JP2002368549A - Automatic gain changeover system for front end amplifier for light receiving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To share one front end amplifier by a plurality of kinds of main signals being mutually different in frequency and also to improve a light reception sensitivity when the main signal with a low frequency is used. SOLUTION: The main signal is converted into a current by a photo diode 5, converted into voltage by gain which is decided by resistances 7 and 8 in the front end amplifier 6, amplified by a succeeding stage amplifier 10 and, then, inputted to a band-pass filter 11. The pass band of the band-pass filter 11 is the same as that of the frequency which is close to the upper limit of the band width of the front end amplifier 6 and a prescribed voltage is outputted when the frequency of the main signal is provided with a frequency component which is the same as that of the pass band. When the frequency of the main signal is close to the lower limit value of the band width of the front end amplifier 6, the prescribed voltage is not outputted from the band-pass filter 11. Then the gate voltage of a transistor 9 drops and the transistor 9 is turned off. Thus, only the resistance 7 is adopted as a feedback resistance in the front end amplifier 6 and gain is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an automatic gain switching method for a front end amplifier for an optical receiving circuit, which automatically switches the gain of the front end amplifier for an optical receiving circuit.

[0002]

2. Description of the Related Art FIG. 3 is a diagram showing a configuration example of a conventional optical receiving circuit.

In FIG. 3, an optical input signal, that is, a main signal is converted into a current by a photodiode 1, converted into a voltage in a front-end amplifier 2 with a gain according to a feedback resistor 3, and amplified by a next-stage amplifier 4. After that, it is output to the main amplifier.

In order to improve the S / N of the front-end amplifier 2, that is, the optical receiving sensitivity, the bandwidth of the front-end amplifier 2 is made sufficiently large, and the gain of the front-end amplifier 2, that is, the resistance value of the feedback resistor 3 is increased. Need to be larger.

However, when a high-frequency main signal is used, the S / N of the front-end amplifier 2 deteriorates.
When the bandwidth of the front-end amplifier 2 is increased for the purpose of using a high-frequency main signal, it is necessary to lower the gain of the front-end amplifier 2, that is, the resistance value of the feedback resistor 3, in order to secure S / N. .

[0006]

In the optical receiving circuit, a main signal having a high frequency f1 near the upper limit of the bandwidth of the front-end amplifier and a main signal having a low frequency f2 near the lower limit of the bandwidth of the front-end amplifier are used. In some cases, it is required to share one front-end amplifier and to improve the optical receiving sensitivity when using the main signal of frequency f2 than the optical receiving sensitivity when using the main signal of frequency f1. .

In order to meet the above demand, the frequency f2
When the main signal is used, it is possible to improve the optical reception sensitivity by increasing the feedback resistance in the front-end amplifier. However, this feedback resistance is set small for the purpose of using the main signal of the frequency f1. Therefore, there is a problem that when the main signal of the frequency f2 is used, only the same optical receiving sensitivity as when the main signal of the frequency f1 is used can be obtained.

It is an object of the present invention to share one front end amplifier for an optical receiving circuit with a plurality of types of main signals having different frequencies, and to improve the optical receiving sensitivity when using a main signal having a low frequency. An object of the present invention is to provide an automatic gain switching method for a front-end amplifier for an optical receiving circuit that can meet the demand when it is required.

[0009]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a main signal, which is an optical input signal converted into a current by a light receiving element, having a gain determined according to a resistance value of a feedback resistor. An automatic gain switching method for a front end amplifier for an optical receiving circuit that converts the voltage into a voltage and outputs the voltage, and an automatic determination unit for automatically determining the frequency of a main signal output from the front end amplifier for the optical receiving circuit. And the resistance value of the feedback resistor is switched according to the output of the automatic determination unit.

A predetermined pass band is set as the automatic discrimination section, and a predetermined voltage is set when the frequency of the main signal output from the front-end amplifier for the optical receiving circuit has the same frequency component as the pass band. And a band-pass filter for outputting the lowest potential voltage when the frequency of the main signal does not have the same frequency component as the pass band.

Further, the front end amplifier for an optical receiving circuit has two ends connected to each other as the feedback resistor.
Two resistance elements, and turns on / off according to the voltage output from the first and second bandpass filters, and turns on and off when the predetermined voltage is output from the bandpass filter. A transistor for connecting the other ends of the two resistance elements to each other.

Further, the pass band of the band-pass filter is a frequency at the upper limit of the bandwidth of the front-end amplifier for the optical receiving circuit.

