Abstract: The principle figures of input stage middle gain stage and output stage for a usual two stage  low-voltage CMOS operational amplifier and their principal performance are presented in the paper.In the input stage the parallel supplementary differential input pair structure by using NMOS tube and PMOS tube is applied in order to enable the input common mode votage range to achieve rail-to-rail.To realize the contant transconductance of input stage,the proportional current mirror technlogy is used.In the middle gain stage,the current mirror load of low voltage wide-swing cascade structure is adopted which is suitable to work in low voltage and increases output resistance,i.e.gain,and realizes rail-to-rail.In the output stage,in order to enhance the efficiency and reach rail-to-raip,it uses the push-pull common soure pole amplifier.To design a norm current source,the differential amplifier with current mirror load is applied which provides the stable bias current and bias voltage to the operational amplifier in order to guarant its stability and accuracy.For preventing op amp oscillation,the Miller compensation technology with adjusting zero  resistance is adopted.The simulation by using Hspice software shows that the performance of the operational amplifier well meets the design requirement.

Key words: operational amplifier, rail-to-rail, transconductance, current mirror, supplementary differential input pair