1-V rail-to-rail CMOS opamp with improved bulk-driven input stage

This paper introduces a CMOS operational amplifier with rail-to-rail input and output voltage ranges, suitable for operation in extremely low-voltage environments. The approach is based on a bulk-driven input stage with extended input common-mode voltage range, in which the effective input transconductance is enhanced by means of a partial positive feedback loop. As a result, a gain and gain-bandwidth product performance similar to that of an amplifier using a gate-driven approach is obtained. Output rail-to-rail operation is achieved by means of a push-pull stage, which is biased in class-AB by using a static feedback loop, thus avoiding frequency limitations inherent in dynamic-feedback tuning schemes. The proposed two-stage operational amplifier was designed to operate with a 1-V supply, and a test chip prototype was fabricated in 0.35-µm standard CMOS technology. The experimental performance features an open-loop DC gain higher than 76 dB and a closed-loop unity-gain bandwidth above 8 MHz when a 1-MOhm//17-pF load is connected to the amplifier output.