A class AB operational amplifier is widely applied to integrated circuits due to rail-to-rail output and high load current driving capability. In a conventional structure, a load tube of a differential input stage works in a critical saturation area, leading to low intrinsic gains of the devices and limiting the open-loop gain of the operational amplifier. A level translation circuit is added, so that the devices are separated from the critical saturation area, and the open-loop gain of the operational amplifier is increased greatly. A circuit in the invention consists of a bias circuit, a differential input stage, a level translation stage and an output stage.