JP2006261866A - Preamplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a preamplifier which enables an extensive dynamic range and a multi-application by removing the influence of a noise based on a supply voltage variation or temperature fluctuation without increasing a circuit scale. <P>SOLUTION: The preamplifier includes a light receiving element 11 for converting an optical signal to a corresponding current signal Iin, a first transimpedance amplifier 13 for inputting the current signal Iin and converting to a voltage signal, and a second transimpedance amplifier (replica amplifier) 14 showing the same input/output characteristics as the first transimpedance amplifier 13, and includes a differential signal formation circuit 15 which forms the difference signal of the first and second transimpedance amplifiers. The current Im corresponding to the mean rate of the current signal Iin from the light receiving element 11 is inputted into the second transimpedance amplifier 14. Moreover, a current mirror circuit can be used for the formation of the current Im corresponding to the mean rate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a preamplifier used in an optical communication system and the like, and more particularly to a preamplifier including a transimpedance amplifier.

  In a receiving apparatus using optical communication, a preamplifier that receives an optical signal received by a light receiving element and changes to a voltage signal is likely to generate noise (also referred to as common-mode noise) based on power supply voltage fluctuation or temperature fluctuation. In order to eliminate the influence of this noise, an auxiliary output including the same component as this noise may be obtained at the same time and canceled using a differential input amplifier or the like. As a specific method, a dummy amplifier (also referred to as a replica amplifier) having the same configuration as an amplifier that amplifies an optical signal from the light receiving element, and the input terminal of the dummy amplifier has the same capacitance value as the capacity of the light receiving element. There is a method of connecting an equivalent capacitance capacitor (see, for example, Patent Document 1).

Also, there is a method of reducing the DC level difference of the differential output signal using a transimpedance type signal amplification amplifier and its replica amplifier (see, for example, Patent Document 2). Further, there is a method of adjusting the optical / electric conversion gain of the preamplifier by feeding back the output to the input side of the optical signal with a preamplifier using a signal amplification amplifier and its replica amplifier (for example, Patent Documents). 3).
JP-A-8-139342 Japanese Patent Laid-Open No. 2002-76793 US Pat. No. 6,778,021

  FIG. 5 is a block diagram showing a configuration example of a preamplifier simulating the above-described Patent Documents 1 to 3, wherein 1 is a light receiving element, 2 is a preamplifier, 3 is a first transimpedance amplifier, and 4 is a second amplifier. , A transimpedance amplifier (replica amplifier), 5 a differential signal generation circuit (output buffer amplifier), 6 a feedback amplifier, and 7 a current source. The current signal Iin generated by the optical signal received by the light receiving element 1 such as a PIN photodiode is input to the first transimpedance amplifier 3 for signal amplification and converted into a voltage signal, and the output voltage signal Vout The signal is input to one terminal of a dynamic signal generation circuit (hereinafter referred to as an output buffer amplifier) 5.

  The second transimpedance amplifier 4 (hereinafter referred to as replica amplifier) is formed in the same configuration as the first transimpedance amplifier 3, and its output is supplied to the other terminal of the output buffer amplifier 5 as a reference voltage Vref. Entered. Further, the output signal from the output buffer amplifier 5 is fed back to the current source 7 on the optical signal input side via the feedback amplifier 6 to stabilize the duty ratio of the output signal.

  However, in the configuration of the preamplifier shown in FIG. 5, for example, when the transimpedance is switched in order to cope with multi-applications with greatly different bands, the gain of the offset compensation loop (AOC) for mark ratio compensation also changes. End up. For this reason, an AOC gain compensation circuit linked with the transimpedance switching is necessary to avoid the fluctuation of the low-frequency cut-off frequency due to the fluctuation of the AOC gain, which inevitably increases the circuit scale.

  In order to realize a wide dynamic range, an automatic gain adjustment circuit (AGC) that detects the output level of the amplifier and automatically adjusts the transimpedance gain may be used with a transimpedance amplifier. At this time, if AGC and AOC are used in combination, feedback control of the double loop is performed. Therefore, if the two cutoff frequencies determined by the AGC and AOC loops are not separated, the two loops interfere with each other, Overshoot etc. will appear due to the response of the low frequency signal.

  For example, when a low-frequency cutoff of several kHz is required as a preamplifier, if the low-frequency cutoff determined by the AGC loop is several kHz, the cutoff determined by the AOC loop needs to be several hundred Hz. Such an extremely low frequency cut-off requires a large external capacitor, making it difficult to reduce the size of the optical receiver module in which the light receiving element and the preamplifier are contained in one package.

  The present invention has been made in view of the above circumstances, and eliminates the influence of noise based on power supply voltage fluctuations and temperature fluctuations without increasing the circuit scale, thereby enabling a wide dynamic range and multi-applications. It is an object to provide a preamplifier.

