A common operational amplifier for a pipeline circuit is provided. The common operational amplifier is used by the stage circuits of the pipeline circuit by turns according to a predetermined timing. The common operational amplifier comprises an operational amplifier circuit, a multiplexer circuit and a demultiplexer circuit. The multiplexer circuit is provided for selecting a signal set of a stage circuit to be amplified to couple to the operational amplifier circuit, and the demultiplexer circuit is provided for transmitting the amplified signal set to the corresponding stage circuit.