JP2003258580A - Optical reception circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical reception circuit wherein no effect is given on a high frequency characteristic and waveform distortion is less caused even in the case of automatic gain control by an AGC (auto gain control) circuit. <P>SOLUTION: The optical reception circuit includes: a preamplifier 202 for converting a current signal into a voltage signal and outputting the converted signal; a low pass filter 103 for smoothing the voltage signal outputted from the preamplifier 202 and outputting a low pass filter signal; and an automatic gain control circuit 205 having a first input receiving the low pass filter signal passing through the low pass filter 103, a second input for receiving the voltage signal from the preamplifier, amplifying the low pass filter signal received at the first input, controlling the high frequency gain with the voltage signal received at the second input, and outputting a feedback signal to the preamplifier 202 to control a current-voltage conversion efficiency of the preamplifier 202. <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 receiving circuit, and more particularly to an optical receiving circuit having an automatic gain adjusting circuit for automatically adjusting the conversion efficiency of a converter for converting an optical signal into an electric signal.

[0002]

2. Description of the Related Art An optical receiving circuit is a circuit for converting an optical signal emitted from an optical fiber into an electric signal, and a light receiving device for converting the optical signal into a current signal and a current signal into a voltage signal. , And an amplifier that amplifies to a voltage required for a circuit connected in the subsequent stage.

FIG. 1 shows the configuration of a conventional optical receiving circuit.
The light receiving circuit 100 includes a light receiving device (PD) 10 that converts an optical signal emitted from the optical fiber 106 into an electric signal.
1 and the output of the PD 101, and the inverting amplifier 121
And a preamplifier 102 composed of a feedback resistor 122,
It has a low-pass filter (LPF) 103 composed of a resistor (R) 131 and a capacitor (C) 132, and a main amplifier 104 composed of two stages of differential amplifiers 141 and 142. The optical receiver circuit 100 also includes an automatic gain control (AGC) circuit 105 that feeds back the output of the preamplifier 102 and adjusts the gain of the preamplifier 102. PD101 is P-type-i-n
PIN-PD (Photo Diode), APD (Avalanche
Photo Diode) or the like can be used.

The PD 101 receives an optical signal emitted from the optical fiber 106 and outputs a current signal (Ph) corresponding to the optical signal.
oto Current). Since the input impedance of the inverting amplifier 121 of the preamplifier 102 is set to be extremely high, most of the current signal flows into the feedback resistor 122 and is sucked into the output stage of the inverting amplifier 121.
A potential difference is generated across the feedback resistor 122, and the output potential of the preamplifier 102 changes correspondingly. Such operation of the preamplifier 102 is called current / voltage conversion. When the light intensity from the optical fiber 106 is large and the current signal from the PD 101 is large, the output potential of the preamplifier 102 decreases. On the contrary, when the light intensity from the optical fiber 106 is low, the output potential of the preamplifier 102 rises.

The output of the preamplifier 102 is guided to one input (positive phase input) of the differential amplifier 141 of the main amplifier 104. The other input (differential phase input) of the differential amplifier 141
A low-frequency signal that has passed through the LPF 103 from the output of the preamplifier 102 is input to the. This low frequency signal is an instantaneous average value (R131 of the signal output from the preamplifier 102.
And below the time constant of C132). Main amplifier 10
Reference numeral 4 amplifies a difference between two signal inputs, that is, a signal corresponding to the data superimposed on the optical signal and its instantaneous average value to generate complementary outputs POUT and NOUT.

The dynamic range of the optical signal input to the light receiving device covers a wide range of -40 dBm to +10 dBm. It is difficult to provide the preamplifier circuit with such a wide dynamic range. When the circuit is designed for a small signal, the circuit is saturated for a large signal of +10 dBm. In the extreme case, the output may decrease with increasing input. Conversely, if the circuit is designed for a large signal, the output may not even appear for a small signal. In order to overcome this problem, it is common to use an AGC circuit that dynamically changes the gain characteristic of the preamplifier according to the magnitude of the signal.

In FIG. 1, the AGC circuit 105 has a single-stage configuration of the inverting amplifier 151. This inverting amplifier 15
A low-frequency signal that has passed through the LPF 103 from the output of the preamplifier 102 is input to 1. The output of the inverting amplifier 151 is connected to the gate of the FET 152 inserted in parallel with the feedback resistor 122 of the preamplifier 102.