A voltage converter for converting a voltage output from the band-pass filter into a DC voltage is provided, and the front-end amplifier for an optical receiving circuit includes a DC-DC converter for converting the voltage output from the band-pass filter into the DC voltage. It is characterized in that it is converted into a DC voltage by a unit and input.

Further, different pass bands are set as the automatic discriminating section, and a predetermined voltage is set when the frequency of the main signal output from the front end amplifier for the optical receiving circuit has the same frequency component as the pass band. And the first and second bandpass filters that output the lowest potential voltage when the frequency of the main signal does not have the same frequency component as the passband are arranged in parallel.

Further, the front end amplifier for an optical receiver circuit has a variable resistance value according to a voltage output from the first and second bandpass filters as the feedback resistor, and When the predetermined voltage is output from both of the bandpass filters, the resistance value becomes the lowest, and when the voltage at the lowest potential is output from both the first and second bandpass filters, the resistance value becomes the highest. And a programmable resistor.

Further, the pass band of one of the first and second band pass filters is the upper limit frequency of the bandwidth of the front end amplifier for the optical receiving circuit, and the other band pass filter is Is a frequency lower than the upper limit frequency of the bandwidth of the front end amplifier for an optical receiving circuit.

Further, first and second voltage converters for respectively converting voltages output from the first and second band-pass filters into DC voltages are provided, and the front-end amplifier for an optical receiver circuit comprises: The voltage output from the first and second band-pass filters is converted into a DC voltage by the first and second DC voltage units and input.

(Function) In the present invention configured as described above, an automatic discriminating section for automatically discriminating the frequency of the main signal is arranged, and an optical receiver circuit The resistance value of the feedback resistor in the front-end amplifier for the optical receiver circuit when the main signal with a low frequency is used because the gain of the front-end amplifier for the optical receiver circuit is switched. Can be increased, and the optical receiving sensitivity can be improved.

Thus, it is required that several types of main signals having different frequencies share one front end amplifier for an optical receiving circuit, and that the light receiving sensitivity is improved when a low-frequency main signal is used. In such a case, it is possible to meet the demand.

[0020]

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 is a diagram showing a first embodiment of an automatic gain switching method for a front-end amplifier for an optical receiving circuit according to the present invention.

In FIG. 1, an optical input signal, that is, a main signal, is converted into a current by a photodiode 5 as a light receiving element, and is converted into a voltage with a gain determined by resistors 7 and 8 in a front-end amplifier 6. After being amplified by the next-stage amplifier 10, a band-pass filter (BP) serving as an automatic determination unit
F) 11 is input.

The band-pass filter 11 outputs a predetermined voltage when the frequency of the main signal has the same frequency component as the predetermined pass band, and outputs the lowest potential voltage when the main signal does not have the same frequency component as the predetermined pass band. And its pass band is the same frequency as the high frequency near the upper limit of the bandwidth of the front-end amplifier 6.

The main signal that has passed through the band-pass filter 11 is converted into a DC voltage by a rectifier circuit 12 and a peak detection circuit 13 which are voltage converters, and then input to the transistor 9.

At this time, whether to use the resistor 8 as the feedback resistor of the front-end amplifier 6 is determined according to the voltage value input to the transistor 9.

More specifically, for example, when the frequency of the main signal is a high frequency close to the upper limit of the bandwidth of the front-end amplifier 6, that is, the band-pass filter 11
, The band-pass filter 11 vibrates to output a sine wave, and the rectifier circuit 1
2 and the DC voltage output from the peak detection circuit 13, that is, the gate voltage of the transistor 9 increases. As a result, the transistor 9 is turned on, and the feedback resistance of the front end amplifier 6 becomes a combined resistance of the resistances 7 and 8,
The resistance value of the feedback resistor of the front end amplifier 6, that is, the gain of the front end amplifier 6 becomes small.

On the other hand, when the frequency of the main signal is a low frequency close to the lower limit of the bandwidth of the front end amplifier 6, that is, when the main signal does not have the same frequency component as the pass band of the band pass filter 11, the band pass filter 11 The voltage of the lowest potential is output, and the output voltage of the peak detection circuit 13, that is, the gate voltage of the transistor 9 decreases. As a result, the transistor 9 is turned off, so that the feedback resistance of the front-end amplifier 6 becomes only the resistance 7, and the gain of the front-end amplifier 6 increases.

As described above, in this embodiment, when the frequency of the main signal is close to the lower limit of the bandwidth of the front end amplifier 6, the frequency of the main signal is
As compared with the case where the bandwidth is close to the upper limit, the resistance value of the feedback resistor of the front end amplifier 6, that is, the gain of the front end amplifier 6 can be increased, and the optical reception sensitivity can be improved.

