Patent ID: 8773831

Claim:
A semiconductor integrated circuit comprising: a circuit comprising a transistor having a thick gate oxide film, a transistor having a thin gate oxide film, a differential amplifier circuit comprising first and second differential input transistors, a first current source transistor having a thin gate oxide film, and a second current source transistor having a thin gate oxide film, wherein the transistor having the thin gate oxide film being configured to operate in a portion of the circuit which is substantially affected by noise, and the thick gate oxide film being thicker than the thin gate oxide film; a protection circuit disposed in the circuit and configured to protect the transistor having the thin gate oxide film from overvoltage, the protection circuit comprising a first protection transistor having a thick gate oxide film and a second protection transistor having a thick gate oxide film, wherein the first protection transistor is connected in series with the first differential input transistor and configured to protect the first differential input transistor from overvoltage, the second protection transistor is connected in series with the second differential input transistor and configured to protect the second differential input transistor from overvoltage; and a bias circuit configured to apply a common bias voltage to gates of the first and second protection transistors, wherein the circuit is configured to receive a power supply voltage corresponding to the transistor having the thick gate oxide film, and wherein the first current source transistor is connected with the first differential input transistor in series through the first protection transistor, the second current source transistor is connected with the second differential input transistor in series through the second protection transistor.