A novel approach to the stability analysis of conditionally stable circuits

This paper presents an intuitive analysis of a class of conditionally stable feedback circuits, that are stable although the Bode stability criteria indicates otherwise: at the frequencies their loop gain phase characteristics first reach -180°, the magnitudes of their magnitude characteristics are far larger than unity. An easy-to-use stability criterion is introduced by means of two circuit examples. Only the usual loop gain Bode plots are required to apply this criterion, but it suits only conditionally stable systems with no right-hand plane (RHP) poles in the open loop transfer function, T(s), and no integrators. A separate, three-step method for assessing the stability of feedback circuits with RHP poles and integrators is also proposed. It applies to all conditionally stable circuits and involves extracting the poles and zeros of T(s) and plotting the corresponding Nyquist contour.