JPS599524A - Photodetector circuit - Google Patents

Abstract

PURPOSE:To omit cumbersome adjustment and an attenuator and to make it possible to perform the detection of light receiving levels in a broad range, by connecting a series circuit of a resistor and a power source and a series circuit of a resistor, a diode, and a power source to a photodetector in parallel. CONSTITUTION:Assume -VB1<-VB2, the bias voltage VB of a photodetector 1 becomes VB1+I0XRB1 when the light receiving level is low and the light current I0 is small; while VB becomes VB2+Vf+I0XRB2 (wherein Vf is a voltage drop in the forward direction of the semiconductor 5) when the receiviing level is high and the current I0 is large. With this regard, when the light receiving level is low, the bias resistance of the photodetector is made large and the degradation in light receiving sensitivity is suppressed. When the light receiving level is high, the bias resistance value is made small, and the degradation in frequency response characteristics can be prevented.

Description

[Detailed description of the invention] The present invention relates to a photodetector circuit.

Compared to conventional telecommunication systems, optical communication systems have the advantage of being able to transmit broadband over long distances, and due to the uniqueness of optical fibers, they are less susceptible to external disturbances.
<, it is possible to configure a transmission system with higher quality.

On the other hand, in addition to the above-mentioned high-quality transmission systems, local optical communications are rapidly becoming popular due to the advantages of optical fibers, such as their light weight, fineness, and low cost (many materials are available). Increasingly, there is a demand for a system with For example, the demands vary from systems with short transmission distances such as intra-factory communications and same-building communications, to subscriber systems and medium-distance systems branched from the main communication network. In order to support such a variety of systems, it is necessary to be able to handle differences in optical reception levels caused by differences in transmission path length.

As a countermeasure for this, the first method is to change the equipment for each system, the second method is to forcibly lower the optical reception level using an optical attenuator in the case of a short-distance transmission system, and the The third step to realize equipment that operates at
There is a method.

In order to provide inexpensive and more versatile equipment for various transmission systems, the third method is superior because it does not require changing equipment for each transmission system and does not use expensive optical attenuators, resulting in a low optical reception level. It is very effective to use equipment that operates stably (with high reception sensitivity) and operates stably even when the optical reception level is high.

This receiving sensitivity largely depends on the optical detection circuit and the subsequent amplifier circuit. Since a photodetector works as a current source in an electrical circuit, the receiving sensitivity is determined by the photodetector's load resistance, that is, the photodetector's power supply resistance (hereinafter referred to as bias resistance) and the feedback resistance of the amplifier circuit. It will be done.

Since the value of the feedback resistor of the amplifier circuit is uniquely determined by the frequency band of the transmission signal, the value of the bias resistor of the photodetector should be selected to be sufficiently larger than the value of the feedback resistor so that the load resistance value of the photodetector does not drop. I have to.

Generally, by using an automatic variable gain circuit or an image width limiting circuit that corrects fluctuations in the optical reception level, it is considered that the equipment can operate even when the optical reception level is relatively high. However, when the optical reception level is very high, for example when a semiconductor junction type photodetector is used, a large voltage drop occurs in the photodetector's piezoelectric resistance due to the photocurrent that flows when detecting the optical signal. , leading to insufficient power source voltage for the photodetector. This shortage of power supply voltage narrows the depletion layer width of the semiconductor junction type photodetector, increases the capacitance between its terminals, and deteriorates the frequency response characteristics, making it impossible to perform normal photodetection.

Since this voltage drop is proportional to the magnitude of the bias resistance of the photodetector, when the optical reception level is high, the bias resistance value can be reduced to prevent deterioration of the frequency response characteristics. However, as mentioned above, in order to obtain sufficient characteristics when the optical reception level is low, it is necessary to increase the bias resistance of the optical detector to suppress deterioration of optical reception sensitivity, and due to these contradictory characteristics, It has been difficult to realize an optical detection circuit that operates over a wide range of optical reception levels.

An object of the present invention is to provide a photodetector circuit which eliminates the above-mentioned drawbacks.The photodetector circuit of the present invention comprises a photodetector means for receiving an optical signal, a first resistor connected to the photodetector means, a first series circuit connected in series with a first power source; and a second series circuit connected in parallel with the first series circuit and consisting of a second resistor, a diode, and a second power source connected in series. It is composed of. (It is assumed that fluctuations in the N signal are compensated for by an amplifier circuit, an automatic variable gain circuit, or an image width limiting circuit after this optical detection circuit.

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

In the figure, the present embodiment includes a semiconductor junction photodetector 1 that receives an optical signal, a first bias resistor 2 connected to the detector 1 and having a resistance value RB1, and a resistor whose positive electrode side is grounded and whose negative electrode side is grounded. 2 and the power supply voltage is -Vllll.
a second resistor 4 connected to the detector l and having a resistance value B2, a diode 5 whose anode is connected to the resistor 4, and a diode 5 whose positive electrode side is grounded and whose negative electrode side is grounded.
The second power supply has a power supply voltage of -vB2 connected to the 0 cathode, 5- an amplifier circuit 8, and a feedback resistor 7 of the amplifier circuit 8.

Cocoa, -VBI <-VB2 (IVBI l >
l VBI21 ), the bias voltage VB of the photodetector is given by equation (1) when the optical reception level is low and the photocurrent is small, and when the optical reception level is high and the photocurrent is large, the bias voltage VB of the photodetector is given by (1). 2) Given by Eq.

vn =VB□+I o X RBI song・
(1) VBeVB □ 10Vf 10K ◎XRB2 °
Concave Curve (2) Here, ■o is the photocurrent %Vf is the forward voltage drop of the semiconductor 5.

That is, as shown in FIG.
When the number is smaller, VBl and RBl in equation (1) and 1
, the bias voltage VB is determined. At this time, since the second power supply voltage vB2 is higher than its bias voltage VB, a reverse voltage is applied to the diode 5, and as a result, the photodetection bias circuit under the equality constraint is the same as one configured only with ``11'' and VBI. , there is no deterioration in optical reception sensitivity due to a decrease in load resistance.

Also, when the photocurrent is large, v in equation (2)
B2 ” f ” ”B2 and l. The bias voltage is determined by This is because as the photocurrent increases, the voltage drop at RB1 increases, and the bias voltage 8 becomes higher than the second power supply voltage vB2, so a forward voltage is applied to the diode 5, causing a current to the light 1. This is because it flows out in the 6 directions of the second power source.

Specific numerical examples are shown below.

vnt=-20(V)+vB□=-5(V), %=
100 (k(1)+”52=1 (kQ) −Rn
2 = 1 (kn) Oyohi V (= 0.7 (v)) becomes lx + = = 036 (mA), and the wavelength of light is 1
．． When a 3 (μm) band photodetector is used, the second power supply 6 will operate at an average optical reception level of approximately -5 (dHm) or higher.

As described above, the present invention has the advantage of being able to provide an optical detection circuit that operates at a wide range of optical reception levels without requiring complicated adjustments or expensive optical attenuators.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing the relationship between the bias voltage VB and the optical current Io of the optical detection circuit of the embodiment. In the figure, l... semiconductor junction type photodetector, 2
...Resistor, 3...'Power supply, 4...
...Resistor, 5...Diode, 6...
...Power supply, 7...Resistor, 8...Amplification circuit. Isamu 4->

Claims (1)

[Claims] a first series circuit connected to the photodetection means and comprising a series connection of a first resistor and a first power source; and a first series circuit connected in parallel with the first series circuit. A photodetector circuit comprising a second series circuit including a second resistor, a diode, and a second power source connected in series.