1. An operational amplifier comprising: an input differential stage comprising one external input receiving an external input voltage and two outputs; two output stages; and a switch section provided between inputs of said two output stages and the two outputs of said input differential stage, and configured to alternately connect the two outputs of said input differential stage and inputs of a positive-only output stage of said two output stages; and the two outputs of said input differential stage and inputs of a negative-only output stage of said two output stages.