JPH02199934A - Conversion gain switching type optical reception circuit - Google Patents

Abstract

PURPOSE:To obtain a stable reception output over a broad input level by connecting a 2nd feedback resistor in parallel with a feedback resistor via a switching element, connecting a capacitor in parallel with the 2nd feedback resistor and closing the switching element when an incoming optical power is large. CONSTITUTION:A 2nd feedback resistor Rf 2 is connected in parallel with a feedback resistor Rf 1 via a switching element 3, and further a capacitor Cf is connected in parallel with the feedback resistor Rf 2. When the incoming optical power given is small, the switching element 3 is turned off. When the incoming optical power is large, the switching element 3 is turned on and the conversion gain is reduced by the decreasing combined feedback resistance Rf 2. By selecting the capacitance of the capacitor Cf properly in this case, both frequency characteristics are made equal. Thus, it is possible to switch the conversion gain without varying the frequency characteristic.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to an optical receiving circuit that converts an optical input signal received by a light receiving element into an electrical signal, and particularly relates to a conversion gain switching type optical receiving circuit with a wide dynamic range. It is something.

[Conventional technology] FIG. 5 shows the configuration of a conventional optical receiver circuit.

This optical receiving circuit has a negative feedback type configuration in which a feedback resistor Rf is connected between the input and output of the inverting amplifier 2, and has a wide band.
Widely used as a low-noise optical receiver circuit. The optical signal incident on the light receiving element 1 is converted into a current signal is and output as a voltage signal v0. At this time, the transimpedance (conversion gain) 2↑ of the receiving circuit is expressed by the following equation.

1S Here, Ao is the voltage gain of the inverting amplifier, C is the sum of the capacitances added to the input terminal of this optical receiving circuit, such as the capacitance between the terminals of the light receiving element and the input capacitance of the inverting amplifier, and ω is the angular frequency represents.

As can be seen from equation (1) above, the conversion gain at low frequencies is approximately equal to Rf, so by increasing the feedback resistance Rf, the conversion gain can be increased and the S/N ratio can be improved. By increasing the voltage gain Ao of the inverting amplifier, the input impedance can be reduced and a wide band can be achieved.

However, it is difficult to increase the dynamic range of an optical receiving circuit having such a configuration.

That is, when the incident optical power is large, the signal current i3 becomes large, so that a large potential difference is generated between the input and output of the inverting amplifier. When this level reaches or exceeds this level, the amplifier goes out of its normal operating range, and the output waveform significantly deteriorates due to saturation of the transistors constituting the amplifier.

In order to prevent such characteristic deterioration at the time of large input, an optical receiving circuit as shown in FIG. 6 has been devised.

In this circuit, a transistor 3 is connected between the input and output of the inverting amplifier 2 of the optical receiving circuit shown in FIG. 5, and the feedback impedance is changed by using the bias dependence of the output impedance of the transistor 3. That is, at the time of a large input, the potential of Vs is raised to lower the feedback impedance, and the conversion gain is reduced to keep the circuit within a normal operating range.

[Problems to be Solved by the Invention] According to the optical receiving circuit shown in FIG. In this case, if the band of the inverting amplifier 2 is sufficiently wide, the transfer characteristic will only become wider by the reduction in Rf, and no major problem will occur. However, in reality, the band of the inverting amplifier 2 is finite, so if the feedback resistor Rf is made small, the frequency characteristics may become unstable, such as peaking in the frequency response or even oscillation. There's a problem.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide a novel conversion gain switching type optical receiver circuit with variable conversion gain, stable frequency characteristics, and a wide dynamic range.

[Means for Solving the Problems] A first form of the conversion gain switching type optical receiver circuit of the present invention connects a feedback resistor between the input and output terminals of an inverting amplifier, and connects a light receiving element to the input terminal of the inverting amplifier. In an optical receiving circuit that connects the light receiving element and converts the signal light incident on the light receiving element into a voltage signal output, a second feedback resistor is connected in parallel with the feedback resistor via a switch element, and the second feedback resistor A capacitor is connected in parallel with the converter, and conversion gain adjusting means is provided to close the switch element when the incident optical power is large.

In a second embodiment, a feedback resistor is connected between the input and output terminals of an inverting amplifier, a light receiving element is connected to the input terminal of the inverting amplifier, and the signal light incident on the light receiving element is converted into a voltage signal output. In the receiving circuit, the inverting amplifier is configured to have a switchable voltage gain, a resistor is connected to the input terminal, the resistor is grounded via a switch element, and the switch element is closed when the incident optical power is large. At the same time, a conversion gain adjusting means for reducing the gain of the inverting amplifier is provided.

[Operation] In the optical receiving circuit of the first form, when the incident optical power becomes large, the switching element is closed by the conversion gain adjusting means. This creates a second resistor in parallel with the previous feedback resistor.
A feedback resistor is connected, lowering the overall feedback resistance value, reducing the conversion gain and widening the dynamic range. In addition, fluctuations in frequency characteristics can be suppressed by using a capacitor connected in parallel with the second feedback resistor, and if the value of the capacitor is selected appropriately, it is possible to maintain the same frequency characteristics as before the switch element was turned on. . Therefore, an optical receiving circuit with stable frequency characteristics and a wide dynamic range is provided.

