Abstract:
A preamplifier for use in an optical signal receiver is provided, which is capable of circumventing a problem of oscillation or shortage of bandwidth even in case an employed photodiode has a capacity value which inherently causes oscillation or shortage of bandwidth. The preamplifier includes a variable gain amplifier connected to a light receiving element for converting a received light signal to electric current; a variable feedback resistor connected to an input and output of the variable gain amplifier; and a band detection circuit for detecting a signal band amplified by the variable gain amplifier, wherein a band control signal obtained from the band detection circuit controls the band of the variable gain amplifier.

Description:
This application is a continuation of international application No. PCT/JP00/01358, filed Mar. 6, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a preamplifier, and more particularly a preamplifier for use in an optical signal receiver. 
     BACKGROUND OF THE INVENTION 
     An example of a conventional preamplifier for use in an optical signal receiver is shown in FIG.  1 . In this FIG. 1, received light is converted into electric current by means of a photodiode. A preamplifier has a function of converting the current having been converted from the optical light into a voltage signal to output. 
     Here, there lies a predetermined relation between a gain A 0  of the preamplifier and a signal bandwidth (f −3dB ). Namely, the signal bandwidth (f −3dB ) is determined by; total input capacity (Cin) consisting of capacity between the photodiode terminals, parasitic capacity caused by mounting, etc., and capacity of an FET constituting the preamplifier, etc., feedback resistance (Rf), and amplifier gain A 0 , in accordance with the following formula (1).                      f       -   3        dB       =                  A   0     /     (     2                 π                 Rf   *   Cin     )                   =                1   /     (     2      π                 Zin   *   Cin     )                     (   1   )                                
     (Here, Zin=Rf/A 0  is an input impedance of the preamplifier.) 
     Now, because the amplifier itself normally has a bandwidth, it is necessary to design the signal bandwidth f −3dB  so that the signal bandwidth is sufficiently smaller than the amplifier bandwidth. However, in the optical signal receiver, there are cases of extremely small capacity between terminals, or on the contrary, extremely large capacity between terminals. Such cases depend on dispersion in the photodiode characteristic. 
     In the case when the capacity between terminals is extremely small, the total input capacity (Cin) becomes small. In this case the signal bandwidth f −3dB  is extended near to the amplifier bandwidth, as can be understood from formula (1). Thus oscillation or ringing is produced. 
     This oscillation or ringing condition will be explained referring to FIG.  2 . FIG. 2A is shown by a scale width of 140 μs. On the other hand, FIGS. 2B,  2 C are shown by scale widths of 200 ns, in which a portion of the amplifier output period shown in FIG. 2A is enlarged. As shown in FIGS. 2B,  2 C, a high-frequency oscillation signal is superposed onto a rectangular wave signal, which illustrates an oscillation or ringing condition. 
     To the contrary, in case the capacity between terminals of the photodiode is extremely large, the total input capacity (Cin) becomes large, producing an extremely reduced signal bandwidth f −3dB . Therefore, as a portion of the preamplifier output period is enlarged in FIG. 3, there appears a condition that a rectangular wave signal component cannot be passed through. Such a condition may be understood from FIGS. 3B,  3 C in which a portion period of FIG. 3A are enlarged with a scale identical to FIG.  2 . The rectangular wave signal disappeared. 
     Meanwhile, when employing an APD (avalanche photodiode) as a photodiode, a photodiode APD has a characteristic of having small capacity when an input light signal is small, while having large capacity when an input light signal is large. There has been a known technology of preventing oscillation using this characteristic. That is, controlling a bias voltage of the photodiode APD by detecting preamplifier output amplitude which may vary in proportion to the input light signal of the photodiode APD (as having been disclosed in the official gazette of Japanese Unexamined Patent Publication No. Hei-7-304922, etc.) 
