On Slew Rate Enhancement in Class-A Opamps Using Local Common-Mode Feedback

The output common-mode voltage in multistage fully differential opamps can be set by using a single common-mode feedback loop around all stages or by independent loops for each stage. The two schemes are analyzed for their impact on the slew rate in a two-stage Miller-compensated class-A opamp. Analysis and simulation results show that using local common-mode feedback loops will lead to higher slew rate. This is because, when slewing, both upper and lower transistors receive signal dependent gate voltages with local common-mode feedback, making the behavior similar to that of a class-AB output stage. Also, with local common-mode feedback, the positive and negative slew rates are symmetric.