When the average value of the optical signal from the optical fiber 106 increases, the output potential of the preamplifier 102 decreases. As the output potential decreases, the input potential of the inverting amplifier 151 of the AGC circuit 105 also decreases, and the output potential of the inverting amplifier 151 rises. Then, since the gate potential of the FET 152 also rises, the current flowing between the drain and source of the FET 152 also rises. The increase in the drain-source current apparently means that the ON resistance of the FET 152 becomes smaller, and the value of the feedback resistor 122 also becomes smaller. Therefore, the current / voltage conversion gain also decreases, and the output potential of the preamplifier 102 increases. This means that the gain decreases as the signal input increases. When the optical signal from the optical fiber 106 becomes smaller, reverse feedback works and the gain increases. In this way, automatic gain control is possible.

[0009]

[Problems to be Solved by the Invention] However, AGC
By the feedback control by the circuit 105, the preamplifier 102
However, there is a problem that the band characteristic of is impaired. FET 15 inserted in parallel with the feedback resistor 122 of the preamplifier 102
This is because the capacitance (Cgs, Cds, Cgd) due to 2 is inserted in parallel with the feedback resistor 122, and the frequency characteristic of the preamplifier 102 is impaired.

Further, the capacitance Cgs caused by the FET 152
Changes when the received optical signal emits light. Therefore, the voltage signal, which is the output of the preamplifier 102, corresponding to the change from the extinction to the light emission of the optical signal is blunted. There is also a problem that waveform distortion occurs because the capacitance changes between light emission and extinction.

Further, as the load of the preamplifier 102,
Since the LPF 103 composed of R131 and C132 is newly connected, there is also a problem that the frequency characteristic of the preamplifier 102, particularly the frequency characteristic in the high frequency range, is affected.

The present invention has been made in view of the above problems, and an object thereof is to not affect the frequency characteristic in the high frequency range even if the automatic gain control by the AGC circuit is performed, and to prevent the waveform distortion. It is to provide a light receiving circuit with less power consumption.

[0013]

In order to achieve such an object, the present invention provides an optical receiver connected to a light receiving device for converting a received optical signal into a current signal. In the circuit, a preamplifier that converts the current signal into a voltage signal and outputs the voltage signal, a low-pass filter that smoothes the voltage signal output from the preamplifier and outputs a low-pass filtered signal, and the low-pass filter A first input for inputting the low-pass filtered signal that has passed through, and a second input for inputting the voltage signal from the preamplifier,
Amplifying the low-pass filtered signal input to the first input;
An automatic gain control circuit for controlling a high-frequency gain by the voltage signal input to the second input, outputting a feedback signal to the preamplifier, and controlling a current-voltage conversion efficiency of the preamplifier. Is characterized by.

According to this configuration, the automatic feedback control circuit operates by the negative feedback through the low-pass filter and the positive feedback by the output of the preamplifier to increase the gain in the high frequency band, and the automatic gain control circuit. It is possible to correct the deterioration of the high frequency characteristic due to.

According to a second aspect of the present invention, the preamplifier according to the first aspect includes an amplifier and a feedback resistor that connects an input and an output of the amplifier, and the feedback from the automatic gain control circuit. A signal is input to a control terminal of a transistor connected in parallel to the feedback resistor to change a transimpedance formed by the feedback resistor and the transistor to control the current-voltage conversion efficiency. Characterize.

According to a third aspect of the present invention, the automatic gain control circuit according to the first or second aspect includes an amplifier for amplifying the voltage signal from the preamplifier and inputting the amplified voltage signal to the second input. The amplitude of the voltage signal of the preamplifier and the amplitude and phase of the feedback signal of the automatic gain control circuit are substantially equal in a frequency band equal to or higher than the cutoff frequency of the low pass filter.

According to a fourth aspect of the present invention, the preamplifier according to the first aspect includes an amplification stage including a voltage gain control transistor connected to a load element connected to the amplification transistor, and an output of the amplification stage. An output stage that is a buffer, and a feedback resistor that connects the input of the amplification stage and the output of the output stage are provided, and the feedback signal from the automatic gain control circuit is input to the control terminal of the gain control transistor. Then, the current-voltage conversion efficiency is controlled by controlling the gain of the amplification stage.

