JPH04367108A - Optical receiving circuit - Google Patents

Abstract

PURPOSE:To simplify the gain changeover operation by providing a comparison circuit comparing the average light receiving signal composed of the dc component of a light receiving element with the reference value given in advance. CONSTITUTION:When a large optical input comes in, the large average light receiving signal can be obtained from a light receiving signal. When this average light receiving signal exceeds the reference value given in advance, the output of a comparison circuit 5 actuates and changes a gain changeover means 4 and the gain is reduced. On the contrary, when the light input is small, the average light receiving signal is lower than the reference value so that the output of the circuit 5 actuates and changes the changeover means 4 and the gain is increased. Thus, since the gain is switched based on the light receiving signal, the gain is small by switching the gain before amplified or saturated when the light input changes from small to large input. Accordingly, the saturation of the amplifier circuit 3 can be prevented.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel optical receiver circuit, and more particularly to an optical receiver circuit whose gain can be switched in response to a received light input and whose gain can be switched without being affected by temperature.

0002

2. Description of the Related Art As shown in FIG. 2, an optical receiving circuit 30 that receives an optical signal and converts it into an electrical signal by a photodetector consisting of a photodetector is generally composed of a photodetector 31 and a photodetector 31. It is composed of an amplifier circuit 32 for amplifying the received light signal obtained from the light reception signal, and a gain switching means 33 for switching the gain of the amplifier circuit 32. Specifically, the output of a photodetector 31 such as a photodiode 34 is connected to an amplifier circuit 32 equipped with a feedback resistor 35, and the output of the amplifier circuit 32 is connected to an optical receiver circuit 30.
The output of the control circuit 36 is connected to the subsequent stage as well as to the control circuit 36, and the output of the control circuit 36 is connected to the gain switching means 33. The control circuit 36 compares a signal detected from the output voltage by peak detection or peak value detection with a preset reference value and turns the gain switching means 33 on and off. The gain switching means 33 includes a switch 37 such as an analog switch and a resistor 38 connected in series.
It is connected in parallel to the feedback resistor 35 to form a feedback path connecting the input and output of the amplifier circuit 32.

When there is a sufficiently large optical input to the photodetector 31 such as the photodiode 34, the output of the amplifier circuit 32 is large, so the control circuit 36 operates to turn on the gain switching means 33, and the amplifier circuit 32 The feedback path is a parallel resistance of feedback resistor 35 and resistor 38. That is, the feedback resistance value of the amplifier circuit 32 becomes smaller, and the gain decreases. In this way, the amplifier circuit 3
Since the gain of 2 is reduced, even if there is a sufficiently large optical input to the photodetector 31, an output signal can be taken out without the amplifier circuit 32 being saturated. Further, when the optical input to the photodetector 31 is small, the control circuit 36 operates to turn off the gain switching means 33 and cut off the resistor 38, contrary to the case where the optical input is large. The feedback path of the amplifier circuit 32 becomes only the feedback resistor 35, and the gain of the amplifier circuit 32 increases. Since the gain of the amplifier circuit 32 is increased in this way, an output signal can be extracted even when the optical input to the photodetector 31 is small.

In this manner, the optical receiving circuit 30 monitors changes in the output signal using the control circuit 36, and operates the gain switching means 33 to switch the gain of the amplifier circuit 32 based on the monitoring results.

[0005]

However, since the optical receiver circuit 30 switches the gain based on its output, when the optical input changes from a small input to a large input, the gain remains large, so the amplifier circuit 32 The problem was that it became saturated.

Furthermore, a dark current flows through the light receiving element and is superimposed on the light receiving signal. This dark current has temperature dependence,
Since the value increases when the temperature is high, it accelerates the saturation of the amplifying circuit 32 described above and causes output fluctuations when there are temperature fluctuations, thus preventing appropriate gain switching.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above-mentioned problems and to provide an optical receiver circuit that makes it possible to switch the gain based on a received light signal and also allows the gain to be switched without being affected by temperature. be.

