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 ar e 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 ol the systematic and the random comm on mode gains to the overall common mode rejection ratio (CMRR) we stu died three on amp topologies, which use N-P complementary input differ ential pairs, A detailed small signal analysis for each of them has be en performed to compare their systematic and random CMRR. The analysis shows that random CMRR caused by mismatch does not depend on the topo logy, while the systematic CMRR is topology dependent. It is also conc luded that the CMRR or low voltage op amps with N-P complementary inpu t pain will be ultimately limited by the process mismatch and that the random CMRR will determine the overall CMRR.