Two Stage Operational Amplifiers: Power and Area Efficient Frequency Compensation for Driving a Wide Range of Capacitive Load

Abstract

Operational amplifiers (OpAmps) have found extensive applications in analog circuits and systems for communications, consumer electronics, controls and signal conversion. Two-stage OpAmps with freguency compensation are popular for driving capacitive loads while ensuring sufficient gain and stability. Frequency compensation technigues have been evolving over the last decades in distinct applications. In particular, power-and-area-efficent two-stage OpAmps capable of driving a wide-range capacitive load are demanded for low-dropout regulators (LDOs) or LCD-panel drivers. Capacitor multiplers (CMs) have emerged as one of the best solutions to implement such kind of OpAmps. This article reviews, for the first time, the state-of the-art CMs for two-stage OpAmps before describing a novel embedded-CM technigue, i.e., the CM as being part of the input stage of the OpAmp, effectively minimizing the physical size of the compensation capacitors while improving the slew-rate with no extra power consumption. Moreover, unlike the classical Miller compensation technique that can lead to an undesired right-half-plane (RHP) zero, a constructive left-half plane (LHP) zero, is created, that can improve the phase margin (PM). Comparing with the state-of-the art current-buffer and CM compensation topologies the proposed solution also features simpler circuitry. The technigue can be further incorporated with a class-AB output stage to speed up the OpAmp's transient re sponses with low guiescent power. A descriptive design example capable of driving capacitive loads ≥ 50 pF is systematically optimized in a 0.35-μm CMOS process. Finally, a few technigues are outlined which allow the combination of current-buffer-based Miller compensation with more sophisticated CMs, or a pole-zero pair (lead network), to further enhance the driving capability of two-stage OpAmps.