JPH0998032A - Light receiving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent errors and to always receive the stable optical signals regardless of the receiving levels of a photodetector. SOLUTION: A light receiving circuit consists of a photodetector 1 which receives the optical signals and converts them into the electric signals, a preamplifier 2 which amplifies the electric signals, a transistor TR 3 which has its collector connected to a power supply that supplies the power voltage and receives the amplified signals at its base, a resistor 6 which is connected to the emitter of the TR 3 via one of its both ends with the other end grounded, and a variable capacitance means 10 which is connected in parallel to the resistor 6 and has the inter-terminal capacitance that is reduced as the inter-terminal voltage level of the resistor 6 rises. The means 10 can use a variable capacitance diode which is connected in parallel to the resistor 6 with its cathode turned to the emitter. The capacitance of the diode 10 varies when the inter- terminal voltage varies in response to the light receiving level. As result, the signal waveform has no ringing despite a high light receiving level.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of characteristics of an optical receiving circuit, and more particularly, to a semiconductor light receiving element and automatic gain control (A).
Automatic Gain Control, hereinafter abbreviated as "AGC". ) Equipped with a preamplifier (hereinafter,
Write as "preamplifier". ) And a transistor peaking circuit.

[0001]

2. Description of the Related Art As an example of a conventional light receiving circuit, as shown in FIG. 7, the cathode of a semiconductor light receiving element 1 is AGC.
A configuration is known in which the output 11 of the preamplifier 2 with AGC is connected to the preamplifier 2 with a built-in amplifier, and the output 11 of the preamplifier with AGC 2 is connected to the base of the transistor 3 to convert the received optical signal into an electric signal, which is amplified and output. In this example, the collector of the transistor 3 also has an automatic threshold control (Automatic Thr).
Eshold Control, hereinafter abbreviated as "ATC". ) Is connected to the inverting input terminal 12 of the comparator 4 having a circuit.

Here, the AGC preamplifier 2 uses two feedback resistors of 2 kΩ and 100 kΩ to form two feedback circuits. In this configuration, when the light receiving level to the semiconductor light receiving element 1 is low, 100 kΩ
The feedback resistor of operates. On the other hand, when the light reception level to the semiconductor light receiving element 1 increases, the feedback resistor of 2 kΩ is switched to operate. The switching of these two feedback resistors is performed at a certain preset level as a boundary.

The preamplifier 2 with AGC has a GB product (Gain).
Band) is constant, so the reception level is low,
When the feedback resistance 100 kΩ is operating, the gain is large but the band is reduced. On the other hand, when the reception level is high and the feedback resistance 2 kΩ is operating, the gain is small but the band is increased. Therefore, the signal waveform of the output 11 of the preamplifier 2 with AGC becomes as shown in FIG.

The rise of the light input waveform of the semiconductor light receiving element 1,
Even if the fall is steep, the waveform of the output 11 of the preamplifier 2 with AGC rises when the reception level is small,
The characteristics are such that the falling edge is gentle, while the rising edge and the falling edge are steep when the reception level is high.

In the conventional optical receiver circuit shown in FIG. 7, the output of the AGC-equipped preamplifier 2 having such characteristics is input to the base of the transistor 3. Transistor 3 is
This is an amplifier circuit with a grounded emitter, and the emitter and ground are connected by a resistor 6 (emitter resistance), and the collector and power source are connected by a resistor 5. Resistor 5
As an example, the constants of the resistor 6 and the resistor 6 are 300Ω and 1
50Ω is used, where the voltage gain is doubled.

FIG. 9 shows another example of the conventional optical receiving circuit. In order to increase the band of the output waveform of the preamplifier 2 with AGC, a capacitor 15 is connected in parallel with the emitter resistor 6 of the transistor 3. It is an example of emitter peaking. This configuration has also been generally implemented conventionally to improve the optical reception characteristic as described later.

In this configuration, in addition to the configuration shown in FIG. 7, the collector of the transistor 3 is connected to the inverting input 12 of the comparator 4. Comparator 4 is AT
Equipped with C circuit, ATC circuit has comparator input 1
An accurate threshold is given to the amplitude of the waveform of No. 2 and the DC level. The output terminal and the non-inverting input terminal of the comparator 4 are connected via the resistor 8, and the non-inverting input terminal and the inverting input terminal are connected via the resistor 7. further,
The capacitor 9 is connected between the non-inverting input terminal and the ground.