     The configuration block diagram in the above disclosure is shown in FIG.  4 . In this FIG. 4, an output of the photodiode APD is input into preamplifier  2 . An output of preamplifier  2  is led to AGC circuit  4 , in which it is controlled so as to produce a constant gain. 
     A peak detection circuit  5  detects a peak value (amplitude) of an output of AGC circuit  4 , to control a bias output of an APD bias control circuit  52  via a phase compensation circuit  51 . 
     Now, a capacity value of the photodiode APD includes dispersion produced by a varying manufacturing condition, etc. In some cases, there is a photodiode APD which inherently has a capacity value causing oscillation or shortage of bandwidth. Therefore, when using a photodiode APD having such a capacity value in the configuration shown in FIG. 4, it is not possible to circumvent oscillation or shortage of bandwidth. The reason is, according to this configuration, that the amplitude detection for use in controlling bias voltage is carried out using a signal after passing through preamplifier  2  and AGC circuit  4 . 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a preamplifier which is capable of circumventing a problem of oscillation or shortage of bandwidth even in case an employed photodiode has a capacity value which inherently causes oscillation or shortage of bandwidth. 
     In order to attain the aforementioned object, according to the present invention, a band detection circuit is employed so as to detect a bandwidth of a preamplifier directly, thereby to control to obtain an appropriate preamplifier band using a band control signal being output from the band detection circuit. 
     As a preferred embodiment of the present invention, the preamplifier includes; a variable gain amplifier connected to a light receiving element for converting a received light signal to electric current; a variable feedback resistor connected to both an input and output of the variable gain amplifier; and a band detection circuit for detecting a signal band being amplified by the variable gain amplifier. Thus the band of the variable gain amplifier is controlled by a band control signal obtained from the band detection circuit. 
     As a preferred embodiment of a preamplifier according to the present invention, the aforementioned band detection circuit includes; a band detector; and a detection signal hold circuit for holding a detection signal obtained from the band detector. The aforementioned band detector includes; a level generation circuit for outputting two internally divided levels by resistance-dividing a bottom value and a peak value of a signal being input to the band detector; a first comparator for comparing an upper level side of the two internally divided levels being output from the level generation circuit with a signal being input to the band detector; a second comparator for comparing a lower level side of the two internally divided levels with a signal being input to the band detector; a first delay circuit for delaying the second comparator output for a predetermined time; and a comparison circuit for comparing the timing of an output of the first delay circuit with the timing of the first comparator output. 
     As another preferred embodiment of a preamplifier according to the present invention, the preamplifier further includes a bitrate detector, and an output terminal of the bitrate detector is connected to a bitrate signal input terminal of the band detection circuit. 
     Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a diagram illustrating one example of a conventional preamplifier in an optical signal receiver. 
     FIG. 2 shows a diagram illustrating occurrence of an oscillation or ringing condition when a preamplifier input capacity is small. 
     FIG. 3 shows a low signal pass band when a preamplifier input capacity is large. 
     FIG. 4 shows a diagram illustrating a conventional art of circumventing oscillation. 
     FIG. 5 shows a diagram illustrating a conceptual configuration of the present invention. 
     FIG. 6 shows one embodiment for detecting a bandwidth of preamplifier  2  using a band detection circuit  1 . 
     FIG. 7 shows an embodiment of band detection using an output of an AGC circuit. 
     FIG. 8 shows an embodiment of band detection circuit  1 . 
     FIG. 9 shows one circuit embodiment of a band detector  7 . 
     FIG. 10 shows an operational waveform in case of large input capacity in preamplifier  2 . 
     FIG. 11 shows an operational waveform in case of small input capacity in preamplifier  2 . 
     FIG. 12 shows an embodiment of a mask circuit configuration. 
     FIG. 13 shows another embodiment of the mask circuit configuration. 
     FIG. 14 shows another embodiment of band detector  7 . 
     FIG. 15 shows an embodiment of a detection signal hold circuit in band detection circuit  1 . 
