Abstract:
When an optical signal that is a wide dynamic range and different in level depending on burst signals is input as in a GPON system, a preamplifier can stably control the gain within a short preamble. The gain changeover of the preamplifier is controlled by the aid of timing information that is extracted from a data signal as data count number, to change over the gain at a high speed and with high precision. A level detector, a preamble recovery, a counter, and a control circuit are disposed within the preamplifier in addition to a TIA main body. In order to suppress the band deterioration or the phase margin reduction which are attributable to the gain changeover, there is provided a bias terminal for conducting a current injection and a current drawing with respect to the signal amplification transistor of the TIA main body.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese application JP 2007-048333 filed on Feb. 28, 2007, the content of which is hereby incorporated by reference into this application. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a preamplifier for a burst receiver of the PON (passive optical network) system, and an optical receiving apparatus using the preamplifier. 
       BACKGROUND OF THE INVENTION 
       [0003]    The GEPON is the standard of a passive optical communication network PON that has been standardized as IEEE 802.3ah in the Electrical and Electric Society in U.S. As one structural example that realizes the above standard, up to now, there has been disclosed an optical receiving apparatus that controls the changeover of a transimpedance by using a level detector and a delayer as a changeover system that changes over the transimpedance of the preamplifier by means of a switch (for example, refer to JP-A No. 2005-86466). 
         [0004]    Also, as one of the burst optical receivers that are the optical receivers which are applied to the optical communication network and transmit a signal beam in a burst form, up to now, there has been the following burst optical receiver. That is, the burst optical receiver includes a mask generator circuit that changes over an amplification gain according to the intensity of an electric signal that has been converted from a received signal beam, turns off a control signal when the rising and falling change points of the electric signal that has been subjected to amplification reach a given count value, and suppresses a process of generating an H level signal with respect to a first bit of the signal cell. There is disclosed a structure in which when N bits are required for changing over the gain of the burst preamplifier, the mask generator circuit sets a control signal to an on-state for an N-bit receiving time period before the gain changes over after a reset pulse signal has been input to the mask generator circuit (for example, refer to JP-A No. 2004-180176). 
         [0005]    Also, as a receiver circuit in a burst multiplex transmission system, more particularly, a burst signal detector circuit that detects the arrival of the burst signal, up to now, there has been a technique by which after a counter circuit inputs the burst signals and completes the counting of a given number of pulses, an output signal from the counter circuit is changed to a high level to detect the burst signal (for example, refer to JP-A No. 06(1994)-318908). 
       SUMMARY OF THE INVENTION 
       [0006]    Before achieving the present invention, the present inventors have started the study of a preamplifier that can be applied in the GPON system that is standardized by ITU-T G984.2 from the control type of the transimpedance. 
         [0007]      FIG. 2  is a schematic diagram of a GPON system. As shown in the figure, the GPON (gigabit-capable passive optical network) system connects plural ONU (optical network units) and one OLT (optical line terminal) to each other, and transmits or receives signals. In an example shown in the figure, a signal  201  that has been transmitted from an ONU  221 , a signal  202  that has been transmitted from another ONU  222 , and a signal  203  that has been transmitted from an ONU  224  that is connected directly or indirectly to a STB  225 , a TV  226 , a PC  227 , and a router  228  in a subscriber  223  are transmitted to an OLT  232  that is disposed in a service provider&#39;s central office  231  through an optical coupler (passive element)  241  as signals  211 ,  212 , and  213 . The optical coupler  241  and the OLT  232  are connected to each other through a communication channel having an upload communication speed of, for example, 1.25 GHz. The service provider&#39;s central office  231  is also connected to a higher communication channel. In general, one side at which the optical coupler  241  exists is called “access system” whereas another side at which the higher communication channel exists is called “backbone system” with the service provider&#39;s central office  231  as a boundary. The OLT  232  is required to receive the burst optical signals  211 ,  212 , and  213  that are different in level from each other which have been transmitted from the respective ONUs  211 ,  222 , and  224 . 
