JPS61177041A - Photodetecting circuit - Google Patents

Abstract

PURPOSE:To execute optical transmission stably by a simple circuit constitution without requiring external adjustment by including a peak value holding circuit for inputting an output from a photodetecting circuit, an LPF for inputting the output of the peak value holding circuit and a means for adjusting the bias current of a photodetector on the basis of the output of the LPF. CONSTITUTION:When an input optical current ip to a pre-amplifier 8 is excessive, the waveform of an output voltage V1 is distorted as shown in Fig.C and the output V1 is turned to an output voltage V2 shown in (b) by the peak value holding circuit 12. The discharge time constant of a capacitor 10 is increased sufficiently larger than the period of an optical pulse signal P and the capacity is selected so that the change of the holding voltage can be neglected at the detection of the rise of the pulse. The output V2 is passed through the LPF17 consisting of a resistor 13(R4), a capacitor 14(C2) and an operational amplifier 15 and having the frequency characteristic of (fc=1/2piR4 C2)-20dB/decade about frequency exceeding fc and outputted as a smoothed voltage V3 regarded as a steady DC voltage for the period of the signal P. The bias current of the photodetector 2 can be adjusted by the voltage V3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a receiving circuit for an optical transmission system such as an optical fiber one-way system or an optical space propagation system, which is realized by a combination of optical transmitting/receiving modules.

[Conventional technology]

Recently, optical 7-eye fiber transmission has the advantages of excellent resistance to electromagnetic induction, eliminates ground loops, and has high insulation properties.In addition, it is possible to configure a system at a low cost, and it requires no optical engineer. It also has the general-purpose characteristics of being easy to handle.
It is attracting attention in fields such as automobile parts and simple LAN. Regarding the performance of the optical transceiver module used for this plastic fiber optical transmission, the transmission distance is around Om to several tens of meters, and the transmission speed is D-C to several M) 1) S or more. There are tremendous market demands.

On the other hand, in order to amplify the light receiving current generated in the light receiving element, as shown in Fig. 4, the power supply (+V) is connected to a resistor 20.
The light receiving element 2 receives the electric current N,i based on the optical signal through an inverting amplifier 3 with a sufficiently large gain and a feedback resistor 5.
A trans-impedance type amplification system consisting of the following is most commonly used due to its high input impedance, wideband operation, and other advantages. Before this trans-impedance type i! A good signal is amplified by an amplifier and then amplified by an appropriate processing section. The characteristics of the preamplifier of this optical receiver module are the minimum amount of 4g for optical transmission systems.
It will have the most influence on the level and frequency band. And the value R of feedback resistor 5? is determined by the sensitivity and frequency band of the preamplifier. However, when the amount of feedback is large, the frequency band is wide but the sensitivity is poor, and when the sensitivity is good, the frequency band is narrow, and it is impossible to bring out the maximum performance of both at the same time. Therefore, the value F of the feedback resistor 5 is determined in consideration of the balance between the two conflicting performances.

[Problem that the invention seeks to solve]

Generally, in order to enable long-distance transmission, a large value is selected for the value R of the feedback resistor 5. The input current of the amplifier becomes excessive, and the elements that make up the amplifier operate in saturation, which can lead to situations such as distortion of the output waveform or the inability to shape the waveform, especially in high-speed pulse operation.

The following method has been used to adjust the amplification sensitivity based on the distance of such a transmission system.

(1) The usable transmission distance 1IIl is limited depending on the sensitivity of the transmitter/receiver module.

(2) Manually adjust the amount of driving current for the light emitting element according to the transmission distance.

(3) One-in-one optical variable attenuator! ! to adjust the intensity of the light according to the transmission distance.

(4) A preamplifier with an AGC function is used, and the amplification sensitivity is determined according to the transmission distance.

There are four ways to do this.

Each of the above-mentioned conventional solutions has the following drawbacks.

Method (1) is the most reliable method in terms of operational stability, but lacks flexibility and cannot be used as a general-purpose component.

Furthermore, methods (2) and (3) require external setup when used, making them difficult to use for general users who do not have interrogation knowledge. Furthermore, since the optical variable attenuator is a precision component, it is expensive and takes up wasted space and money. Finally, method (4) has the problem that the AGC function has a large variable gain range, which makes the circuit extremely complex and, like the variable optical attenuator, expensive.

[Means for solving problems]

The optical receiving circuit of the present invention includes a light receiving element that converts an optical signal into an electrical signal, a preamplifier that amplifies the light receiving current generated in the light receiving element, and a peak value hold that detects and holds the output peak value of the preamplifier. a low-pass filter that smoothes the output value of the peak value hold circuit to increase the DC voltage; and a low-pass filter that adjusts the DC output voltage of the low-pass filter to adjust the light-receiving element's bias current R and the light-receiving current of the light-receiving element. and a light receiving circuit having means for applying negative feedback to the light receiving circuit.

[Implementation row]

Next, with reference to the drawings, we will explain the manufacturing details I/cd of the present invention.
I'll tell you my brother.

FIG. 1 is a block diagram showing an embodiment of the present invention. When the light receiving element 2Kg2 is incident on the light pulse No. 100 shown in FIG. 2(a), as shown in FIG. A photocurrent i flows such that The photocurrent 50 is the trans-impedance type inverting amplifier 3.4'f:2 which was mentioned earlier.
Current-voltage conversion and common-mode amplification are performed in the preamplifier 8 directly connected to the stage, and the output voltage vl is determined by the feedback resistor 5 (reduced), the resistor 6 (Rs), and the feedback resistor 7 (approximately 3 degrees). It is known that the following formula is determined by ・It should be noted that Avl and Ay2 are the voltage amplification arm of the inverting amplifier, Ay
is the product of both.

