JPH10223922A - Photodetective circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photodetective circuit for generating a reproducing signal with no pulse distortion, even when an optical signal with high power is received. SOLUTION: An optical signal received is changed into a current signal through a photo-diode 11 and changed into a voltage signal through a pre-amplifier 12. When an optical signal with high power is received, a bypass transistor 131 is put in continuity, and part of current signal from the photo-diode 11 is bypassed. At this time, a voltage level divided by resistors 171 and 172 is shifted lower from an amplitude center of the output of the pre-amplifier 12. By the shifting into a lower-side direction, a pulse width distortion is canceled. As the bypass current is increased, the shifting amount is proportionally increased. As a result, an output from a comparator 18 has no pulse width distortion in a wide range of light receiving power level.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical receiving circuit, and more particularly, to an optical receiving circuit that receives a pulse-modulated optical signal, converts the signal into an electric signal, and amplifies and reproduces the signal.

[0002]

2. Description of the Related Art FIG. 5 is a block diagram showing a configuration of an optical receiver disclosed in Japanese Patent Publication No. 7-107943. FIG. 6 is a diagram showing voltage levels of respective parts of the optical receiver shown in FIG. Hereinafter, a conventional optical receiver will be described with reference to FIGS.

In FIG. 5, a conventional optical receiver includes a photodiode 51, a current-to-voltage converter 52, and a voltage source 53.
, A peak detector 54, a comparator 55, and a resistor 5
6 and 57. Photodiode 51
Converts the received optical signal into an electric signal. The current-voltage converter 52 converts a current signal into a voltage signal. Voltage source 5
3 generates the same voltage as the output voltage of the current-to-voltage converter when there is no input. The peak detection unit 54 includes a current-voltage conversion unit 52
Of the output signal (in this case, the minimum value) is detected and held. The resistors 56 and 57 have the same resistance value and are prepared to output an intermediate value between the output of the peak detector 54 and the output voltage of the voltage source 53.

[0006] In FIG. 6, V 50 denotes a current-to-voltage converter 5.
2, V51 is the output of the peak detector 54, V52 is the output of the voltage source 53, and V53 is the voltage level generated by the resistors 56 and 57 and applied to the reference input of the comparator 55.

[0005] The pulse-modulated received optical signal is converted into a current signal by a photodiode 51. The converted current signal is converted into a voltage signal (V5
0). The converted voltage signal has a minimum peak value (V51) of the pulse signal as a peak detector 5.
4 is detected and held.

On the other hand, the voltage source 53 is
The output (V52) is the same as the output level of the current-to-voltage converter 52 when no optical signal is received, since the circuit configuration is the same as that of FIG.

The voltage level V53 generated by the voltage dividing circuit of the resistors 56 and 57 indicates an intermediate value between the output V51 of the peak detecting section 54 and the output V52 of the current / voltage converting section 53. That is, the voltage level V53 is set to an intermediate value of the output signal V50 of the current / voltage converter 52. Then, in the comparator 55, the output signal V50 is discriminated using the voltage level V53 as a threshold, and a pulse signal is reproduced.

In the above-mentioned conventional optical receiver, the maximum light receiving level is determined by the maximum value of the output amplitude range of the circuit in the current-voltage converter 52. Further, the minimum light receiving level is determined according to the noise level in the current-voltage converter 52. That is, this optical receiver can reproduce an optical signal having a light receiving level between the maximum light receiving level and the minimum light receiving level.

However, in recent years, application to a system with a longer transmission distance or application to a system with a large number of branches has been studied, and it is required to further expand the dynamic range as an optical receiver.

In view of this, as an attempt to extend the dynamic range of a receivable optical signal, the present applicant has previously proposed a new optical receiver (Japanese Patent Application No. 08-281964;
(Not disclosed at the time of filing of the present application). FIG. 7 is a block diagram showing the configuration of the optical receiver of the optical receiver proposed earlier. FIG. 8 is a waveform diagram showing an output waveform of the optical receiver shown in FIG.

In FIG. 7, the optical receiver in the optical receiver proposed by the present applicant includes a photodiode 61, a current-voltage converter 62, and a current source 63 for bypass.

When the signal power of the received optical signal is small, the bypass current source 63 does not operate, and its current value becomes 0.
It is. Therefore, all the current signals converted by the photodiode 61 are supplied to the current-voltage converter 62, and an output voltage signal is obtained. Output voltage signal waveform 6 in FIG.
01 represents the output in this case.

