A current supply circuit supplies a bias current to operational amplifiers which constitute a pipelined AD converter. A current switching circuit switches between output currents in response to a current control signal from current control means, thereby switching between currents to be supplied from a bias circuit to the operational amplifiers via transistors. A sufficiently large current enough for the amplifiers to operate at a high frequency is supplied, while the current is switched to a lower supply current for the amplifier to operate at lower frequencies.