According to a fifth aspect of the present invention, the optical receiving circuit according to any one of the first to fourth aspects includes a main amplifier for amplifying the voltage signal output from the preamplifier, the preamplifier and the preamplifier. The main amplifier and the automatic gain control circuit are integrally integrated and configured.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. (First Embodiment) In this embodiment, the AGC control circuit is a double feedback circuit. Conventional AGC shown in FIG.
The input signal of the circuit 105 was controlled only by the signal that passed through the LPF 103. In addition to this, in the present embodiment, in order to compensate for the high frequency characteristic of the AGC circuit, the output of the preamplifier is directly input to the AGC circuit. Feedback through the low-pass filter is negative feedback, and feedback by the output of the preamplifier is positive feedback. That is, the conventional AG
In order to correct the deterioration of the high frequency characteristic due to the C circuit, it operates so as to increase the gain of the AGC circuit in the high frequency region. Hereinafter, a specific circuit configuration will be described.

FIG. 2 shows the configuration of the optical receiving circuit according to the first embodiment of the present invention. The optical receiver circuit 200 is connected to the light receiving device (PD) 101 that converts an optical signal emitted from the optical fiber 106 into an electrical signal, and the output of the PD 101, and is a preamplifier 202 that includes an inverting amplifier 221 and a feedback resistor 222. And a low pass filter (LP) including a resistor (R) 131 and a capacitor (C) 132.
F) 103 and a main amplifier 104 composed of two stages of differential amplifiers 141 and 142. Further, the optical receiving circuit 200 feeds back the output of the preamplifier 202,
It has an AGC circuit 205 for adjusting the gain of the preamplifier 202.

The PD 101 is an InGaAs PIN-
PD or APD can be used. The light receiving wavelength band can be arbitrarily selected to be 1.3 μm band or 1.55 μm band depending on the composition of InGaAs. The diameter of the sensitive area of the light receiving surface is 30 μm to 100 μm. If it is small, the capacity will be small and high speed operation is possible,
The light receiving sensitivity is lowered, and the alignment work with the optical fiber becomes difficult.

FIG. 3 shows the circuit configuration of the preamplifier of the optical receiving circuit according to the first embodiment of the present invention. The preamplifier 202 includes an FET (Tr1) one amplification stage and an FET
Inverting amplifier 22 having (Tr2, Tr3) output stage
1 and a feedback resistor 222 of 1 kΩ. The value of the feedback resistor 222 is appropriately selected according to the current / voltage conversion efficiency. If the resistance value is increased, the conversion efficiency is increased, but the frequency characteristic of the preamplifier 202 is deteriorated.
When the bandwidth of the preamplifier 202 is fw and the resistance value of the feedback resistor 222 is Rf, fw × Rf has a fixed relationship.

In order to self-bias the gate of the FET (Tr1), a diode D3 is inserted in the source.
Since one diode has a voltage drop of about 0.75V, the gate is biased by about -0.75 with respect to the source. The load resistance RL is 1 kΩ. The gain of the amplification stage is determined by RL × Wg × gm, where Wg is the gate width of the FET (Tr1) and gm is the mutual conductance. If Wg is increased, the gain of the preamplifier 202 increases, but the input capacitance of the FET (Tr1) increases, so that the band characteristic of the preamplifier 202 deteriorates. The values of RL, Wg, Rf, and gm are determined by comprehensively judging the gain and the band.

The output stage has a source follower structure composed of FETs (Tr2, Tr3) and diodes D1, D2. The FET (Tr3) has a gate and a source short-circuited and has a function of a current source. The diodes D1 and D2 are
Insert to drop the potential. Diodes D1 and D
The capacitor C1 may be connected in parallel with 2. The high frequency signal functions as a speed-up capacitor because the capacitor C1 bypasses the diodes D1 and D2. Although the capacitance value of the capacitor C1 depends on the signal frequency, it is preferable to select several pF for a signal of several GHz.

The output of the preamplifier 202 is the diode D
From the cathode of 2 to the positive phase input of the main amplifier 104 via the LPF 103, from the source of the FET (Tr2),
It is connected to the positive feedback input of the AGC circuit 205. These two outputs are exactly the same in phase except for the DC level.

FIG. 4 shows a circuit configuration of the LPF of the optical receiving circuit according to the embodiment of the present invention. The output of the preamplifier 202 is directly connected to the positive phase input of the main amplifier 104. On the other hand, the output of the preamplifier 202 is connected to the negative phase input of the main amplifier 104 via the LPF 103 composed of the resistor 131 and the capacitor 132. The capacitor 132 may be configured inside the integrated circuit that constitutes the optical receiving circuit, or may be connected to an external capacitor.

