JPH10173449A - Optical receiver circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical receiver circuit capable of receiving an optical signal without a saturated diode even if photoelectric current increases. SOLUTION: The optical receiver circuit 20 is composed of the optical receiving diode D1, an operational amplifier AMP1, a feedback resistor Rr, a low-pass filter 26, a current detecting part 28, a current source 29 and a constant-voltage source 31. In the case where sunlight directly irradiates the optical receiving diode D1, a photoelectric current flowing in the cathode of the optical receiving diode D1 is to be the sum a DC current by sunlight components and an AC current by optical signal components. Since a collector current which is equivalent to the above DC current is flown in the collector of an NPN transistor forming the current source 29, only signal current flows in one terminal side of the operational amplifier AMP1 from the anode of the light receiving diode D1. As a result, voltage drop in the feedback resistor Rr is restrained at a small, and the optical receiving diode D1 does not reach a saturating region.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical receiving circuit.

[0002]

2. Description of the Related Art Conventionally, an optical transmission / reception system includes a transmitter and a receiver (see FIG. 1). The transmitter has a plurality of modulation circuits that frequency-modulate the input audio signal,
A mixing circuit that mixes the modulated signals from the plurality of modulation circuits and an optical transmission circuit that converts the mixed electric signal into an optical signal are provided. On the other hand, the receiver is provided with an optical receiving circuit for converting an optical signal into an electric signal, a mixing circuit for mixing the converted electric signal, and a plurality of demodulating circuits for demodulating the mixed signal. FIG. 10 is a circuit diagram showing a basic configuration of a conventional optical receiving circuit. The light receiving circuit is a light receiving diode D1
a, operational amplifier AMP1a and feedback resistor R
The reference voltage Vr is provided to the non-inverting input terminal (+ terminal) of the operational amplifier AMP1a. In this light receiving circuit, the light receiving diode D1a is reverse-biased and operates at high speed by reducing its internal capacitance.

FIG. 11 is a circuit diagram showing a configuration of a conventional optical receiving circuit to which a low-frequency cut capacitor is added. Also,
FIG. 12 is a diagram showing general operating characteristics of the light receiving diode. FIG. 3A shows the connection of the light receiving diode Di, the resistor R and the power supply (power supply voltage E), and FIG. 3B shows the characteristic curve of the light receiving diode Di. In the drawing, a straight line m indicates a resistance load line. When there is no light incident on the light receiving diode Di, only a very small leak current flows through the light receiving diode Di, so that the voltage across the both ends is the same as the power supply voltage E. When the photocurrent Id flows, the voltage between both ends of the light receiving diode Di is changed to a voltage (E-Id
× R) (b in the figure). When the light amount increases and the photocurrent Id increases accordingly, the voltage across the light receiving diode Di further decreases and finally reaches 0 V (c in the figure).
Further, when the photocurrent Id increases, the forward voltage of the diode (about 0.6 V for Si) is reached (d in the figure), and a saturation region where no voltage change occurs even if the photocurrent Id is further increased. Therefore, in the optical receiving circuit, the optical signal is received in a region where the light receiving diode does not enter a saturation region, that is, in a reverse-biased linear region. On the other hand, in the optical receiving circuit, when the feedback resistance Rra is increased, the S / N of the optical signal is increased.
It is known that the ratio can be improved. This is because the output signal of the negative feedback operational amplifier AMP1a increases in proportion to the resistance Rra (output voltage = input signal current × resistance Rra), whereas the output noise voltage increases in proportion to the square root of the resistance Rra. It depends. For example, when the resistance Rra is quadrupled, the output signal is quadrupled, but the output noise is only doubled, so that the S / N ratio can be doubled.

