Finding phase margin of feedback systems from closed loop AC simulations

The stability of a feedback system is generally characterized with a point variable, the system phase margin. Phase Margin is a function of the system loop gain and is not always well defined. In this paper we show how to obtain the phase margin from the closed loop system response by translating the phase of this function with a simple algebraic relation. As the method utilizes a closed loop result and an exact algebraic translation, the results fully account for all loading and effects of feed-forward through the feedback and any reverse transmission through the active elements.