The cutoff frequency of the low pass filter LPF103 is about 800 from 1 / (2πCpRp).
It becomes kHz. A low-pass filtered signal having a frequency lower than the cutoff frequency is input to the negative phase input terminal of the main amplifier 104, but a high frequency signal is grounded by the capacitor 132 and is not input to the negative phase input terminal. Since the low-pass filtered signal that has passed through the LPF 103 is input to the AGC circuit 205, the negative feedback of the AGC circuit 205 is 800 k.
It will not operate above Hz.

FIG. 5 shows a circuit configuration of the main amplifier of the optical receiving circuit according to the embodiment of the present invention. The main amplifier 104 is composed of a two-stage differential amplifier. The first stage differential amplifier is composed of FETs (Tr4 to Tr10), resistors R1 to R3, and diodes D4 to D7, and the last stage differential amplifier is FET (Tr11 to TR1).
7) and resistors R4 to R6. Further, capacitors C2 and C3 for connecting the first stage differential amplifier and the last stage differential amplifier are provided.

Load resistors R1 and R2 of the first stage differential amplifier
Is 650Ω, and the resistance value of Vref and the resistance R3 is 5
A current I1 determined by 0Ω flows. The gain of the first stage differential amplifier is the resistance value of the load resistors R1 and R2 and the FET (T
It is determined by the gate width of r4, Tr5). FET (Tr
4 or Tr5), or increase the load resistance R1, R
If the resistance value of 2 is increased, the gain is increased, but the band characteristic is deteriorated. Further, depending on the magnitude of the input signal, the product of the flowing current I1 and the resistance values of the load resistors R1 and R2 becomes larger than the power supply voltage, and the amplification stage is saturated and the signal is distorted.
In consideration of these, the gate width, the resistance value and the current I are determined.

The final stage differential amplifier has the same circuit configuration as the first stage differential amplifier. However, the resistance value of the resistor R6 is 25Ω, and the flowing current I2 is almost twice the flowing current I1. The load resistances R4 and R5 are 300Ω in order to cancel the magnitude of the flowing current I2. As a result,
The magnitudes of the signal waveforms generated at both ends of the load resistors R4 and R5 are almost the same in the first stage differential amplifier and the last stage differential amplifier. The output source follower circuit does not use a diode for voltage drop. This is because the outputs POUT and NOUT of the main amplifier 104 are input to the signal processing circuit of the next stage by capacitive coupling.

Positive feedback is provided by capacitors C2 and C3 between the first stage differential amplifier and the last stage differential amplifier. This is because capacitors C2 and C3 have the same in-phase signal between the first stage differential amplifier and the last stage differential amplifier.
By connecting via, only high-frequency signals are bypassed.

FIG. 6 shows the circuit configuration of the AGC circuit of the optical receiver circuit according to the first embodiment of the present invention. The amplifier 251 of the AGC circuit 205 is composed of a two-stage differential amplifier. The first stage differential amplifier is a FET (Tr18-
Tr24), resistors R7 to R10, and diode D8 to
D10 and the final stage differential amplifier is a FET
(Tr25 to Tr30), resistors R11 to R12, and diodes D12 to D13.

The input of the first stage differential amplifier is the LPF 103.
And the same signal as the negative phase input of the main amplifier 104 is input. As mentioned above, the low pass filtered signal is 80
Since the first stage differential amplifier does not include a signal component of kHz or higher, it is not necessary to pay attention to the frequency characteristic. The load resistors R7 and R8 are set to a relatively high value of 2.2 kΩ. A reference voltage obtained by resistance-dividing the power supply is input to the other input of the first stage differential amplifier.

The differential amplifier at the final stage has one load resistor R1.
2 is set to pure resistance, the other load is pure resistance R11 and FE
It was connected in series with T (Tr28). FET (Tr2
The output of the preamplifier 202 is directly input to the gate of 8). That is, F of the preamplifier 202 shown in FIG.
It is connected to the source of ET (Tr2). FET (Tr2
With respect to the output of the preamplifier 202 input in 8), the differential amplifier at the final stage does not have an amplifying action and acts as a source follower without correction of frequency characteristics.

The output of the FET (Tr25) is the FET (T
FET 252 connected to the feedback resistor 222 of the preamplifier 202 via the source follower of r29 to Tr30).
Input to the gate. Since the signal frequency passes through the FET (Tr28) and the source follower, it is necessary to pay attention to the frequency characteristic. A speed-up capacitor may be inserted in parallel with the diodes D12 and D13.