[0008]

FIG. 10 shows the signal waveform of each part of the optical receiving circuit when the receiving level is small in the above conventional optical receiving circuit. FIG. 9A shows an optical input waveform to the semiconductor light receiving element 1. The band is relatively wide and the rising and falling edges are steep. However, FIG. 9 (b)
As shown in (1), in the output waveform of the preamplifier with AGC, since the feedback resistor 100 kΩ is operating, the band is reduced, and the rising and falling edges of the waveform are gentle.

FIG. 10 (c) is a diagram showing the input waveform to the comparator 4 which is doubled by the transistor 4 and the threshold value of the comparator 4. As shown in FIG. 10C, the threshold value 13 of the ATC circuit changes depending on whether the rising edge of the input waveform is crossed or the falling edge of the input waveform is crossed. This allows the input waveform DC
Operates stably against offset.

Here, in the conventional configuration shown in FIG. 9, a predetermined time constant is set by the capacitor 9 so that the rising and falling of the threshold value 13 are gentler than the rising and falling of the input waveform 12. Has been done. The reason for this is
This is because the threshold value 13 follows the input waveform and is prevented from changing, and an error occurs unless the threshold value is delayed enough to cross the rising and falling edges of the input waveform 12.

However, if it is set too slow, the threshold value 13 will not return to a predetermined potential within 1 bit of the pulse of the input waveform, and this will cause jitter or duty fluctuation in the output waveform after comparison due to the pattern effect of the input waveform. Therefore, there is a limit in increasing the time constant. In the conventional optical receiving circuit, the time constant of the threshold 13 is determined under the conditions as described above, but as shown in FIGS. 10 (c) and 10 (d), the optical receiving level is When it is small, the rising and falling edges of the input waveform are relatively close to the rising and falling edges of the threshold value 13, and an error such that the input waveform is not crossed is apt to occur when crossing the input waveform. This causes a restriction on the minimum optical reception level (minimum reception level) that is error-free. This is largely due to the fact that the band decreases when the optical reception level, which is a characteristic of the preamplifier 2 with AGC, is small. On the other hand, the AGC is an indispensable circuit for ensuring a wide dynamic range of the optical reception level. is there.

FIG. 11 is a diagram showing the receiving characteristic when another example of the conventional optical receiving circuit shown in FIG. 9 is used.
In the configuration shown in FIG. 9, since the capacitor 15 is provided, as shown in FIGS. 11 (a) and 11 (b), when the optical reception level is low, the waveform band increases and the minimum The reception level is further improved.

However, FIG. 11C and FIG.
As shown in (d), when the optical reception level is high,
The output band of the preamplifier 2 with AGC is increased. As a result, if emitter peaking is further applied to the transistor 3, excessive peaking will occur, causing waveform distortion and causing an error. Therefore, there arises a problem that the maximum reception level is greatly deteriorated.

FIG. 12 is a diagram showing, in comparison, the gain-frequency characteristics of the amplifier circuit using the respective transistors 3 in the two examples of the conventional optical receiving circuit described above. It can be seen that increasing the capacitance of the capacitor 15 lowers the low cutoff frequency of the circuit. Conventionally, since the capacitance of the capacitor 15 is constant, the low cutoff frequency of the circuit is always constant.

In the conventional optical receiving circuit, as described above, the cathode terminal of the semiconductor light receiving element is input to the preamplifier having the AGC by switching the two feedback resistors. The output of the preamplifier is input to the base of a transistor amplifier circuit in which a collector and a power source and the emitter and ground are respectively connected through a resistor, and the collector of the transistor amplifier circuit is connected to an inverting input terminal of a comparator. The comparator has an ATC (non-inverting input terminal and output terminal, a non-inverting input terminal and an inverting input terminal connected via a resistor, respectively, and a capacitor connected between the non-inverting input terminal and ground). Auto
The device includes a physical threshold control circuit. With such a configuration, when the optical reception level is low, the rise and fall of the input waveform to the comparator become relatively close to the rise and fall of the threshold value, and there is an error that the input waveform is not crossed where it crosses. As described above, there is a problem that the minimum reception level is greatly restricted due to the occurrence of the above.

