1-V Rail-to-Rail CMOS OpAmp With Improved Bulk-Driven Input Stage

Abstract

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> 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-<formula formulatype="inline"><tex>$\mu{\hbox {m}}$</tex></formula> 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-M<formula formulatype="inline"><tex>$\Omega\Vert $</tex></formula>17-pF load is connected to the amplifier output. </para>