CMOS LOW-VOLTAGE OPERATIONAL-AMPLIFIERS WITH CONSTANT-G(M) 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 trans conductance (g(m)) ensures a constant unity-gain frequency within the whole common-mode input range. Two new methods to control the g(m) are introduced. Both operational amplifiers use the same rail-to-rail out put stage. The operational amplifiers have been integrated in a CMOS s emicustom process with transistor lengths of 10 mum. The common-mode i nput voltage swing extends beyond the positive supply rail by 400 mV a nd beyond the negative supply rail by 200 mV. The output voltage is ab le to reach within 130 mV of the supply rails. The output current of t he operational amplifiers is 2 mA and the voltage gain is 85 dB. The u nity-pin frquency is 165 kHz, which is mainly limited by the relativel y long transistor lengths of 10 mum. In another process with channel l engths of 2 mum, simulation results showed that a unity-gain frequency of 4 MHz can easily be obtained.