An operational amplifier includes: a first amplifier stage, configured to generate first output voltages according to first input voltages; a second amplifier stage, configured to generate second output voltages according to the first output voltages; a second output stage circuit, configured to replicate an equivalent or a scaled-down version of the first output stage circuit; a first common-mode feedback circuit, configured to keep an output common-mode voltage of the second output stage circuit at a predetermined value; a logic loop circuit configured to, when the operational amplifier operates in a direct current calibration phase, adjust a difference between the first output voltages; a bias circuit, configured to generate a voltage close to a common-mode voltage of the first output voltages produced after the operational amplifier is turned on, the voltage serving as a reference voltage of a second common-mode feedback circuit.

An exemplary embodiment of the present invention relates to an electrical amplifier comprising a differential preamplifier having a first output port and a second output port; a first output unit connected to the first output port of the differential preamplifier and a second output unit connected to the second output port of the differential preamplifier, the first and second output units being electrically arranged in parallel relative to each other; and a positive feedback loop that couples the first and second output units and comprises a first capacitor and a second capacitor; wherein each of the first and second output units comprises an emitter-follower unit and a bias transistor that is connected in series with the emitter-follower unit of its output unit; wherein an emitter of the emitter-follower unit of the first output unit is connected to a base of the bias transistor of the second output unit through the first capacitor of the positive feedback loop; and wherein an emitter of the emitter-follower unit of the second output unit is connected to a base of the bias transistor of the first output unit through the second capacitor of the positive feedback loop.

An operational amplifier includes: a first amplifier stage, configured to generate first output voltages according to first input voltages; a second amplifier stage, configured to generate second output voltages according to the first output voltages; a second output stage circuit, configured to replicate an equivalent or a scaled-down version of the first output stage circuit; a first common-mode feedback circuit, configured to keep an output common-mode voltage of the second output stage circuit at a predetermined value; a logic loop circuit configured to, when the operational amplifier operates in a direct current calibration phase, adjust a difference between the first output voltages; a bias circuit, configured to generate a voltage close to a common mode voltage of the first output voltages produced after the operational amplifier is turned on, the voltage serving as a reference voltage of a second common-mode feedback circuit.