         [0008]      FIG. 3  is a structural diagram showing an optical receiver module  232 . The optical receiver module  232  is made up of a PD module  302  that incorporates an APD (avalanche photo diode)  110  and a preamplifier  101  that is called “a TIA (trans-impedance amplifier) circuit that conducts current to voltage conversion, and an ATC (auto threshold control)-IC (post amplifier)  302 . The APD  110  has one terminal connected to a reference voltage (for example, ground potential)  111 , and another terminal connected to an input port  112  of the preamplifier  101 . In the PD module  301 , 1) the optical signals that have been transmitted from the ONUs  221 ,  222 , and  224  are subjected to photoelectric conversion by the APD  110 , and extracted as current signals, and 2) the current signal is converted into a voltage signal by the preamplifier  101 , and then transmitted to the post-stage ATC-IC  302 . Then, the signal waveform is shaped by the ATC-IC 302 , and the signal is transmitted to a signal processor circuit  303  having a CDR (clock and data recovery) function at a poststage. The optical signal that is input to the APD  110  can be, for example, −32 to −6 dBm in the optical input intensity, and 10 dB or more in the extinction ratio. However, the present invention is not limited to those numeric values. Also, the optical current that is output from the APD  110  can have a value, for example, ranging from 2 μA to 1.8 mA. However, the present invention is not limited to those numeric values. In this example, the preamplifier  101  is of a gain changeover type in which its gain can be changed by changeover, and the number of changeovers can be set to, for example, three. The output of the preamplifier  101  is connected to the input of the ATC-IC  302 , and, for example, preferably differentially connected. However, the present invention is not limited to this configuration. Both of the preamplifier  101  and the ATC-IC  302  are controlled by a bidirectional Reset. The output of the ATC-IC  302  is connected to the input of a signal processor ASIC  303  with a CDR, and, for example, preferably differentially connected. However, the present invention is not limited to this configuration. The ATC-IC  302  is controlled according to a reset signal from the signal processor ASIC  303 . 
         [0009]      FIG. 4  is a structural diagram showing a preamplifier obtained by reconstituting the preamplifier disclosed in  FIG. 3  of JP-A No. 2005-86466 from the inventors&#39; unique viewpoint. The main function of the preamplifier  401  is to output the optical current signal that has been input to the PD  410  to a DOUTP/M as a voltage signal. The APD  410  has one terminal connected to a reference level (for example, ground potential)  411  and another terminal connected to an input port  412  of the preamplifier  401 . A main TIA  407  subjects the optical current that has been input from the PD  410  through an input port  412  to current/voltage conversion. The output of the main TIA  407  is fed back to the input of the main TIA  407  through a feedback resistor  409 . The conversion coefficient (gain) of the current/voltage conversion is called “transimpedance”, and can be controlled in the level according to a control signal from the control circuit  402 . A main path outputs the signal that has been converted into the voltage from the main amplifier  407  to output ports  413  and  414  through an output amplifier  408 . The output amplifier  408  outputs a result obtained by comparing the output of the main amplifier (main TIA)  407  with the output of a reference level generator  406  to output ports  413  and  414 . A voltage level detector  403  is connected to the output of the main TIA  407 , and detects the level of an output signal from the main TIA  407 . A signal corresponding to the signal level that has been detected by the voltage level detector  403  is input to a delayer  404 . The signal is subjected to delay processing by the delayer  404 , and thereafter output to the control circuit  402  as a timing control signal. The control circuit  402  outputs a control signal to the main TIA  407  on the basis of the output of the voltage level detector  403  and the timing control signal from the delayer  404 , and controls the transimpedance of the main TIA  407 . 
         [0010]      FIG. 7  shows the simplest structural example of the TIA. Reference i denotes an input port, o is an output port, and QN 0  is a main transistor  701 . Also, reference R L  denotes a load resistor, R F  is a feedback resistor  409 , and the respective resistances are represented by R L  and R F . 
         [0011]    When a band (fc) of the circuit shown in the figure is represented by the following expression when it is assumed that an open loop gain is a. 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     c 
                   