Next, a case where the signal power of the received optical signal is large will be described. At the beginning of the fall of the pulse signal, the current source 63 does not operate and the photodiode 61
All the current signals converted by are supplied to the current-to-voltage converter 62 and are extracted as voltage signals. When the amplitude value exceeds a certain value as an output from the current-voltage conversion unit 62 in accordance with the falling of the pulse signal, the transistor constituting the current source 63 starts conducting, and the current signal from the photodiode 61 starts. Is bypassed to the current source 63. As a pulse signal, a part of the current signal is bypassed when the output voltage signal from the current-voltage converter 62 exceeds a certain value, so that a signal having a waveform with a limited end of the pulse signal is obtained. Become. The output voltage signal waveform 602 represents the output in this case. As the received signal light level increases and the amount of bypassed signal current increases, the pulse width distortion at the center of the output pulse increases.

In this way, it is possible to receive even when the received signal has a large power, and the receivable dynamic range can be expanded. However, when a part of the current is bypassed by the current source 63, However, the waveform of the output signal from the current-voltage converter 62 has a large pulse width distortion at the center of its amplitude.

[0015]

The optical receiver having the structure shown in FIG. 7 is replaced with an optical receiver comprising a photodiode 51 and a current-voltage converter 52 shown in FIG.
When realized as an optical receiver having a wide dynamic range, there is no problem if the signal power level of the received optical signal is small, but the signal power level increases and the tip of the pulse signal is limited from the current-voltage converter 62. When the output waveform is output, a large pulse width distortion appears at the center of the amplitude. Therefore, there is a problem that the output signal of the comparator 55 becomes a signal having large pulse width distortion. This means that the duty ratio of the output signal of the comparator 55 fluctuates, and an accurate clock cannot be reproduced by a PLL (phase locked loop) (not shown) at the subsequent stage. As a result, erroneous information may be reproduced.

Therefore, an object of the present invention is to provide an optical receiving circuit capable of obtaining a reproduced signal without pulse distortion even when a high-power optical signal is received.

[0017]

A first aspect of the present invention is an optical receiving circuit for receiving and reproducing a pulse-modulated optical signal, comprising: a photodiode for converting the optical signal into a current signal; A current-voltage conversion circuit for converting a current signal extracted by the photodiode into a voltage signal, and a current source for bypassing a part of the current signal extracted from the photodiode when an optical signal having a predetermined power or more is received. A reference control unit whose output level is controlled in accordance with a current value bypassed by the current source, receiving an output of the current-voltage conversion circuit as a signal input, receiving an output of the reference control unit as a reference input, and A comparator that discriminates the signal input using the input as a threshold. According to the first aspect, the output level of the reference control unit changes according to the value of the current bypassed by the current source and, accordingly, the power value of the received optical signal, so that the received signal is a high power optical signal. If
The reference input level of the comparator, that is, the threshold level automatically changes to an appropriate value, and a reproduced signal without pulse width distortion can be obtained.

In a second aspect based on the first aspect, the reference control section outputs the center value of the output amplitude of the current-voltage conversion section when the value of the current bypassed by the current source is zero. If the current value bypassed exceeds 0, a value that changes in a direction approaching the extreme value of the output signal from the amplitude center of the output signal of the current-voltage converter is output in proportion to the bypassed current value. It is characterized by the following.

According to the second aspect, the reference control unit shifts from the center value of the output amplitude of the current-voltage conversion unit only when the current value bypassed by the current source exceeds 0. Even when a signal is received, a reproduced signal without pulse width distortion can be obtained.

A third invention is characterized in that, in the second invention, the output of the reference control unit does not exceed the range of the amplitude value of the output signal of the current-voltage conversion circuit.

In a fourth aspect based on the third aspect, the current-voltage conversion circuit is constituted by a transimpedance type preamplifier, and the reference control section detects and holds the minimum value of the output of the preamplifier. A value holding circuit, a voltage source for generating an output voltage of the same level as the output level of the preamplifier when there is no input, a current mirror circuit for extracting a current having a current value equivalent to the current bypassed by the current source, A voltage drop resistor connected between the source and the current mirror circuit, a second minimum value holding circuit for detecting and holding the lowest value of the terminal voltage of the voltage drop resistor, and first and second minimum values A voltage dividing circuit for extracting an intermediate output voltage of the holding circuit.

[0022]

FIG. 1 is a block diagram showing a configuration of an optical receiving circuit according to an embodiment of the present invention. In FIG. 1, an optical receiving circuit according to the present embodiment includes a photodiode 11 for converting an optical signal into a current signal, a transimpedance preamplifier 12 for performing current-voltage conversion, and a part when a high-power optical signal is received. A current bypass unit 13 for bypassing a current; a voltage source 14 for generating a voltage having the same level as the output level of the preamplifier 12 when there is no input; a voltage drop resistor 15; and minimum value holding circuits 161 and 162
And the resistors 171 and 172 for obtaining an intermediate value by resistance division, the output of the preamplifier 12 as a signal input, the divided output obtained from the resistors 171 and 172 as a reference input, and the reference input as a threshold. And a comparator 18 for discriminating the signal input.
The current bypass unit 13 includes a transistor 131 for determining a bypass current and transistors 132 and 1 forming a current mirror for extracting a current value to be bypassed.
33.

