A constant-voltage circuit includes a first transistor, a first control circuit, and a second control circuit having a second transistor and a differential amplifier. The first transistor controls an output current according to a first control signal output by the first control circuit such that an output voltage is substantially equal to a predetermined voltage. The second control circuit has a response property faster than the first control circuit to a variation of the output voltage, and causes the first transistor to increase the output current for a predetermined time period, regardless of the first control signal, when the output voltage varied to an extent greater than a predetermined output voltage variation value. The second transistor controls an operation of the first transistor according to a second control signal output by the differential amplifier such that a voltage at an inverting input terminal is substantially equal to the bias voltage.
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No. 11/332,163, filed Jan. 17, 2006 now U.S. Pat. No. 7,196,504 which is based on Japanese Patent Application No. 2005-018337, filed on Jan. 26, 2005 in the Japanese Patent Office, the entire contents of each of which are incorporated by reference herein.