As another example of the conventional optical receiving circuit,
In order to increase the band of the output waveform of the preamplifier with AGC, there is a configuration example of emitter peaking in which a capacitor is connected in parallel with the emitter resistor of the transistor. In this method, when the optical reception level is low, the band of the waveform is increased and the minimum reception level is improved. However, when the optical reception level is high, the output band itself of the preamplifier with AGC is increased. When emitter peaking is applied, excessive peaking occurs, waveform distortion occurs, and the maximum reception level deteriorates significantly.

In view of the above-mentioned conventional drawbacks, the optical receiving circuit of the present invention is designed to prevent an error from occurring regardless of the receiving level of the light receiving element and to always enable stable reception. To aim.

[0018]

In order to solve the above-mentioned problems of the conventional optical receiving circuit, the optical receiving circuit of the present invention includes a light receiving element which receives an optical signal and converts it into an electric signal, and an electric signal. A preamplifier that amplifies and outputs an amplified signal, a transistor whose collector is connected to a power supply that supplies a power supply voltage and whose amplified signal is input to the base, and a resistor whose one end is connected to the emitter of the transistor and whose other end is grounded. And a capacitance varying means that is connected in parallel to the resistor and has a capacitance between the terminals that decreases as the voltage between the terminals of the resistor increases.

The capacitance varying means is a variable capacitance diode connected in parallel with the resistor with the cathode facing the emitter.

Further, the preamplifier is provided with automatic gain adjusting means for automatically adjusting the gain so as to maintain the output level of the amplified output within a predetermined range. further,
The preamplifier includes two feedback control circuits for adjusting the gain according to the output of the electric signal. The feedback control circuit has at least two feedback resistors that have resistance values different from each other and are connected in parallel to the preamplifier that amplifies the electric signal and that are switched and connected according to the level of the optical signal. It is characterized by including.

On the other hand, a PIN photodiode or an avalanche photodiode can be adopted as the light receiving element of the optical receiving circuit of the present invention.

Further, the optical receiving circuit of the present invention further comprises a comparator which is connected between the collector and the power supply and which compares the level of the amplified signal with a predetermined reference level and outputs a waveform of the level of the reference level. It is characterized by

In the conventional optical receiver circuit, as described above,
Even if a capacitor is provided in parallel with the resistor arranged between the emitter of the transistor and the ground, a new problem arises when the reception level is high, and it cannot be an effective means after all. Therefore, in the optical receiver circuit of the present invention, in view of the fact that the problem that occurs when the reception level is high is due to the constant capacitance of the capacitor, the capacitance changes in accordance with the terminal voltage instead of the capacitor. It is supposed to use the capacity varying means for performing this. That is, when the reception level is low, the capacity is high, and as the reception level increases, the capacity decreases.

By using such a capacitance varying means, it is possible to expect an operation similar to that of the configuration using a conventional capacitor when the reception level is high, while the capacitance decreases when the reception level is high, so that the signal waveform It becomes possible to prevent ringing from occurring.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a semiconductor laser module of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing the configuration of an embodiment of the optical receiving circuit of the present invention. The cathode of the semiconductor light receiving element 1 is
It is connected to the preamplifier 2 with AGC, and the output 11 of the preamplifier 2 with AGC is connected to the base of the transistor 3. The transistor 3 is an amplifier circuit including a peaking circuit.

FIG. 2 shows an AT between the power supply and the collector of the transistor in the embodiment of the present invention shown in FIG.
The example of the optical receiving circuit to which the comparator 4 including the C circuit is connected is shown. Here, the collector of the transistor 3 is connected to the inverting input terminal 12 of the comparator 4. Since the principle is common to both configurations, both examples will be described below.

The feedback resistance of the preamplifier 2 with AGC is 2k.
Two of Ω and 100 kΩ are used. When the reception level of the optical signal to the semiconductor light receiving element 1 is low, 100k
Ω feedback resistor works. On the other hand, PIN-PD
When the optical reception level to 1 becomes large, it is switched to the feedback resistance of 2 kΩ, and this operates. The switching of these two feedback resistors is performed with a preset reception level as a boundary.

