JP2003264434A - Optical signal receiving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical signal receiving circuit having a wide dynamic range capable of responding to a optical burst signal at a high speed. <P>SOLUTION: In the optical signal receiving circuit having an inverting amplifier 12 connected with an output circuit of a light receiving element 10 that converts the optical signal into a current signal and a feedback resistor 13 connected between input and output of the inverting amplifier, it is provided with a transistor 14 for current control formed by connecting a collector on the input side of the inverting amplifier, connecting the emitter on the output side of the inverting amplifier, and driving the base at a constant voltage. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal receiving circuit, and more particularly to a current-voltage conversion preamplifier used for converting an optical input signal into an electric signal in an optical communication system.

[0002]

2. Description of the Related Art In an optical communication system, an optical input signal is converted into an electric (current) signal by a light receiving element, and an output current of the light receiving element is converted into a voltage by a preamplifier. Normally, the preamplifier is a transformer with a feedback resistor.
Impedance type is adopted. Since a typical preamplifier has a circuit configuration in which a fixed voltage is applied to the signal input terminal, the product of the output current of the light receiving element and the feedback resistance is calculated from the input terminal voltage at the output terminal of the preamplifier. An output voltage corresponding to the subtracted value appears.

The light receiving element outputs a larger current as the input optical signal is stronger. When the optical signal intensity changes, the output signal of the preamplifier changes due to the change in the voltage drop that occurs in the feedback resistor. However, when the optical signal strength exceeds a certain value, the circuit operation of the preamplifier is saturated,
The output amplitude does not follow the input change. That is, in the preamplifier, the dynamic range capable of responding to the intensity change of the optical input signal is limited by the reference potential, the power supply voltage, or the ground side potential (GND). Therefore, there is a demand for a circuit system capable of raising the saturation point of the amplifier circuit and expanding the dynamic range capable of responding to the optical signal intensity without changing the applied power supply voltage or the reference potential.

As one method for expanding the dynamic range of the preamplifier, for example, as shown in FIG. 6, a feedback resistor 13 is provided between the input and output terminals of the inverting amplifier 12.
A circuit configuration in which a diode 20 and a diode 20 are connected in parallel is known. As the intensity of the optical input signal increases, the output current of the light receiving element 10 increases and the voltage drop generated in the feedback resistor 13 also increases. When the intensity of the optical signal further increases and the voltage across the feedback resistor 13 exceeds the threshold value of the diode 20, a part of the output current of the light receiving element 10 is bypassed by the diode 20 and the saturation of the amplifier circuit is prevented. However, in this circuit system, as a result, the output signal amplitude of the light receiving element is clipped at a certain value, and the output waveform of the amplifier 12 is accompanied by ringing and waveform distortion.

In Japanese Patent Laid-Open No. 10-126167 (Prior Art 1), as shown in FIG. 7, a source and a drain of a PMOS transistor 30 are connected between an output terminal of a light receiving element 100 and a ground (GND) potential. A circuit configuration has been proposed in which the gate of the PMOS transistor is controlled by the output voltage of the inverting amplifier 12.

In the circuit of the prior art 1, the amplitude of the output signal appearing at the output terminal OUT of the inverting amplifier 12 is determined by the light receiving element 1
The higher the intensity of the optical signal input to 00,
When it increases to the negative voltage side and the peak value of the output signal of the inverting amplifier 12 approaches the GND potential, the PMOS transistor 30 is turned on. Therefore, the output current of the light receiving element 100 branches to GND and the current flowing through the feedback resistor 13 decreases, so that the saturation of the amplifier is avoided and the dynamic range for the optical signal intensity is expanded.

FIG. 8 shows a circuit configuration of a preamplifier described in Japanese Patent Laid-Open No. 2001-127560 (Prior Art 2). Here, the load resistance R of the transistor Q1
The output extracted from L is input to the base of the transistor Q2 at the next stage, the output of the transistor Q2 is divided by the emitter resistors R1 and R2, and the potential of the resistor R2 is fed back to the base of the transistor Q1 via the feedback resistor Rf. As a result, a current feedback type transimpedance circuit with a grounded emitter is formed. Light receiving element (photodiode P
The output current of D) is connected to the base of the transistor Q1, and the emitter output of the transistor Q2 is taken out to the output terminal OUT via the emitter follower circuit composed of the transistor Q3 and the resistor R3. Transistor Q4 ~
The current mirror circuit composed of Q6 and the resistor R4 constitutes a current control circuit that controls the input current to the transistor Q1.

