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 usesgₘenhancement based on local common-mode feedback to provide class-AB operation with enhanced effectivegₘ, 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 negativeSRs. 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-μ mCMOS 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-μ Atotal static current.