                   = 
                   
                     
                       1 
                       + 
                       a 
                     
                     
                       2 
                        
                       
                         π 
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                           R 
                           F 
                         
                       
                        
                       C 
                     
                   
                 
               
               
                 
                   Ex 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0012]    Also, the loop gain is represented by the following expression. 
         [0000]    
       
         
           
             
               
                 
                   
                     β 
                     · 
                     
                       R 
                       L 
                     
                   
                   
                     R 
                     F 
                   
                 
               
               
                 
                   Ex 
                    
                   
                       
                   
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                   2 
                 
               
             
           
         
       
     
         [0000]    For that reason, when a feedback resistance RF is larger, it is estimated that the band is narrower, and when a feedback resistance RF is smaller, it is estimated that the phase margin is lower. Also, in general, a factor for limiting the minimum input of the amplifier is a noise. For that reason, it is necessary to increase the feedback resistance R F  in order to reduce the thermal noise that is the main noise component of the preamplifier  401 . On the other hand, the maximum input is to saturate the operation of the transistor within the circuit, and it is necessary to reduce the feedback resistance R F . Under the circumstance, in order to deal with the input of a wide dynamic range, plural gains are prepared, and the feedback resistance R F  and the load resistance R L  are controlled by the control circuit according to the optical reception level to select an appropriate gain. 
         [0013]      FIG. 14  shows a changeover type of the transimpedance. The changeover types of the transimpedance include a cramp type, a step changeover type, and a nonstep changeover type. The cramp type cannot realize the extinction ratio resistance in principle, and the nonstep changeover type requires a sufficient time in order to achieve the stabilizing operation of the circuit. As a result, it is concluded that in order to ensure the extinction ratio resistance and achieve the stabilizing operation of the circuit, it is most effective to realize the changeover of the transimpedance by the aid of the step changeover type. 
         [0014]    Under the above circumstances, it has been studied from the inventors&#39; unique viewpoint whether the preamplifier configuration shown in  FIG. 4  which is used in the GEPON standard realization can be applied, or not. 
         [0015]      FIG. 5  shows the data structure of the burst signal. One frame of the burst optical signal includes 1) a prebias section, 2) a preamble section, 3) a delimiter section, and 4) a payload section. 
         [0016]    1) The prebias section is a section for stabilizing the optical output at the time of transmission, and 0 to 16 bits are allocated to the prebias section in the standard. What bit number among 0 to 16 bits are allocated vary depending on the environments. 2) The preamble section is a section for conducting clock synchronization, and 44 bits are allocated to the preamble section in the standard. 3) The delimiter section is a section for detecting the frame, and 20 bits are allocated to the delimiter section in the standard. 4) The payload section is allocated for data body. 
         [0017]    Because the above data structure is applied, it is necessary to complete both of the output stabilization of TIA/ATC and clock synchronization at CDR in the preamble section. 
         [0018]    In the GEPON that applies the conventional system, the preamble section is 256 bits or more, and even if the device or the environmental conditions vary, the gain changeover of the TIA can be conducted on the basis of the timing control signal generated by the delayer  404 , which originates with the input level determination as shown in  FIG. 6 , by the aid of a circuit shown in  FIG. 4 . 
         [0019]    However, in the specification of 1.24 Gbps which is regulated in ITU-T G.984.2, 44 bits need to be distributed by TIA/ATC/CDR. It is necessary to complete that the output is stabilized, that is, the gain changeover is completed when TIA is within 16 bits from the data head even if estimation is largely made, and thereafter the gain changeover is not conducted. 