If the input to the preamplifier 8 is excessive, the waveform of the output voltage v1 will be distorted, resulting in a waveform as shown in FIG. ) and an operational amplifier 11, the peak value hold circuit 12 detects and holds the maximum value of the output signal of the device amplifier 8, resulting in the output voltage v shown in FIG. 2(d). Here, when the input signal to the peak value hold circuit 12 becomes less than the peak value, the charge in the capacitor 10 is discharged due to leakage of the transistor constituting the diode 9 and the operational amplifier 11, but this discharge time constant Make the periodic operation 0 of Melf No. 16 sufficiently large,
The capacitance C1t of the capacitor 10 is selected so that the change in the holding voltage can be ignored when detecting the rise of the next pulse.

The output from this peak value hold circuit 12 is composed of a resistor 13 (几4), a capacitor 14 (Cz), an operational amplifier 15, and a variable power supply 19, and the output exceeds fot
For a pair, (, f, = 1/2πR<Cz) ~ 20 d
It passes through a low-pass filter 17t having a frequency characteristic of B/f, and if it is compared with the period of the pulse signal, it can be regarded as a steady DC voltage and is smoothed as a specific voltage 3 as shown in Figure 2. The output is as shown in (e).

This output voltage v3 is applied to the gate of the P-channel FET 1 through a resistor 17t, and the bias I of the light receiving element 2 is
Adjust the E flow. Incidentally, here, the variable power supply 19 is FET1
t- Adjust the potential of the specific voltage v3 which is smoothed when not in the cut-off state.

And this error potential V3[) transistor band
It can be easily created using a gap.

In this way, a constant voltage v3 proportional to the light flow i is obtained, and when the light receiving element 2 is a photodiode, the drain and source are connected between the cathode of the photodiode and the resistor 18. It is fed back to the gate of P-channel FETI. The resistor 17 has the effect of smoothing the change in the amount of feedback to the PET 1 in response to a sudden change in optical signal intensity, and the resistor 18 has the effect of smoothing the change in the amount of feedback to the PET 1 in response to a sudden change in optical signal intensity.
One theory is that it has the effect of preventing electrostatic damage to the light receiving element 2.

Now, if the source potential of FETI is 5, the gate-source voltage is V. 8 is given by the following equation.

V, 5=V3-V, =AVtp-V, -・-・-
123 Furthermore, knowing the conductance of the FETI, the current "D8" flowing between the drain and source is given by the following equation.

Ins=gmVes=gm(Avip-v3) ++
H+HH (3JHere, ENG., is photoreceptor fit,
It represents the allowable amount of '%'1) -ID, -c al,
Therefore, in order to know i max at which the input of the preamplifier 80 is saturated, in equation (3), l p 1718
Considering ip” I D6, ipmax=gm(Avipmax-V3)・”・・
(4) Solving equation (4) for ipmax yields ipm
ax = and to determine an error gm that satisfies equation (5), the input to the preamplifier 8 is ip<ip
It becomes possible to limit it to rmx.

For ease of understanding, the correlation between the incident light iP on the light receiving element 2 and the input current iin to the preamplifier is shown in FIG. The dotted line shows an example of a receiving circuit in the conventional method, and the solid line shows an example of the receiving circuit according to the present invention.

〔Effect of the invention〕

As explained above, the present invention is very effective in limiting excessive input current with a simple circuit configuration for optical transmission typified by plastic fibers, without impairing the band characteristics or sensitivity of the light receiving circuit. This has the effect of stably and reliably transmitting light from one distance to another without requiring external adjustment. Further, the circuit according to the present invention can be easily made monolithic, and when applied to plastic fiber optical transmission, it is expected to have great commercial value for medium and short distances. Furthermore, as a side effect, the effect of absorbing output deterioration of the light emitting element can be expected.

Furthermore, the present invention is not limited to the above embodiments, but
The P-channel FET can be replaced with a pi-polar balance transistor, and if the output waveform of the preamplifier is in reverse phase, the peak value hold circuit in the latter stage is changed to the minimum value hold circuit, and the P-channel FETt-N-channel FET is used. You can expect the same effect as L9 if you do this.

[Brief explanation of drawings]

FIG. 1 is a circuit configuration diagram of an optical receiving circuit according to an embodiment of the present invention, i! 2(a) to 2(e) are waveform diagrams of various parts of the optical receiver circuit in FIG. 1, and FIG. 3 is a correlation diagram between the ninth incident light iP and the input light receiving current iin, which clearly represents the young fruit according to the present invention. It is. FIG. 4 is a circuit diagram of a conventional optical receiving circuit. 1... P-channel PET, 2... Light receiving element, 3.4... Inverting amplifier, 5, 6.7...
...Resistor, 8...Preamplifier, 9...
...Diode, 10...Capacitor, 1
1...Operation amplifier, 12...Peak value hold circuit, 13...Resistor, 14...
... Capacitor, 15 ... Operational amplifier, 16.
...Low pass filter, 17, 18...
・Resistor, 19...Variable power supply, 20...
・Signal processing system.

Claims (1)

[Claims] A light receiving element that converts an optical signal into an electrical signal, a light receiving circuit that inputs the current from the light receiving element, a peak value hold circuit that inputs the output voltage of the light receiving circuit, and an input voltage of the peak value hold circuit. 1. An optical receiving circuit comprising: a low-pass filter; and means for adjusting a bias current flowing to the light-receiving element according to an output voltage of the low-pass filter.