In the prior art 2, the average value detection circuit 4 connected to the output terminal OUT extracts the average value of the output voltage and supplies the average value to the base of the transistor Q6 to obtain the output current of the light receiving element PD. , A current proportional to the average output value is shunted to the transistor Q4.

[0009]

The circuit configuration of the prior art 1 uses the PMOS transistor 30 for shunt control of the output current of the light receiving element. However, since the MOS type transistor generally has a large variation in the characteristics of the drain or source current with respect to the gate voltage, the PMO
There is variation in the current value branched to the S-transistor 30, and it is not easy to guarantee uniform characteristics for mass-produced preamplifiers.

Further, the circuit configuration of the prior art 2 is effective when the optical signal input to the light receiving element PD is a continuous signal, but a burst-type input in which the optical signal intensity changes greatly in a short time. When converting a signal into an electric signal, there is a problem that a time delay occurs in detecting the average value of the amplifier output, and the control operation of the output current of the light receiving element cannot follow the change of the optical signal.

An object of the present invention is to provide an optical signal receiving circuit having a wide dynamic range capable of responding to bursty optical signals at high speed. Another object of the present invention is to provide an optical signal receiving circuit which has little variation in characteristics and is excellent in mass productivity by being integrated into an IC.

[0012]

To achieve the above object, the present invention comprises a light receiving element for converting an optical signal into a current signal, and an inverting amplifier connected to an output circuit of the light receiving element. In an optical signal receiving circuit having a feedback resistance between the input and output of an inverting amplifier, the collector is connected to the input side of the inverting amplifier, the emitter is connected to the output side of the inverting amplifier, and the base is driven with a constant voltage for current control. It is characterized by including the transistor of.

Another feature of the present invention is that the inverting amplifier has a voltage dividing circuit for dividing the output voltage, and the output of the feedback resistor current control transistor divided by the voltage dividing circuit is applied to the emitter of the transistor. The feature is that a voltage is applied.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of an optical signal receiving circuit (preamplifier circuit) according to the present invention. The optical signal receiving circuit constitutes a current-voltage conversion circuit by an inverting amplifier 12 connected to the output circuit of the light receiving element 10 and a feedback resistor 13 connected in parallel between the input and output terminals of the inverting amplifier. The collector and the emitter are connected to the input terminal and the output terminal of the current-voltage conversion circuit, respectively, and the base is connected to the constant voltage source 15. The transistor 14 for feedback resistance current control is provided.

If the input impedance of the inverting amplifier 12 is set to a value that is sufficiently larger than the resistance value of the feedback resistor 13, most of the output current I1 of the light receiving element will be the feedback resistor 13.
Is the inflow current I2. Therefore, the feedback resistor 13
A voltage drop that is proportional to the output current I1 occurs, and as the intensity of the optical signal increases, the voltage at the output terminal (OUT) of the current-voltage conversion circuit decreases and approaches the saturation point of the dynamic range.

In this embodiment, by setting the base to the fixed potential Vth by the constant voltage source 15, the current controlling transistor 14 operates as a grounded base transistor, and the collector current I3 causes the base-emitter voltage. Is controlled by. Emitter voltage is fixed potential Vth
When it is higher than the above, the transistor 14 is in the off state, so most of the output current I1 of the light receiving element flows to the feedback resistor 13. When the output voltage of the current-voltage conversion circuit decreases and the emitter voltage of the transistor 14 becomes lower than the fixed potential Vth, a voltage is applied in the forward direction between the base and the emitter, and the transistor 14 becomes conductive.

Therefore, if the base potential (fixed potential Vth) of the transistor 14 is set so that the transistor 14 for current control becomes conductive just before the saturation point of the dynamic range, the current branched as the collector current I3. An increase in the current I2 flowing into the feedback resistor 13 can be suppressed by that much. If the increase amount of the current I2 of the feedback resistor is reduced with respect to the increase amount of the output current I1 of the light receiving element 10, the arrival at the saturation point is delayed, and as a result, the dynamic range can be expanded.

