Design techniques for cascoded CMOS op amps with improved PSRR and common-mode input range

Internally compensated CMOS op amps have been widely used in sampled-analog signal processing applications over the past several years. However, the popular two-stage op amp suffers from poor AC power supply rejection to one of the power rails. Two circuits are presented that overcome the power-supply rejection ratio (PSRR) problems of the earlier amplifier: one for virtual ground applications such as switched-capacitor integrators, the other for buffer applications requiring wide common-mode input range. Small signal analysis is developed for the open-loop and PSRR responses of the two amplifiers. In addition, design guidelines are suggested and test results are presented.