A Highly Efficient Composite Class-AB-AB Miller Op-Amp With High Gain and Stable From 15 pF Up To Very Large Capacitive Loads

In this paper, a highly power-efficient class-AB-AB Miller op-amp is discussed. The structure uses gm enhancement based on local common-mode feedback to provide class-AB operation with enhanced effective gm, open-loop gain, unity-gain frequency, and slew rate (SR) without significant increase in quiescent power consumption. Utilization of a nonlinear load leads to large symmetric positive and negative SRs. Stability over an extremely wide range of capacitive loads is achieved through a combination of Miller and phase-lead compensations. The unity-gain frequency does not show sensitivity to capacitive load values. A test chip prototype fabricated in 0.18-mu m CMOS technology shows 90.8-dB open-loop gain, 12.5-MHz bandwidth for a 25-pF load capacitance, and a factor 60 SR enhancement with maximum output current close to 1-mA and 43-mu A total static current.