FIG. 2A shows a current control transistor 1
4 shows the relationship between the collector current and the emitter voltage of No. 4. Since the fixed voltage Vth is applied to the base of the transistor 14, the current control transistor 14 has an emitter voltage lower than the base voltage Vth and a forward voltage equal to or higher than a predetermined value Vbe in the base-emitter voltage. It becomes conductive when applied. Therefore, by changing the value of the fixed voltage Vth applied to the base, the shunt start position to the transistor 14 can be arbitrarily set before the saturation point of the dynamic range.

When the base-emitter voltage changes within a predetermined range 300 exceeding the threshold value Vbe, as shown in the enlarged view of FIG. 2B, as the base-emitter voltage increases, the collector When the current gradually increases and the base-emitter voltage exceeds the range 300, the collector current exponentially increases rapidly.

FIG. 3 shows the output DC characteristic of the preamplifier circuit according to the present invention. A region 310 is a dynamic range of the amplifier circuit having the conventional structure shown in FIG. 6, and a region 320 is a dynamic range of FIG.
5 shows a dynamic range of the amplifier circuit of the present invention shown in FIG.

By controlling part of the current I2 flowing through the feedback resistor 13 as the collector current I3 of the base-grounded transistor 14, the amplifier circuit of the present invention has the dynamic range of the conventional circuit as shown by the solid line 300. The dynamic range with respect to changes in the optical signal can be expanded by logarithmically extending the optical signal intensity near the saturation point (broken line portion). Therefore, even when an optical signal having a high signal intensity is input to the light receiving element 10, it is possible to obtain an output voltage that follows the change in the optical signal.

FIG. 4 shows a second embodiment of the optical signal receiving circuit according to the present invention. In the present embodiment, a voltage divider 16 is connected to the output side of the current-voltage conversion circuit shown in the first embodiment, and 1 /
It is characterized in that the divided output voltage is applied to n.

The present embodiment can extend the dynamic range for the same reason as in the first embodiment. If the voltage divider 16 is not provided, the collector-emitter capacitance Cob of the transistor 14 is connected in parallel to the feedback resistor 12, and the grounding (GND) capacitance seen from the light receiving element 10 = (1 + A) ×
Cob is a limiting factor on the operating speed. On the other hand, as in this embodiment, the voltage divider 16 is used to
When the base-emitter voltage of 4 is set to 1 / n, the capacitance to GND seen from the light receiving element 10 can be reduced to (1 + A) × Cob / n, so that the operation speed can be increased.

FIG. 5 shows an example of a concrete circuit configuration of the second embodiment. Light receiving element (photodiode) 100
Output current I1 is input to the base of the emitter follower type first transistor 121 including the load resistor 122, and the output thereof is input to the base of the grounded emitter type second transistor 123 including the load resistor 124. . The output of the second transistor is the third emitter follower type which is provided with series load resistors 131 and 132 which serve as a voltage dividing circuit.
The change in the emitter potential of the third transistor is input to the base of the transistor (output transistor) 130.
The output voltage is output to the terminal OUT. These transistor circuits form the inverting amplifier 12 shown in FIGS. Further, by feeding back the output of the third transistor 130 to the base of the first transistor 121 via the feedback resistor 13, a current-voltage conversion circuit is configured.

The transistor 151 having the load resistor 152 in the collector functions as a constant voltage diode. By connecting the base of the transistor 14 for current control to the base (collector) of the transistor 151, the transistor 14 is connected. It operates as a base-grounded transistor. Further, by coupling the emitter of the current control transistor 14 to the connection point of the resistors 131 and 132, the transistor 14 is driven by the divided output voltage.

The branch current I3 to the current control transistor 14 can be adjusted by changing the value of the load resistor 152 of the constant voltage circuit and the resistance ratio of the resistors 131 and 132 in the voltage dividing circuit. If the resistor 131 is omitted, the circuit configuration of the first embodiment is obtained.

