Distortion in CMOS operational amplifier circuits

The paper presents a nonlinear distortion analysis of three basic CMOS inverter circuits, namely PMOS load, constant-current source load and push-pull inverter, and compares their performance experimentally. The analysis is based on a small-signal MOSFET model including the channel-length modulation effect. The theoretical predictions are compared with measurements made on inverters, and incorporated in an operational amplifier, fabricated in an n-well CMOS polysilicon-gate 6 ¿m technology. The results confirm that the distortion depends on the signal level, quiescent bias, channel-length modulation effect, device geometries and processing parameters. The push-pull inverter has higher gain and lower distortion than the other output circuits. Only third-harmonic distortion exists in the operational amplifier using the push-pull inverter as its output stage.