JP2003168933A - Photoreceiving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-noise and wide-band photoreceiving circuit by keeping 50% in duty ratio of output waveform at all times to a wide range of optical input signal level. <P>SOLUTION: This photoreceiving circuit is equipped with a preamplifying means 2 which amplifies a received signal, a differential amplifying circuit 4 which linearly differentially amplifies the output signal of the preamplifying means 2, a signal limit amplifying means 5 which amplifies the limit of input signals through a plurality of cascaded limit amplifying circuits, and a feedback circuit which includes an offset detecting circuit 6 for detecting the quantity of offset of the limit amplifying circuit at the final stage. The feedback circuit converts the output signal of the offset detecting circuit 6 into a signal for reducing the quantity of offset, and feeds it back to the point between the output end of the differential amplifying circuit and the input end of the limit amplifying circuit at the latter stage. <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 receiver circuit, and more particularly to an optical receiver circuit having a differential amplifier circuit for offset compensation.

[0002]

2. Description of the Related Art Optical receiving circuits used in optical communication systems are required to have characteristics such as low noise, wide dynamic range, high gain and wide band.

As an example of a conventional optical receiving circuit, for example,
As shown in FIG. 5, the optical receiving circuit described in Japanese Patent Application Laid-Open No. 7-79257 includes a light receiving element 21 that receives an optical signal and generates a current signal according to the received intensity, and a light receiving element 21.
Preamplifier 2 for amplifying and converting the current signal from the amplifier into a voltage signal
2, an amplitude limiter 25 that limits the amplitude of the voltage signal to a certain limit value, and a comparator 26 that slices the output signal of the amplitude limiter 25 with a certain threshold value to convert it into a binary signal.
And the comparator 26 according to the output level of the amplitude limiter 25.
And a threshold value setting device 27 that gives a threshold value for ensuring optimum discrimination.

FIG. 6 shows the operation of the basic structure of a transimpedance type preamplifier which has been often used conventionally.
A description will be given with reference to (a). The current input from the terminal 207 is divided into the inverting amplifier 301 and the resistor 205. The inverting amplifier 301 operates so that the potential Vo of the output terminal 208 decreases as the input current increases.
When the output voltage Vo decreases, the current flowing through the resistor 205 increases and the input current to the inverting amplifier 301 decreases. In this case, the output voltage Vo rises, reducing the current flowing through the resistor 205 and increasing the input current of the inverting amplifier 301 again. By such a feedback operation, balance as a preamplifier is achieved.

Next, the operating principle of a specific configuration example of the above transimpedance type preamplifier will be described with reference to FIG. 6 (b). The transistor 201 and the resistor 203 configure an inverting amplifier, the transistor 202 and the resistor 204 configure an output buffer circuit, and the resistor 205 configures the output terminal 2
Current feedback is performed from 08 to the input terminal 207. That is,
The current photoelectrically converted by the light receiving element input from the terminal 207 becomes the base current of the transistor 201,
It is amplified and converted as a voltage across the resistor 203.
This is taken out to the terminal 208 via the transistor 202, while being fed back to the input terminal 207 via the resistor 205, and functions to cancel the increase / decrease in the input current.
The circuit is balanced. So in this preamplifier,
Input terminal 207 current Iin and output terminal 208 voltage V
The relationship with o is Vo = −Iin · Rf (1) when the feedback resistor 205 is Rf, and the transimpedance gain is approximately equal to the feedback resistor Rf. Also, the thermal noise Irf due to the feedback resistance
Is expressed by the following equation.

Irf = 4 kTB / Rf (2) where k is Boltzmann's constant, T is absolute temperature, B is frequency band, frequency band B is junction capacitance of light receiving element,
When the sum of the input capacitance of the amplifier and the stray capacitance added in mounting is Ct and the voltage gain of the inverting amplifier is A, B = A / (2π · Rf · Ct) (3) Therefore, in order to increase the transimpedance gain and reduce the thermal noise, it is necessary to increase the value of the feedback resistor.

FIG. 7 shows the feedback resistance dependence of the input current and output voltage characteristics in the preamplifier shown in FIG. When the feedback resistance Rf is small, a wide range of linear amplification operation is possible, but when the feedback resistance Rf is large,
The linear amplification operation is performed up to the input current c point in FIG. 7, and the transistor 201 in FIG. 6 is saturated at the input current d point,
The output is distorted and the duty ratio changes.

Now, the variation of the duty ratio of the preamplifier will be described. FIG. 8 is an example showing the output waveform with respect to the input current of the preamplifier. As shown in the figure, as the input current increases, the output amplitude increases, the cross point approaches the lower limit of the output, and the duty ratio changes.

