Generalized Relationship between Frequency Response and Settling Time of CMOS OTAs: Toward Many-Stage Design

Abstract

The presence of Pole-Zero (P-Z) pairs in the open-loop frequency response of CMOS OTAs has always been considered detrimental to the closed-loop operation of OTAs. In this work, a new proposed theory is presented showing how to reduce the impact of such P-Z pairs on the settling time of CMOS OTAs - using low-frequency zeros and cascaded-gain stages - consequently revealing un-tapped opportunities for many-stage CMOS OTA design. The proposed theory will be validated and verified through a design example that also demonstrates how the generalized theory unveils opportunities for many-stage OTA design. The presented example is a 2- to 8-stage CMOS OTA based on the TSMC 65 nm CMOS process, verified through simulations (schematic and post-layout) as well as some measurement results.