[0004]

However, in the conventional optical receiving circuit, if the resistance Rra is set large, when sunlight or the like directly enters the light receiving diode, the voltage at the resistance Rra is increased due to the large photocurrent Id. The drop is large and the light receiving diode D1a is saturated. Referring to FIG. 12, in the case of a resistance load line p having a large resistance R, when the photocurrent Id increases, the light receiving diode Di immediately reaches a saturation region and becomes a short-circuit state, making it impossible to detect an optical signal. In addition, not only in the case where the photocurrent increases due to the direct incidence of sunlight, but also when the temperature is high or when a high voltage is applied to the light-receiving diode and a part of the PN junction is broken, the leakage current increases and the same problem occurs. Occurs. Therefore, an object of the present invention is to provide an optical receiving circuit that can receive an optical signal without saturating a light-receiving diode even when a photocurrent increases.

[0005]

To achieve the above object, an optical receiving circuit according to the present invention comprises a reverse-biased photo diode, a negative feedback amplifier having an input terminal connected to the anode of the photo diode, and An optical receiving circuit including an output terminal of the negative feedback amplifier and a feedback resistor interposed between the input terminal and a smoothing unit for smoothing a photocurrent flowing to a cathode side of the light receiving diode; It is characterized by comprising a current detection unit for detecting a photocurrent, and a current source for flowing a current substantially equal to the detected photocurrent from the anode side of the light-receiving diode to the ground side. Further, the optical receiving circuit of the present invention includes a reverse-biased light-receiving diode, a negative feedback amplifier having an input terminal connected to the cathode of the light-receiving diode, and an output terminal of the negative feedback amplifier interposed between the input terminal and the input terminal. An optical receiving circuit including a feedback resistor, a smoothing unit for smoothing a photocurrent flowing to the anode side of the light-receiving diode, a current detecting unit for detecting the smoothed photocurrent, and the detected light A current source for flowing a current substantially equal to the current from the cathode side of the light receiving diode to the ground side.

In the optical receiving circuit of the present invention, the photocurrent flowing to the cathode side of the light receiving diode is smoothed by the smoothing section, the photocurrent smoothed by the current detecting section is detected, and the current substantially equal to the detected photocurrent is detected. Is received from the anode side of the light receiving diode by a current source. Also, the light receiving circuit of the present invention smoothes the photocurrent flowing to the anode side of the light receiving diode by the smoothing unit, detects the smoothed photocurrent by the current detecting unit, and detects the photocurrent by the current source. A current substantially equal to the photocurrent is supplied to the cathode side of the light receiving diode.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the optical receiving circuit according to the present invention will be described. The optical receiving circuit according to the present embodiment is applied to an optical transmitting and receiving system. FIG. 1 is a block diagram illustrating a configuration of the optical transmission and reception system according to the first embodiment.
The optical transmission / reception system 1 includes a transmitter 5 and a receiver 8. The transmitter 5 includes a plurality of modulation circuits 11 for frequency-modulating the input audio signals CH1 and CH2 of a plurality of channels, a mixing circuit 13 for mixing the modulation signals from the plurality of modulation circuits 11, and a mixed electric signal. Transmission circuit 1 for converting light into an optical signal
5 are provided. The optical transmission circuit 15 converts an electric signal into an optical signal using a light emitting diode (LED) or a laser diode. On the other hand, the receiver 8 has an optical receiving circuit 20 for converting an optical signal into an electric signal, a mixing circuit 22 for mixing the converted electric signal, and demodulates the mixed signal into output signals CH1 and CH2 for each channel. A plurality of demodulation circuits 25 are provided.

FIG. 2 is a block diagram showing a basic configuration of the optical receiving circuit 20. The light receiving circuit 20 has a light receiving diode D
1, an operational amplifier AMP1, a feedback resistor Rr, a low-pass filter (LPF) 26, a current detector 28, a current source 29, and a constant voltage source 31. FIG. 3 is a circuit diagram showing a specific configuration of the optical receiving circuit 20. The low-pass filter 26 is composed of a capacitor C1 connected between the line of the power supply voltage (+5 V) and the cathode of the light receiving diode D1. Further, the current detection unit 28 includes resistors r1, r2,
r3, a PNP transistor Q1, Q2 and an NPN transistor Q3. A current equal to the current (averaged DC current Idc) smoothed by the low-pass filter 26 flowing to the cathode side of the light receiving diode D1 is applied to the collector side of the transistor Q1. Shed. The current source 29 includes a resistor r4 and an NPN transistor Q4. The NPN transistor Q4 draws a current equal to the current flowing through the collector of the NPN transistor Q3 from the collector connected to the anode of the light receiving diode D1, and grounds the ground GND through the resistor r4. Pour into The resistances r1, r2, r
3, r4 are provided to reduce the change in the current amount even if the characteristics of the transistors Q1 to Q4 vary.
The resistances r1 and r2 and the resistances r3 and r4 are set to the same value.

