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A Constant-gm CMOS operational amplifier with rail-to-rail input common-mode voltage range has been designed. The input stage is based on two pairs of single p-channel differential pairs and two identical n-channel source followers as DC level shifters, which realizes rail-to-rail input swing. To attain a constant-gm over the whole common-mode input range, the compensation circuit which is based on processing signal currents has also been designed. The technique is universal in that the constant-gm can be obtained independent of I-V characteristic and is compatible with deep submicron CMOS devices. Using BSIM3v3 model of a standard 0.6 mum CMOS process, simulation result shows that the input /output swing of the op amp circuit is rail-to-rail. The op-amp achieves open-loop gain of 113.57 dB, unity-gain bandwidth of 11.9 MHz and phase margin of 53deg.