The preamplifier according to the present invention includes a light receiving element that converts an optical signal into a corresponding current signal, a first transimpedance amplifier that receives the current signal and converts it into a voltage signal, and the same input as the first transimpedance amplifier. And a second transimpedance amplifier having output characteristics, and a preamplifier including a differential signal generation circuit that generates a differential signal of the first and second transimpedance amplifiers. A current corresponding to the average value of the current signal is input to the second transimpedance amplifier.
The preamplifier further includes a current mirror circuit having two current output terminals, wherein one of the two current terminals is connected to the light receiving element, and the other is connected to the second transimpedance amplifier, Is input to the second transimpedance amplifier. Further, an equivalent capacitance capacitor having a capacitance value equivalent to the capacitance of the light receiving element may be inserted on the input side of the second transimpedance amplifier.

  According to the present invention, it is possible to eliminate the influence of noise due to power supply voltage fluctuations and temperature fluctuations by the replica amplifier, and to maintain the reference voltage to the differential signal generation circuit at or near the center level of the amplitude of the output voltage signal. Thus, the operating characteristics of the preamplifier can be stabilized. Further, since a negative feedback configuration of AOC is unnecessary, wide dynamic range and multi-rate can be easily realized by adjusting or switching transimpedance with a simple circuit configuration.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration example of a preamplifier according to the present invention, FIG. 2 is a diagram showing a specific example of an average value detection and voltage control current source of the preamplifier according to the present invention, and FIG. It is a figure explaining the operating characteristic of a preamplifier. In the figure, 11 is a light receiving element, 12 is a preamplifier, 13 is a first transimpedance amplifier, 14 is a second transimpedance amplifier (replica amplifier), 15 is a differential signal generation circuit (output buffer amplifier), 16 Is an average current detection circuit, 17 is a voltage controlled current source, and 18 is a current mirror circuit.

  The preamplifier for optical reception according to the present invention is a first transimpedance amplifier for inputting a current signal Iin generated by an optical signal received by a light receiving element 11 such as a PIN photodiode and converting it into a voltage signal, as in the prior art. 13 and a second transimpedance amplifier 14 (hereinafter referred to as a replica amplifier) having the same configuration as that of the first transimpedance amplifier 13. The output voltage signal Vout converted into a voltage by the first transimpedance amplifier 13 is input to one input terminal of a differential signal generation circuit (hereinafter referred to as an output buffer amplifier) 15.

  In the present invention, a current Im corresponding to the average value of the signal current Iin from the light receiving element 11 is input to the replica amplifier 14. The current Im is generated by the average current detection circuit 16 and the voltage control current source 17 and input to the replica amplifier 14. In the replica amplifier 14, the voltage is converted to a voltage corresponding to the average value of the input current signal Iin, and input to the other terminal of the output buffer amplifier 15 as a reference voltage Vref in reverse phase. The in-phase components of the voltage signal Vout of the first transimpedance amplifier 13 and the reference voltage Vref of the replica amplifier 14 are canceled by the output buffer amplifier 15 and output as a signal from which the in-phase noise is removed.

  FIG. 2 is a specific example of the preamplifier of FIG. 1 and shows an example of forming an average current detection circuit and a voltage controlled current source. This configuration example can be formed, for example, by providing a current mirror circuit 18 including resistors R1 and R2 and transistors Tr1 and Tr2 in a system path including the light receiving element 11. The capacitor C1 is connected to the bias circuit side of the resistor R1 and the transistor Tr1 to form a CR filter (LPF) circuit, thereby stabilizing the operation of the light receiving element 11. The resistor R1 and the capacitor C1 at this time can be shared as an integrator for detecting the average value current. As a result, an additional external capacitor or a large-area chip capacitor for setting the time constant becomes unnecessary.

  On the system path side of the resistor R2 and the transistor Tr2, a current Im corresponding to the average value of the input signal current Iin by the light receiving element 11 is generated and input to the replica amplifier 14. Accordingly, the replica amplifier 14 generates a reference voltage Vref having a magnitude corresponding to the input signal current Iin and is input to the output buffer amplifier 15. Further, an equivalent capacitance capacitor C <b> 2 equivalent to the capacitance of the light receiving element 11 may be inserted on the input side of the replica amplifier 14. By inserting the equivalent capacitor C2, the characteristics of the transimpedance amplifier 13 and the replica amplifier 14 on the signal amplification side can be perfectly balanced. As a result, an improvement in power supply ripple resistance can be expected. The equivalent capacitor can also be applied to the configuration example of FIG.

  3A and 3B are diagrams for explaining the operation state of the preamplifier 12 described above. FIG. 3A shows a case where the input signal current is relatively small, and FIG. 3B shows a case where the input signal current is relatively large. Is shown. As shown in FIG. 3A, when the input signal current Iin is input to the first transimpedance amplifier 13 on the signal amplification side, the output voltage signal Vout is output. The reference voltage Vref by the replica amplifier 14 is a value proportional to the average value of the input signal current Iin. For example, the reference voltage Vref is set to be equal to or close to the center level Va of the signal amplitude of the output signal voltage Vout of the transimpedance amplifier 13.

