JPH0738139A - Optical receiver - Google Patents

Abstract

PURPOSE:To provide an optical receiver which is simple in circuit structure and capable of amplifying a photocurrent well without distorting it. CONSTITUTION:A first photodetector 2 and a second photodetector 3 both generating a photocurrent are provided in an optical receiver, a second photocurrent i2 outputted from the second photodetector 3 is compared with a reference current iS through a current comparison amplifier, and when it is found that the second photocurrent i2 is smaller than the reference current iS, an output current is set to zero, and when the second photocurrent i2 is larger than the reference current iS, the second photocurrent i2 is amplified as much as a current gain K and then outputted. In a differential amplifier 6, a difference between a first photocurrent i1 outputted from the first photodetector 2 and an output current from the current comparison amplifier is amplified and outputted. Therefore, when a photocurrent is small, only the first photocurrent is amplified, and when a photocurrent becomes large, a difference between two photocurrents is amplified, so that an excessive current large enough to make a differential amplifier saturated is prevented from flowing into it.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical receiver used in an optical space signal transmission device for wirelessly transmitting light modulated by a voice signal, a video signal, etc. by infrared rays.

[0002]

2. Description of the Related Art Generally, in an infrared space transmission device, the intensity of a received light signal fluctuates greatly depending on the condition of the space in which infrared rays are transmitted and the distance, and the fluctuation range reaches several tens of dB. As shown in FIG. 4, a photodetector of an optical receiver that detects this light is a very small amount (nW) of the incident light to the order of 10,000 to 100,000 times that of the incident light (nW) as shown in FIG. mW), and outputs a photocurrent proportional to the input light amount over 80 to 100 dB in decibel conversion. Note that FIG. 4 is a diagram showing the relationship between the incident light amount and the photocurrent in a logarithmic log graph.

However, since the above-mentioned ordinary amplifier for amplifying the photocurrent cannot amplify the photocurrent changing over a wide range as described above with a good S / N ratio, the light level is generally large to some extent. At this stage, the current level of the output or the voltage level obtained by current / voltage conversion is detected and automatic gain adjustment (AGC) is applied to the amplifier so that the amplifier does not saturate and output a distortion current. .

Examples of automatic gain control devices of this type include Japanese Patent Publication No. 62-34169 and Japanese Patent Laid-Open No. 3671/1992.
As disclosed in Japanese Patent Publication No. 09-09, a current bypass using a non-linear resistance element is used to prevent an excessive current of a light receiving element from saturating an amplifier, a FET using a current adjusting FET element, and Japanese Patent Laid-Open Publication No. Hei 5 (1998) -58. As disclosed in Japanese Patent No. 114887, a variable attenuator is provided in front of a preamplifier to appropriately attenuate a photocurrent to limit an inflow current;
As shown in JP-A-10345, JP-A-5-91051 and JP-A-3-2666577, the gain of the amplifier is automatically adjusted according to the amount of input light by cutting the AGC for an excessive photocurrent. It is known to do things.

[0005]

However, in any of the above-described conventional configurations, after amplifying the input photocurrent, the current level is detected to determine whether the input light amount is appropriate, and the determination result. Is fed back to the input stage as a control signal to adjust the bypass current and the amplification factor of the first stage amplifier, so various elements other than the signal amplifier element are required for this signal detection and control, and the cost is high. There was a problem of becoming.

Further, since a rectifying circuit is generally provided for detecting the signal level, when the signal level becomes small, not only a problem occurs in the linearity of the control signal and the S / N ratio, but also the time constant of the rectifying circuit causes However, there are problems such as a time delay in control and a case where the amplifier cannot operate at its best operating point. The present invention has been made in view of the above problems and was devised in order to effectively solve them, and an object thereof is to enable good amplification without distortion of photocurrent with a simple circuit configuration. It is to provide an optical receiver.

