This article presents a rail-to-rail CMOS operational amplifier of constant gm. It offers simple topology and low power consumption. It uses a simplified shift level circuit at the input, a self-biased folded cascode as the intermediate stage and a class AB, or push-pull output stage to provide large gain. The amplifier, implemented in IBM 130nm CMOS technology, works on a 1.5V power supply voltage and consumes just 215 mu W, and offers a 125dB gain. A comparison to other work in the literature reveal a large gain, low power, rail-to-rail amplifier suitable to many applications.