Optimized Active Single-Miller Capacitor Compensation with Inner Feedforward for Very High-Load Three-Stage OTAs

This paper introduces a new effective single-Miller capacitor compensation topology for three-stage amplifiers with very large capacitive loads, realized through an active-feedback capacitor together with an inner half-feedforward stage. Moreover, an optimized design strategy which profitably exploits the two left half-plane zeros is presented. To improve the amplifier large signal transient response, the topology also includes an external feedforward path, that only marginally affects the frequency compensation, and a novel slew-rate enhancer section. To validate the solutions presented, a three-stage OTA driving a 10-nF load has been designed and implemented in a standard 0.35-μm CMOS technology. The amplifier occupies less than 0.003-mm2 of die area, provides 2.7-MHz gain-bandwidth product and 0.55-V/μs average slew-rate, while consuming only 25-μA quiescent current.