Thus, the main signal whose frequency is close to the upper limit of the bandwidth of the front-end amplifier 6 and the main signal whose frequency is close to the lower limit of the bandwidth of the front-end amplifier 6 constitute one front-end. Even when it is required to improve the optical receiving sensitivity when the main signal is shared with the amplifier 6 and the frequency is close to the lower limit of the bandwidth of the front-end amplifier 6, the requirement can be met.

(Second Embodiment) This embodiment is different from the first embodiment in which one front-end amplifier is shared by two kinds of main signals having different frequencies, and three kinds of frequencies different from each other. The main signal shares one front-end amplifier.

The definition in the present embodiment is 3
The frequencies of the two main signals are f1, f2, f3 [H
z], f1 is four times the frequency of f2, f2 is four times the frequency of f3, and the upper limit of the bandwidth of the front-end amplifier is close to f1.

As a further definition, the S / N of the front-end amplifier when the main signal having the frequency f3 is used, that is, the optical receiving sensitivity is improved as compared with the optical receiving sensitivity when using the main signal having the frequency f2. It is assumed that the optical reception sensitivity when the main signal having the frequency f2 is used is required to be higher than the optical reception sensitivity when the main signal having the frequency f1 is used.

In the first embodiment, a transistor is used to switch the resistance value of the feedback resistor of the front-end amplifier. However, in this embodiment, three kinds of resistance values are required for the feedback resistor. Therefore, a 2-bit programmable resistor is used.

FIG. 2 is a diagram showing a second embodiment of the automatic gain switching method of the front end amplifier for an optical receiving circuit according to the present invention.

In FIG. 2, an optical input signal, that is, a main signal, is converted into a current by a photodiode 14 as a light receiving element, and then input to a front end amplifier 15.

As the front end amplifier 15, a 2-bit programmable resistor 16 is used to switch the resistance value of the feedback resistor of the front end amplifier 15.

The programmable resistor 16 is a component in which four types of resistors can be selected by changing the logic of a 2-bit input signal input to the input terminals A and B.

Table 1 shows the correspondence between the resistance of the programmable resistor 16 when the logic of the input signal input to the input terminals A and B of the programmable resistor 16 is changed, that is, the gain of the front end amplifier 15. Show.

[0039]

[Table 1]

Table 1 shows that the logic of the input signals input to the input terminals A and B is High when the frequency of the main signal is f1, and is Low when the frequency of the main signal is f2. , And High, and shows that both become Low when the frequency of the main signal is f3, which will be described in detail later.

A next-stage amplifier 17 for signal amplification is arranged at a stage subsequent to the front-end amplifier 15, one of the two outputs of the next-stage amplifier 17 is connected to the main amplifier, and the other output is an automatic discriminator. A certain bandpass filter (BP
F) 18 and 19 are connected.

The band-pass filter 18 outputs a predetermined voltage when the frequency of the main signal has the same frequency component as the predetermined pass band, and outputs the lowest potential voltage when the main signal does not have the same frequency component as the predetermined pass band. And the pass band is f1.

The band-pass filter 19 outputs a predetermined voltage when the frequency of the main signal has the same frequency component as the predetermined pass band, and outputs the lowest potential voltage when the main signal does not have the same frequency component as the predetermined pass band. And the pass band is f2.

The output of the band-pass filter 18 is connected to a rectifier circuit 20 and a peak detection circuit 21 which are voltage converters, and the output of the peak detection circuit 21 is connected to an input terminal A of the programmable resistor 16.

The output of the band-pass filter 19 is connected to a rectifier circuit 22 and a peak detection circuit 23 which are voltage converters, and the output of the peak detection circuit 23 is connected to an input terminal B of the programmable resistor 16.

The operation of the automatic gain switching system of the optical receiving circuit front-end amplifier configured as described above will be described below.

The optical input signal, that is, the main signal is converted into a current by the photodiode 14, converted into a voltage with a gain determined by the programmable resistor 16 in the front-end amplifier 15, and amplified by the next-stage amplifier 17.

After that, the main signal amplified by the next-stage amplifier 17 is input to the band-pass filters 18 and 19.

Here, when the frequency of the main signal is f3, since the frequency component is different from the pass band of the band-pass filter 18, the voltage of the lowest potential is output from the band-pass filter 18. On the other hand, the bandpass filter 19
Since the frequency component is different from the pass band of the band pass, the voltage of the lowest potential is also output from the band pass filter 19.

