Design of a Constant-Gm Rail-to-Rail Operational Amplifier with Chopper Stabilization

In order to improve the detection accuracy of current singal, a low offset, input/output rail-to-rail operational amplifier is proposed. To ensure that the transconductance of the input stage is constant in the entire common-mode range, the operational amplifier adopts three pairs of differential tubes as the input stage; a folded-cascode is used to improve the circuit gain; and the feed-forward Class AB output stage controlled by the floating current source realizes the output swing from rail to rail. At the same time, the chopper technology is used to reduce the input offset and noise. Based on a 0.18μm CMOS process, the operational amplifier has an input transconductance varistion of 5.14%, a GBW of 1.37MHz, an equivalent DC gain of 115.4dB, and the standard deviation of the input offset voltage is 6.9μV under the condition of 3.3 V supply voltage and 25°C.