     FIG. 16 shows another embodiment of the detection signal hold circuit in band detection circuit  1 . 
     FIG. 17 shows an embodiment of a configuration in which the embodiment shown in FIG. 16 is expanded. 
     FIG. 18 shows another embodiment of the present invention. 
     FIG. 19 shows a typical example of FIG.  18 . 
     FIG. 20 shows a typical example corresponding to FIG.  19 . 
     FIG. 21 shows an effect of the present invention corresponding to FIG.  2 . 
     FIG. 22 shows an effect of the present invention corresponding to FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the present invention is described hereinafter referring to the charts and drawings, wherein like numerals or symbols refer to like parts. 
     FIG. 5 is a diagram illustrating a conceptual configuration of the present invention. A band of a preamplifier  2  is detected by a band detection circuit  1  to output a band control signal. Using this, it is possible to control so that the band of preamplifier  2  becomes constant. 
     Input impedance is controlled as one means to control the band of preamplifier  2 . More specifically, this is enabled by altering either a resistance value of a feedback resistor  3  or a gain of a variable gain amplifier  6 . 
     FIG. 6 shows one embodiment of detecting the band of preamplifier  2  by means of band detection circuit  1 , by which it becomes possible to detect the band directly from an output of preamplifier  2 . In other words, because it is an object to control the band of preamplifier  2 , it is direct and most effective to detect the band from the output of preamplifier  2 . 
     Also, according to the band detection method shown in FIG. 7, an output signal of preamplifier  2  is linearly amplified by an AGC (automatic gain control) circuit  44  to a certain value irrespective of the output amplitude of preamplifier  2 . This signal is input to band detection circuit  1 . Therefore, band detection circuit  1  has a signal for detection having sufficient amplitude. Thus band detection with a small error can be performed. 
     FIG. 8 shows a configuration example of band detection circuit  1 . Band detection circuit  1  is constituted by a band detector  7  and a detection signal hold circuit  8 . One circuit embodiment of band detector  7  is shown in FIG.  9 . In this embodiment, by detecting a rise time of an input signal of band detector  7 , it is determined whether or not the band is appropriate. 
     Namely, in FIG. 9, a level generation circuit  11  includes a bottom detection circuit  19  and a peak detection circuit  20 . Both a bottom value and a peak value of the signal output from preamplifier  2  are respectively detected by bottom detection circuit  19  and peak detection circuit  20 . The detected values of bottom detection circuit  19  and peak detection circuit  20  are resistance-divided by resistors  21 ,  22  and thus two arbitrary internally divided levels can be obtained. 
     Here, for the sake of easy understanding, waveforms of various portions in band detector  7  are shown in comparison in FIGS. 10,  11 . FIG. 10 shows a case of either large input capacity of preamplifier  2 , or low band frequency, while FIG. 11 shows a case of either small input capacity of preamplifier  2 , or high band frequency. 
     Referring back to FIG. 9, two internally divided levels {circle around ( 1 )}, {circle around ( 2 )} are respectively input to comparators  10 ,  9 , to compare to an input signal of band detector  7 . Comparator  9  outputs a period during which the input signal (FIGS. 10A,  11 A) exceeds the lower internally-divided level {circle around ( 2 )} (FIGS. 10B,  11 B). 
     Meanwhile, comparator  10  outputs a period during which the input signal exceeds the upper internally-divided level {circle around ( 1 )} (FIGS. 10D,  11 D). A delay circuit  12  produces a delay for a predetermined period against the output of comparator  9  (FIGS. 10C,  11 C). 
     Thereafter, the signal after the delay is produced and the output of the aforementioned period in which the input signal exceeds the upper internally-divided level {circle around ( 1 )} are compared. In this comparison, if the delayed signal appears earlier, then it is determined the band is too low. To the contrary, if the signal rise timing at which the input signal exceeds the upper internally-divided level {circle around ( 1 )} appears earlier, then it is determined the band is too high. 