The lowest potential voltages output from the band-pass filters 18 and 19 are respectively supplied to the rectifier circuit 20 and the peak detector 21, the rectifier circuit 22 and the peak detector 23.
Are input to the input terminals A and B of the programmable resistor 16 as Low logic while maintaining the lowest potential voltage.

That is, when the frequency of the main signal is f3, both the A and B inputs of the programmable resistor 16 are L
Since the logic becomes ow, as shown in Table 1, the resistance value of the programmable resistor 16, that is, the gain of the front-end amplifier 15 is increased, and the optical reception sensitivity is improved as compared with the case where the main signal of frequency f2 is used. be able to.

When the frequency of the main signal is f2, the bandpass filter 19 has the same frequency component as the pass band of the bandpass filter 19, and the bandpass filter 19 vibrates to output a sine wave. On the other hand, the bandpass filter 18
Since the frequency component is different from the passband of the bandpass filter 18, the lowest potential voltage is output from the bandpass filter 18.

The sine wave output from the band-pass filter 19 is converted into a DC voltage corresponding to the High level input of the programmable resistor 16 by the rectifier circuit 20 and the peak detection circuit 21, and the input terminal B of the programmable resistor 16.
Is input as High logic. On the other hand, the lowest potential voltage output from the band-pass filter 18 is applied to the input terminal A of the programmable resistor 16 as Low logic while maintaining the lowest potential voltage via the rectifier circuit 20 and the peak detection circuit 21. Is entered.

That is, when the frequency of the main signal is f2, the A input of the programmable resistor 16 is logic low,
As shown in Table 1, the resistance of the programmable resistor 16, that is, the gain of the front-end amplifier 15 is medium, and the light reception sensitivity is higher than when the main signal having the frequency f1 is used. Can be improved.

When the frequency of the main signal is f1, since the frequency of the main signal is the same as the pass band of the band-pass filter 18, the band-pass filter 19 vibrates and a sine wave is output. Here, f1 is four times the frequency of f2 and has a frequency component of f2, so that a sine wave is also output from the bandpass filter 19.

The sine waves output from the band-pass filters 18 and 19 are converted into DC voltages corresponding to the High level input of the programmable resistor 16 by the rectifier circuit 20 and the peak detector 21, the rectifier circuit 22 and the peak detector 23, respectively. The signals are input to the input terminals A and B of the programmable resistor 16 as High logic.

That is, when the frequency of the main signal is f1, both the A input and the B input of the programmable resistor 16 have a logic of High, so that the resistance value of the programmable resistor 16 as shown in Table 1, that is, The gain of the front-end amplifier 15 becomes small, and an appropriate S / N can be secured.

As described above, in the present embodiment, as the frequency of the main signal decreases to f1, f2, and f3,
It is possible to increase the resistance value of the feedback resistor of the front-end amplifier 15, that is, the gain of the front-end amplifier 15, and improve the optical reception sensitivity.

As a result, one front-end amplifier 15 is composed of three types of main signals having frequencies f1, f2, and f3.
Can be satisfied even when it is required to improve the optical receiving sensitivity as the frequency of the main signal decreases to f1, f2, and f3.

[0060]

As described above, in the present invention,
An automatic discriminating unit for automatically discriminating the frequency of the main signal is provided, and the resistance value of the feedback resistor in the front-end amplifier for the optical receiving circuit according to the output of the automatic discriminating unit, that is, Since the configuration is such that the gain of the amplifier is switched, when a main signal having a low frequency is used, the resistance value of the feedback resistor in the front-end amplifier for an optical receiving circuit can be increased, and the optical receiving sensitivity can be improved.

Thus, it is required that several types of main signals having different frequencies share one front-end amplifier for an optical receiving circuit, and that the light receiving sensitivity is improved when a low-frequency main signal is used. In such a case, the request can be met.

As described above, in the present invention, several types of main signals having different frequencies can share one optical receiving circuit front-end amplifier, so that the versatility of the optical receiving circuit front-end amplifier is high. Is obtained.

Further, unlike the related art, it is not necessary to individually adjust the feedback resistance of the front-end amplifier for the optical receiving circuit with a fixed resistance according to the frequency of the main signal and manage the products separately, so that the number of management steps is reduced. Can be reduced, thereby improving the productivity of the front-end amplifier for the optical receiving circuit.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of an automatic gain switching method for a front-end amplifier for an optical receiving circuit according to the present invention.

