Improvements in biasing and compensation of CMOS opamps

In this paper, we present modifications to the constant-gm bias circuit and the Miller-lead compensation technique which eliminate or minimize some of their shortcomings. First, we demonstrate how parasitic pad capacitance can cause instability in the constant-gm bias circuit, and show that the transconductance is constant only for specific bias conditions. Next, we suggest a new circuit topology that requires 75% less compensation capacitance to achieve stability. We also discuss problems with Miller-lead compensation that arise from temperature, process and load variations. Finally, we present a new biasing technique to correct these problems, and, through simulation, demonstrate a 40° improvement in phase margin over load current variations.