The preamplifier 2 with AGC has a GB product (Gain
Band) is constant, so the reception level is low,
When the feedback resistor 100 kΩ is operating, the gain is large but the band is reduced. On the other hand, when the reception level is high and the feedback resistor 2 kΩ is operating, the gain is low but the band rises. Therefore, the waveform of the output 11 of the preamplifier 2 with AGC is as shown in FIG. Semiconductor light receiving element 1
Even if the rising and falling edges of the optical input waveform of
The signal waveform of the output 11 of the C-equipped preamplifier 2 has characteristics that the rising and falling are gentle when the reception level is low, while the rising and the falling are steep when the reception level is high.

The output of the preamplifier with AGC 2 having such characteristics is input to the base of the transistor 3.
The transistor 3 is a grounded-emitter amplifier circuit, and the emitter and ground are connected by a resistor 6 (emitter resistance), and the collector and power supply are connected by a resistor 5, respectively. For example, the constants of the resistor 5 and the resistor 6 are 300Ω and 150Ω, respectively. The voltage gain is double.

A variable capacitance diode 10 is connected in parallel with the emitter resistor 6. The variable-capacitance diode 10 is connected so that the cathode thereof faces the emitter of the transistor 3 and the cathode thereof faces the ground, and a reverse bias voltage is applied thereto.

In another embodiment shown in FIG. 2,
The collector of the transistor 3 is connected to the inverting input 12 of the comparator 4. The comparator 4 includes an ATC circuit. The ATC circuit is a circuit which gives an accurate threshold to the amplitude and DC level of the waveform of the comparator input 12, and the output terminal and the non-inverting input terminal of the comparator 4 are the resistors 8 and the non-inverting input terminal. The inverting input terminals are connected by resistors 7, respectively. Further, the capacitor 9 is connected between the non-inverting input terminal and the ground.

FIG. 3 shows a signal waveform of each part of the optical receiving circuit when the optical receiving level is low. FIG. 3 (a)
Is an optical input waveform to the semiconductor light receiving element 1. The band is relatively wide and the rising and falling edges are steep. However, as shown in FIG. 3B, in the output waveform of the preamplifier with AGC, the band is reduced because the feedback resistor 100 kΩ is operating, and the rising and falling edges of the waveform are gentle. Therefore, in the present invention, by applying emitter peaking by the variable capacitance diode 10 in the transistor 3, a steep rising and falling signal waveform as shown in FIG. 3C is obtained.

FIG. 4 shows an example of the characteristic of capacitance change between the anode and cathode terminals when a reverse bias voltage is applied to the variable capacitance diode 10. It can be seen that the inter-terminal capacitance decreases as the reverse bias voltage increases.
The optical receiving circuit of the present invention utilizes this characteristic to perform variable emitter peaking.

In the emitter of the transistor 3, a waveform having the same phase as the input waveform appears at a voltage level that is lower than the base input waveform of the transistor 3 by a voltage drop VBE (about 0.7 V) between the base and the emitter. That is, when the amplitude of the input waveform to the base is large, the amplitude of the waveform appearing at the emitter also becomes large, and accordingly, the inter-terminal capacitance of the variable capacitance diode 10 decreases and the peaking amount decreases.

FIG. 5 shows the signal waveform of each part of the optical receiving circuit by this operation. FIG. 5B shows the waveform of the output 11 of the preamplifier with AGC when the optical reception level is low. Since the peaking is applied from the transistor 3, the waveform of the comparator input 12 has steep rising and falling edges, and the comparator 4 operates in a stable manner and becomes error-free up to a place where the optical reception level is lower.

On the other hand, FIGS. 5C and 5D show the comparators 4 when the optical reception level is low and when it is high.
Of the input 12, the threshold 13, and the output 14 of the comparator 4. As shown in FIGS. 5C and 5D,
Even when the optical reception level is high, the waveform of the comparator input 12 does not cause excessive peaking, the comparator 4 operates stably, and the error is free even when the optical reception level is higher.

6A to 6C are gain-frequency characteristics of the output of the preamplifier 2 with AGC, gain-frequency characteristics of a transistor peaking circuit in which a variable capacitance diode is connected to the emitter of the transistor 3, and preamplifier 2 with AGC. The gain-frequency characteristics of the circuit in which the variable capacitance diode 10 is connected to the emitter of the transistor 3 at the output terminal of FIG. From these figures, it can be seen that the high cutoff frequency is constant regardless of the magnitude of the optical reception level.

