Fully Differential Rail-to-rail Input Opamp With Novel Constant-transconductance Control In 120nm CMOS Technology

A fully differential operational amplifier featured with rail-to-rail input and output performance, realized by a novel method of constant small- and large-signal behavior control as well as a new symmetrical differential class-AB output is proposed. The small-signal deviation is suppressed within only 1.6%, which is the best result achieved among all the constant behavior rail-to-rail input-stage designs. With suitable compensation, the opamp is able to drive heavy ohmic load down to 100ohm or capacitive load higher than 50pF. Also the total harmonic distortion is reduced owing to the symmetrical push-pull structure for the output stage. With a 1.5V power supply and a capacitive load of 10pF, the low-frequency gain and unity-gain bandwidth of the opamp are 77dB and 4.7MHz, respectively. The chip was fabricated in 120nm digital CMOS technology