FIG. 9 is an example showing changes in the output amplitude and the duty ratio with respect to the input current of the preamplifier. As the input current increases, the output amplitude increases and the duty ratio decreases. In this case, the variation of the duty ratio is about 10%. When the duty ratio of the preamplifier changes, the output of the amplitude limiter 25 as shown in FIG. 5 has a cross point shift between the positive phase output and the negative phase output. In order to suppress the deviation of the cross points, conventionally, a circuit including a circuit for controlling the offset of the limit amplifier as shown in Japanese Patent Laid-Open No. 62-15909 as shown in FIG. 10 has been applied.

In the figure, the current photoelectrically converted by the light receiving element 31 is converted into a voltage by the preamplifier 32, and then the limit amplifier 3 is passed through an offset controller 33.
Input to 5. Incidentally, reference numeral 32 'indicates the preamplifier 3
2 is a reference preamplifier having the same circuit configuration as that of No. 2. The signal amplified by the limit amplifier 35 is output to the output terminals C and D. The offset detector 36 compares the average value of the output signal of the terminal C with the center value of the output signal of the terminal D, and amplifies the difference signal by a differential amplifier. Limit amplifier 3
The average value of the output signals of No. 5 is obtained by the integrating circuit of the capacitance and the resistance. The center value of the output signal of the limit amplifier 35 is obtained by resistance-dividing the positive-phase output C and the negative-phase output D. When the optical signal is a signal having an average mark ratio of 50% and a signal having no cross point offset, the average value of the output signals of the limit amplifier 35 and the central value of the output signals are equal. On the other hand, in the case of a signal having an offset at the cross point of the output signal, the offset detector 36 produces two outputs having different potentials. In response to this output, the offset controller 33 controls the input potential of the first stage differential amplifier of the limit amplifier 35 so that the offset becomes normal.

[0011]

The conventional optical receiving circuit disclosed in Japanese Patent Laid-Open No. 62-15909 shown in FIG.
With the configuration as described above, when the input optical signal level increases and the input current of the preamplifier 32 is large,
As in the output waveform shown in FIG. 11, the output signals at the terminals C and D have a lot of jitter. This is due to the small input dynamic range and the presence of distortion due to non-linear amplification in the first stage differential amplifier of the limit amplifier 35. In this case, the output signal of the first stage of the limit amplifier 35 has a difference between the rising time and the falling time in the positive phase output and the negative phase output in the signal side amplification and the reference voltage side amplification. When this signal is received by a limit amplifier connected in multiple stages in the subsequent stage, it becomes jitter. When this output signal is received by the comparator or discriminator in the subsequent stage, the margin in the phase direction is reduced, which causes deterioration of the reception sensitivity.

Further, in the optical receiving circuit described in the above publication,
Since the feedback is applied so as to change the output level of the preamplifier, the base resistance is inserted so as not to affect the feedback resistance of the preamplifier that determines the receiving sensitivity. Therefore, the band may deteriorate.

Further, as shown in FIG. 12, when the shot noise from the light receiving element is added to the high level of the signal at the input side of the preamplifier, the preamplifier goes low at the output side of the preamplifier. Gain noise and electrical noise from the preamplifier are generated. In this case, in the optical receiving circuit described in the above publication, the reference voltage value at the point h is near the cross point of the output signal at the point g, and is close to low level noise, which may cause a code error.

Therefore, the present invention has been made in view of the problems in the above-mentioned conventional optical receiving circuit.
An object of the present invention is to provide an optical receiving circuit which can always maintain a duty ratio of 50% for a wide range of optical input signal levels without deteriorating the receiving sensitivity and obtain an output waveform without jitter.

[0015]

In order to achieve the above object, an invention according to claim 1 is an optical receiving circuit, which is a preamplifying means for amplifying a received signal, and an output signal of the preamplifying means. A differential amplification circuit that linearly differentially amplifies the signal, a signal limit amplification unit that cascades a plurality of limit amplification circuits to limit-amplify an input signal, and an offset detection circuit that detects the offset amount of the final-stage limit amplification circuit A feedback circuit, the feedback circuit converting an output signal of the offset detection circuit into a signal for reducing the offset amount, and connecting the output end of the differential amplification circuit and the input end of the limit amplification circuit in the subsequent stage. It is characterized by returning in the meantime.

According to the first aspect of the present invention, the differential amplifier circuit for linearly differentially amplifying the latter stage of the preamplifying means is used, so that when the input optical signal level is large or the distorted optical signal is generated. When input, the offset amount of the preamplification means becomes clear, the offset amount is extracted by the offset detection circuit, converted into a current signal by the feedback circuit, and the duty ratio is 50 at the output of the differential amplification circuit. It is possible to change the output level so that it becomes%.

