Abstract:
An amplifier implementing with a common base circuit is disclosed. The amplifier includes the common base circuit, a current shunt, and a current supplement. The common base circuit receives an input current. The current shunt shunts the input current based on the average of the output of the pre-amplifier. The current supplement supplements a current shunted by the current shunt.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a common base circuit with output compensation, a current-to-voltage circuit configured with the common base circuit, and an optical receiver implementing with the current-to-voltage converter. 
     2. Related Prior Art 
     A Japanese Patent Application published as JP-H09-008534A has disclosed an amplifier with the common base arrangement.  FIG. 8  shows a fundamental portion of the common base circuit disclosed therein. The amplifier  100  includes three (3) transistors,  101  to  103 . The transistor  103  has the common base configuration where the emitter thereof receives the photocurrent generated in a photodiode by illuminated with an optical signal; the base thereof is fixedly biased; and the collector generates an output. The other two transistors,  101  and  102 , have an arrangement of, what is called, the tandem connection, namely, the collector of the transistor  101  is directly connected to the base of the other transistor  102 , while, the base of the transistor  101  is connected to the emitter of the other transistor  102 . These two transistors,  101  and  102 , connected in tandem operate as a load of the last transistor  103 . 
     Another Japanese Patent Application published as JP-H11-205047A has discloses, what is called, a trans-impedance amplifier (hereafter denoted as TIA) used in an optical receiver that converts a photocurrent into a voltage signal. The TIA disclosed therein includes a transistor with the common base arrangement, a variable current source coupled with the emitter of the transistor, a load resistor, a fixed bias source for the base of the transistor, and a controller connected between the input and the output of the TIA to adjust the magnitude of the current generated in the current source depending on the output voltage. 
     Still another Japanese Patent Application published as JP-2009-246823A has disclosed a type of TIA. The TIA disclosed therein has a plurality of power supplies dynamically switched depending on the magnitude of the input photocurrent. 
     An optical receiver generally includes a photodiode (hereafter denoted as PD), and a pre-amplifier to convert the photocurrent into a voltage signal and amplifies this voltage signal. A TIA is generally applicable to such a pre-amplifier. A TIA has an arrangement including an inverting amplifier with high input impedance and a trans-impedance element, typically a resistor, connected between the input and the output of the inverting amplifier. In such an arrangement, a substantial portion of a current input to the TIA flows in the trans-impedance to cause a voltage drop thereat. Thus, the voltage drop, which may be evaluated by a product of the input current with the impedance of the trans-impedance element, becomes proportional to the input current. 
     The TIA with the arrangement above described is necessary to set the input impedance of the inverting amplifier high enough, which equivalently enhance the input capacitance of the amplifier and resultantly degrades the high frequency performance of the pre-amplifier. 
     Another type of the pre-amplifier of an optical receiver has been known as the common base circuit. The common base circuit receives the photocurrent generating in the PD at the emitter of the transistor, and outputs a voltage signal form the collector. The common base circuit has an inherent feature of the low input impedance, which may eliminate the influence of the input capacitance of the device. Moreover, the output of the common base circuit, which is drawn from the collector, has a phase same with that of the input; accordingly, the common base circuit may reduce the miller effect between the output and the input. 
     However, the common base circuit has a subject explained in  FIG. 9  that shows a fundamental circuit of a conventional common base circuit. The common base circuit  200  includes a transistor  201  whose base is fixedly biased by a voltage Va determined by a ratio of resistance of two resistors,  202   a  and  202   b ; the collector thereof is connected to the power supply Vcc through a load resistor  205 ; and the emitter is grounded through the constant current source  206 . The input current Iin is given to the emitter of the transistor  201 , while, the output Vout thereof is given at the collector. 
     When no input current Iin is input, the current flowing in the transistor  201  and the resistor  205  is given by I E , substantially equal to the constant current determined by the current source  206 . When a substantial current Iin is input, which flows into the current source  206 , the current flowing in the load resistor  205  becomes I E −Iin. Thus, as the input current increases, which decreases the current flowing in the load resistor  205 , the voltage drop by the load resistor  205  becomes smaller and the output level Vout approaches the power supply Vcc and saturates thereto. 