In addition, in the optical receiver circuit of the second form, when the incident optical power increases, the conversion gain adjustment means closes the switch element and at the same time lowers the gain of the inverting amplifier to a small value. While the range becomes wider, the input terminal of the inverting amplifier is grounded via a resistor, and the additional input impedance of the circuit is reduced, so fluctuations in frequency characteristics are suppressed. By appropriately selecting the value of the resistor, it is possible to obtain the same frequency characteristics as before the switching element was turned on. Therefore, similarly, amplification with stable frequency characteristics and a wide dynamic range can be achieved.

[Example] Hereinafter, the present invention will be explained according to illustrated embodiments.

Figure 1 shows the basic circuit configuration.

5, this optical receiving circuit connects a light receiving element 1 biased by a voltage V to the input terminal of an inverting amplifier 2 with a voltage gain Ao, and also has a feedback resistor between the input and output terminals of the inverting amplifier 2. In the optical receiving circuit connected to Rf1,
A second feedback resistor Rf2 is connected in parallel with the feedback resistor Rfl via a switch element 3, and a capacitor Cf is further connected in parallel with the feedback resistor Rf2. C is the terminal capacitance of the photodetector, the input capacitance of the inverting amplifier, etc.

This is the total sum of capacitances added to the input terminals of this optical receiving circuit.

When the incident optical power is small, the switch element 3 is turned off. The operation of the optical receiving circuit in this off state is similar to that shown in FIG. 5, and the transimpedance Zr of the optical receiving circuit is equal to the equation (1) in which the feedback resistor Rf is replaced with Rfl.

On the other hand, when the incident optical power is large, the switch element 4 is turned on. The transimpedance Zr at this time is expressed by the following equation.

1+A.

However, the symbol j represents calculation of a parallel circuit of circuit elements before and after it.

From equation (2) above, the conversion gain is 1 +A.

Therefore, by reducing the feedback resistance Rf2,
The conversion gain when the switch element 4 is turned on can be reduced. Furthermore, +1) From the comparison of equation (2), the value of capacitor Cf at this time is 1+Ao (R
f1/Rf2), the frequency characteristics of both become equal. Therefore, it is possible to switch the conversion gain without changing the frequency characteristics.

FIG. 2 shows a specific example of the configuration of the optical receiving circuit.

The inverting amplifier 2 is composed of an intensifying transistor Q1 and an output huffer (emitter follower) transistor Q2. Diode D1 corresponds to switch element 4 in FIG. 1, and transistor Q3. A constant current source constituted by Q4 supplies a current flowing to diode D1. Transistor Q5 operates as a switching element that turns on and off this constant current source. Note that Vc is a DC power supply for the entire circuit.

When the incident optical power is small, a conversion gain adjusting means (not shown) sets the voltage of the control signal Vs to the transistor Q5 to H level. At this time, transistor Q5 is turned on, transistor Q3. Since Q4 is off, diode D
No current flows through Rf1, it is in a high impedance state, and the conversion gain is approximately equal to the feedback resistor Rf1.

On the other hand, when the incident optical power is large, the control signal Vs becomes L level by the conversion gain adjustment means.

At this time, transistor Q5 is off, transistor Q3.
Since Q4 is turned on, current flows through diode D1,
It becomes a low impedance state. The conversion gain at this time is
It becomes approximately Rf1//Rf2.

Therefore, the conversion gain at the time of switching can be reduced by reducing the feedback resistor Rf2. Furthermore, by setting the capacitor Cf to the value described above, the frequency characteristics hardly change even when the conversion gain is switched.

As mentioned above, the feedback resistor Rf2 of the receiving circuit is switched by a switch element depending on the magnitude of the incident optical power, and a capacitor C for characteristic compensation is connected in parallel with this feedback resistor Rf2.
By connecting f, conversion gain switching and frequency response stabilization are achieved. Furthermore, since the capacitor Cf for characteristic compensation can be as small as or smaller than the input additional capacitance C, it can be easily formed within an IC and is suitable for integrated circuit implementation.

In the example shown in FIG. 2, the diode D1 is used as the switch element 3, but the same characteristics can be achieved using an FET.

FIG. 3 shows another basic circuit configuration of the optical receiving circuit of the present invention.

The inverting amplifier 4 changes its voltage gain A0 to a high gain (Ad =
AOhigl+) and low gain (A0=A01ow).

Further, one end of a resistor Ri is connected to the input terminal of the inverting amplifier 4, and the other end of this resistor Ri is grounded via a switch element 5. On/off of the switching element 5 and switching of the voltage gain of the inverting amplifier 4 are simultaneously controlled by the same control signal Vs.

When the incident optical power is small, this circuit operates in the same manner as in FIG. 5 by turning off the switching element 5 and simultaneously increasing the voltage gain A0 of the inverting amplifier 4 (A o =A o h+gh).

