Sizing of multi-stage Op Amps by combining design equations with the gm/ID method.

Analog integrated circuit design has as integral parts both analytical reasoning and numerical validation in the process from topology construction to sizing. Given a circuit topology, different circuit sizing results can be obtained from different processes of sizing inference. Sizing methods by simulation-based numerical searching have been a continuously studied subject. However, almost all approaches in this category require an overwhelming number of circuit simulations to arrive at an optimized sizing result. On the other hand, many published manual sizing methods by using the conventional device equations also require repeated SPICE simulations to correct the equation-based sizing results. This paper proposes a systematic gm/ID-based initial sizing method specifically customized for designing multiple-stage operational amplifiers (Op Amps). A main feature of the proposal is to use circuit-level design equations as constraints on the gm/ID table lookup method to substantially reduce the uncertainty in the sizing calculations. As a result, a significant amount of SPICE based correction work can be reduced to complete an initial sizing. The proposed sizing procedure includes a few regular sizing rules customized to the configuration of multi-stage Op Amps. We validate the proposed sizing method by application to several multi-stage Op Amp examples with a capacitive load or Miller compensation. Simulations have justified that the produced initial sizing results can achieve most of the prespecified design targets. • This paper makes a systematic study of using the gm/ID method for initial sizing of the class of multi-stage Op Amps with Miller compensations. • We propose methods to derive constraints from design equations to make the gm/ID-based sizing more accurate and less simulation hungry. • We further propose heuristic sizing rules that are suited for application to multi-stage Op Amps for quickly working out acceptable sizing. • We have validated the effectiveness of the initial sizing method by Op Amp examples containing different number of stages. • The proposed initial sizing method can be potentially integrated in an auto-sizing tool. [ABSTRACT FROM AUTHOR]