CMOS low-voltage operational amplifiers with constant-gm rail-to-rail input stage

Two 3.3-V operational amplifiers with constant-g _m rail-to-rail input stage and rail-to-rail output stage are presented. The constant transconductance (g _m ) ensures a constant unity-gain frequency within the whole commonmode input range. Two new methods to control theg _m are introduced. Both operational amplifiers use the same rail-to-rail output stage. The operational amplifiers have been integrated in a CMOS semicustom process with transistor lengths of 10µm. The common-mode input voltage swing extends beyond the positive supply rail by 400 mV and beyond the negative supply rail by 200 mV. The output voltage is able to reach within 130 mV of the supply rails. The output current of the operational amplifiers is 2 mA and the voltage gain is 85 dB. The unity-gain frquency is 165 kHz, which is mainly limited by the relatively long transistor lengths of 10µm. In another process with channel lengths of 2µm, simulation results showed that a unity-gain frequency of 4 MHz can easily be obtained.