According to the characteristics of the output buffer of IRFPA readout circuit with large output swing and large voltage swing rate,a new output buffer for IRFPA readout circuit is designed.Because the out put buffer needs a large output swing,the full differential operational amplifier with rail to rail input and class AB output is adopted to improve the bandwidth and output without loss of gain In order to make the circuit more stable,a constant transconductance circuit is in serted into the rail to rail structure to make the circuit more stable; under the requirement of high swing rate of output buffer,the voltage s wing rate is inserted into the output stage to improve the circuit performance.The output buffer adopts TSMC 0.18μ m CMOS process,50pF capacitance load,100kΩ resistance load and 5V working voltage.The simulation results show that when the input swing is 1~4.8V ,the output swing can reach 1~4.8V,the gain can reach 100.34dB,the bandwidth is 58MHz,and the positive and negative voltage sw ing rates are 89V/μs and －92V/μs,respectively.