Constant- $g_{m}$ Rail-To-Rail Input/Output Operational Amplifier

A constant- $\boldsymbol{g}_{\boldsymbol{m}}$, rail-to-rail input/output dual channel operational amplifier (op-amp) was implemented in a $0.18 \mu ~\mathrm{m}$ CMOS process. Each channel is a two-stage operational amplifier consisting of input stage, output stage circuit, and compensation circuit. Parallel complementary differential pairs were adopted to realize rail-to-rail input in the first stage. A circuit containing dummy differential pairs is proposed to obtain constant transconductance value over the entire common-mode input voltage range. A class-AB output stage is used to implement rail-to-rail output. The operating voltage range of the op-amp is 2.7 $\mathrm{V} \sim 5.5 ~\mathrm{V}$. The sample test results show that the input voltage range and output voltage range of the proposed op-amp are rail-to-rail, input voltage range is $0 ~\mathrm{V} \sim 5 ~\mathrm{V}$, the high output voltage is 4.996 V, the low output voltage is 5.213 mV, the open-loop voltage gain is $110.1 ~\text{dB} \sim 118.1 ~\text{dB}$ and the rate of change is 7.0 % over the common-mode input voltage range, the gain bandwidth product is 22.0 MHz, the common-mode rejection ratio is 127.5 dB, and the power supply rejection ratio is 135.8 dB.