The limitation of CMRR in low voltage operational amplifier with N-P input pairs

Abstract

This paper discusses the Common Mode Rejection Ratio (CMRR) degradation problem in low voltage Operational Amplifiers using N-P complementary input pairs. The CMRR degradation in a low voltage opamp is due to the significant increase in both systematic and mismatching common mode gains which are, in turn, due to the tail current switching between the N and P pairs. Small signal analysis revealed that both systematic common mode gain and mismatching common mode gain are proportional to the rate of change of the tail current source of the input differential pairs. It is concluded that the systematic CMRR degradation could be overcome by topology modification but the mismatching remains to be the dominant factor which limits the improvement of CMRR in low voltage opamp with N-P differential pairs.