Next, the operation of the optical receiving circuit according to the first embodiment will be described. Consider a high frequency band in which the LPF 103 does not operate, for example, a signal frequency band. In this case, L
The low-pass filtered signal that has passed through the PF 103 is equivalently a direct current,
That is, since the frequency is sufficiently smaller than the signal frequency, it can be regarded as a signal that hardly changes. Figure 6
Input FET of the amplifier 251 of the AGC circuit 205 shown in FIG.
When an equivalent DC signal is input to (Tr18), it simply functions as a current source FET.

On the other hand, the FET (Tr28) of the amplifier 251 which directly inputs the output of the preamplifier 202 operates as a source follower for the output of the preamplifier 202.
Therefore, the output of the AGC circuit 205 is the preamplifier 202.
It will directly reflect the output of.

When the optical signal becomes large and the output potential of the preamplifier 202 drops, the output potential of the LPF 103 drops. In the amplifier 251 of the AGC circuit 205, the output potential of the input FET (Tr18) rises and the reverse phase input F
The output potential of ET (Tr19) decreases. FET (Tr
21, Tr22) and FETs (Tr25, Tr2)
When it is guided to the gate of 6), the potential of the gate of the FET (Tr25) decreases. Since the current flowing through the FET (Tr25) decreases, the level shift amount of the source follower composed of the FET (Tr28), the resistor R11, and the FET (Tr25) also decreases. Therefore, FET (Tr29)
In comparison with the output of the preamplifier 202, an output having a higher direct current level and a substantially equal amplitude is obtained.

As a result, the feedback resistor 2 of the preamplifier 202
Since the gate potential of the FET 252 inserted in parallel with 22 increases, the equivalent resistance between the drain and the source of the FET 252 decreases. In this way, the preamplifier 2
The current / voltage conversion gain of 02 can be changed. Since such positive feedback always follows the change of the optical signal, it compensates the high frequency characteristic of the preamplifier 202 which is deteriorated by inserting the AGC circuit 205.

(Second Embodiment) In the present embodiment, the output of the preamplifier 202 is passed through a differential amplifier that is not deteriorated in the high frequency characteristics, and then input to the amplifier of the AGC circuit.

FIG. 7 shows the configuration of an optical receiving circuit according to the second embodiment of the present invention. The difference from the first embodiment shown in FIG. 2 is the configuration of the AGC circuit 305. The AGC circuit 305 includes an amplifier 351, an FET 352 inserted in parallel with the feedback resistor 222 of the preamplifier 202, and a differential amplifier 353. The configuration of the amplifier 351 is the same as the configuration of the amplifier 251 shown in FIG. 6, and the output of the differential amplifier 353 is connected to the gate of the FET (Tr28) of the final stage differential amplifier.

FIG. 8 shows the circuit configuration of the differential amplifier in the AGC circuit of the optical receiving circuit according to the second embodiment of the present invention. The main amplifier 1 is connected to the input of the differential amplifier 353.
The same signal as the positive-phase input and the negative-phase input of 04 is introduced. The load resistances R13 and R14 of the differential amplifier 353 are 650Ω. FET (Tr3 that is the current source of the differential amplifier 353)
Vref, which is separately generated as a reference potential, is input to 3).

The output of the opposite phase side is connected to the gate of the FET (Tr28) in the differential amplifier at the final stage of the amplifier 351. The output on the opposite phase side means that the positive phase signal is input in terms of phase. Since the phase inversion does not occur in the source follower of the final stage differential amplifier, the preamplifier 20
Since positive phase feedback is applied to 2, the frequency characteristics in the high frequency band can be improved.

The differential amplifier 353 has the same configuration as the differential amplifier at the first stage of the main amplifier 104 shown in FIG. 5, including the circuit constants. Therefore, the differential amplifier 353 and the first stage differential amplifier of the main amplifier 104 can be shared. That is, the differential amplifier 35 shown in FIG.
By omitting 3, the connection point between the FET (Tr5) and the load resistor R2 in the first stage differential amplifier of the main amplifier 104, and the FET in the last stage differential amplifier of the amplifier 351.
You may connect with the gate of (Tr28).

