Rail-to-rail constant- gm operational amplifier for video applications

Abstract

This paper presents an operational amplifier (op-amp) with input/output rail-to-rail capability suitable for video applications. The input stage, which is based on two complementary differential pairs, provides a constant transconductance over the entire input common-mode voltage range, whereas a class-AB output stage with an accurate control of the quiescent current ensures rail-to-rail output swing even in the presence of a heavy load. Both the input stage transconductance and the output stage quiescent current are controlled by means of static loops, which allows high-frequency operation of the amplifier. In addition, the proposed constant-transconductance technique does not depend on the I– V characteristics of the input transistors—it is therefore compatible with modern submicron CMOS devices—and does not rely on any accurate matching between the complementary input pairs. The op-amp was designed to operate with 3 V of supply voltage and fabricated in 0.8-μm CMOS technology. The measured open-loop gain and unity gain-bandwidth for a 100-Ω∥100-pF load are 60 dB and 32 MHz, respectively, while the output swing with the same output load reaches 83% of the total supply voltage.