Abstract: A rail-to-rail operational amplifier with low voltage constant transconductance and high performance is introduced with TSMC 0.18 μm CMOS process. The input stage uses NMOS and PMOS complementary differential pairs to make the common-mode input voltage rail-to-rail. Meanwhile the input stage uses current switches with constant common-mode output current to keep the effective transconductance constant over the whole input range. A class-AB output stage with folded mesh feedback is used to achieve the rail-to-rail output range. And miller compensation is adopted to guarantee the stability of the operational amplifier by controlling the zero and pole. The whole circuit is simulated in Cadence Spectre with a single 1 V supply. The simulation results show that the dc open-loop gain of the op-amp is more than 127 dB, and the unity-gain frequency is 6.4 MHz with phase margin of 67.58° while driving 20 pF. The total harmonic distortion of a unity-gain connected amplifier is -107 dB.