Il A 图P6-13 1=-mB A 2(R3+r2) 2=1+ RB.U 0 R1 R yff f u f i i u u u u b e u U U A R U I R U I A A A R R A R r R A     = − = = = + = + = − 0 9 0 1 1 2 7 8 2 1 2 1 , , , 1 , 2( ) 