       FIG. 10  shows eye diagrams of the output Vout of the common base circuit  200  as varying the input current Iin from 100 μA to 2000 μA. As shown in  FIG. 10 , the cross point of the eye diagrams shifts to the higher level as the input current Iin increases. This is because the output Vout rises faster and saturates as the input current Iin increases. Thus, the conventional common base circuit  200  has an inherent subject that the output Vout degrades the shape thereof as the input current Iin increases. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to an amplifier circuit that includes a common base circuit, a current shunt, and a current supplement. The common base circuit includes the first transistor that operates in the common base arrangement. The first transistor may receive an input current in the emitter thereof and generates the first current flowing therein. The current shunt may shunt the second current from the first current depending on the average of the output of the amplifier. The current supplement may supply the third current to the first current. The third current is equal to the second current that is shunt from the first current by the current shunt. 
     In the amplifier according to the present invention, the reduction of the first current flowing in the common base transistor due to the shunting by the current shunt may be supplied by the third current generated in the current supplement. Accordingly, the signal deformation inherently appeared in the output of the conventional common base amplifier may be effectively suppressed to be independent of the input current. 
     Another aspect of the present invention relates to a current-to-voltage converter with the common base arrangement. The current-to-voltage converted includes an amplifier with the common base arrangement above described and a detector to detect the output of the current-to-voltage converter. Even in the current-to-voltage converter, the decrease of the current flowing in the common base transistor due to the current shunt depending on the input current may be compensated by the current supplement. Accordingly, the distortion of the output signal, in particular, the shift of the cross point in the eye diagram of the output, may be effectively suppressed. 
     Still another aspect of the present invention relates to an optical receiver that implements the PD, the current-to-voltage converter with the common base arrangement to convert the photocurrent generated in the PD into a voltage signal, a series of differential amplifiers, and an offset adjustor. Because the current-to-converter has an arrangement same as those described above, the distortion appeared in the output of the current-to-voltage converter when the optical input is strengthened and the photocurrent becomes large may be effectively suppressed. Moreover, the common base arrangement of the input of the current-to-voltage converter inherently has characteristics of the restricted input impedance and the phase of the output signal common to that of the input signal; accordingly, the current-to-voltage converter is hard to be affected by the input capacitance and the miller effect, which may suppress degradations in the high frequency performance of the optical receiver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
         FIG. 1  is a functional block diagram of an optical receiver according to an embodiment of the present invention; 
         FIG. 2  is a circuit diagram of the pre-amplifier according to an embodiment of the present invention; 
         FIG. 3  only displays a fundamental part of the common base circuit, the current shunt, and the current supplement; 
         FIG. 4  shows variation of the collector currents flowing in the first and second transistors, respectively; 
         FIG. 5  shows the variation of the collector current of the first transistor in the common base circuit, that of the third transistor in the current shunt, and the current flowing in the road resistor; 
         FIG. 6  shows eye diagrams of the output of the pre-amplifier according to the embodiment of the preset invention as varying the input photocurrent Iin from 100 μA to 2000 μA; 
         FIG. 7  shows the amplitude of the output of the pre-amplifier in a peak-to-peak unit as varying the input photocurrent; 
         FIG. 8  shows a fundamental portion of the common base circuit disclosed in a prior art; 
         FIG. 9  that shows a fundamental circuit of a conventional common base circuit; and 
         FIG. 10  shows eye diagrams of the output Vout of the common base circuit  200  as varying the input current Iin from 100 μA to 2000 μA. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Next, some preferred embodiments of an amplifier, a current-to-voltage converted, and an optical receiver according to the present invention will be described as referring to accompanying drawings. In the description of the drawings, elements same with or similar to those will be referred by the numerals same with or similar to each other without overlapping explanations. 
