Rail-to-rail constant-g m input stage and class AB output stage for low-voltage CMOS op amps

Abstract

In this paper an input stage and an output stage are presented for application in low-voltage CMOS operational amplifiers. The input stage operates in strong inversion and has a rail-to-rail common-mode input voltage range. The transconductance (g m ) is insensitive to the common-mode input voltage. The class AB output stage has a rail-to-rail output range. A class AB control circuit prevents any transistors in the output stage from switching off. This improves the large-signal high-frequency behavior and the step response of the amplifier. A complete two-stage Op Amp employing the proposed input and output stages was realized in a semi-custom CMOS process with minimum channel lengths of 10µm and transistor threshold voltages of approximately 0.7 V. The measured minimum supply voltage is 2.5 V. The measured input voltage range exceeds the supply rails and the output voltage reaches both rails within 130 mV. The unity-gain bandwidth of the complete Op Amp is severely limited by the long channel lengths. Simulations show that a unity-gain bandwidth of 7 MHz is feasible if 2.5µm channel lengths are used.