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.

本文提出了一种恒定g _m轨至轨运算放大器。通过控制并联的nMOS和pMOS输入差分对的尾电流，可以减少由于输入共模电压变化而引起的跨导变化。此外，对电路应用了凹凸平滑技术，以降低过渡区域中的亚阈值效应，因此降低了总的g _m变化。此外，在输出级采用一种稳定技术，通过固定共源共栅晶体管的偏置电流，将直流增益和相位裕度的波动降至最低。由于在整个输入共模电压范围内都稳定了放大器的DC增益，因此该电路可用作外部反馈环路中的精确小信号电压放大器，用于具有各种共模电压的输入信号。该电路采用标准的0.18μmCMOS技术进行仿真。仿真结果清单，该克_米偏差约为±0.85％，增益带宽，低频电压增益（DC增益）和相位裕度的偏差分别为±1.20％，±3.75％和±3.00％。所展示的运算放大器在5pF容性负载下具有81.3 dB的低频电压增益和50.9 MHz的增益带宽，在室温和典型条件下，在1.8 V电源电压下的功耗为1.48 mW。