R. Hogervorst et al., CMOS LOW-VOLTAGE OPERATIONAL-AMPLIFIERS WITH CONSTANT-G(M) RAIL-TO-RAIL INPUT STAGE, Analog integrated circuits and signal processing, 5(2), 1994, pp. 135-146
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.