As described above, in both of the above embodiments of the optical receiving circuit of the present invention, the circuit in which the power source voltage is single and the positive power source is described, but it is also possible to use both the positive and negative power sources and the negative power source. Yes, comparators used are TTL and ECL
It goes without saying that it does not matter. Further, the light receiving element can be applied to an avalanche photodiode as well as a PIN photodiode.

[0040]

As described above, in the optical receiving circuit of the present invention, in the optical receiving circuit including a preamplifier, a transistor, etc., between the emitter of the transistor amplifier circuit and the ground, the anode is in the ground direction and the cathode is the transistor. The variable capacitance diode is connected so that it is in the emitter direction, and the emitter peaking is applied by the variable capacitance diode in the transistor amplifier circuit, improving both the minimum reception level and the maximum reception level, and the input dynamic range of the optical reception level. Will be able to expand.

[Brief description of drawings]

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

FIG. 2 is a diagram showing another embodiment of the optical receiving circuit of the present invention.

FIG. 3 is a diagram showing a signal waveform of each part when the optical reception level is low in one embodiment of the optical reception circuit of the present invention.

FIG. 4 is a diagram showing a relationship between a reverse bias voltage and a terminal capacitance of a variable capacitance diode used in the optical receiver circuit of the present invention.

FIG. 5 is a diagram showing signal waveforms of respective parts of a comparator input, a threshold, and a comparator output in another embodiment of the optical receiving circuit of the present invention.

FIG. 6 is a diagram showing gain-frequency characteristics of a preamplifier with AGC and an amplifier circuit of a transistor in the optical receiving circuit of the present invention.

FIG. 7 is a diagram showing an example of a configuration of a conventional optical receiving circuit.

FIG. 8 is a diagram showing an optical input waveform of a semiconductor light receiving element and a signal waveform of an output of a preamplifier with AGC.

FIG. 9 is a diagram showing another example of the configuration of a conventional optical receiving circuit.

FIG. 10 is a diagram showing a signal waveform of each part when an optical reception level is low in an example of a conventional optical receiving circuit.

FIG. 11 is a diagram showing signal waveforms of a comparator input, a threshold value, and each unit of a comparator output unit in another example of the conventional optical receiving circuit.

FIG. 12 is a diagram showing a gain-frequency characteristic of a transistor in a conventional optical receiving circuit.

[Explanation of symbols]

 1 Semiconductor light receiving element 2 Preamp with AGC 3 Transistor 4 Comparator 5 Resistor 6 Resistor 7 Resistor 8 Resistor 9 Capacitor 10 Variable capacitance diode 11 Preamp output with AGC 12 Comparator input 13 Comparator threshold 14 Comparator output

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location H04B 10/14 H04B 9/00 Y 10/04 S 10/06 H04L 25/03

Claims (8)

[Claims] 1. A light receiving means for receiving an optical signal and converting it into an electric signal, a preamplifying means for amplifying the electric signal and outputting an amplified signal, and a collector connected to a power supply means for supplying a power supply voltage. A transistor to which the amplified signal is input to the base; a resistor whose one end is connected to the emitter of the transistor and whose other end is grounded; and a resistor which is connected in parallel to the resistor and has a higher terminal voltage of the resistor, An optical receiving circuit, comprising: a capacitance varying means for reducing the capacitance between the terminals. 2. The optical receiving circuit according to claim 1, wherein the capacitance varying means is a variable capacitance diode having a cathode directed toward the emitter and connected in parallel to the resistor. 3. The preamplifying means comprises an automatic gain adjusting means for automatically adjusting the gain so as to maintain the output level of the amplified output within a predetermined range. The optical receiving circuit described. 4. The preamplification means comprises two feedback control means for performing gain adjustment according to the output of the electric signal.
The optical receiving circuit described. 5. The feedback control means has a preamplifier that amplifies the electric signal and resistance values different from each other, is connected in parallel to the preamplifier, and is switched and connected according to the level of the optical signal. The optical receiving circuit according to claim 4, further comprising at least two feedback resistors. 6. The light receiving circuit according to claim 5, wherein the light receiving means is a PIN photodiode. 7. The optical receiving circuit according to claim 5, wherein the light receiving means is an avalanche photodiode. 8. The light receiving circuit is further connected between the collector and the power supply means, compares the level of the amplified signal with a predetermined reference level, and outputs a waveform of high and low with respect to the reference level. The optical receiver circuit according to claim 5, further comprising: