Patent ID: 7476839

Claim:
An amplifier circuit, for use in an optical disk drive operating with optical disks of a plurality of media types, for converting, into a voltage, a photoelectric current generated by a photodiode based on light from a loaded optical disk, comprising: an operational amplifier including a pair of differential input transistors; and a variable current source for selecting a first current when said loaded optical disk is a first type optical disk which requires a first frequency band and a second current when said loaded optical disk is a second type optical disk which requires a second frequency band as a common bias current supplied to said pair of differential input transistors, wherein the second current is smaller than the first current; and, the second frequency band is narrower than the first frequency band, the amplifier circuit further comprising a feedback resistor switching circuit for selecting a first resistor having a first resistance value when said loaded optical disk is a first type optical disk which has a first reflectance and a second resistor having a second resistance value when said loaded optical disk is a second type optical disk which has a second reflectance value, as a feedback resistor of said operational amplifier, wherein the second resistance value is larger than the first resistance value; and, the second reflectance value is lower than the first reflectance value.