THEORY OF FEEDBACK AMPLIFIERS I

In the preceding chapter, we demonstrated that by introducing physical feedback loops externally to an active device, we can produce a particular change in the performance of the network. Specifically, we showed that a three-terminal device can be unilateralized by a lossless reciprocal imbedding. In this and following chapters, we shall study the subject of feedback in detail and demonstrate that feedback may be employed to make the gain of an amplifier less sensitive to variations in the parameters of the active components, to control its transmission and driving-point properties, to reduce the effects of noise and nonlinear distortion, and to affect the stability or instability of the network.

We first discuss the conventional treatment of feedback amplifiers, which is based on the ideal feedback model, and analyze several simple feedback networks. We then present Bode's feedback theory in detail. Bode's theory is based on the concepts of return difference and null return difference and is applicable to both simple and complicated feedback amplifiers, where the analysis by conventional method for the latter breaks down. We show that return difference is a generalization of the concept of the feedback factor of the ideal feedback model and can be interpreted physically as the returned voltage. The relationships between the network functions and return difference and null return difference are derived and are employed to simplify the calculation of driving-point impedance of an active network.