On the common mode rejection ratio in low voltage operationalamplifiers with complementary N-P input pairs

摘要

Low voltage op amps with complementary N-P input differentialnpairs are known to suffer from low common mode rejection ratio due tonmismatch errors and the tail current switching between the N and P inputnstage. To understand the contribution of the systematic and the randomncommon mode gains to the overall common mode rejection ratio (CMRR) wenstudied three op amp topologies, which use N-P complementary inputndifferential pairs. A detailed small signal analysis for each of themnhas been performed to compare their systematic and random CMRR. Thenanalysis shows that random CMRR caused by mismatch does not depend onnthe topology, while the systematic CMRR is topology dependent. It isnalso concluded that the CMRR of low voltage op amps with N-Pncomplementary input pairs will be ultimately limited by the processnmismatch and that the random CMRR will determine the overall CMRR