Further, the constant voltage source 31 is composed of resistors r5 and r6 and a capacitor C2. The reference voltage Vx divided by the resistor is supplied to the non-inverting input terminal (+ terminal) of the operational amplifier AMP1.
Give to. A feedback resistor R is connected between the inverting input terminal (-terminal) and the output terminal of the operational amplifier AMP1.
r is provided, and a low impedance output signal is obtained from the output terminal. FIG. 4 is a timing chart showing signal waveforms of various parts in the optical receiving circuit 20. When sunlight directly hits the light-receiving diode D1, the amount of light incident on the light-receiving diode D1 is the sum of the light signal component from the transmitter 5 and the sunlight component (FIG. 7A), and flows through the light-receiving diode D1. The photocurrent is the sum of the DC current Idc due to the sunlight component and the signal current Iac due to the optical signal component (FIG. 2B). Also, the NPN transistor Q4
Has a collector current I equal to the DC current Idc.
(c), the signal current Iac obtained by removing the DC current Idc from the photocurrent Id flows from the anode of the light receiving diode D1 to the inverting input terminal (-terminal) of the operational amplifier AMP1. The voltage drop at the feedback resistor Rr is kept small. As a result, the light receiving diode D1 does not reach the saturation region, and the operational amplifier AMP
1 has an output terminal Vo corresponding to the signal current Iac.
(= -Iac × Rr) is obtained (FIG. (D)).

Here, the calculation of the collector current Ic of the NPN transistor Q4 will be described. FIG. 5 is a diagram showing a simplified circuit by removing the emitter resistance in the optical receiving circuit 20 of FIG. 3 and the current value of each part. here,
The DC current Idc caused by sunlight or leak current flows at point A in the figure, and the DC amplification factor H of each transistor.
FE (= Ic / Ib) is assumed to be equal. According to the current flow shown in the figure, the collector current Ic of the NPN transistor Q4 is represented by the equation (1), and is substantially equal to the DC current Idc. Note that the DC amplification factor HFE is generally a large value compared to the value 2.

Ic = Idc · (HFE / (2 + HFE)) ² As described above, when an optical signal or sunlight enters the light receiving diode D1, a photocurrent Id flows, and the smoothed DC current (average value) Idc flows to ground GND,
Since the current does not flow to the feedback resistor Rr side of the operational amplifier AMP1, the average potential at the point A does not change and becomes the reference voltage Vx, and the operational amplifier AMP1 outputs a voltage corresponding to the optical signal.

FIG. 6 is a graph showing the relationship between the light signal output of the light receiving circuit 20 and the amount of sunlight. Optical receiving circuit 20
Then, as described above, the DC current Idc due to the sunlight component of the photocurrent that has passed through the light receiving diode D1 flows to the NPN transistor Q4 side and does not flow to the feedback resistor Rr, so that the light receiving diode D1 does not reach the saturation region. Therefore, even if sunlight directly enters the light receiving diode D1, the optical signal output does not change (solid line a in the figure). On the other hand, in the conventional light receiving circuit, when the light amount of sunlight exceeds a predetermined light amount, the light receiving diode reaches a saturation region and enters a short-circuit state, so that an optical signal output cannot be obtained (broken line b in the figure).

