A new implementation of constant-gm op amp input stage for CMOS low voltage applications

In this paper we propose a new implementation method for the design of op amp input stage with rail-to-rail constant-g_m. The new implementation is based on processing signal currents rather than handling DC tail currents. As a result, it is universal in that the constant-g_m can be obtained independent of input transistor types (MOS or bipolar) and their operating regions (weak or strong inversion for MOS and active for bipolar). To demonstrate the idea, a new MOS-type input stage has been designed with a 2-μm CMOS process and simulation results of input stage transconductance are provided. The common mode input voltage can even exceed the positive and the negative rails by 300 mV