A Constant-$g_{m}$ Constant-Slew-Rate Rail-to-Rail Input Stage With Static Feedback and Dynamic Current Steering for VLSI Cell Libraries

A constant-g_m input stage featuring both constant small signal and large signal behaviors over the entire input common-mode range is proposed in this brief. A novel static feedback loop is employed to minimize the N and P transconductance mismatch due to process and temperature variations. The output currents of the N and P differential pairs are dynamically steered to keep a constant g_m and a constant slew rate. The overall technique is independent of the operation regions of input transistors, and does not rely on the quadratic characteristics of input MOS transistors. A prototype chip was designed for a 0.35-mum CMOS technology with 3-V power supply. The experimental results demonstrate that a constant-g_m (plusmn3% variations) and a constant slew rate over the entire input common-mode range have been achieved

I. Introduction

For low-voltage and high dynamic range mixed signal integrated circuits (ICs), rail-to-rail input operational amplifiers are of great interests due to their wide input common-mode range. Rail-to-rail operations can be achieved by complementary input stages with both p-channel and n-channel differential pairs in parallel [1]. When the input common-mode voltage is close to either of the power rails, at least one input differential pair remains active. However, when the input common-mode voltage is in the middle, both of the two differential pairs operate, and the total doubles. Large variations prevent optimal frequency compensations and introduce severe signal distortions [1]–[3].