Head amplifier and an optical disk device using the same

An optical disk device for recording a signal onto or reproducing a signal from a disk having a track of land/groove structure, a pre-formatted area, and a mirror area, said optical disk including a head amplifier, said head amplifier comprising:

• a photodiode for performing light-current conversion and producing a detected current;
• a current-voltage conversion amplifier for converting the detected current into a voltage;
• again setting circuit for setting a plurality of gain values;
• a circuit for applying a plurality of offset voltages;
• a circuit for detecting a direct current level of a track and a direct current level of a mirror level on said disk;
• and
• a circuit for supplying an offset voltage obtained from a result detected by said detecting circuit, to said current-voltage conversion amplifier by using said offset voltage applying circuit;
• wherein when a detected current having a plurality of different direct current levels in time series is supplied, an offset voltage and a gain is set, so as not exceed a dynamic range of said current-voltage conversion amplifier, for each of intervals respectively having said different direct current levels.

An optical disk device comprising:

• a head amplifier for converting reflected light supplied from an optical recording medium having a pre-formatted area and a recording field into a voltage;
• and
• again switchover circuit for switching over a gain of said head amplifier;
• wherein said gain switchover circuit switches a gain for the reflected light from said recording field over to a gain for the reflected light from said pre-formatted area, and vice versa.