A Two Stage Op-Amp Phase Margin Symbolic Expression

Abstract

A phase margin symbolic expression of a two stage Miller compensated operational amplifier is computed in this paper. Using this expression, an analysis to evaluate the influence of the Miller and load capacitance on phase margin is performed. This way, a designer can rapidly choose the optimal set of values to fulfil an imposed phase margin. The phase margin expression is based on poles/zeros symbolic expressions obtained using a symbolic LR algorithm able to compute both the numerical values and the approximate symbolic expressions of poles and zeros of a circuit. The numerical values obtained with this algorithm are compared with those computed by SPECTRE. The example is a two stage Miller compensated operational amplifier designed in a 180nm CMOS technology.