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
$g_{m}$
enhancement based on local common-mode feedback to provide class-AB operation with enhanced effective
$g_{m}$
, 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
$SR\text{s}$
. 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-
$\mu \text{m}$
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-
$\mu \text{A}$
total static current.