageAAnalogCircuitPassageBBinarySystemandLogicSystemsPassageCMagnitudeLockedLoopUnit3ElectricalTechniquePassageAAnalogCircuit1.PrinciplesofCommonEmitterCircuitAnNPNtransistorisshowninFigure3.1withaloadresistor(RL=10kΩ)inserieswiththecollectorterminal.Acollectorsupplyvoltage(VCC=20V)isprovidedwithapolaritythatreversebiasesthecollectorbasejunction.AbasecurrentIBisalsoprovidedviaRB,andthisisresultsinaforwardbias(VBE)atthebaseemitterjunction.Unit3ElectricalTechniqueFigure3.1CommonEmitterCircuitUnit3ElectricalTechniqueAsignalvoltageVShavingasourceresistanceRSiscapacitorcoupledviaC1tothetransistorbase.TheoutputisderivedviaanothercapacitorC2connectedtothetransistorcollector.Bothcapacitorsareopencircuittodirectcurrents,butofferverylowimpedancetoACsignals.Ifthesignalsourceweredirectconnectedinsteadofcapacitorcoupled,therewouldbealowresistancepathfromthebasetothenegativesupplyline,andthiswouldaffectthecircuitbiasconditions.Similarly,anexternalloaddirectlyconnectedtothetransistorcollectormightalterthecollectorvoltage.Unit3ElectricalTechniqueAssumethatRBisselectedtogiveabasecurrentofIB=20μA.Also,lettheDCcurrentgainfactorofthetransistorbeβ=50.ThenIC≈βIB=50×20×10-6=1mAThevoltagedropacrossRLisIC·RL=1mA×10kΩ=10V,andthecollectortoemittervoltageVCEisVCC-(IC·RL)=20V-10V=10V.ThecircuitDCconditionshavebeenestablishedasIB=20μA,IC=1mA,VCE=10V,VCC=20V.IfVBEisincreaseduntilIB=25μA,thenIC≈βIB=50×25×10-6=1.25mAThevoltagedropacrossRLisIC·RL=1.25mA×10kΩ=12.5V,andVCE=VCC-(IC·RL)=20V-12.5V=7.5V.Unit3ElectricalTechniqueWhenIBis20μA,VCE=10V,andwhenIBis25μA,VCE=7.5V.Hence,foranincreaseinIBof5μA,VCEdecreasedby2.5V(i.e.,VCEchangedbythesameamountasthevoltagechangeacrossRL).Similarly,ifVBEisdecreaseduntilIBis15μA,ICbecomes50×15×10-6=0.75mAandIC·RL=0.75mA×10kΩ=7.5V.Thus,VCE=20V-7.5V=12.5V.Therefore,fora5μAdecreaseinIB,VCEincreasesby2.5V.Unit3ElectricalTechniqueThevariationinbaseemittervoltagecouldbeproducedbytheACsignalVS.Thismightrequiresignalamplitudeofperhaps±10mV.IfVS=±10mVproducesVO=±2.5V,thesignalmaybesaidtobeamplifiedbyafactorofVO/VS=2.5V/10mV=250,orcircuitamplificationis250.Thetransistorcurrentandvoltagevariationshavenoeffectonthesupplyvoltage(VCC).So,whenassessingtheACperformanceofthecircuit;VCCcanbetreatedasashortcircuit.ThecouplingcapacitorC1alsobecomesashortcircuittoACsignals.RedrawingthecircuitofFigure3.1withVCCandC1shortedgivestheACequivalentcircuitshowninFigure3.2.Unit3ElectricalTechniqueFigure3.2CommonEmitterACEquivalentCircuitUnit3ElectricalTechniqueInFigure3.2thecircuitinputterminalsarethebaseandtheemitter,andtheoutputterminalsarethecollectorandtheemitter.Thus,theemitteriscommontobothinputandoutput,andthecircuitisdesignatedcommonemitter,orsometimesgroundedemitter.ItisalsoseenfromthefigurethatresistorsRBandRLareinparallelwiththecircuitinputandoutputterminals,espectively.Unit3ElectricalTechnique2.BasicOp-ampsAconventionaloperationalamplifier(op-amps)canbesimplydescribedasahigh-gaindirect-coupledvoltageamplifierthathasasingleoutputterminal,andbecauseithasbothinvertingandnon-invertinginputterminals,thedevicecanfunctionasaninverting,non-inverting,ordifferentialamplifiers,filters,oscillators,levelswitches,comparators,etc.Threebasictypesofoperationalamplifiersarecurrentlyavailable.Wearegoingtotakeanin-depthlookattheoperatingprinciplesandpracticalapplicationofthemostcommontype,theconventional“voltage-involtage-out”op-amp(typifiedbytheLM741andCA3140).Theothertwobasictypesofop-ampsarethecurrent-differencingorNortonop-amp,andtheoperationaltransconductanceamplifierorOTA.Unit3ElectricalTechnique·Op-ampBasicsInitssimplestform,aconventionalop-ampconsistsofadifferentialamplifier(bipolarorFFT)followedbyoffsetcompensationandoutputstages,asshowninFigure3.3.AllofthoseelementsareintegratedonasinglechipandhousedinanICpackage.Thedifferentialamplifierhasahigh-impedance(constant-current)“tail”togiveitahighinputimpedanceandahighdegreeofcommon-modesignalrejection.Italsohasahigh-impedancecollector(ordrain)load,togiveitalargeamountofsignal-voltagegain(typicallyabout100dB).Unit3ElectricalTechniqueFigure3.3SimplifiedOpampEquivalentCircuit(Thebasicoperationofanopampcanbesimulatedusingdiscreetcomponentsasshown.)Unit3ElectricalTechniqueTheoutputofthedifferentialamplifierisfedtothecircuit’soutputstageviaanoffset-compensationnetwork,whichcausestheop-amp’soutputtocenteratzerovolts.Theoutputstagetakestheformofacomplementaryemitterfollower,andprovidesalow-impedanceoutput.[1]Op-ampsarenormallypoweredfromasplitsupplyproviding+V,-V,andacommonground,enablingtheop-amp’soutputtoswingtoeithersideofgroundandtakeonavalueofzerovoltswhenthedifferentialinputvoltageiszero.Unit3ElectricalTechnique·BasicConfigurationsWehaveseenthattheop-amp’sisahigh-gaindirectcoupledvoltageamplifierwithhighinputimpedanceandlowoutputimpedance.Inpractice,theoutputvoltageofanop-ampisproportionaltothedifferentialvoltagebetweenitstwoinputs,andisequalto,VOUT=AVO(V1-V2)WhereAVOisequaltoopen-loopvoltagegainoftheop-amp(typically100,000),V1isthevoltageatthe