Next, the operation of the optical receiving circuit according to the second embodiment will be described. In the first embodiment, the amplifier 251 of the AGC circuit 205 functions as a source follower in the high frequency range, so that the gain never exceeds 1. Actually, it is about 0.5 to 0.7. Preamplifier 2
The control signal which multiplied the output of 05 by 0.5 to 0.7 is FET
It is input to the gate of 252. At this time, FET 252
Since the gate-source voltage of the above also changes in accordance with the change of the signal, the gate-source capacitance Cgs affects the frequency characteristic in the high frequency range.

In order to suppress the effect of Cgs,
Output of preamplifier 202 and amplifier 3 of AGC circuit 305
An amplifier 353 for compensating for the gain reduction due to the source follower is inserted between the amplifier 51 and the amplifier 51. By setting the ratio of the output of the preamplifier 202 and the output of the AGC circuit 305 to approximately 1.0, the variation of Vgs of the FET 352 is suppressed, and the FET3
The effects of 52 Cgs can be offset.

As described above, the optical receiving circuit includes the preamplifier 202, the main amplifier 104, and the AGC circuit 2.
05 and 305 and the Vref voltage generation circuit can be manufactured as one integrated circuit.

FIG. 9 shows the output characteristic of the optical receiving circuit according to this embodiment. The horizontal axis represents frequency, and the vertical axis represents transimpedance Zt, which is represented by 20 · log (Zt). The transimpedance Zt is the preamplifier 205.
Feedback resistor 222 and FE of AGC circuits 205 and 305
It is determined by the parallel circuit of T252 and 352. For example, Zt = 1 kΩ for 60 dbΩ and Zt = 1 for 40 dbΩ.
Corresponds to 00Ω.

The point of -3 dB lower than the low band which is the pass band is the transimpedance band. It is less than 1.2 GHz in a conventional circuit that does not perform positive feedback at all. On the other hand, in the first embodiment in which the positive feedback is applied, the band increases up to 1.7 GHz, and
In the above embodiment, a band exceeding 2.0 GHz is realized.

(Third Embodiment) FIG. 10 is a block diagram showing the configuration of an optical receiving circuit according to the third embodiment of the present invention. The difference from the first embodiment shown in FIG. 2 is that
This is a configuration of the preamplifier 302 and the AGC circuit 405. The gain control of the preamplifier 302 is performed by changing the on resistance of a transistor connected in series with the load resistance of the amplification stage to the AGC circuit 40.
This is done by dynamically controlling the gain by setting 5.

FIG. 11 shows the circuit configuration of the preamplifier of the optical receiving circuit according to the third embodiment of the present invention. In the preamplifier 202 shown in FIG. 3, the FET (Tr
As the load of 1), in addition to the load resistance RL, a gain control FET (Tr35) is connected in parallel. AGC circuit 40
The control signal from 5 is input to the gate of the FET (Tr35).

With such a configuration, when the optical signal becomes large, the DC level of the control signal from the AGC circuit 405 rises. Then, since the gate bias comes into contact with the + side, the ON resistance (RO of the FET (Tr35) (RO
N) decreases. Since the load of the FET (Tr1) has a parallel resistance value of the load resistances RL and RON, the parallel resistance value of RL and RON decreases as the optical signal increases. As a result, the gain of the amplification stage is reduced and negative feedback works. The control on the high frequency side by the configuration in which the output of the preamplifier 302 is directly input to the AGC circuit 405 also has the same operation as in the case of the first embodiment.

[0053]

As described above, according to the present invention,
A first input for inputting the low-pass filtered signal that has passed through the low-pass filter, and a second input for inputting the voltage signal from the preamplifier
Of the low-pass filtered signal input to the first input, the high-frequency gain is controlled by the voltage signal input to the second input, and the feedback signal is output to the preamplifier.
By the negative feedback through the low pass filter and the positive feedback by the output of the preamplifier, the automatic gain control circuit operates to increase the gain in the high frequency band, and corrects the deterioration of the high frequency characteristic by the automatic gain control circuit. It becomes possible.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a conventional optical receiving circuit.

FIG. 2 is a block diagram showing a configuration of an optical receiving circuit according to the first embodiment of the present invention.

FIG. 3 is a circuit diagram showing a configuration of a preamplifier of the optical receiving circuit according to the first embodiment of the present invention.

FIG. 4 is an LP of an optical receiving circuit according to an embodiment of the present invention.
It is a circuit diagram showing the configuration of F.

