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【正文】 + e b c I b I i ? I b V i Vo 1. What condition can we get the smallsignal equivalent circuit? end Review Questions Smallsignal model CommonEmitter Amplifiers 1. Voltage Divider Biasing Circuit i B R C v O i C R B1 V CC + ? R E + ? v s R B2 C B1 C B 2 R S + ? v I R L I B R C I C R TH V CC R E + ? V TH I E (1) Find Qpoint Values CCB2B1B2TH VRRRV ???21TH // BB RRR ?BC II ??? Forming Thevenin equivalent circuit EECCCCCE RIRIVV ??????Qpoint ETHBETHB)1( RRVVI?????1. Voltage Divider Biasing Circuit (2) Sketch smallsignal equivalent circuit + I b R C I c ? I b R B ? + ? V s V o r ? R E R B1 R B2 RL R S + ? V i (3) Determine smallsignal parameter. EQTBQTπ IVIVr ???? is given. Voltage gain Output loop )//( LCbo RRIV ??? ?Input loop EbπbEeπbi )1( RIrIRIrIV ??????(4) Determine AC parameters. 1. Voltage Divider Biasing Circuit Voltage gain If considering Rs EπLCEπbLCbio)1()//( ])1([)//(RrRRRrIRRIVVA v???????????????vvs ARRRVVVVVVAiSiiosiso????? + I b R C I c ? I b R B ? + ? V s V o r ? R E R B1 R B2 RL R S + ? V i 1. Voltage Divider Biasing Circuit input resistance ])1(/ / [// EπB2B1iii RrRRIVR ?????bi B III R ??EbπbEeπbi )1( RIrIRIrIV ??????)//( B2B1i B RRIV R?We get equations ? Input resistance Ri does not include internal resistance Rs of ac voltage source vs . iii IVR ? + I b R C I c ? I b R B ? + ? V s V o r ? R E R B1 R B2 RL R S + ? V i 1. Voltage Divider Biasing Circuit + I b R C I c ? I b R B ? + ? V x r ? R E R B1 R B2 R S r o I x 1 2 R’ o R o Output resistance ? Get equivalent circuit of finding output resistance Ro ? The KVL equations of loop 1 , 2: ro can be neglected 0)()( ????? EcbSπb RIIRrI0)()( Ebcobc ????? RIIrIIV x ?where B2B1SS //// RRRR ??oCo RRR ?? //Co RR ??? Co RR ?generally Coo RrR ????（ ） )1(ESπEoco RRrRrIVR x???????? ?when CommonEmitter Amplifiers 2. Circuit with emitter by bypass capacitor EπLC)1()//( RrRRAv ???????? Voltagedivider biasing stabilizes Q， but degrade ac voltage gain, how can we improve it？ i B R C v O i C R B1 V CC + ? R E + ? v s R B2 C B1 C B 2 R S + ? v I C E R L πLC )//( rRRAv???? ?AC parameters πB2B1i //// rRRR ?Co RR
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