ABSTRACT

In Chapter 27.2, we used the ideal feedback model to study the properties of feedback amplifiers. The model is useful only if we can separate a feedback amplifier into the basic amplifier

µ

(

s

) and the feedback network

β

(

s

). The procedure is difficult and sometimes virtually impossible, because the forward path may no be strictly unilateral, the feedback path is usually bilateral, and the input and output coupling networks are often complicated. Thus, the ideal feedback model is not an adequate representation of a practical amplifier. In the remainder of this section, we shall develop Bode’s feedback theory, which is applicable to the general network configuration and avoids the necessity of identifying the transfer functions

µ

(

s

) and

β

(

s

). Bode’s feedback theory [2] is based on the concept of return difference, which is defined in terms of

network determinants. We show that the return difference is a generalization of the concept of the feedback factor of the ideal feedback model, and can be measured physically from the amplifier itself. We then introduce the notion of null return difference and discuss its physical significance. Because the feedback theory will be formulated in terms of the first-and second-order cofactors of the elements of the indefinite-admittance matrix of a feedback circuit, we first review briefly the formulation of the indefinite-admittance matrix.