FIG. 2 is a diagram showing a second embodiment of the automatic gain switching method of the front-end amplifier for an optical receiving circuit according to the present invention.

FIG. 3 is a diagram illustrating a configuration example of a conventional optical receiving circuit.

[Explanation of symbols]

 Reference Signs List 5 photodiode 6 front-end amplifier 7, 8 resistor 9 transistor 10 next-stage amplifier 11 band-pass filter (BPF) 12 rectifier circuit 13 peak detection circuit 14 photodiode 15 front-end amplifier 16 programmable resistor 17 next-stage amplifier 18, 19 band Pass filter (BPF) 20,22 Rectifier circuit 21,23 Peak detection circuit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) H04B 10/14 10/26 10/28 F term (Reference) 5J091 AA01 AA56 CA62 CA87 FA17 FA18 HA19 HA25 HA26 HA44 KA00 KA44 KA51 MA11 SA13 TA01 5J092 AA01 AA56 CA00 CA87 FA14 FA17 FA18 HA09 HA19 HA25 HA26 HA39 HA44 KA00 KA44 KA55 MA11 SA13 TA01 UL02 5J100 JA01 KA05 LA00 LA03 LA10 QA01 SA02 5K002 AA03 CA10

Claims (9)

[Claims] 1. A front-end amplifier for an optical receiving circuit, which converts a main signal, which is an optical input signal converted into a current by a light receiving element, into a voltage with a gain determined according to a resistance value of a feedback resistor and outputs the voltage. An automatic gain switching system, wherein an automatic determination unit for automatically determining the frequency of a main signal output from the optical receiving circuit front-end amplifier is arranged, and the feedback is performed according to an output of the automatic determination unit. An automatic gain switching method characterized by switching a resistance value of a resistor. 2. A predetermined pass band is set as the automatic discrimination section, and a predetermined voltage is set when a frequency of a main signal output from the front end amplifier for an optical receiving circuit has the same frequency component as the pass band. 2. The automatic gain switching system according to claim 1, further comprising a band-pass filter that outputs the lowest voltage when the frequency of the main signal does not have the same frequency component as the pass band. . 3. The front-end amplifier for an optical receiving circuit includes two resistance elements, one ends of which are connected to each other, as the feedback resistance, and according to a voltage output from the first and second bandpass filters. 3. The transistor according to claim 2, further comprising a transistor that turns on and off when the predetermined voltage is output from the band-pass filter, and turns on and connects the other ends of the two resistance elements to each other. 4. Automatic gain switching method. 4. The pass band of the band-pass filter is
4. The automatic gain switching method according to claim 2, wherein the frequency is an upper limit frequency of a bandwidth of the front end amplifier for the optical receiving circuit. 5. A front end amplifier for an optical receiving circuit, comprising: a voltage converter for converting a voltage output from the bandpass filter into a DC voltage, wherein the voltage output from the bandpass filter is a DC voltage. 5. The automatic gain switching method according to claim 2, wherein the voltage is converted into a DC voltage by a unit and input. 6. A pass voltage different from each other is set as the automatic discriminating unit, and a predetermined voltage is set when a frequency of a main signal output from the front end amplifier for an optical receiving circuit has the same frequency component as the pass band. And first and second band-pass filters for outputting the lowest potential voltage when the frequency of the main signal does not have the same frequency component as the pass band are arranged in parallel. 2. The automatic gain switching method according to 1. 7. The front end amplifier for an optical receiving circuit, wherein the feedback resistor has a variable resistance value according to a voltage output from the first and second band pass filters, and When the predetermined voltage is output from both of the bandpass filters, the resistance value becomes the lowest, and when the voltage at the lowest potential is output from both the first and second bandpass filters, the resistance value becomes the highest. The automatic gain switching method according to claim 6, further comprising a programmable resistor. 8. A pass band of one of the first and second band-pass filters is a frequency of an upper limit of a bandwidth of the front-end amplifier for the optical receiving circuit, and the other band-pass filter. 8. The automatic gain switching method according to claim 6, wherein the pass band of the frequency is lower than the upper limit frequency of the bandwidth of the front-end amplifier for the optical receiving circuit. 9. A first converter for converting voltages output from the first and second band-pass filters into DC voltages, respectively.
And a second voltage conversion unit, wherein the front-end amplifier for an optical receiving circuit is configured so that the voltages output from the first and second bandpass filters are applied to the first and second DC voltage units, respectively. 9. The automatic gain switching method according to claim 6, wherein the signal is converted into a DC voltage and input.