ROMJIST Volume 25, No. 2, 2022, pp. 205-223
Valentin BELECA, Cosmin-Sorin PLESA, Raul ONET, Marius NEAG Methods for Assessing the Stability of Conditionally Stable Circuits by Using Small-signal Simulations
ABSTRACT: The popular Bode stability criterion is not suitable for assessing the stability of feedback systems for which the phase characteristic of their loop gain, T(s), crosses the -180° horizontal more than once. This fact is illustrated by four simple examples of such circuits, that also help demonstrate an extension of the Bode criterion proposed in this paper: the stability of a sub-class of conditionally stable circuits – whose T(s) comprises neither RHP poles nor poles in the origin, and the module frequency characteristic of their T(s) crosses the 0dB horizontal only once – can be assessed by analyzing the phase characteristic of their loop gain. The relationship between the phase margin of these circuits and their frequency and step response is also analyzed. Finally, the paper introduces a three-step method for assessing the stability of all feedback systems, including the conditionally stable ones, by using small-signal analysis. It involves extracting the poles and zeros of T(s), then plotting the corresponding Nyquist contour by using MATLAB scripts developed for this purpose. KEYWORDS: Bode stability criterion, conditionally stable circuits, multiple critical frequencies, Nyquist contour, Nyquist stability criterion, pole-zero analysisRead full text (pdf)