[0007]

In order to solve the above problems, the present invention provides first and second light receiving elements which receive light and generate a photocurrent, and a second light receiving element of the second light receiving element. And a reference current, and a current comparison amplifier that amplifies the second photocurrent with a predetermined current gain when the second photocurrent exceeds the reference current; a first photocurrent of the first light receiving element; The differential amplifier for amplifying the difference current from the output current from the current comparison amplifier is provided.

[0008]

Since the present invention is configured as described above, the second photocurrent from the second light receiving element is compared with the reference current preset by the current comparison amplifier, and the second photocurrent is compared with the reference current. The output current is zero when the output current is lower than the reference current, but is amplified and output with a predetermined current gain when the output current is higher than the reference current. The difference between this output current and the first photocurrent from the first light receiving element is taken by the differential amplifier, and the value is amplified. Therefore, in the range where the photocurrent is small, only the photocurrent from one light emitting element is amplified, and when the photocurrent becomes large, the difference between the photocurrent from one light receiving element and the amplified photocurrent from the other light receiving element. Since the current is amplified, an excessive current does not flow into the differential amplifier, and distortion-free amplification is possible.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an optical receiver according to the present invention will be described in detail below with reference to the accompanying drawings. 1 is a circuit configuration diagram showing an example of an optical receiver according to the present invention, FIG. 2 is a graph showing an output current of a current comparison amplifier, and FIG. 3 is a graph showing a photocurrent amplified by a differential amplifier.

As shown in the figure, the optical receiver 1 is a first receiver such as a photodiode which receives an infrared ray and causes a photocurrent to flow.
The light emitting element 2 and the second light receiving element 3 are used, and these two light receiving elements are the elements having the same characteristics in this embodiment, and are arranged in parallel so as to receive the same light. The output of the second light receiving element 3 is connected to one input terminal of the current comparison amplifier 4 having a predetermined current gain, for example K.
A preset reference current, for example, 1 is applied to the other input terminal.
A reference current source 5 that flows 0 μA is connected. And
The current comparison amplifier 4 has a current comparison function, compares the second photocurrent i2 from the second light receiving element 3 with the reference current is, and outputs no output when the reference current is is large. However, when the second photocurrent i2 becomes larger than the reference current is, the value obtained by adding the current gain K is output as the output current i3 (= K · i2). There is. In this embodiment, the current gain K
Is determined within the range of 0 <K <1, and a current having a value smaller than the input current is output.

The output of the current comparison amplifier 4 is connected to the-terminal of the differential amplifier 6 having a current amplification degree of A, while the output of the first light receiving element 2 is connected to the + terminal. Therefore, the first photocurrent i1 from the first light receiving element 2 is input to the + terminal. Therefore, in the differential amplifier 6, the difference between the first photocurrent i1 and the output current i3 from the current comparison amplifier 4 is taken and this difference current is amplified A times (A> 1). There is.

The output of the differential amplifier 6 is connected to the normal processing circuit 7 in the subsequent stage. still,
In this embodiment, the first and second light receiving elements 2 and 3 are
Since the elements having the same characteristics are used, the first and second photocurrents i1 and i2 have substantially the same value.

Next, the operation of this embodiment configured as described above will be described. First, for example, it is assumed that the maximum input current at which the differential amplifier 6 does not saturate is set to 100 μA and the reference current is is set to 10 μA. Also, the first and second
As the light receiving elements 2 and 3, elements having a relationship between incident light and photocurrent as shown by a straight line X1 in FIG. 3 are used. A range where the amount of incident light is very small, for example, 10 lux (Lu
x) to 10 ³ lux, the value of the second photocurrent i2 from the second light receiving element 3 is the value of the reference current is,
Since it is smaller than 10 μA, the output current i3 of the current comparison amplifier 4 becomes zero.