According to a second aspect of the present invention, as a preferred form of the optical receiving circuit according to the first aspect, the feedback circuit detects a difference between the outputs of the offset detection circuit and a current output from the comparator. A conversion circuit for converting the signal into a signal, and generating a signal for reducing the offset amount based on an output value of the conversion circuit.

According to a third aspect of the present invention, as a preferred form of the optical receiving circuit according to the second aspect, the output value of the comparator is converted into the current signal by a semiconductor differential pair.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a specific example of an embodiment of an optical receiving circuit according to the present invention will be described with reference to the drawings.

FIG. 1 shows an embodiment of a light receiving circuit according to the present invention. This light receiving circuit comprises a light receiving element 1, a preamplifier 2, a voltage-current conversion circuit 3, and a differential amplifier for linear amplification. An amplifier circuit 4, a limit amplifier 5 in which differential amplifier circuits are connected in multiple stages, an offset detector 6, a comparator 7,
And a reference voltage generator 8.

As shown in FIG. 2, the differential amplifier circuit 4 includes transistors 101, 102 and resistors 105, 106, 1.
07 and 108 and a constant current source 109, which is a differential amplifier circuit for linear amplification. The differential amplifier circuit 4 is connected to a semiconductor differential pair as the voltage-current converter circuit 3 shown by the dotted frame, and the output offset compensation signal is output from the differential amplifier circuit 4 and the limit amplifier 5 at the subsequent stage. Is fed back to the input terminal of. The voltage-current conversion circuit 3 includes a transistor 1
03 and 104, and a constant current source 110.

Next, the operation of the optical receiving circuit having the above configuration will be described. In the following description, the case where the feedback resistance of the preamplifier 2 is set large in order to increase the transimpedance gain of the optical receiving circuit and reduce the noise will be considered.

FIG. 4 is an example comparing the high level, the low level, the center value, the crosspoint value of the output e point with respect to the input current of the preamplifier 2 and the output f point of the reference voltage generator 8. As the input current increases, the cross point value approaches the low level, but the reference voltage generator 8
The output of is approximately equal to the central value. This is because in the integrating circuit composed of resistors and capacitors, the decrease of the average value due to the decrease of the duty ratio and the increase of the average value due to the increase of the output amplitude are offset.

From the above description, when the light input signal level of the light receiving element 1 is small, the preamplifier 2, the differential amplifier circuit 4 and the limit amplifier 5 operate in the linear amplification region, and FIG. An output waveform as shown in is obtained. Differential amplifier circuit 4
When the base current of the transistor in the first stage in is set to be small, the potential drop due to the resistance of the reference voltage generator 9 becomes small. Therefore, the potential at the point f is substantially centered with respect to the output waveform at the point e, there is no cross point offset in the outputs of the differential amplifier circuit 4 and the limit amplifier 5, and the duty ratio is 50%.

On the other hand, when the optical input signal level of the light receiving element 1 becomes large, as shown in FIG. 3B, at the output point e of the preamplifier 2, a cross point offset occurs and the duty ratio fluctuates. In this case, as shown in FIG. 3B, the potential at the point f is substantially at the center of the output waveform at the point e, and the differential amplifier circuit 4 in the subsequent stage performs linear amplification.
The output waveform at the point is a signal with a DC offset.

When the offset detector 6, the comparator 7 and the feedback loop of the voltage-current conversion circuit 3 are not provided, the positive-phase output and the negative-phase output of the limit amplifier 5 have waveforms having the same amplitude value but different duty ratios. Is output.

On the other hand, when there is a feedback loop, a DC signal corresponding to the difference in duty ratio between the positive phase output and the negative phase output of the limit amplifier 5 is output to the output of the offset detector 6. This DC signal is amplified by the comparator 7, converted into a current signal by the voltage-current conversion circuit 3, and automatically controlled so that the potentials at the cross points of the points g and h become equal. Since this signal is received by the limit amplifier 5, the output of the limit amplifier 5 always has a duty ratio of 50%.

The operation of the voltage / current conversion circuit 3 and the differential amplifier circuit 4 will be described with reference to FIG.

The input terminal 1 connected to the bases of the transistors 103 and 104 which form the voltage-current conversion circuit 3.
When the output potential from the comparator 8 is input from 14, 115, the current output from the collectors of the transistors 103, 104 changes, and the output signal of the differential amplifier circuit 4 changes. The changing direction is a direction in which the offset is reduced. As a result, the signal level after the output terminals 116 and 117 changes, the offset of the input crosspoint of the limit amplifier 5 is compensated, and the output of the limit amplifier 5 has a duty ratio of 50%.

