Design for Slew-Rate in Multi-Stage CMOS OTAs

Cascading gain stages in CMOS Operational Transconductance Amplifiers (OTAs) has become a necessity in applications with high gain requirements, where the contribution of each stage to the overall gain is well-known and carefully designed. Many of these applications also impose requirements on speed, including a minimum Slew-Rate (SR) to ensure signal fidelity, however the impact of individual gain stages on the overall SR in multi-stage OTAs has been difficult to characterize–let alone carefully design. The difficulty arises due to the complexity of the compensation networks involved in these OTAs. This paper presents a systematic design approach for achieving a target SR in multi-stage CMOS OTAs, enabled through the utility of a novel analytical model for estimating the lower-bound Slew-Rate in multi-stage OTAs. The model evaluates individual currents and equivalent capacitances at the output node of each stage, providing insights on the dominant node slowing down the overall SR. For generality, the model establishes the SR analysis based on N-stage designs, and considers widely employed compensation networks. Example designs, with post-layout simulations and measurements of a 3- and a 4-stage CMOS OTA, and with post-layout simulations of a 5-stage CMOS OTA, are presented for validating the model’s utility. The results show strong agreement between theoretical, simulated, and measured SR values, confirming the model’s reliability in estimating the lower-bound SR, and its utility in a systematic design-for-SR approach in multi-stage CMOS OTAs.