FIG. 7 is a circuit diagram showing a configuration of an optical receiving circuit according to the second embodiment. The same components as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted. In the first embodiment, the transistor Q4
Is (HFE / (2 + HFE)) ^{2 according} to equation (1), and the collector current Ic of the transistor Q4 is slightly smaller than the DC current Idc. In the second embodiment, the PNP transistor Q5 and the NPN transistor Q6 are
By providing the transistor between the base and the collector of the NP transistor Q2 and the base and the collector of the NPN transistor Q3, the collector current Ic of the transistor Q4 approaches the DC current Idc. That is, PNP is applied from the bases of the PNP transistors Q1 and Q2.
By reducing the current flowing into the collector of the transistor Q2 and the current flowing from the collector of the NPN transistor Q3 to the bases of the NPN transistors Q3 and Q4, the collector current Ic of the transistor Q4 can be reduced by the DC current Idc as shown in the equation (2). Can be close.

[0013]

(Equation 2)

FIG. 8 is a circuit diagram showing a configuration of another optical receiving circuit. The same components as those of the optical receiving circuit 20 of FIG. 3 are denoted by the same reference numerals. In this optical receiving circuit,
A capacitor C3 is provided between the light receiving diode D1 and the inverting input terminal of the operational amplifier AMP1 to cut low frequency components. This prevents a DC component from flowing into the operational amplifier AMP1 due to sunlight, leak current, or the like.

FIG. 9 is a circuit diagram showing a configuration of another optical receiving circuit. In this optical receiving circuit, the light receiving diode D1 is provided on the ground GND side from the minus terminal of the operational amplifier AMP1, and a capacitor C1 is provided between the collector terminal of the NPN transistor Q4 and the ground GND. In this case, NPN transistors Q4, Q3 and P
The NP transistor Q1 forms a current detector, and the PNP transistor Q2 forms a current source. Thus, even when the light receiving diode D1 and the capacitor C1 are provided on the ground GND side, the same operation and effect as those of the first embodiment are obtained.

[0016]

According to the optical receiving circuit of the present invention, the photocurrent flowing to the cathode side of the light receiving diode is smoothed by the smoothing section, and the smoothed photocurrent is detected by the current detecting section.
Since a current substantially equal to the detected photocurrent is caused to flow from the anode side of the light-receiving diode to the ground by the current source, an optical signal can be received without saturation of the light-receiving diode even when the photocurrent increases. Therefore, it is possible to realize an optical receiving circuit that is hardly affected by direct incidence of sunlight, increase in leak current, and the like. According to the optical receiving circuit of the second aspect, the smoothing unit is formed from the low-pass filter having the capacitor connected between the power supply and the cathode, so that the configuration of the smoothing unit can be simplified. Further, according to the optical receiving circuit of the present invention, the current detecting section includes a first PNP transistor having a collector and an emitter connected in parallel with the capacitor, and the first PNP transistor and a base connected to each other. A second PNP transistor; and a first NPN transistor having a collector connected to the second PNP transistor and having an emitter grounded, and the current source has a base connected to the first NPN transistor. And a second NPN transistor having a collector connected to the anode and an emitter grounded, so that the first and second PNP transistors and the first and second NPN transistors having the same characteristics are employed. By doing so, the current of the current source can be approximated to the smoothed photocurrent.

According to the optical receiving circuit of the present invention, the third PNP in which the emitters are connected to the bases of the first and second PNP transistors, and the bases are connected to the collectors of the first PNP transistors. Transistors and
A third NPN transistor having an emitter connected to the base of the first and second NPN transistors and a base connected to the collector of the first NPN transistor, so that the bases of the first and second PNP transistors are The current flowing from the current source to the collector of the first PNP transistor is reduced, and the current flowing from the collector of the second NPN transistor to the bases of the first and second NPN transistors is reduced. The value can be made closer to the photocurrent.
Further, according to the light receiving circuit of the present invention, a low-frequency cut capacitor is provided between the input terminal of the negative feedback amplifier and the anode of the light-receiving diode. DC components can be prevented from flowing into the feedback amplifier. Further, according to the optical receiving circuit of the present invention, the photocurrent flowing to the anode side of the light receiving diode is smoothed by the smoothing unit, the smoothed photocurrent is detected by the current detecting unit, and the detection is performed by the current source. Since a current substantially equal to the generated photocurrent is supplied to the cathode side of the light receiving diode, even if the photocurrent increases, the light receiving diode can receive an optical signal without being saturated.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an optical transmission / reception system.

