Optical semiconductor device and infrared data communication apparatus eliminating low frequency component

1. 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.

2. The optical semiconductor device according to claim 1 ,

wherein said first photodetector and said second photodetector are adjacently disposed.

3. The optical semiconductor device according to claim 1 ,

wherein said first photodetector is disposed around said second photodetector.

4. The optical semiconductor device according to claim 1 ,

wherein said second photodetector is either a polygon or a circle in shape.

5. The optical semiconductor device according to claim 1 ,

wherein an effective light receiving area of said first photodetector is smaller than an effective light receiving area of said second photodetector.

6. The optical semiconductor device according to claim 1 ,

wherein said current amplifying circuit further includes first and second resistors, and

an output terminal of said first photodetector and an end of said first resistor are connected to a negative input terminal of said first operational amplifier, another end of said first resistor and an end of said second resistor are connected to an output terminal of said first operational amplifier, and another end of said second resistor and an output terminal of said second photodetector are connected to a positive input terminal of said first operational amplifier.

7. The optical semiconductor device according to claim 1 ,

wherein said current-voltage conversion circuit includes a second operational amplifier and a third resistor, and

an output terminal of said second photodetector, the output terminal of said current amplifying circuit, and an end of said third resistor are connected to a negative input terminal of said second operational amplifier, and another end of said third resistor is connected to the output terminal of said second operational amplifier.

8. The optical semiconductor device according to claim 1 ,

wherein said current amplifying circuit further includes a capacitor which is connected to a negative input terminal of said first operational amplifier, and

a capacitance value of the capacitor is greater than a junction capacitance value of said second photodetector.

9. The optical semiconductor device according to claim 6 ,

wherein a resistance value of said first resistor is greater than a resistance value of said second resistor.

10. The optical semiconductor device according to claim 6 ,

wherein a ratio of an effective light receiving area of said first photodetector to an effective light receiving area of said second photodetector is equal to a ratio of a resistance value of said second resistor to a resistance value of said first resistor.

11. An infrared data communication apparatus, comprising:

an optical semiconductor device that converts a light signal into an electric signal and amplifies the converted electric signal; and

a signal processing circuit which processes the electric signal outputted from said optical semiconductor device,

wherein said optical semiconductor device includes:

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 amplifying an output current of 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 to be 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 for eliminating a current component equal to or lower than the first frequency, so that another current component equal to or higher than the first frequency is inputted into said current-voltage conversion circuit.

12. The infrared data communication apparatus according to claim 11 ,

wherein said first photodetector and said second photodetector are adjacently disposed.

13. The infrared data communication apparatus according to claim 11 ,

wherein said first photodetector is disposed so as to surround said second photodetector, and

said second photodetector is either a polygon or a circle in shape.

14. The infrared data communication apparatus according to claim 11 ,

wherein an effective light receiving area of said first photodetector is smaller than an effective light receiving area of said second photodetector.

15. The infrared data communication apparatus according to claim 11 ,

wherein said current amplifying circuit further includes first and second resistors, and

an output terminal of said first photodetector and an end of said first resistor are connected to a negative input terminal of the first operational amplifier, another end of said first resistor and an end of said second resistor are connected to an output terminal of the first operational amplifier, and another end of said second resistor and an output terminal of the second photodetector are connected to a positive input terminal of said first operational amplifier.

16. The infrared data communication apparatus according to claim 11 ,

wherein said current-voltage conversion circuit includes a second operational amplifier and a third resistor, and

an output terminal of said second photodetector, the output terminal of said current amplifying circuit, and an end of said third resistor are connected to a negative input terminal of said second operational amplifier, and another end of said third resistor is connected to the output terminal of said second operational amplifier.

17. The infrared data communication apparatus according to claim 11 ,

wherein said current amplifying circuit further includes a capacitor connected to a negative input terminal of said first operational amplifier, and

a capacitance value of the capacitor is greater than a junction capacitance value of said second photodetector.

18. The infrared data communication apparatus according to claim 15 ,

wherein a resistance value of said first resistor is greater than a resistance value of said second resistor, and

a ratio of an effective light receiving area of said first photodetector to an effective light receiving area of said second photodetector is equal to a ratio of a resistance value of said second resistor to a resistance value of said first resistor.