例4求电压放大倍数Aau/u; u=u4=0 F b+ F 0 R b tIR2 RI RE R P R uFup=u。u F 3 F R R 2 3 Rp=r//(rF+r2lr3) RrR RER RE+r R+R2+ F F 2 R A R R例4 求电压放大倍数Au=uo /ui 。 i i b+ =0 b- =0, u-= u+=0 _ +  +  RF R1 RP ui uo i1 iF ib￾ib+ R2 R3 i2 i3 uF i1 =iF = i2 +i3 ui R1 -uF RF = = -uF RF uF -uo R2 uF R3 + uo = - R ui 1 RF + R2 + RF R2 R3 R1 RF + R2 + RF R2 A R3 u= – RP=R1 //(RF + R2 //R3 )