FIG. 2 is a block diagram showing a basic configuration of an optical receiving circuit 20.

FIG. 3 is a circuit diagram showing a specific configuration of an optical receiving circuit 20.

FIG. 4 is a timing chart showing signal waveforms at various parts in the optical receiving circuit 20.

FIG. 5 is a diagram showing a simplified circuit by removing an emitter resistor in the optical receiving circuit 20 of FIG. 3 and a current value of each part.

FIG. 6 is a graph showing the relationship between the optical signal output of the optical receiving circuit 20 and the amount of sunlight.

FIG. 7 is a circuit diagram illustrating a configuration of an optical receiving circuit according to a second embodiment.

FIG. 8 is a circuit diagram showing a configuration of another optical receiving circuit.

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

FIG. 10 is a circuit diagram showing a basic configuration of a conventional optical receiving circuit.

FIG. 11 is a circuit diagram showing a configuration of a conventional optical receiving circuit to which a low-frequency cut capacitor is added.

FIG. 12 is a diagram showing general operating characteristics of a light receiving diode.

[Explanation of symbols]

20: optical receiving circuit, 26: low-pass filter, 28
... Current detector, 29 current source, D1 light receiving diode, Rr feedback resistance, AMP1 operational amplifier, C1, C3, capacitor, Q1, Q2, Q
3, Q4, Q5, Q6 ... transistors.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/14 10/04 10/06

Claims (6)

[Claims] A negative feedback amplifier having an input terminal connected to an anode of the light receiving diode; a feedback resistor interposed between an output terminal of the negative feedback amplifier and the input terminal; In the optical receiving circuit, a smoothing unit for smoothing a photocurrent flowing to the cathode side of the light receiving diode, a current detecting unit for detecting the smoothed photocurrent, and a current substantially equal to the detected photocurrent And a current source for causing a current to flow from the anode side of the light-receiving diode to the ground side. 2. The optical receiving circuit according to claim 1, wherein said smoothing unit comprises a low-pass filter having a capacitor connected between a power supply and said cathode. 3. The current detector, wherein a collector and a base of a first PNP transistor are connected to a base of a second PNP transistor and a cathode of a light-receiving diode.
The emitters of the first and second PNP transistors are connected to a power supply. The collector of the second PNP transistor, the collector and the base of the first NPN transistor, and the second NPN transistor
The bases of the PN transistors are connected, the emitters of the first and second NPN transistors are grounded, and the second NP
3. The light receiving circuit according to claim 2, wherein a collector of the N transistor is connected to an anode of the light receiving diode. 4. A third PN transistor having an emitter connected to the bases of the first and second PNP transistors and a base connected to the collectors of the first PNP transistors.
A P transistor (collector is grounded) and a third NPN transistor (collector connected to a power supply) whose emitter is connected to the bases of the first and second NPN transistors and whose base is connected to the collector of the first NPN transistor 4. The optical receiving circuit according to claim 3, further comprising: 5. The optical receiving circuit according to claim 1, wherein a low-frequency cut capacitor is provided between an input terminal of the negative feedback amplifier and an anode of the light receiving diode. 6. A reverse-biased light-receiving diode, a negative feedback amplifier having an input terminal connected to a cathode of the light-receiving diode, and a feedback resistor interposed between an output terminal of the negative feedback amplifier and the input terminal. In the optical receiving circuit, a smoothing unit for smoothing a photocurrent flowing to the anode side of the light receiving diode, a current detecting unit for detecting the smoothed photocurrent, and a current substantially equal to the detected photocurrent And a current source for causing a current to flow from the cathode side of the light-receiving diode to the ground side.