[0008]

[Means for Solving the Problems] In order to achieve the above object, a first invention provides a photodetector comprising a light receiving element, an amplifier circuit for amplifying a signal obtained from the detector, and a gain switch for switching the gain of the amplifier circuit. an optical receiving circuit comprising means, a comparison circuit for comparing an average light reception signal consisting of a DC component of the light receiving element with a predetermined reference value, and an output of the comparison circuit being connected to a gain switching means. .

[0009] In the second invention, two or more light receiving elements of the same shape are formed on the same chip, and the light receiving surface of any one of the light receiving elements is shielded from light.

[0010] The third invention constitutes a photodetector by forming two or more light receiving elements of the same shape on the same chip and shielding the light receiving surface of any one of the light receiving elements, and blocking the light of the photodetector. It is equipped with a difference circuit that detects an average light reception signal obtained by subtracting the DC component of a light receiving element whose light receiving surface is shielded from light from the DC component of a light receiving element whose light receiving surface is shielded from light.

[0011] A fourth aspect of the invention is that a comparison means is provided for comparing the average light reception signal detected by the difference circuit with a predetermined reference value.

0012

[Operation] With the configuration of the first invention described above, a large average received light signal is obtained from the received light signal obtained when the optical input is large, and when this average received light signal exceeds a predetermined reference value, the output of the comparison circuit becomes a gain. The gain is reduced by switching the switching means. Therefore, the amplifier circuit is not saturated. Conversely, when the optical input is small, the average received light signal is small and falls below the reference value, so the output of the comparison circuit switches the gain switching means to increase the gain. Therefore, a sufficient output signal can be obtained. In addition, since the gain is switched based on the received light signal, when the optical input changes from a small input to a large input, the gain is switched and becomes smaller before it is amplified or saturated. saturation is avoided.

[0013] According to the configuration of the second aspect of the invention, in the light-receiving element whose light-receiving surface is shielded from light, even when light is incident, no light-receiving current is generated and only a dark current is generated. Further, light receiving elements formed in the same shape on the same chip have the same electrical characteristics and the same temperature characteristics. Therefore, the dark current of the light-receiving element whose light-receiving surface is not shielded from light and the dark current of the light-receiving element whose light-receiving surface is shielded from light are approximately equal and undergo approximately equal temperature changes.

According to the configuration of the third aspect of the present invention, even if light is incident on the light receiving element whose light receiving surface is shielded from light, no light receiving current is generated and only a dark current is generated. On the other hand, when a light-receiving element of a photodetector has an unshielded light-receiving surface, a light-receiving current and a dark current are superimposed and generated when light is incident. At this time, both dark currents are approximately equal. The differential circuit can detect the average light reception signal obtained by subtracting the DC component of the light receiving element with the light receiving surface of the photodetector from the DC component of the light receiving element with the light receiving surface of the photodetector with no light shielding. . Therefore, this average light reception signal is approximately equal to the average light reception signal of the pure light reception current of a light receiving element having an unblocked light reception surface of the photodetector.

According to the configuration of the fourth invention, the comparison means compares the average light reception signal detected by the difference circuit with a predetermined reference value, and there is no influence of dark current.
Therefore, it operates based only on the pure light-receiving current of the light-receiving element without being affected by temperature changes.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the accompanying drawings.

As shown in FIG. 1, the optical receiving circuit 1 mainly includes a photodetector 2, an amplifier circuit 3, gain switching means 4, and a comparison circuit 5.

The photodetector 2 is a photodetector 2 according to the second aspect of the present invention, and in this embodiment, a photodetector 6 having an unshielded light-receiving surface and a light-receiving surface having a shielded light-receiving surface are used. Each light receiving element 7 is provided with one light receiving element 7. These light receiving elements 6 and 7 are formed in the same shape on the same chip. In this embodiment, the light receiving elements 6 and 7 are constituted by photodiodes 8 and 9. Photodiode 8
, 9 have their outputs connected to the stage before the amplifier circuit 3 and to the comparison circuit 5 under a common bias Vcc.

The amplifier circuit 3 includes an input capacitor 10, an amplifier 11, and a fixed feedback resistor 12. The input of the amplifier 11 is AC connected to the photodiode 8 via the input capacitor 10. The input and output of the amplifier 11 are connected by a fixed feedback resistor 12. The output of the amplifier 11 is also the output of the optical receiving circuit 1. That is, the configuration is such that the alternating current component of the current Ir of the photodiode 8 is amplified and obtained as the output of the optical receiver circuit 1.

