A low-voltage CMOS op amp with a rail-to-rail constant-gm input stage and a class AB rail-to-rail output stage

A low-voltage two-stage operational amplifier (op-amp) is presented. The op-amp features rail-to-rail operation and has an input stage with a constant transconductance (g_m) over the entire common-mode input range. The input stage consists of an n- and a p-MOS differential pair connected in parallel. The constant g_m is accomplished by regulating the tail-currents with the aid of an MOS translinear (MTL) circuit. The resulting g_m is constant within 5%. The common-source output stage employs a feedback circuit which also contains an MTL circuit. This feedback circuit ensures class AB operation and prevents the transistors in the output stage from cutting off. The op-amp isi realized in a semi-custom CMOS process with minimum channel lengths of 10μm. Simulations show that the minimum supply voltage is less than 2.5 V. A unity gain bandwidth of 550 kHz and a DC voltage gain larger than 80 dB are feasible. The input range exceeds the supply rails, whereas the output range reaches the rails within 130 mV