     In FIG. 10, the rise timing of a signal in which an output of comparator  9  is delayed for a predetermined time, that is an output signal of delay circuit  12  (refer to FIG.  10 B), appears earlier than the rise timing of a signal exceeding the upper internally-divided level {circle around ( 1 )} of the input signal, that is an output signal of comparator  10  (refer to FIG.  10 D). 
     Therefore, FIG. 10 corresponds to large input capacity of preamplifier  2 . 
     The fact that the rise timing of the output of delay circuit  12  appears earlier than the signal exceeding upper internally-divided level {circle around ( 1 )}, or the rise timing of the output of comparator  10  (refer to FIG.  10 D), is determined in the following way using signal outputs of mask circuits  17 ,  18 . 
     In FIG. 10, the output of delay circuit  12  is input to one input terminal of a NAND gate  15 , and the output of comparator  10  is input to the other input terminal of NAND gate  15  via a NOT circuit  13 . Thus an output of NAND gate  15  becomes as shown in FIG.  10 F. 
     Meanwhile, in FIG. 10, the output of delay circuit  12  is input to one input terminal of a NAND gate  16  via a NOT circuit  14 , and the output of comparator  10  is input to the other input terminal of NAND gate  16 . Thus an output of NAND gate  16  becomes as shown in FIG.  10 G. 
     In FIG. 10F, during the time difference from the rise timing of the output of delay circuit  12  to the rise timing of the output of comparator  10 , an output logic level of NAND gate  15  is kept low, while an output logic level of NAND gate  16  is continuously high. 
     Meanwhile, in an example shown in FIG. 11, the output of delay circuit  12  appears later than the output of comparator  10 . Therefore, in this case, an input capacity of preamplifier  2  is small, which is not possible to circumvent oscillation. 
     This is determined from the outputs of NAND gates  15 ,  16 , as having been explained in FIG. 10, in which the output logic level of NAND gate  15  is kept high (FIG.  11 F), while the output logic level of NAND gate  16  is low. 
     Additionally, in FIG. 10F, the logic level is low in the period “X” also. This may possibly be determined as the period of time difference from the rise timing of the output of delay circuit  12  to the rise timing of the output of comparator  10 . Similarly, in FIG. 11F, a low level appears in the period of “Y” during which the logic level is originally high. 
     To solve this problem, there are provided mask circuits  17 ,  18  shown in FIG.  9 . FIG. 12 shows configurations of mask circuits  17 ,  18 , in which mask circuit  17  is constituted by a NOR gate  32  and a NOT circuit  33 , while mask circuit  18  is constituted by a NOR gate  34 . The output of delay circuit  12  is further delayed by a delay circuit  31 , so that a control signal is not output when this delay output signal is high in logic level. 
     Here, in case of oscillation or near oscillation condition, the oscillation or large ringing is produced at higher frequency than the intended frequency band. Therefore the high band is easily determined and it is possible to restore to the normal band in which oscillation does not occur. 
     However, in case of oscillation or a similar condition, it may possibly occur that a signal is output in which low band is determined by mistake because it is not possible to mask an unnecessary signal produced at the fall timing. To avoid such a condition, when a signal indicating the determination of high band is output, a signal output is masked so as not to output a signal indicating the determination of low band for a certain period. 
     An embodiment to realize this is shown in FIG.  13 . Namely, an output of mask circuit  17  is controlled through a mask signal generation circuit  36  having a plurality of stages of delay circuits  37  to  39  (as an example, three stages are shown here). Mask circuit  17  includes a NOR gate  32  and a NAND gate  35 , and an output of NOR gate  32  is delayed for the delay time produced by a mask signal generation circuit  36 . 
     When the amplitude of preamplifier  2  is decreased because of variation of the input signal amplitude, disappearance of ringing which was existing previously, or any other similar reasons, it becomes necessary to decrease the level of peak detection circuit  20  or increase the level of bottom detection circuit  19  in the embodiment shown in FIG.  19 . 
