A low-voltage CMOS op amp with rail-to-rail constant-gm input stage and high-gain output stage

A 2 V low-voltage CMOS op amp with a new rail-to-rail constant-g _m input stage and a high-gain, high-swing output stage is presented. The input stage consists of two complementary differential pairs whose tail currents are controlled by the common-mode input signal in such a way that the total transconductance (g_m) of the input transistors remains as flat as possible across the input common-mode range. The output stage employs composite transistors based on regulated-cascode circuits (RGC) with large feedback gains. DC measurements were performed on an experimental op amp fabricated in a standard 2 μm, p-well CMOS process. The low-frequency gain is larger than 80 dB. The g_m variation is about 15% from rail-to-rail. Output signal can swing within 200 mV of each supply rail and input range exceeds both rails. The power dissipation is about 540 μW. Simulated unity-gain frequency is 5 MHz with 15 pF load capacitance