Photodetector

• US 20080217516A1
• Filed: 07/22/2005
• Published: 09/11/2008
• Est. Priority Date: 07/22/2004
• Status: Active Grant

• Current Assignee: Hamamatsu Photonics KK
• Original Assignee: Hamamatsu Photonics KK
• Inventors: Suzuki, Takayuki, Asahara, Takashi, Meguri, Shintaro
• Granted Patent: US 7,605,358 B2
• Time in Patent Office: Days
• US Class Current: 250/214A
• CPC Class Codes: H01L 31/02327 the optical elements being integrated or being directly associated to the device, e.g. back reflectors optical coatings H01L31/0216
• CPC Class Codes: H04B 10/6931 Automatic gain control of the preamplifier

Abstract

There is provided a photodetector applicable to communications requiring a high speed and broad dynamic range. When a photocurrent output of a photodiode 14 for detecting optical power is lower than a specified value, the output of a comparator COMP1 is rapidly switched, however, a low pass filter SC removes high harmonic components, so that the voltage of a gain control terminal VG as the output of the low pass filter SC is gradually lowered. Therefore, a current amount flowing between the source and drain of a gain adjusting N-type MOS transistor GCT to which the voltage is applied is gradually reduced, the current amount flowing via a resistor R1 is reduced, the resistance value Rx of the composite resistor of the gain adjusting amplifier AMP is increased, so that the gain of the gain adjusting amplifier AMP gradually increases to a specified gain. In this case, the time period from the increase in the output voltage until the saturation thereof is within 3 μs. However, by slowly varying the gain of the output voltage as described above, an adverse effect on a PHY chip connected at the rear stage can be suppressed.

Claims

1. A photodetector comprising:

a first photodiode for light receiving;

a light-shielded second photodiode;

a first differential amplifier to which the outputs of the first and second photodiodes are input;

gain adjusting amplifiers that are interposed between the first photodiode and the first differential amplifier and the second photodiode and the first differential amplifier respectively;

an optical power detector for detecting optical power on the basis of an output of a comparator; and

a low pass filter interposed between an output terminal of the optical power detector and a gain adjusting terminal of the gain adjusting amplifier.

2. The photodetector according to claim 1, wherein the optical power detector comprises:

a third photodiode for light receiving;

a light-shielded fourth photodiode;

a second differential amplifier to which the outputs of the third and fourth photodiodes are input; and

the comparator to which the output of the second differential amplifier is input.

3. The photodetector according to claim 1, wherein the gain adjusting amplifier comprises:

a plurality of resistors interposed between the input and output terminals of an operational amplifier in parallel; and

a transistor connected to the resistor in series, wherein the control terminal of the transistor is set as the gain adjusting terminal.

4. The photodetector according to claim 2, wherein the gain adjusting amplifier comprises:

a plurality of resistors interposed between the input and output terminals of an operational amplifier in parallel; and

a transistor connected to the resistor in series, wherein the control terminal of the transistor is set as the gain adjusting terminal.