A Modified Nyquist Stability Criteria that Takes into Account Input/Output Circuit Loading Effects

H. S. Black in 1928 described the principle of negative feedback using two functions denoted as A(s) and β(s). While negative feedback has many important practical benefits, circuits that utilize this principle can go unstable. In 1932 H. Nyquist described the conditions that maintain stable behavior based on the complex behavior of the product of A(s) times β(s) culminating into what is referred to as the Nyquist Stability Criterion. When applied to electronic circuits, however, there is uncertainty in identifying A(s) and β(s) due to circuit loading effects. In this paper, a theory of stability will be introduced that takes into account the influence that input and output circuit loading effects have on the operation of the circuit. An example will be used to illustrate the significance of these findings.