An operational amplifier, any of a pair of amplification circuits of its output-stage amplification circuit unit comprises: a first and second transistors, a capacitor and a DC bias circuit; a control electrode of the first transistor is connected with a corresponding output terminal of a preceding-stage amplification circuit unit, a first electrode thereof is connected with a first power terminal and a second electrode thereof is connected with an output terminal of an amplification circuit of the output-stage amplification circuit unit; an output terminal of the DC bias circuit is connected with a control electrode of the second transistor, a first electrode of which is connected with a second power terminal, and a second electrode thereof is connected with the output terminal; both ends of the capacitor are respectively connected with the control electrodes of the first and second transistors; and the first and second transistors are of opposite polarities.