Error analysis in Miller´s theorems

Miller´s theorems are important tools to analyze amplifiers with feedback, and also to analyze the oscillation conditions of analog oscillators. The rigorous application of Miller´s theorems depends on the previous knowledge of the closed-loop gain which, in turn, allows the calculation of the closed-loop input impedance/admittance. However, this gain is usually not known a priori, and the use of the open-loop gain to estimate the closed-loop gain is frequently found in the literature. This approximation is not always valid. Hence, we present a rigorous analysis of the error associated to this approach