818-3 Small signal analysis R i=f(o) 丿/R i=f(o) N--a large constant voltage source u (t)-a small time-varying voltage source(small signal) 2>>(3)|bmn +i=(1)2+N:N(0)t=:A anion gnitn19go oiiDk2--(oo)<0=(1)2 i9u
§18-3 Small signal analysis R i i = f () − + − + s − + Vs i • Vs / R Vs P V0 0 I 0 i = f () s V-- a large constant voltage source s V s | )t( | and (t) s -- a small time-varying voltage source (small signal) + Ri =)t( s +s V : KVL ) ( f =i : KCL int po operating static ) I, V( P ) t ( Let − − 0 0 →0 = s • Q
R U/R f({U) 0 tniog gniitn19goiint2--lo)>(3)|m0年(3)sh (△+0N=(3) Q i△+0=(0) i(t)=f(U=f()+f"(V0)(U-1)+f(0)(U-V0)2+ or i(t)=lot duo △1d2i △2+ 2 du
R i i = f () − + − + s − + Vs i • s / R s P V0 0 I 0 i = f () int ) , ( 0 ) ( po operating 0 0 static I V P t Let − − → = s s V s s | )t( | and 0 )t( when + = + = − − i I t i V t Q 0 0 ) ( ) ( • Q = = 0 + 0 − 0 + ''( 0 )( − 0 ) 2 + 2 1 i(t) f () f (V ) f '(V )( V ) f V V = + + 2 + 0 2 2 0 0 2 1 ( ) d V d i d V di or i t I 0 0
d .i(t)=10+ △D=I+G△ 00 ∵()=lo+M∴M=G,△DOr△U=R,A 0 (+i=(3)2+2:NA(△+0)+(+01)=()2+N 0+0=N0=(3)i R △+△=(3)∴ U,(t) △U=Ry △U ∴D,()=R△i+Ra,,△ 0 (Small-signalequivalent circuit) U,(t) (R+R )Ai △i R+Rd
= + = + 0 0 0 0 ( ) V I G d V di i t I d i V or R V i t = I + i i = Gd = d 0 0 0 ( ) + Ri =)t( s +s V : KVL ) ( ) ( ) ( +0 V + i +0 I R= t s +s V 0 0 V RI V , 0 )t( at + = s = s +i R=)t( s i V = Rd 0 i V R R i V t R i R d s d = + = + ( ) ( ) 0 0 d s R R t i + = ( ) R i V0 Rd − + − + (t) s (Small-signal equivalent circuit)