  Here, as shown in FIG. 3B, if a large input signal current Iin having a high light receiving intensity is input, the large output signal voltage Vout corresponding to the input signal current Iin has a signal amplitude from the peak level Vp. It changes greatly downward. For this reason, the center level Va of the signal amplitude of the output voltage signal Vout largely fluctuates downward. On the other hand, the reference voltage Vref also greatly changes downward with a value corresponding to the input signal current Iin. As a result, regardless of the magnitude of the input signal current Iin, it is possible to follow the center level Va of the signal amplitude of the reference voltage Vref and the output voltage signal Vout so that they always coincide with each other, thereby stabilizing the operation. Can be planned.

  FIG. 4 is a diagram for explaining an example of widening the preamplifier according to the present invention or making it multirate. In the figure, 19 denotes a level detection circuit, 20 denotes a decoder circuit, and the other reference numerals are the same as those in FIG. In this example, when the amplitude of the input signal current increases, the output amplitude is saturated and the output waveform is distorted by decreasing the feedback resistor R and increasing the signal current that is shunted to the feedback resistor side. Is to ensure a dynamic range.

  In this case, a feedback resistor R that can be used as a variable resistor by combining FETs or the like is used. The output signal of the transimpedance amplifier 13 is detected by the level detection circuit 19 and fed back to the feedback resistor R, and the impedance value of the feedback resistor R is varied. In addition, the output signal of the level detection circuit 19 is made to have the same variable impedance value in addition to the feedback resistor R on the replica amplifier 14 side, and the feedback characteristic is made to match that of the transimpedance amplifier 13 on the signal amplification side. Is desirable.

  Also, multiple applications with significantly different signal transmission speeds and reception sensitivities, such as US standard SONET (Synchronous Optical NET) or international standard SDH (Synchronous Digital Hierarchy) May have to deal with. In such a case, it is necessary to greatly change the impedance value of the feedback resistor R so as to be optimal for each application. For this reason, transmission rate information is input to the decoder circuit 20 from the outside, and from the decoder circuit 20 It is preferable that the impedance value of the feedback resistor R can be switched by a switch based on the transmission speed information. In this case also, it is desirable to switch the impedance value for the feedback resistor R on the replica amplifier 14 side.

  Note that the feedback resistor R can be varied by combining both the detection signal of the level detection circuit 19 and the transmission speed information of the decoder circuit 20, or may be used independently. In the present invention, since the circuit that generates the reference voltage for the output buffer amplifier 15 does not have a negative feedback loop configuration, the feedback resistor R of the replica amplifier 14 is detected by the level detection circuit 19 as shown in the configuration example of FIG. It is compatible with automatic gain control (AGC) based on the above, and a wide dynamic range can be easily achieved. At this time, the low-frequency cutoff wavelength as the preamplifier can be determined only by the time constant of the AGC loop. Further, even if the impedance of the feedback resistor R changes greatly due to the operation mode switching based on the transmission speed information, the low-frequency cutoff of the preamplifier does not change, so that it is easy to cope with the multi-rate.

It is a figure which shows the structural example of the preamplifier by this invention. It is a figure which shows a specific example of the average value detection of a preamplifier in this invention, and a voltage control current source. It is a figure explaining the operating characteristic of the preamplifier in this invention. It is a figure explaining the example which makes the preamplifier by this invention wide dynamic and multi-rate. It is a figure explaining a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Light receiving element, 12 ... Preamplifier, 13 ... 1st transimpedance amplifier, 14 ... 2nd transimpedance amplifier (replica amplifier), 15 ... Differential signal generation circuit (output buffer amplifier), 16 ... Average current Detection circuit, 17... Voltage controlled current source, 18... Current mirror circuit, 19... Level detection circuit, 20.

Claims (3)

A light receiving element that converts an optical signal into a corresponding current signal, a first transimpedance amplifier that receives the current signal and converts it into a voltage signal, and a second input / output characteristic that is the same as the first transimpedance amplifier A pre-amplifier comprising a differential signal generation circuit that generates a differential signal of the first and second trans-impedance amplifiers,
A preamplifier, wherein a current corresponding to an average value of the current signal is input to the second transimpedance amplifier. A light receiving element that converts an optical signal into a corresponding current signal, a first transimpedance amplifier that receives the current signal and converts it into a voltage signal, and a second input / output characteristic that is the same as the first transimpedance amplifier A pre-amplifier comprising a differential signal generation circuit that generates a differential signal of the first and second trans-impedance amplifiers,
A current mirror circuit having two current output terminals, wherein one of the two current terminals is connected to the light receiving element, the other is connected to the second transimpedance amplifier, and an average value of the current signals is A preamplifier for inputting to a second transimpedance amplifier.   3. The preamplifier according to claim 1, wherein an equivalent capacitance capacitor having a capacitance value equivalent to the capacitance of the light receiving element is inserted on the input side of the second transimpedance amplifier.

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