FIG. 2 shows waveforms of current and voltage of each part when the optical receiving circuit shown in FIG. 1 receives a small power signal. In FIG. 2, a waveform 20 is a current waveform extracted by the current bypass unit 13, a waveform 21 is an output signal waveform of the preamplifier 12, a waveform 22 is an output signal of the minimum value holding circuit 162, and a waveform 24 is a minimum value holding circuit 161. , The waveform 23 indicates the output signal divided by the resistors 171 and 172, and the waveform 25 indicates the output signal of the comparator 18.

FIG. 3 shows the current and voltage waveforms of each part when the optical receiving circuit shown in FIG. 1 receives a large power signal. In FIG. 3, a waveform 30 is a current waveform extracted by the current bypass unit 13, a waveform 31 is an output signal waveform of the preamplifier 12, a waveform 32 is an output of the minimum value holding circuit 162, and a waveform 320 is a signal of the minimum value holding circuit 162. A waveform 34 indicates an output signal of the minimum value holding circuit 161, a waveform 33 indicates an output signal divided by the resistors 171 and 172, and a waveform 35 indicates an output signal of the comparator 18.

FIG. 4 is a diagram showing an operating characteristic curve of the transistor 133. In FIG. 4, the horizontal axis represents the drain-source voltage of the transistor 133, the vertical axis represents the drain current, and the operating characteristic curve is described using the gate-source voltage as a parameter. Further, in FIG.
5 are overlapped.

Hereinafter, referring to FIGS. 2 to 4, FIG.
The operation of the optical receiving circuit shown in FIG. First, a case where a low-power optical signal is received will be described. The current signal converted by the photodiode 11 flows into the preamplifier 12, and is converted into a voltage signal. Since the preamplifier 12 is configured by a negative feedback circuit, when an optical signal is input, the voltage waveform appears as a downwardly convex waveform from the level when there is no input.

In this case, the amplitude of the voltage signal generated by the preamplifier 12 is relatively small, and the current bypass transistor 131 is not turned on.
Are supplied to the preamplifier 12.
Therefore, the output signal waveform 21 of the preamplifier 12 has no pulse width distortion at the center of the amplitude.

The voltage source 14 generates an output potential level equal to the output of the preamplifier 12 when no optical signal is input. Since there is no bypass current flowing through the transistor 133, the current taken out by the current mirror is 0, and the potential difference between both ends of the potential dropping resistor 15 is also 0. Therefore, the input of the minimum value holding circuit 162 becomes the same as the output level of the voltage source 14, and the output of the minimum value holding circuit 162 becomes
As shown as a waveform 22, the level is the same as the level at the upper end of the output signal waveform of the preamplifier 12.

On the other hand, the output waveform 21 from the preamplifier 12
The input of the minimum value holding circuit 161 having the input
, The lowest level of the signal amplitude.
Therefore, as the potential level divided by the resistors 171 and 172, an intermediate value of the output signal amplitude of the preamplifier 12 is obtained as shown by the waveform 23. Therefore, if threshold value discrimination is performed by the comparator 18 using the level of the intermediate value as a threshold value, an output (waveform 25) having substantially no pulse width distortion is obtained from the comparator 18.

Next, a case where a high power optical signal is received will be described. The current signal converted by the photodiode 11 flows into the preamplifier 12, and is converted into a voltage signal.

In this case, the amplitude of the voltage signal generated by the preamplifier 12 becomes large, and when the voltage signal exceeds a certain threshold value, the current bypass transistor 131 is turned on. As a result, a part of the current signal from the photodiode 11 is bypassed, and the remaining current signal is
Supplied to The waveform of the bypassed current at this time is shown as a waveform 30. By partially bypassing the signal current, the waveform 31 of the output signal of the preamplifier 12 is distorted.

Here, the amount of bypassed current is set as ib. Since a current mirror is formed by the transistors 132 and 133, the same amount of current as the bypassed current flowing through the transistor 131 flows through the transistor 133. For this reason, the current ib
Flows. Assuming that the resistance value of the resistor 15 is R, the potential difference between both ends of the resistor 15 is ib · R. Therefore, the input signal waveform of the minimum value holding circuit 162 becomes as shown by the waveform 320, and the output of the minimum value holding circuit 162 becomes a level lower than the level at the time of no input by ib · R as shown as a waveform 32. .

