Patent ID: 8035072

Claim:
An optical semiconductor device that converts a light signal into an electric signal and amplifies the converted electric signal, said optical semiconductor device comprising: a first photodetector which converts a light signal into a current signal; a second photodetector which converts a light signal into a current signal; a current amplifying circuit which includes a first operational amplifier which amplifies an output current from said first photodetector; and a current-voltage conversion circuit, wherein an output terminal of said current amplifying circuit is connected to an input terminal of said current-voltage conversion circuit, and said current-voltage conversion circuit converts a current into a voltage, the current being an output current of said current amplifying circuit subtracted from an output current of said second photodetector, wherein said current amplifying circuit has a low-pass characteristic determined to be a first frequency as a cut-off frequency, said current-voltage conversion circuit has a low-pass characteristic determined to be a second frequency as a cut-off frequency, the second frequency being greater than the first frequency, and the output current from said current amplifying circuit is inputted into said second photodetector so as to negate a current component equal to or lower than the first frequency, so that another current component higher than the first frequency is inputted into said current-voltage conversion circuit.