电子信息类专业英语(西电第二版)Unit-3--Electrical-Technique课件.ppt

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1、Unit3ElectricalTechniquePassageAAnalogCircuitPassageBBinarySystemandLogicSystemsPassageCMagnitudeLockedLoopPassageAAnalogCircuit1.PrinciplesofCommonEmitterCircuitAnNPNtransistorisshowninFigure3.1withaloadresistor(RL=10kΩ)inserieswiththecollectorterminal.Acollectorsupp

2、lyvoltage(VCC=20V)isprovidedwithapolaritythatreversebiasesthecollectorbasejunction.AbasecurrentIBisalsoprovidedviaRB,andthisisresultsinaforwardbias(VBE)atthebaseemitterjunction.Figure3.1CommonEmitterCircuitAsignalvoltageVShavingasourceresistanceRSiscapacitorcoupledvi

3、aC1tothetransistorbase.TheoutputisderivedviaanothercapacitorC2connectedtothetransistorcollector.Bothcapacitorsareopencircuittodirectcurrents,butofferverylowimpedancetoACsignals.Ifthesignalsourceweredirectconnectedinsteadofcapacitorcoupled,therewouldbealowresistancepat

4、hfromthebasetothenegativesupplyline,andthiswouldaffectthecircuitbiasconditions.Similarly,anexternalloaddirectlyconnectedtothetransistorcollectormightalterthecollectorvoltage.AssumethatRBisselectedtogiveabasecurrentofIB=20μA.Also,lettheDCcurrentgainfactorofthetransist

5、orbeβ=50.ThenIC≈βIB=50×20×10-6=1mAThevoltagedropacrossRLisIC·RL=1mA×10kΩ=10V,andthecollectortoemittervoltageVCEisVCC-(IC·RL)=20V-10V=10V.ThecircuitDCconditionshavebeenestablishedasIB=20μA,IC=1mA,VCE=10V,VCC=20V.IfVBEisincreaseduntilIB=25μA,thenIC≈βIB=50×25×10-6=1.2

6、5mAThevoltagedropacrossRLisIC·RL=1.25mA×10kΩ=12.5V,andVCE=VCC-(IC·RL)=20V-12.5V=7.5V.WhenIBis20μA,VCE=10V,andwhenIBis25μA,VCE=7.5V.Hence,foranincreaseinIBof5μA,VCEdecreasedby2.5V(i.e.,VCEchangedbythesameamountasthevoltagechangeacrossRL).Similarly,ifVBEisdecreasedunti

7、lIBis15μA,ICbecomes50×15×10-6=0.75mAandIC·RL=0.75mA×10kΩ=7.5V.Thus,VCE=20V-7.5V=12.5V.Therefore,fora5μAdecreaseinIB,VCEincreasesby2.5V.ThevariationinbaseemittervoltagecouldbeproducedbytheACsignalVS.Thismightrequiresignalamplitudeofperhaps±10mV.IfVS=±10mVproducesVO=±2.

8、5V,thesignalmaybesaidtobeamplifiedbyafactorofVO/VS=2.5V/10mV=250,orcircuitamplificationis250.Thetransistorcurrentandvoltagev