On impedance evaluation in feedback circuits

One of the main benefits of negative feedback is the ability to control the impedances in linear active networks (LANs). Traditionally in undergraduate courses, the design of a feedback circuit is cast in terms of a two-port analysis, since the closed-loop properties of the circuit are expressed in terms of the corresponding open-loop ones. Therefore, the designer can directly identify the proper feedback topology and adjust the amount of feedback based on the specified closed-loop performances. However, there are practical situations in which a two-port analysis may lead to incorrect results if applied inappropriately or cannot be applied at all. In this paper, the authors show how it is possible to achieve a better insight of these problems in the light of Blackman´s method. Moreover, after a derivation of the formula as a consequence of the superposition principle applied to a LAN, they discuss, with reference to some typical examples, the conditions that must be met for the correct application of the two-port method.