       FIG. 1  is a functional block diagram of an optical receiver  10  according to an embodiment of the present invention. The optical receive  10  may convert a photocurrent Iin generated by a PD  11  into a faint voltage signal and amplify this voltage signal to a substantial level. The optical receiver  10 , as shown in  FIG. 1 , includes the PD  11  and an optical receiving circuit  12  that comprises a pre-amplifier  13 , three (3) differential amplifiers,  14  to  16 , and an offset compensator  17 . 
     The PD  11  may generate the photocurrent Iin by receiving an optical signal with high frequency components to the optical receiving circuit  12 . The photocurrent Iin corresponds to the optical signal. The photocurrent Iin is provided to the pre-amplifier  13  through an input terminal  12   a  of the optical receiving circuit  12 . The pre-amplifier  13  may not only amplify the signal but also convert the photocurrent Iin into the faint voltage signal Vout. This voltage signal Vout is transferred to the differential amplifier  14 , and so on. The signals, which are complementary to each other and output from the last differential amplifier  16 , are externally output from the output terminals,  12   a  and  12   b , of the optical receiving circuit  12 . The offset compensator  17  may adjust the output offset of the first differential amplifier  14  by receiving the outputs from the last differential amplifier  16  and adjusting the input of the first differential amplifier  14 . 
       FIG. 2  is a circuit diagram of the pre-amplifier  13  according to an embodiment of the present invention. The pre-amplifier  13  includes a common base circuit  20 , a current shunt  30 , a current supplement  40 , a detector  50 , and an emitter follower  51 .  FIG. 3  only displays a fundamental part of the common base circuit  20 , the current shunt  30 , and the current supplement  40  for explanation sake. 
     The common base circuit  20  includes a transistor  21 , which is the first transistor of the current embodiment, and a load resistor  22 . The transistor  21  has the common base configuration, that is, a signal Iin to be amplified is given in the emitter thereof and output from the collector thereof as the base is fixedly biased by a reference Vbias. The emitter of the transistor  21  is grounded through the current source  61 , while, the collector in the node A is biased by the power supply Vcc ( 18 ) through the load resistor  22 , and the base thereof receives the reference Vbias determined by two (2) resistors,  62   a  and  62   b , and a current source  65  where they are connected in series between the power supply Vcc ( 18 ) and the ground. 
     The output Vout of the common base circuit  20  is drawn from the node A, the collector of the transistor  21 . Specifically, the node A is connected to the output terminal  13   b  of the pre-amplifier  13 . 
     The current shunt  30  includes a transistor  31 , which is the second transistor of the present embodiment and connected in parallel to the common base circuit  20 . Specifically, the emitter of the transistor  31 , which is connected to the emitter of the first transistor  21 , is grounded through the current source  61 . The emitter of this transistor  31  may also receive the input current Iin from the input terminal  13   a . The collector of the transistor  31  is biased from the power supply Vcc ( 18 ) through two transistors,  32  and  33 , connected in series and having the diode connection. The base of the transistor  31  receives a gain control signal Vagc output from the detector  50 . 
     The current supplement  40  includes two transistors,  41  and  42 . The former transistor  41 , which is the third transistor of the present embodiment, connected in the collector thereof to the node A. The latter transistor  42 , which is the fourth transistor of the present embodiment, is connected in parallel to the third transistor  41  and the load resistor  22 . Specifically, the collector of the fourth transistor  41  is the node A and has the load resistor  22  common to the first transistor  21 . The emitter of the transistor  41  is grounded through the current source  63 . The base of the transistor  41  receives the gain control signal Vagc from the detector  50 . 
     On the other hand, the collector of the fourth transistor  42  is biased by the power supply Vcc ( 18 ) through two transistors,  43  and  44 , each connected in series to the others and having the diode connection. The emitter of the transistor  42  grounded through the current source  63  common to the emitter of the third transistor  41 . The base of the fourth transistor  42  receives the fixed bias Vbias. 
     The current source  63  is the second current source of the present embodiment and may generate a constant current Ibias 2  substantial equal to the constant current Ibias 1  generated by the first current source  61 . 
