Analysis on the effect of MOSFET threshold voltage mismatch in CMOS operational amplifiers for RF application

Abstract

CMOS operational amplifiers (op amps) are used in RF circuit blocks as low noise or power amplifiers. One of the critical issues in the performance and operation of these op amps is its threshold voltage (Vth) mismatch between adjacent MOSFET devices. Different Vth values between the op amps' input MOSFETs of the differential stage causes it to deviate in its output specification, which include among others, the AC and DC gain, bandwidth, and most notably, the offset voltage. First order analysis of offset voltage approximation is performed with 30% Vth variation, and the obtained result is verified through a BSIM3 MOSFET model, with parametric sweep SPICE simulation of a 2-stage large gain and bandwidth op amp. The study is found to be crucial in order to develop a technique that could detect Vth variation of transistors, and compensate op amps output specifications accordingly