ABSTRACT
We now study the effects of feedback on amplifier impedance and gain and obtain some useful relations among the return difference, the null return difference, and impedance functions in general.
Refer to the general feedback configuration of Figure 3.6. Let w be a transfer function. As before, to emphasize the importance of the feedback element x, we write w¼w(x). To be definite, let w(x) for the time being be the current gain between the output and input ports. Then, from Equation 3.24 we obtain
w(x) ¼ Ipq Is ¼ Y2Vpq
Is ¼ Yrp,sq(x)
Yuv(x) Y2 (4:1)
yielding
w(x) w(0)
¼ Yrp,sq(x) Yuv(x)
Yuv(0) Yrp,sq(0)
¼ F^(x) F(x)
(4:2)
provided that w(0) 6¼ 0. This gives a very useful formula for computing the current gain:
w(x) ¼ w(0) F^(x) F(x)
(4:3)
Equation 4.3 remains valid if w(x) represents the transfer impedance zrp,sq¼Vpq=Is instead of the current gain.