The gain switching means 4 is constructed by connecting a resistor 14 in series to a switching element 13 such as an analog switch, and is connected in parallel to the fixed feedback resistor 12 of the amplifier circuit 3 to control the feedback of the amplifier circuit 3. forming a road. When the switch element 13 is on, the feedback resistance of the amplifier circuit 3 is a parallel resistance of the fixed feedback resistor 12 and the resistor 14, and when the switch element 13 is off, the feedback resistance of the amplifier circuit 3 is a parallel resistance of the fixed feedback resistor 12 and the resistor 14. It is configured to include only a fixed feedback resistor 12.

The comparator circuit 5 has a signal input 15 and a reference input 16 as inputs, and the signal input 15 is connected to a photodiode 8 which is a light receiving element 6 having an unblocked light receiving surface of the photodetector 2. , a reference input 16 is connected to a photodiode 9, which is a light-receiving element 7 of the photodetector 2 having a light-shielded light-receiving surface. Signal input 15 is connected to the input of amplifier 20 via an RC circuit formed by resistors 17, 18 and capacitor 19. The output of the amplifier 20 has a voltage V corresponding to the DC component of the current Ir of the photodiode 8.
r appears. Similarly, reference input 16 is connected to the input of amplifier 24 via an RC circuit constituted by resistors 21, 22 and capacitor 23. At the output of the amplifier 24, a voltage Vb corresponding to the DC component of the current Ib of the photodiode 9 appears. These two RC circuits and amplifier 20,
24 are circuits each having equivalent characteristics. amplifier 2
At the subsequent stage of 0 and 24, there is provided an arithmetic unit 25 which has two inputs and outputs a voltage corresponding to the difference between them, and the output voltage Vd of the arithmetic unit 25 changes from the DC component of the current Ir to the DC component of the current Ib. The difference circuit 26 is configured to produce an average light reception signal that corresponds to a current obtained by subtracting the current, that is, a pure light reception current of a light reception element having an unshielded light reception surface. Arithmetic unit 25
The output of is connected to one input of comparator 27. A comparison reference voltage Vt is given to the other input of the comparator 27 as a reference value given in advance, and a comparison means 28 is configured to compare the difference signal detected by the difference circuit 26 with the reference value given in advance. has been done. The output of the comparator 27 constituting the comparison means 28 becomes the output of the comparison circuit 5.

The output of the comparison circuit 5 is connected to gain switching means 4. Specifically, the output of the comparator 27 is connected to a driving terminal of a switch element 13 such as an analog switch forming the gain switching means 4, and the switch element 13 is turned on and off by turning on and off the comparator 27. has been done.

Next, the operation of the embodiment will be described.

When there is a sufficiently large optical input to the photodetector 2,
A current Ir flows through the photodiode 8, which is the light-receiving element 6, which has an unblocked light-receiving surface, and a current I, which flows through the photodiode 9, which is the light-receiving element 7, which has a light-receiving surface, which is shielded.
b is the DC component extracted by the RC circuit,
The amplifiers 20 and 24 produce voltages Vr and Vb. Vb is subtracted from the voltage Vr by the difference circuit 26, and the light receiving element 6
A difference Vd corresponding to the average received light signal consisting of the DC component is obtained. Since the difference Vd corresponding to this average light reception signal is sufficiently large compared to the reference value Vt, the output of the comparison circuit 5 is turned on. When the output of the comparator circuit 5 is turned on, the switch element 13 is turned on and conductive, and the amplifier circuit 3
The feedback resistance becomes a parallel resistance of the fixed feedback resistance 12 and the resistance 14. That is, the gain switching means 4 operates and the gain becomes smaller. On the other hand, the input capacitor 1 is connected to the amplifier circuit 3.
Although a sufficiently large alternating current component of the current Ir is inputted through the amplifier circuit 3, the amplifier circuit 3 is not saturated because the gain is small as described above.

