A Constant Gm CMOS Op-Amp with Rail-to-Rail input/output stage

A Constant-gm CMOS op-amp with Rail-to-Rail input and output stage is proposed in this paper. It is based on a novel configuration that consists of four MOSFETS as dummy differential pairs to select different differential pair as input pair, according to the different common-mode voltage. The constant gm is accomplished by avoiding the input NMOS and PMOS differential pairs operating synchronously, if they operate at the same time, the tail current will be halved. The gate of the dummy differential pairs is controlled by a fixed dc voltage V_B. The resulting gm is constant within 4.65%. In addition, a class-AB output stage is employed to obtain a wide output range. The scheme can work properly for 1.5V~3.3V or 2.5V~5.5V depended on the process selected. The Op Amp performs 106.6-dB dc gain, 13.85-MHz bandwidth (PM=63.86°) at 10pF//10KΩ output load and 5V power supply.