         [0020]    For that reason, it is concluded that the system using the delayer  404  originating with the level detection is difficult to deal with the short overhead that is the feature of the GPON system. 
         [0021]      FIG. 8  is a structural diagram showing a conventional TIA. The TIA is configured in such a manner that a load resistor R L  portion and a feedback resistor R F  portion are connected to an amplifier  801  including a main transistor QN 0 . When the gain control function is introduced, both of the lower band at the time of the high gain and the reduced phase margin at the time of the low gain must be suppressed. To achieve this, up to now, the TIA is so designed as to change over the negative resistance R L  according to the feedback resistance R F  as shown in  FIG. 8 , thereby achieving the above suppression. 
         [0022]    However, it is also concluded that when a rising of the output load which is higher than the conventional output load with dealing with the short overhead is estimated with dealing with the short overhead, and a new countermeasure against the lower band and the reduced phase margin is required. 
         [0023]    JP-A No. 2004-180176 discloses a technique by which the control signal is set to an on-state after a predetermined number of bits have been counted up. In this technique, it appears that data per se is not used for counting, but control is conducted from an external ASIC. 
         [0024]    Also, JP-A No. 06(1994)-318908) discloses a technique by which a counter is used for detection of the burst signal. In this technique, the counting operation conducted by the counter is not used for changing over the gain of the transimpedance amplifier. 
         [0025]    A representative example of the present invention is represented as follows. That is, the changeover control system of the present invention subjects a PD current to current/voltage conversion by a main TiA, converts a voltage signal into plural level detection signals and plural data detection signals, and changes over a transimpedance according to the signal level detection signal and the data detection signal state. Also, the plural level detection signals are determined on the basis of the extinction ratio and the assumed noise. 
         [0026]    Also, the injection/drawing of a current into/from a main transistor within the main TiA is controlled for each of the transimpedances. 
         [0027]    More specifically, according to the present invention, there is provided a preamplifier comprising an input port that inputs a current signal according to the intensity of an optical signal; a first amplifier that converts the current signal that is input from the input port into a voltage signal; a second amplifier that inputs the voltage signal that is output from the first amplifier and amplifies the voltage signal; an output port that outputs the voltage signal that is output from the second amplifier; and a counter that counts the number of voltage signals and outputs a counted result, wherein the gain of the first amplifier is changed over on the basis of the count value that is output from the counter. 
         [0028]    The representative advantage of the present invention with the above configuration resides in that the transimpedance is appropriately changed over within the short overhead of the GPON system, to obtain a stable output waveform. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is a block diagram showing a preamplifier according to the present invention; 
           [0030]      FIG. 2  is a schematic diagram showing a GPON system; 
           [0031]      FIG. 3  is a schematic diagram showing an OLT side receiver module; 
           [0032]      FIG. 4  is a block diagram showing an example of the preamplifier that satisfies the GEPON standard; 
           [0033]      FIG. 5  is a diagram showing an example of one frame that is standardized by ITU-T G984.2; 
           [0034]      FIG. 6  is a diagram showing a transimpedance changeover image in the GEPON; 
           [0035]      FIG. 7  is a schematic circuit diagram showing the TIA that is most simply configured; 
           [0036]      FIG. 8  is a circuit diagram showing an example of a conventional transimpedance amplifier corresponding to the changeover of a transimpedance; 
           [0037]      FIG. 9  is a circuit diagram showing an example of a transimpedance amplifier according to the present invention; 
           [0038]      FIG. 10  is a diagram showing the contents of a comparison threshold level; 
           [0039]      FIG. 11  is a diagram showing an example of a level diagram when the number of changeovers is three; 
           [0040]      FIG. 12A  is a diagram showing a changeover truth table in each of states; 
