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

Low voltage op amps with complementary N-P input differential pairs are known to suffer from low common mode rejection ratio due to mismatch errors and the tail current switching between the N and P input stage. To understand the contribution of the systematic and the random common mode gains to the overall common mode rejection ratio (CMRR) we studied three op amp topologies, which use N-P complementary input differential pairs. A detailed small signal analysis for each of them has been performed to compare their systematic and random CMRR. The analysis shows that random CMRR caused by mismatch does not depend on the topology, while the systematic CMRR is topology dependent. It is also concluded that the CMRR of low voltage op amps with N-P complementary input pairs will be ultimately limited by the process mismatch and that the random CMRR will determine the overall CMRR.