Implementation of constant gm CMOS op-amp input stage using overlapping of transition region at 0.18 um technology

Abstract

Operational amplifier is the backbone of most of analog circuit design. For low voltage applications, op-amp should have a rail-to-rail common mode input voltage. This report describes the implementation of a constant gm rail-to-rail CMOS op-amp using complementary input pairs, at 0.18µm MOS technology. The concept used to make the input transconductance (gm) constant is the overlapping of transition regions of n-pair and p-pair tail transistors using a DC level shifter [2]. A constant gm input stage insures a uniform frequency response for the entire common mode input range. It also improves the Common Mode Rejection Ratio (CMRR). The results of the designed op-amp show that it has a rail-to-rail input common mode range and a rail-to-rail output voltage swing. For rail-to-rail output voltage swing a Class AB output stage is used. Layout of the chosen architecture is made using 0.18µm technology. Comparisons of pre-layout and post-layout simulation results are done.