FIG. 5 is a circuit diagram showing a configuration of a main amplifier of the optical receiving circuit according to the embodiment of the present invention.

FIG. 6 is a circuit diagram showing a configuration of an AGC circuit of the optical receiving circuit according to the first embodiment of the present invention.

FIG. 7 is a block diagram showing a configuration of an optical receiving circuit according to a second embodiment of the present invention.

FIG. 8 is a circuit diagram showing a configuration of a differential amplifier in the AGC circuit of the optical receiving circuit according to the second embodiment of the present invention.

FIG. 9 is a graph showing output characteristics of the optical receiving circuit according to the present embodiment.

FIG. 10 is a block diagram showing a configuration of an optical receiving circuit according to a third embodiment of the present invention.

FIG. 11 is a circuit diagram showing a configuration of a preamplifier of an optical receiving circuit according to a third embodiment of the present invention.

[Explanation of symbols]

100, 200 Optical receiver circuit 101 Light receiving device (PD) 102, 202, 302 Preamplifier 103 Low pass filter (LPF) 104 Main amplifier 105, 205, 305, 405 Automatic gain adjustment (A
GC) circuit 106 optical fiber 121, 151, 221, 321 inverting amplifier 122, 222, 322 feedback resistor 131 resistance (R) 132 capacitor (C) 141, 142, 353 differential amplifier 152, 252, 352 FET 251, 351 451 amplifier

Continued front page    F-term (reference) 5J090 AA01 AA56 CA00 CA21 FA17                       FA20 GN08 HA09 HA19 HA25                       HA44 KA00 KA02 KA04 KA42                       MA08 MA11 MA21 MN01 NN13                       SA13 TA01 TA03                 5J092 AA01 AA56 CA00 CA21 FA00                       FA17 HA09 HA19 HA25 HA44                       KA00 KA02 KA04 KA42 MA08                       MA11 MA21 SA13 TA01 TA03                       UL02                 5J100 JA01 LA02 QA01 QA04 SA01                 5J500 AA01 AA56 AC00 AC21 AF00                       AF17 AF20 AH09 AH19 AH25                       AH44 AK00 AK02 AK04 AK42                       AM08 AM11 AM21 AS13 AT01                       AT03 LU02 NM01 NN13

Claims (5)

[Claims] 1. An optical receiving circuit connected to a light receiving device for converting a received optical signal into a current signal, a preamplifier converting the current signal into a voltage signal and outputting the voltage signal, and the voltage signal output from the preamplifier. A low-pass filter for smoothing and outputting a low-pass filtered signal, a first input for inputting the low-pass filtered signal passed through the low-pass filter, and the voltage signal from the preamplifier. A second input, amplifies the low-pass filtered signal input to the first input, controls a high-frequency gain by the voltage signal input to the second input, and outputs to the preamplifier. Outputs a feedback signal, and the current of the preamplifier −
An optical receiving circuit, comprising: an automatic gain control circuit for controlling voltage conversion efficiency. 2. The preamplifier includes an amplifier and a feedback resistor that connects an input and an output of the amplifier, and a transistor in which the feedback signal from the automatic gain control circuit is connected in parallel to the feedback resistor. 2. The optical receiving circuit according to claim 1, wherein the current-voltage conversion efficiency is controlled by inputting the signal to the control terminal to change the transimpedance composed of the feedback resistor and the transistor. 3. The automatic gain control circuit includes an amplifier for amplifying the voltage signal from the preamplifier and inputting the voltage signal to the second input, and the amplitude of the voltage signal of the preamplifier and the automatic gain control circuit are provided. The optical receiving circuit according to claim 1 or 2, wherein the amplitude and the phase of the feedback signal have substantially equal values in a frequency band equal to or higher than a cutoff frequency of the low pass filter. 4. The preamplifier includes an amplification stage including a voltage gain control transistor connected to a load element connected to the amplification transistor, an output stage which is an output buffer of the amplification stage, and an input of the amplification stage. A feedback resistor that connects the output of the output stage, and inputs the feedback signal from the automatic gain control circuit to the control terminal of the gain control transistor to control the gain of the amplification stage. The optical receiver circuit according to claim 1, wherein the current-voltage conversion efficiency is controlled. 5. The optical receiving circuit includes a main amplifier that amplifies the voltage signal output from the preamplifier, and the preamplifier, the main amplifier, and the automatic gain control circuit are integrally integrated and configured. The optical receiving circuit according to any one of claims 1 to 4, wherein: