A class AB operational amplifier includes: a first transistor, for generating a first current; a second transistor, where a second source voltage of the second transistor is equal to a first source voltage of the first transistor, for generating an output stage quiescent current; and an output stage quiescent current controller, coupled to a gate and a source of the first transistor, for controlling a first drain-to-source voltage of the first transistor to be equal to a second drain-to-source voltage of the second transistor. A ratio of the output stage quiescent current to the first current is equal to a ratio of a second W/L ratio of the second transistor to a first W/L ratio of the first transistor.