Comparative Analysis of Simulation-Based Methods for Deriving the Phase- and Gain-Margins of Feedback Circuits With Op-Amps

Ten methods for finding through simulations the small-signal phase and gain margins of feedback circuits based on op-amps are described and analyzed in this mostly tutorial paper. The testbenches employed by these methods are presented and the corresponding analytical expressions of the return ratio are derived and compared against their “ideal” counterpart, obtained with standard circuit analysis; the requirement that the return ratio should not depend on the point it was measured at is also verified. These analyses are performed on a fairly general case: a generic reciprocal two-port network that closes a feedback loop around an op-amp acting as the forward amplifier. The four main types of op-amps were considered. The limitations of some of the tested methods are then highlighted by simulations. Besides the detailed analysis of previously reported methods, the paper proposes a novel method for deriving the return ratio of feedback circuits, that employs only current stimuli; it is demonstrated analytically that this method can be used for bilateral circuits, not only for op-amp-based (unilateral) ones. Also, a recent method for deriving directly the phase margin of a circuit is extended to estimating the gain margin, too. Conclusions on the accuracy and suitability of the analyzed methods for practical circuit cases are drawn. These results are then extended to other circuit topologies.