Patent ID: 7133346

Claim:
A photo-detector amplifier circuit for an optical disk device, comprising: a gain setting section for suitably setting an amplifier gain for each of a writing mode, a low-reflective disk reproducing mode, and a high-reflective disk reproducing mode; a two-stage amplifier, which includes a first-stage amplifier and a second-stage amplifier, said gain setting section including: a first gain switching section, provided for the first-stage amplifier, for selecting the writing mode, the low-reflective disk reproducing mode, or the high-reflective disk reproducing mode; and a second gain switching section, provided for the second-stage amplifier, for selecting the low-reflective disk reproducing mode or the high-reflective disk reproducing mode; wherein the first-stage amplifier is a grounded amplifier, and said first gain switching section includes two feedback loops that are connected to a photo-detector element, said first gain switching section switching the feedback loops between the writing mode and the reproducing modes, so as to select a gain; and wherein one of the feedback loops corresponding to the writing mode includes a clamping circuit with a clamping transistor whose collector is connected to a positive power supply, whose base is connected to a reference voltage, and whose emitter is connected to a junction of an input of the first-stage amplifier and the photo-receptor element.