Patent ID: 6903607
Filing Date: 2005-06-07
Classification: H03F

Abstract:
1. An operational amplifier having a differential amplifier stage for performing amplification in response to a difference between a pair of input signals input thereto via a noninverting input terminal and an inverting input terminal respectively, wherein said differential amplifier stage includes a first constant current source for use in current limitation, said operational amplifier comprising: a pair of first MOS transistors, sources of which are commonly connected with a first voltage supply via the first constant current source, and gates of which receive the pair of input signals respectively; a pair of second MOS transistors of a high voltage resistant type having a same conduction type that is identical to a conduction type of the pair of first MOS transistors, wherein the second MOS transistors are respectively arranged on current paths between drains of the first MOS transistors and a second voltage supply; a pair of load circuits that are arranged on current paths between drains of the second MOS transistors and the second voltage supply; and a bias circuit for biasing gates of the second MOS transistors at a prescribed voltage, wherein the bias circuit includes a third MOS transistor in which a source is connected with the first voltage supply via the first constant current source and in which a gate and a source are connected together, a fourth MOS transistor of a high voltage resistant type in which a source is connected with the drain of the third MOS transistor and in which a gate and a drain are commonly connected with the gates of the second MOS transistors, and a second constant current source that is connected between the drain of the fourth MOS transistor and the second voltage supply.