Next, when there is a small optical input to the photodetector 2, the voltage Vd corresponding to the difference between the current Ir and the current Ib is small compared to the reference value Vt, so the output of the comparator circuit 5 is turned off. . When the output of the comparator circuit 5 is turned off, the switch element 13 is turned off and cut off, and the feedback resistance of the amplifier circuit 3 becomes only the fixed feedback resistance 12. That is, the gain switching means 4 works to increase the gain. On the other hand, a small alternating current component of current Ir is input to the amplifier circuit 3 via the input capacitor 10, but since the gain is large as described above, the amplifier circuit 3 can amplify this and obtain a sufficient output signal.

Furthermore, when the optical input changes from a small input to a large input, the gain is switched and reduced before it is amplified or saturated, so that saturation of the amplifier circuit 3 can be avoided.

In the light-receiving element 7 whose light-receiving surface of the photodetector 2 is shielded from light, even when light is incident, no light-receiving current is generated and only a dark current is generated. Further, the light receiving elements 6 and 7 formed in the same shape on the same chip have the same electrical characteristics and the same temperature characteristics. Therefore, the dark current of the light-receiving element 6 whose light-receiving surface is not shielded from light and the dark current of the light-receiving element 7 whose light-receiving surface is shielded from light are approximately equal and undergo approximately equal temperature changes.

In the light receiving element 7 whose light receiving surface of the photodetector 2 is shielded from light, no light receiving current is generated even when light is incident, and only a dark current Ib is generated. On the other hand, when light is incident on the light receiving element 6 of the photodetector, which has a light receiving surface that is not blocked, the light receiving current and the dark current are superimposed to generate a current Ir. At this time, both dark currents are approximately equal. The difference circuit 26 detects an average light reception signal obtained by subtracting the DC component of the light receiving element 7 having a shielded light receiving surface of the photodetector from the DC component of the light receiving element 6 having an unshielded light receiving surface of the photodetector. be able to. Therefore, this average light reception signal is approximately equal to the average light reception signal of the pure light reception current of the light receiving element 6 of the photodetector 2 having an unblocked light reception surface.

The comparison means 28 compares the average light reception signal detected by the difference circuit 26 with a predetermined reference value, and the purity of the light reception element 6 is determined without being affected by dark current and therefore unaffected by temperature changes. It operates based only on the light-receiving current.

In this embodiment, the photodetector 2 is formed as a pair of light receiving elements 6 and 7, but it goes without saying that a large number of light receiving elements may be formed as required.

[0031]

[Effects of the Invention] The present invention exhibits the following excellent effects.

(1) Since the gain is switched based on the received light signal, the gain can be switched quickly in response to changes in the received light signal.

(2) By including a light-shielded light-receiving element in the photodetector, dark current components can be canceled out by a simple circuit.

(3) Since the dark current component is canceled by the differential circuit, temperature dependence is eliminated, and gain switching corresponding to pure optical input becomes possible.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an optical receiving circuit showing an embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional optical receiving circuit.

[Explanation of symbols]

1 Optical receiver circuit 2 Photodetector 3 Amplification circuit 4 Gain switching means 5 Comparison circuit 6, 7 Photo-receiving element 26 Differential circuit 28 Comparison means

Claims (4)

[Claims] Claim 1: In an optical receiving circuit comprising a photodetector including a photodetector, an amplifier circuit for amplifying a signal obtained from the detector, and gain switching means for switching the gain of the amplifier circuit, the DC component of the photodetector is 1. An optical receiving circuit comprising: a comparison circuit for comparing an average received light signal with a predetermined reference value; and an output of the comparison circuit is connected to gain switching means. 2. A photodetector characterized in that two or more light-receiving elements of the same shape are formed on the same chip, and the light-receiving surface of any one of the light-receiving elements is shielded from light. 3. A photodetector is constructed by forming two or more light-receiving elements of the same shape on the same chip and shielding the light-receiving surface of any one of the light-receiving elements; An optical receiving circuit characterized by having a difference circuit that detects an average light reception signal obtained by subtracting a DC component of a light receiving element having a shielded light receiving surface of the photodetector from a DC component of a light receiving element having a light receiving surface. 4. The optical receiving circuit according to claim 3, further comprising comparison means for comparing the average light reception signal detected by the difference circuit with a predetermined reference value.