           [0041]      FIG. 12B  is a waveform diagram showing the contents of the changeover truth table shown in  FIG. 12A ; 
           [0042]      FIG. 13  is a diagram showing a waveform example when the noise environments are low; and 
           [0043]      FIG. 14  is a diagram showing a control system of the transimpedance value. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]    Now, a description will be given in more detail of preferred embodiments of the present invention with reference to the accompanying drawings. 
       First Embodiment 
       [0045]    Elements that constitute the respective blocks in a first embodiment are formed on one semiconductor substrate through a known integrated circuit technology. A transistor element is not limited to BJT, but can be realized by any elements. 
         [0046]      FIG. 1  shows a preamplifier circuit according to this embodiment. 
         [0047]    A main TIA  107  subjects an optical current that has been input from a PD  110  to current/voltage conversion. The output of the main TIA  107  is fed back to the input of the main TIA  107  through a feedback resistor  109 . The conversion coefficient (gain) at the time of conducting the current/voltage conversion is called “transimpedance”, and can be controlled in level according to a control signal from a control circuit  102 . It is preferable that the number of gain steps is smaller from the viewpoint of increasing the changeover timing speed whereas the number of gain steps is larger from the dynamic range regulation of input and output. Taking the high speed changeover and the ease of the control into consideration, for example, the provision of three steps of high/middle/low is preferable. However, the present invention is not limited to the number of gain steps. Because an initial state corresponds to a fine input signal, the transimpedance is normally set to be high. 
         [0048]    A main path is a path that outputs the signal that has been converted into the voltage to output ports  113  and  114  from the main amplifier  107  through an output amplifier  108 . The output amplifier  108  compares the output of the main amplifier (main TIA)  107  with the output of a reference  106 , and outputs the comparison result to the output ports  113  and  114 . 
         [0049]    A voltage level detector  103  is connected to the output of the main TIA  107 , and compares the signal that has been converted into the voltage by the main TIA  107  with three signal levels shown in  FIG. 10 . When the signal level that has been converted into the voltage is equal to or higher than a comparison threshold value  1  (i.e., Vdet as shown in  FIG. 11  and LEVEL 1  as shown in  FIG. 10 ) (level  1 : V DET ) by one degree or higher, a flag (counter enable) that transits the counter  104  to the operation enable state is output to the counter  104 . When the signal level that has been converted into the voltage is equal to or higher than a comparison threshold value  2  (level  2 : V CH ) of the voltage level detector  103 , a flag that decreases the transimpedance by one step is output to the control circuit  102 . When the signal level that has been converted into the voltage is equal to or higher than a comparison threshold value  3  (level  3 : V FRC ) of the voltage level detector  103 , a flag that decreases the transimpedance to the minimum value is output to the control circuit  102 . 
         [0050]    A preamble recovery  105  is connected to the output of the main TIA  107 , converts the signal that has been converted into the voltage into a voltage that is logically meaningful in an internal circuit, and outputs the converted voltage to a poststage counter  104 . It is preferable that the preamble recovery  105  is made up of, for example, a BPF (band pass filter). 
         [0051]    The counter  104  is connected between a counter enable output of the voltage level detector  103  and the output of the preamble recovery  105 , counts the number of signals that is 1 or higher in the comparison threshold value, and outputs its result to the control circuit  102  as timing information (timing control signal). 
         [0052]    The control circuit  102  selects an optimum gain from the appropriate gains of high/middle/low according to the comparison threshold values  1 ,  2 , and  3  and the timing information on the basis of a table shown in  FIG. 12A . The details of the gain selection will be described below.  FIG. 12B  is a waveform diagram showing the contents of the changeover truth table of  FIG. 12A . 
         [0053]    (1) In the case where the signal level that has been converted into the voltage is smaller than all of the comparison threshold values  1 ,  2 , and  3  when the state of the transimpedance is high, the counting operation of the counter  104  and the changeover operation of the transimpedance do not occur. 
