A high-constant g m rail-to-rail operational amplifier using bump-smoothing technique with stabilized output stage

In this paper, a constant-g m rail-to-rail operational amplifier is proposed. The transconductance variation due to the changes in the input common-mode voltage is reduced by controlling the tail-current of the parallel nMOS and pMOS input differential pairs. In addition, a bump-smoothing technique is applied to the circuit to decline sub-threshold effects in transition regions, and therefore lower the overall g m variations. Furthermore, a stabilizing technique is employed in the output stage to minimize the fluctuation of the DC-gain and the phase margin by fixing the bias current of the cascode transistors. Thanks to stabilizing the DC-gain of the amplifier in the entire range of the input common-mode voltage, the circuit can be used as an accurate small-signal voltage amplifier in an external feedback loop for input signals with various common-mode voltages. The circuit is simulated in a standard 0.18 μm CMOS technology. The simulation results manifest that the g m variation is around ± 0.85% and the variations of the Gain–Bandwidth, the low-frequency voltage gain (DC-gain), and the phase margin are ± 1.20%, ± 3.75%, and ± 3.00%, respectively. The presented operational amplifier with the low-frequency voltage gain of 81.3 dB and the Gain–Bandwidth of 50.9 MHz for a 5-pF capacitive load consumes 1.48 mW with a 1.8 V supply voltage at the room temperature and in the typical condition.