Rail-to-rail CMOS complementary input stage with alternating active differential pairs

A simple and power efficient scheme for input rail-to-rail Op-Amp operation with constant g_m and CMRR over the common mode input range is introduced. The input stage uses complementary differential pairs but only one pair is active at a time. Three implementations of the scheme are discussed. They use very compact control circuitry to turn only one of the differential pairs at a time. Simulations and experimental verification of the proposed scheme are provided from a test chip prototype in 0.5μm CMOS technology.