Therefore, in the differential amplifier 6, this +
A value obtained by amplifying only the first photocurrent i1 from the first light receiving element 2 input to the terminal by A times, the output current i4 of A · i1
Will be output to the processing circuit 7. That is, the current value defined by the straight line X1 is amplified by the differential amplifier 6 when the amount of incident light is 10 to 10 ³ lux. On the other hand, when the incident light becomes stronger and the amount of incident light exceeds 10 ³ lux, the second photocurrent i2 of the second light receiving element 3 becomes larger than the reference current is (10 μA). The amplifier 4 has a value i obtained by multiplying the second photocurrent by K.
The output current i3 of 2 · K is output.

As a result, in the differential amplifier 6, the first
The difference current obtained by subtracting the previous output current i3 (= i2 · K) from the photocurrent i1 is amplified, and the value of the output current i4 becomes A · (i1−i2 · K). As a result, as shown in FIG. 3, in the range of the incident light whose photocurrent exceeds 10 μA is 10 ³ lux or more, the characteristics shown by the broken line X2 apparently change, and the difference current (i1-i2
・ K) will be amplified. As described above, when the photocurrent increases to some extent or more, the characteristics are shifted to the right in FIG. 3 to suppress the increase of the photocurrent, and as a result, even when the incident light amount is significantly increased. It is possible to suppress the generation of distortion output without the differential amplifier 6 being saturated. In this way, the automatic gain adjustment circuit used in the conventional device can be dispensed with, and the simple circuit configuration without using it causes the amplifier to saturate and the distortion even when the incident light amount is greatly increased. It can be eliminated.

Therefore, since the structure can be simplified as compared with the conventional optical receiver, not only a great cost reduction can be achieved but also an IC can be easily formed. Further, since the automatic gain adjustment circuit can be dispensed with, the time delay of the gain adjustment due to this time constant can be eliminated, and quick control can be performed. With the above-described configuration, a wide range, for example, 8
It is possible to adjust the gain over a range of 0 dB or more.

In the above embodiment, the first and second
The case where the light receiving elements 2 and 3 are separately provided has been described, but if these are arranged side by side on the same substrate, for example, the dark current characteristics of the elements depending on the temperature are uniform, so that the adverse effect due to the temperature is not affected. It is possible to reduce the amount, and it is possible to obtain better characteristics. Further, although the current gain K of the current comparison amplifier 4 is set to be smaller than 1 in the above embodiment, the present invention is not limited to this. For example, in the above embodiment, the first and second light receiving elements 2 and 3 are provided. Since the same light receiving area is used, the multi-divided light receiving element is used. For example, when the light receiving area of the second light receiving element is smaller than the light receiving area of the first light receiving element. Sets the current gain K to a value greater than 1.

[0018]

As described above, according to the optical receiver of the present invention, the following excellent operational effects can be exhibited. If the photocurrent is increased to some extent by using two light receiving elements, the difference current between them is amplified.
Amplification can be performed over a wide range without causing distortion with a simple circuit configuration without using an automatic gain adjustment circuit that has been used conventionally. Further, since various elements forming the automatic gain adjustment circuit are not required, not only the cost can be significantly reduced but also the control delay due to the time constant of the circuit element can be eliminated.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an example of an optical receiver according to the present invention.

FIG. 2 is a graph showing an output current of a current comparison amplifier.

FIG. 3 is a graph showing a photocurrent amplified by a differential amplifier.

FIG. 4 is a graph showing general characteristics of a light receiving element.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical receiver, 2 ... 1st light receiving element, 3 ... 2nd light receiving element, 4 ... Current comparison amplifier, 5 ... Reference current source, 6 ... Differential amplifier, i1 ... 1st photocurrent, i2 ... Second photocurrent, i
3, i4 ... Output current, is ... Reference current.

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Claims (1)

[Claims] 1. The first and second light receiving elements that receive light to generate a photocurrent, and a second photocurrent of the second light receiving element and a reference current are compared to determine a second photocurrent. A current comparison amplifier that amplifies with a predetermined current gain when the reference current is exceeded,
An optical receiver comprising a differential amplifier for amplifying a difference current between a first photocurrent of the first light receiving element and an output current from the current comparison amplifier.