Constant-gm rail-to-rail CMOS op-amp input stage with overlapped transition regions

Conventional techniques to achieve a constant-g_m rail-to-rail complementary N-P differential input stage require complex additional circuitry. In addition, the frequency response and common-mode rejection ratio (CMRR) are degraded. An economical but efficient design technique to overcome these problems is proposed. The proposed technique strategically overlaps the transition regions of the tail currents for the n- and p-pairs to achieve constant overall transconductance. Experimental results demonstrate that g_m variation can be restricted to within ±4% with improved CMRR and frequency response