A Robust Rail-to-Rail Input Stage with Constant-gm and Constant Slew Rate Using a Novel Level Shifter

A constant-g_m input stage, which features both constant small-signal and large-signal behavior over the entire input common-mode range, is proposed in this paper. The technique is based on a single differential pair, avoiding the issues associated with complimentary pair techniques, such as the degradation of common-mode rejection ratio and signal-dependent input referred offset. A novel capacitive level shifter keeps the effective input common-mode voltage constant, without attenuating the differential-mode voltage. The overall technique is independent on the operation regions of input transistors; it also does not rely on the quadratic characteristic of input MOS transistors. A prototype chip was designed for 0.35-mum CMOS technology of 3-V supply voltage. The simulation results showed constant-gm (plusmn0.2% variations) and constant slew rate over the entire input common-mode range