As described above, in the optical receiver circuit according to the present embodiment, since feedback is applied at the output of the differential amplifier circuit 4 so that the potentials of the cross points become equal, the rise as in the conventional circuit of FIG. There is no difference between the time and the fall time, and there is no output jitter in the limit amplifier 5 in the subsequent stage.

Further, in the optical receiving circuit according to the present embodiment,
Since the differential amplifier circuit 4 that linearly amplifies is provided after the preamplifier 2 and the output of the voltage-current conversion circuit 3 is fed back between the output of the differential amplifier circuit 4 and the first stage of the limit amplifier 5, The feedback resistance of the preamplifier 2 is not affected, and low noise can be maintained.

[0032]

As described above, according to the present invention,
It is possible to increase the feedback resistance of the preamplifier to realize low noise, always maintain the duty ratio at 50% for a wide range of optical input signal levels, and obtain an output waveform without jitter.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an optical receiving circuit according to the present invention.

FIG. 2 is a circuit configuration diagram of a differential amplifier circuit and a voltage / current conversion circuit of the optical receiver circuit of FIG.

3A and 3B are diagrams showing output waveforms of respective nodes of the optical receiving circuit of FIG. 1, where FIG. 3A is a case where the light input signal level of the light receiving element is small, and FIG. 3B is a light input signal of the light receiving element. The case where the level is large and the operation of the preamplifier is close to the non-linear operation region is shown.

FIG. 4 is a diagram comparing a high level, a low level, a crosspoint value, a center value of an output with respect to an input current of a preamplifier of the optical receiving circuit of FIG. 1, and an output of a reference voltage generator.

FIG. 5 is a block diagram showing an example of a conventional optical receiving circuit.

6A and 6B are diagrams showing a conventional preamplifier, in which FIG. 6A is a block diagram and FIG. 6B is a circuit configuration diagram.

FIG. 7 is a diagram showing transfer characteristics at input and output of a conventional preamplifier.

FIG. 8 is a comparison diagram of the output waveform with respect to the input current of the conventional preamplifier.

FIG. 9 is a diagram showing an output amplitude and a duty ratio with respect to an input current of a conventional preamplifier.

FIG. 10 is a block diagram showing another example of a conventional optical receiving circuit.

FIG. 11 is a diagram showing an output waveform of each node in the conventional optical receiving circuit.

FIG. 12 is a diagram showing noise at the input and output of a conventional preamplifier.

[Explanation of symbols]

1 Light receiving element 2 Preamplifier 3 voltage-current conversion circuit 4 Differential amplifier circuit 5 Limit amplifier 6 Offset detector 7 comparator 8 Reference voltage generator 101 transistor 102 transistor 103 transistor 104 transistor 105 resistance 106 resistance 107 resistance 108 resistance 109 constant current source 110 constant current source 114 input terminals 115 input terminals 116 output terminal 117 output terminal

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H04B 10/26 10/28 F term (reference) 5J092 AA01 AA56 CA41 CA62 FA00 HA02 HA19 HA25 HA29 HA44 KA00 KA02 KA05 KA11 KA17 KA20 MA11 SA13 TA01 TA02 TA06 UL02 5J500 AA01 AA56 AC41 AC62 AF00 AH02 AH19 AH25 AH29 AH44 AK00 AK02 AK05 AK11 AK17 AK20 AM11 AS13 AT01 AT02 AT06 LU02 5K002 AA03

Claims (3)

[Claims] 1. A preamplifier for amplifying a received signal, a differential amplifier circuit for linearly differentially amplifying an output signal of the preamplifier, and a plurality of limit amplifier circuits connected in series to limit-amplify an input signal. And a feedback circuit that includes an offset detection circuit that detects the offset amount of the final stage limit amplification circuit, and the feedback circuit reduces the offset amount of the output signal of the offset detection circuit. An optical receiving circuit, which is converted into a signal and fed back between an output end of the differential amplifier circuit and an input end of a limit amplifier circuit at a subsequent stage. 2. The feedback circuit includes a comparator that detects a difference between the outputs of the offset detection circuit and a conversion circuit that converts the output of the comparator into a current signal, and the feedback circuit includes the comparator based on the output value of the conversion circuit. The optical receiving circuit according to claim 1, wherein a signal for reducing the offset amount is generated. 3. The optical receiving circuit according to claim 2, wherein the output value of the comparator is converted into the current signal by a semiconductor differential pair.