A Rail-To-Rail Op-Amp with Constant Gm Using Current Switches

A scheme to achieve constant Gm for rail-to-rail CMOS op-amps is presented. It is based on a novel concept that employs four transistors as current switch to select different differential pairs as input pairs for different common-mode voltage. The operation of the two differential pairs is separated, so they can be regulated for constant Gm for the whole input common-mode voltage range. An OTA stage is used at the output for better frequency response and precisely controlling of output current. There is no requirement for any precise analytic relationship between drain current and transconductance, this is particularly important as it makes the circuit useful for deep submicron CMOS technologies. And it can work for supply power of 2.5V~5.5V. With GSMC 0.18um CMOS process, the simulation results show that the gain can achieve 80 dB. And the CMRR is improved also.