The invention discloses an operational amplifier. The operational amplifier comprises an input-stage circuit, a bias circuit and an output-stage circuit; the input-stage circuit comprises a differential input module, a first gain bootstrapping module and a second gain bootstrapping module; the bias circuit comprises a bias module and a common-mode feedback module; the output-stage circuit comprises a differential-to-single-ended module; the common-mode feedback module provides a bias voltage for the differential input module, and has the common-mode feedback function; the feedback signal controls the tail current source of the differential input module to eliminate the offset voltage; and the differential-to-single-ended module overlaps two differential signals output by the input-stage circuit in a phase reversal way, and then output with a single port. The gain of the operational amplifier disclosed by the invention can be effectively improved.