         [0054]    (2) In the case where the signal level that has been converted into the voltage is equal to or higher than the comparison threshold value  1  (i.e., Vdet as shown in  FIG. 11  and LEVEL 1  as shown in  FIG. 10 ) and lower than the comparison threshold value  2  when the state of the transimpedance is high, the counter  104  conducts the counting operation, but the changeover operation of the transimpedance does not occur. Further, when two pulses are counted in this state, the changeover operation is suppressed so that the changeover operation does not occur, and the state of the transimpedance is locked to a high state because malfunction caused by the noise signal is prevented. 
         [0055]    (3) In the case where the voltage converted signal level is equal to or higher than the comparison threshold value  2 , and the count state of the counter  104  is lower than two pulses when the state of the transimpedance is high, the count state of the transimpedance is reduced to middle from high by one step. 
         [0056]    (4) In the case where the signal level of a third pulse of the voltage converted signal is smaller than the comparison threshold value  2  when the state of the transimpedance is transited to middle from high, the changeover operation is suppressed, and the state of the transimpedance is locked to a middle state because malfunction caused by the noise signal is prevented. 
         [0057]    (5) In the case where the signal level of the third pulse of the voltage converted signal is equal to or larger than the comparison threshold value  2  when the state of the transimpedance is transited to middle from high, the state of the transimpedance is further reduced to the low state from the middle state by one step. 
         [0058]    As described above, since control is conducted on the basis of the comparison threshold values  1 ,  2 , and  3 , and the timing information, the setting of the comparison threshold values  1 ,  2 , and  3  is also characteristic, which will be described in more detail below. Hereinafter, for simplification of description, when it is assumed that the feedback resistance=transimpedance value, and the feedback resistances are R H , R M , and R L  from the higher gain side, the following relationship is essential in the comparison threshold values  1 ,  2 , and  3  (V DET , V CH , V FRC ) in order to prevent malfunction when it is assumed that the noise or overshoot quantity is 20% of the main signal. 
         [0000]      1.2× V   DET   &lt;V   CH    (1) 
         [0000]      1.2× V   CH   &lt;V   FRC    (2) 
         [0059]    Also, the minimum amplitude regulation must be kept. 
         [0060]      FIG. 11  shows a level diagram. As shown in the level diagram, since the transimpedance is transited to the lower transimpedance at the output V CH , when it is assumed that the minimum output regulation is V MIN , and the maximum output regulation is V MAX , the following expressions are satisfied. 
         [0000]      V FRC &lt;V MAX    (3) 
         [0000]        V   MIN   &lt;V   CH   *R   M   /R   H    (4) 
         [0000]        V   MIN   &lt;V   CH   *R   L   /R   M    (5) 
         [0000]    The comparison threshold values  1 ,  2 , and  3  that satisfy the above expressions (1) to (5) are set. 
         [0061]    As described above, appropriate threshold values and appropriate counter control are set, thereby enabling the gain changeover at a high speed and with high precision to be realized. 
         [0062]      FIG. 9  shows one structural example of the TIA that constitutes the preamplifier  101  which is an example of the preamplifier according to the present invention. The TIA is configured in such a manner that the load resistor R L  portion and the feedback resistor R F  portion are connected to the amplifier  901 . As compared with the conventional art shown in  FIG. 8 , a cascode transistor QN 1  is connected to an upper stage of a main transistor QN 0 , and a current control terminal ibias is disposed at the collector terminal of the main transistor QN 0  to constitute a pair of amplifiers  901 , unlike the conventional art shown in  FIG. 8 . 
         [0063]    When the transimpedance state is high, R F  is larger. As a result, since the band is lowered as can be understood from Expression A, it is necessary to suppress the band from being lowered. Therefore, a current that flows in the main transistor is injected from the terminal ibias and increased, and the open loop gain is increased. As a result, the band of the circuit is increased without changing the circuit constant. On the other hand, when the transimpedance state is low, the stability of the circuit is reduced since the loop band is large, and the band is wide. Under the circumstances, the current that flows in the main transistor is drawn from the terminal ibias so that the loop gain is reduced, the band is lowered, and the phase margin is ensured, thereby enhancing the stability of the circuit. The current that flows in the main transistor is controlled in each of the transimpedance states thus switched over, and the countermeasure is conducted against the lowered band at the time of the high gain and the lowered phase margin at the time of the low gain. 