The circuit shown in FIG. 5 can form a high-speed optical signal receiving circuit by applying NPN transistors to all the transistors. Further, the emitter output of the third transistor 130 is input to the base of the further added fourth transistor, and the collector and the emitter of the fourth transistor are connected to Vcc and the ground potential via the load resistors, respectively, It is also possible to adopt a circuit configuration in which the output voltages of both phases are extracted from the collector and the emitter of the fourth transistor.

[0028]

As can be understood from the above description, according to the present invention, the grounded base type transistor is applied as the transistor for controlling the signal current flowing into the feedback resistance of the inverting amplifier, so that the optical signal intensity is increased. It is possible to provide an optical signal receiving circuit that can respond to bursty optical input signals at high speed by expanding the dynamic range of the optical signal.

[Brief description of drawings]

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

FIG. 2 is a diagram for explaining operating characteristics of a current controlling transistor 14 in the optical signal receiving circuit.

FIG. 3 is a diagram for explaining the amplification characteristic of the optical signal receiving circuit according to the present invention.

FIG. 4 is a diagram showing a second embodiment of the optical signal receiving circuit according to the present invention.

FIG. 5 is a diagram showing an example of a specific circuit configuration of the second embodiment circuit.

FIG. 6 is a diagram showing a conventional basic circuit configuration of an optical signal receiving circuit.

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

FIG. 8 is a circuit configuration diagram showing another known example of an optical signal receiving circuit.

[Explanation of reference numerals] 10: light receiving element, 12: inverting amplifier, 13: feedback resistor,
14: current control transistor, 16: voltage divider.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Masataka Tanaka             292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa             Ceremony company Hitachi Image Information System (72) Inventor Takaki Arai             216 Totsuka-cho, Totsuka-ku, Yokohama-shi, Kanagawa             Inside this Opnext Co., Ltd. F term (reference) 5J092 AA01 AA46 AA56 CA15 CA32                       CA65 CA91 FA04 HA02 HA10                       HA19 HA25 HA44 KA00 KA04                       KA09 KA11 KA27 MA01 MA11                       MA21 SA01 TA01 TA02 UL02                 5J500 AA01 AA46 AA56 AC15 AC32                       AC65 AC91 AF04 AH02 AH10                       AH19 AH25 AH44 AK00 AK04                       AK09 AK11 AK27 AM01 AM11                       AM21 AS01 AT01 AT02 LU02

Claims (4)

[Claims] 1. A light receiving element for converting an optical signal into a current signal,
In an optical signal receiving circuit having an inverting amplifier connected to the output circuit of the light receiving element and having a feedback resistance between the input and output of the inverting amplifier, the collector is connected to the input side of the inverting amplifier, and the emitter is the inverting amplifier. An optical signal receiving circuit comprising a current control transistor connected to the output side of an amplifier and having a base driven at a constant voltage. 2. The inverting amplifier has a voltage dividing circuit for dividing the output voltage, and the output voltage divided by the voltage dividing circuit is applied to the emitter of the feedback resistance current controlling transistor. The optical signal receiving circuit according to claim 1, which is characterized in that. 3. An emitter follower type first transistor circuit in which the inverting amplifier is base-connected to an output circuit of the light receiving element, and a grounded emitter type second transistor circuit for amplifying an emitter output of the first transistor circuit. An emitter follower type third transistor circuit for amplifying a collector output of the second transistor circuit, wherein a collector of the current control transistor and one end of the feedback resistor are connected to a base of the first transistor circuit, The optical signal receiving circuit according to claim 1, wherein the emitter of the current controlling transistor and the other end of the feedback resistor are connected to the emitter of the third transistor circuit. 4. An emitter follower type first transistor circuit in which the inverting amplifier is base-connected to an output circuit of the light receiving element, and a grounded-emitter type second transistor circuit for amplifying an emitter output of the first transistor circuit. An emitter follower type third transistor circuit for amplifying a collector output of the second transistor circuit, the first and second load resistors in which the emitters of the third transistor circuit are connected in series, The collector of the control transistor and one end of the feedback resistor are connected to the base of the first transistor circuit, the other end of the feedback resistor is connected to the emitter of the third transistor circuit, and the emitter of the current control transistor is The optical signal according to claim 2, wherein the optical signal is connected to a connection point of the first and second load resistors. Shin circuit.