On the other hand, when the incident optical power is large, a control signal Vs is output from a conversion gain adjusting means (not shown), thereby turning on the switch cable 5 and simultaneously reducing the voltage gain A0 (AOAo IOW). At this time, the conversion gain 2 is approximately expressed by the following equation.

ZT = AO(LOW)Ri ・10JωcRt As is clear from comparing the above equation 1(3), 1 mo Ao
(high), both can have substantially the same frequency characteristics. Further, by adjusting the value of Ao(low), a desired conversion gain can be obtained without affecting the frequency characteristics.

FIG. 4 shows a specific configuration of the optical receiving circuit shown in FIG. 3.

The inverting amplifier 4 is composed of an amplification stage transistor Q9 and an output buffer (emitter follower) transistor Q10. Transistor Q6. Q7 serves as a switching element 5 between the input resistor Ri and ground, and also as a transistor Q.
8 operates as a gain changeover switch element of the inverting amplifier 4.

When the incident optical power is small, the output voltage of the control signal Vs from a conversion gain adjusting means (not shown) is set to H level.

At this time, transistor Q6. Q8 is on, transistor Q7 is off, and no current flows through diodes D3 and D4 connected to the emitter of transistor Q7.
The input impedance of the optical receiver circuit becomes high, and the voltage gain of the inverting amplifier becomes approximately R3/(R1/R2). Therefore, in this case, it normally operates as a transimpedance type circuit, and R3#R1/R2> If is large enough, the conversion gain is approximately equal to Rf.

Also, the -3dB band is approximately I R3 2πRfCR1/R2 It is.

On the other hand, when the incident optical power is large, the voltage of the control signal Vs of the conversion gain adjustment means is set to L level.

At this time, transistor Q6. Q8 is turned off, transistor Q7 is turned on, and diode D3. Since a current flows through D4, the additional input impedance of the optical receiving circuit becomes low. Also, the voltage gain of the inverting amplifier is approximately R3/R2
becomes. The conversion gain in this case is approximately R1(R3/R2
), and the -3dB band is 1/(2πRiC),
Therefore, by doing so, even when switching the conversion gain, it is possible to obtain approximately the same stable frequency characteristics.
Furthermore, by changing R3/R2, it is possible to change the conversion gain at the time of switching.

In this way, when the incident optical power is large, by reducing the input additional impedance of the optical receiving circuit and at the same time reducing the voltage gain of the inverting amplifier, switching of the conversion gain and stabilization of the frequency response can be achieved.

[Effects of the Invention] Since the optical receiving circuit of the present invention is configured as described above, the following effects can be obtained.

(1) By switching the conversion gain, stable reception output can be obtained over a wide range of input levels.

(2) Fluctuations in frequency characteristics due to conversion gain switching are small, and a stable frequency response can be obtained over a wide range of optical input levels.

(3) The above effects can be achieved with a simple circuit configuration.

(4) A high-performance optical receiver circuit that is suitable for integrated circuits can be realized.

[Brief explanation of the drawing]

Fig. 1 is a basic circuit diagram showing one embodiment of the present invention, Fig. 2 is a circuit diagram showing a specific configuration of Fig. 1, and Fig. 3 is a basic circuit diagram showing another embodiment of the invention. FIG. 4 is a circuit diagram showing the specific configuration of FIG. 3, and FIGS. 5 and 6 are circuit diagrams showing conventional optical receiving circuits. In the figure, 1 is a light receiving element, 2 is an inverting amplifier circuit, 3°5 is a switch element, 4 is a variable gain inverting amplifier, Rf, Rfl
, Rf2 is a feedback resistor, Ri is a resistor, Cf is a capacitor,
C indicates input additional capacitance. Patent Applicant Hitachi Cable Co., Ltd. Representative Patent Attorney Nobuo Kinutani No. 1 Figure 3 Rf Figure 2 Figure 4

Claims (1)

[Claims] 1. A feedback resistor is connected between the input and output terminals of an inverting amplifier, a light receiving element is connected to the input terminal of the inverting amplifier, and the signal light incident on the light receiving element is converted into a voltage signal output. In the optical receiving circuit, a second feedback resistor is connected in parallel with the feedback resistor via a switch element, and a capacitor is connected in parallel with the second feedback resistor, so that when the incident optical power is large, the switch element A conversion gain switching type optical receiving circuit characterized in that a conversion gain adjustment means for closing the conversion gain is provided. 2. In an optical receiving circuit that connects a feedback resistor between input and output terminals of an inverting amplifier, connects a light receiving element to the input terminal of the inverting amplifier, and converts signal light incident on the light receiving element into a voltage signal output, The inverting amplifier is configured such that the voltage gain can be switched, and a resistor is connected to the input terminal, and the resistor is grounded via a switch element, so that when the incident optical power is large, the switch element is closed and the inverting amplifier is closed at the same time. 1. A conversion gain switching type optical receiving circuit, characterized in that a conversion gain adjustment means for reducing the gain is provided.