         [0064]    As described above, according to this embodiment, the transimpedance is appropriately changed over within the short overhead of the GPON system, thereby making it possible to obtain a stable output waveform. 
       Second Embodiment 
       [0065]    In the operations (1) to (5) of selecting the gain shown in the first embodiment, it is assumed that the count operation of the counter  104  occurs due to the input of the preamble signal portion as a precondition. Accordingly, in noisier environment, and the counter  104  conducts the miscount operation in the prebias portion, it is difficult to transit to the appropriate transimpedance. 
         [0066]    For example, as shown in  FIG. 13 , it is difficult to transit to the appropriate transimpedance with respect to the waveform in noisier environment such that the low level of the prebias is about 0.1 times of the main signal level, the overshoot of the prebias is about 0.1 times of the main signal level, and the main signal level is five times or more of the maximum value of the prebias input. 
         [0067]    Under the circumstances, more particularly, as a bailout effective in the case where the noise environment is low, the following operation (6) and (7) can be added after the above operation (1) to (5). 
         [0068]    (6) When the state of the transimpedance is high, the number of count pulses of the counter  104  is equal to or higher than 3 but lower than 8, and the level of the signal that has been converted into the voltage is equal to or higher than the comparison threshold value  3 , the state of the transimpedance is transited from high to low by two steps at a time. 
         [0069]    (7) When the state of the transimpedance is middle, the number of count pulses of the counter  104  is equal to or higher than 4 but lower than 8, and the level of the signal that has been converted into the voltage is equal to or higher than the comparison threshold value  3 , the state of the transimpedance is transited from middle to low. 
         [0070]    Since the control is conducted on the basis of the comparison threshold values  1 ,  2 , and  3 , and the timing information, the setting of the comparison threshold values  1 ,  2 , and  3  is characteristic, like the above first embodiment. However, the following points are different from the first embodiment. 
         [0071]    In the level diagram shown in  FIG. 11 , that the noise environment is low, and the prebias portion conducts the miscount means that the extinction ratio and the noise input exceed the comparison threshold value  1  (i.e., Vdet as shown in  FIG. 11  and LEVEL 1  as shown in  FIG. 10 ), the counter  104  transits to the operation enable state, and the miscount is conducted by the waveform of the signal that is transmitted from the main amplifier  107  to the preamble recovery  105  due to the waveform fluctuation caused by the noise input. The high frequency noises and the low frequency noises can be removed by some degree by the BPF of the preamble recovery  105 . However, the noises of the signal band cannot be removed. 
         [0072]    Under the above circumstances, when the signal level exceeds the comparison signal  1  in the prebias portion provided that the removal of the noises is disenabled, even if the transimpedance transits to the lowest transimpedance, the transimpedance is so set as not to violate the minimum output voltage regulation. 
         [0073]    Taking that the extinction ratio specification 10 dB of the GPON is the signal level of about 10% of the main signal at the low level into consideration, when the overshoot of the prebias portion and the noises are about 10% of the main signal, the prebias portion main signal level of the comparison signal  1  is represented by the following expression. 
         [0000]        I=V   DET   /R   H *5 
         [0000]    Since the voltage that operates the optical current by the low gain (R L ) needs to be larger than the minimum output regulation (V MIN ), the following expression is satisfied. 
         [0000]        V   MIN   &lt;V   DET   *R   L   /R   H *5   (6) 
         [0000]    The comparison threshold values  1 ,  2 , and  3  are set which satisfy the expressions (1) to (6) obtained by adding the above expression (6) to the expressions (1) to (5) of the first embodiment. 
         [0074]    As described above, according to this embodiment, in particular, even in noisier environment, the transimpedance is appropriately changed over within the shorter overhead of the GPON system, thereby making it possible to obtain the stable output waveform.