     However, in either peak detection circuit  20  or bottom detection circuit  19 , there is no other way than relying on leak current. When a certain degree of fast response is required for peak detection circuit  20  and bottom detection circuit  19 , there are required a peak envelope detector  24  and a bottom envelope detector  25  in level generation circuit  11 , as shown in the embodiment in FIG.  14 . 
     Additionally, band detection circuit  1  shown in FIG. 8 includes detection signal hold circuit  8  for holding the detection signal (output of mask circuits  17 ,  18 ) obtained from band detector  7 . FIGS. 15,  16 , and  17  are embodiment configurations of detection signal hold circuit  8 . 
     In the configuration shown in FIG. 15, an output of a charge pump  26  is charged into a capacitor  27 . According to this configuration, an input impedance control signal is entirely analog-processed. Therefore a delicate adjustment can be attained with a simple configuration. 
     Meanwhile, according to the embodiment configurations shown in FIGS. 16,  17 , the detection signal of band detection circuit  1  (i.e. the output of mask circuits  17 ,  18 ) is once converted into a logic signal using an up-down counter  28 . The logic signal is then converted into an analog signal using a digital-to-analog converter  29 . Thus, it becomes easy to set control accuracy by means of an input impedance control signal at the conversion accuracy of up-down counter  28  and digital-to-analog converter  29 . 
     In the configuration shown in FIG. 17, a non-volatile memory  30  is provided in addition to the configuration shown in FIG. 16, in which an adjustment level before shipping the product is stored. This enables to ship the produce at appropriate impedance, by which the band adjustment becomes unnecessary at the time of actual operation. It becomes possible to vary to an appropriate band against a signal bitrate. 
     Here, as another embodiment, it is possible to obtain an appropriate band timely by inputting a control signal corresponding to a bitrate into a bitrate signal input terminal of band detection circuit  1 . FIG. 18 shows a conceptual configuration diagram based on the above-mentioned method. A bitrate detector  43  is provided and the output signal thereof is input to band detection circuit  1 . 
     An embodiment of this conceptual configuration is shown in FIG.  19 . In this embodiment shown in FIG. 19, a bitrate is detected from an output of preamplifier  2  and the detected signal thereof is input to band detection circuit  1 . In the configuration shown in FIG. 20, a bitrate is further detected to control a delay amount produced by delay circuit  12  using the detected bitrate signal. 
     The aspects of the effects according to the embodiment of the present invention are shown in FIGS. 21,  23  in contrast to FIGS. 2,  3 , respectively. 
     FIGS. 21D,  22 D show two cases of a band control signal being output from band detection circuit  1 . FIG. 21, which corresponds to FIG. 2, is a case of small input capacity of preamplifier  2  and a signal band becomes near the band of preamplifier  2 . Therefore, a band control signal becomes large at an early period of measurement range period to feed back so as to control the band of preamplifier  2 . Accordingly, a normal band range is obtained thereafter and the band control signal is settled at a constant level. 
     FIG. 22, which corresponds to FIG. 3, shows a case of large input capacity of preamplifier  2  resulting in shortage of the band. Therefore, the band control signal is small at an early period of the measurement range period to feed back so as to control the band of preamplifier  2 . Accordingly, a normal band range is obtained thereafter and the band control signal is settled at a constant level. 
     APPLICATION FIELD IN THE INDUSTRY 
     As the present invention has been described, by employing the present invention, it becomes possible to cope with not only capacity dispersion existent in an input of a preamplifier in an optical reception circuit, but also dispersion of either a preamplifier gain or a feedback resistance. 
     Moreover, it becomes possible to retain a required band as well as to prevent oscillation even when an input capacity of the preamplifier is varied possibly caused by a capacity between the terminals of a light receiving diode PD or an implementation pattern, etc.