On the other hand, the output from the preamplifier 12 (waveform 3
The output of the minimum value holding circuit 161 having 1) as an input becomes the lowest level of the output signal amplitude of the preamplifier 12 as shown as a waveform 34. Therefore, as the voltage level divided by the resistors 171 and 172, an intermediate value between the output levels 32 and 34 of the minimum value holding circuits 161 and 162 is obtained as shown in a waveform 33. The level of this waveform 33 is the output of the preamplifier 12 (waveform 31).
Is lower than the amplitude center by (ib · R) / 2.

As described above, the voltage level of the waveform 33 is
The output of the pudding amplifier 12 is shifted downward from the center of the amplitude, and this shift is a direction in which the pulse width distortion is canceled. Further, as the bypass current increases, the shift amount can be increased in proportion. As a result, an output (waveform 35) with almost no pulse width distortion is obtained from the comparator 18 over a wide range of the received optical power level.

With the above-described configuration, as the bypass current increases, the reference input level of the comparator 18 falls relatively below the center with respect to the output amplitude of the preamplifier 12.

The drain current of the transistor 133 is represented by Ib
And the drain-source voltage is V _DSAnd the resistance 15
When the resistance value is R and the output voltage of the voltage source 14 is Vt
In this case, the following equation (1) holds. V_DS+ Ib · R = Vt (1)

In the graph shown in FIG. 4, where the drain-source voltage of the transistor 133 is plotted on the abscissa and the drain current is plotted on the ordinate, a straight line represented by the above equation (1) is superimposed. When a current value that is biased when an optical signal of power flows is received, the gate voltage of the transistor 133 forming the current mirror is set to V2
In this case, the drain-source voltage at the intersection with the straight line represented by the above equation (1) is VL. This potential level VL becomes the output voltage level of the minimum value holding circuit 162. By determining the parameters of the transistor 133 so that the potential level VL does not fall below the lowest level of the output amplitude value of the preamplifier 12, it is possible to reliably reproduce the pulse signal up to the maximum light receiving level.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an optical receiver according to an embodiment of the present invention.

FIG. 2 is a diagram showing current and voltage waveforms of each unit when the optical receiving circuit shown in FIG. 1 receives a small power signal.

FIG. 3 is a diagram showing current and voltage waveforms of each unit when the optical receiving circuit shown in FIG. 1 receives a large power signal.

FIG. 4 is a diagram showing an operation characteristic curve of a transistor 133 in FIG.

FIG. 5 is a block diagram showing a configuration of a conventional optical receiver.

FIG. 6 is a diagram showing voltage levels of respective parts of the optical receiver shown in FIG.

FIG. 7 is a block diagram showing a configuration of an optical receiver of an optical receiver proposed by the present applicant.

8 is a waveform chart showing an output waveform of the optical receiver shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Photodiode 12 ... Transimpedance type preamplifier 13 ... Current bypass part 14 ... Voltage source 15,171,172 ... Resistance 161,162 ... Minimum value holding circuit 18 ... Comparator

Claims (4)

[Claims] 1. An optical receiving circuit for receiving and reproducing a pulse-modulated optical signal, comprising: a photodiode for converting the optical signal into a current signal; and a current signal extracted by the photodiode as a voltage signal. A current-voltage conversion circuit for converting, a current source for bypassing a part of a current signal taken out of the photodiode when an optical signal having a predetermined power or more is received, and a current value bypassed by the current source. A reference control unit whose output level is controlled, receiving the output of the current-voltage conversion circuit as a signal input, receiving the output of the reference control unit as a reference input, and using the reference input as a threshold as the An optical receiving circuit comprising: a comparator that discriminates an input. 2. The method according to claim 1, wherein the current value bypassed by the current source is 0.
Outputting the center value of the output amplitude of the current-to-voltage converter, and when the current value bypassed by the current source exceeds 0, the output signal of the current-to-voltage converter is proportional to the bypassed current value. The optical receiving circuit according to claim 1, wherein a value that changes from the amplitude center of the output signal in a direction approaching an extreme value of the output signal is output. 3. The optical receiving circuit according to claim 2, wherein an output of said reference control unit does not exceed a range of an amplitude value of an output signal of said current-voltage conversion circuit. 4. The current-voltage conversion circuit is configured by a transimpedance type preamplifier, the reference control unit detects and holds a minimum value of an output of the preamplifier, and the preamplifier includes: A voltage source for generating an output voltage of the same level as the output level at the time of no input; a current mirror circuit for extracting a current having a current value equivalent to a current bypassed by the current source; and the voltage source and the current A voltage drop resistor connected to the mirror circuit; a second minimum value hold circuit for detecting and holding the lowest value of the terminal voltage of the voltage drop resistor; and first and second minimum value hold circuits 4. The optical receiving circuit according to claim 3, further comprising: a voltage dividing circuit that extracts an intermediate output voltage.