Abstract: A constant span rail-to-rail operational amplifier based on charge pump high voltage internal power supply is designed. The input stage adopts a simple PMOS differential pair of the input stage, and the power supply of the input stage is increased through the charge pump, so that the transport is placed to realize the rail-to-rail input while keeping the transconductance constant within the working range. The external clock input of the charge pump used in this paper is the output clock of the comparator, which generates the non-overlapping signal required by the charge pump switch through the time interleaving circuit and the voltage bootstrap circuit, and charges and discharges the capacitor through the conversion of the switching signal, and finally, output the high-voltage internal power supply alternately. The structure of auxiliary switch with following operational amplifier is adopted in charge pump implementation to reduce the burr of main switch during switching. The amplifier also uses the gain bootstrap structure in the foldable common source common gate structure to improve the overall gain, and the output stage adopts the Class AB output structure to achieve rail-to-rail output. Simulation results show that the open-loop gain of the operational amplifier is 150.76 dB, the unity-gain bandwidth is 53.407 MHz, and the phase margin is 96.1° under the 5V power supply voltage. The rate of change of the input stage transconductance within the rail-to-rail input is 0.00125%.