     The emitter follower  51  includes a transistor  51   a  and a current source  51   b  connected in series between the power supply Vcc ( 18 ) and the ground. The transistor  51   a  in the base thereof receives the output Vout at the node A, while, the transistor  51  outputs a signal in the emitter thereof to the detector  50 . 
     The detector  50  may detect an average of the output Vout to generate the gain control signal Vagc. The gain control signal Vagc thus generated in the detector  50 , as described above, is provided to the base of the second and third transistors,  31  and  41 . Specifically, the detector  50  includes an integrator  52  including a resistor  52   a  and a capacitor  52   b , which generates an average of the output Vout. The integrator  52  receives the output of the emitter follower  51 , while, the integrator  52  outputs the averaged signal to the comparator  53 . The comparator  53  may compare thus generated average of the output Vout with a reference, and transfers a difference between the average and the reference to the second and third transistors,  31  and  41 . 
     In a case where the output Vout increases the level thereof, the detect enhances the gain control signal Vagc, which increases the current flowing in the second and third transistors,  31  and  41 , and resultantly the current flowing in the first and fourth transistors,  21  and  42 , decreases. The integrator  52  of present embodiment has the resistor  52   a  of resistance  20   k Ω, and the capacitor  52   b  of capacitance 0.1 μF, where the cut-off frequency of the integrator  52  becomes about 80 Hz far smaller than the fundamental frequency of the optical signal. Thus, the detector  50  may detect the average, or the DC component of the output Vout. 
     The operation of the pre-amplifier  13  having the arrangements above described will be described in two extreme cases, where the second transistor  31  in the current shunt  30  turns on and turns off. 
     When the second transistor  31  turn on, namely, the average of the output Vout is insufficient to turn the second and third transistors,  31  and  41 , on; the operating of the pre-amplifier  13  becomes substantially equal to those of a conventional common base circuit. Specifically, the current flowing in the first transistor  21  and the load resistor  22  becomes a current subtracted by the photocurrent Iin from the current of the current source Ibias 1 , I c1 =Ibias 1 −Iin. Accordingly, the variation of the photocurrent Iin is directly reflected in the change of the current I c1  flowing in the load resistor  22  and the change of the output Vout. 
     When the average of the output Vout becomes greater than the reference, the gain control signal Vagc may turn the transistor  31  on. Moreover, the gain control signal Vagc may increase the current I C2  flowing in the second transistor  31  as the average of the output Vout increases. The first and second transistors,  21  and  31 , operate as the common base circuit; then, two currents, I C1  and I C2 , have the phase thereof common to each other. Accordingly, the magnitude of respective collector currents, I C1  and I C2 , are proportionally divided from the differential current, Ibias 1 −Iin, by respective base levels, Vbias and Vagc. That is, the variation of the input photocurrent Iin is not fully reflected in the collector current I C1 , but only a portion thereof the variation may be appeared in the collector current I C1  and the output Vout, which means that the current gain of the common base circuit  20  decreases compared with the case where the second transistor  31  turns off. Although the gain control signal Vagc is generated based on the average of the output Vout, in other words, the DC component of the output Vout; the current gain of the common base circuit  20  in high frequencies is also reduced because the second transistor  31  has the operating point similar to that of the first transistor  21 . 
     When a portion of the difference current Ibias 1 −Iin is shunt by the current shunt  30 , the voltage drop at the load resistor  22  decreases, which enhances the collector level, namely, the average of the output Vout of the first transistor  21 . The pre-amplifier  13  of the present embodiment may provide the current supplement  40  to suppress the increase of the average level of the output Vout. 
     Specifically, the base  41  of the third transistor  41  receives the gain control signal Vagc same with the second transistor  31 . Moreover, the current source  63  in the current supplement  40  generates the current Ibias 2  substantially equal to the current of the first current source  61 , then, the transistor  41  may flow the current substantially equal to the current flowing in the second transistor  31 . Because the collector of the third transistor  41  is commonly connected to the node A, then the current, flowing in the transistor  41  flows also in the load resistor  22 . Accordingly, the current supplement  40  may compensate the current flowing in the load resistor  22 , which is decreased by the current flowing in the second transistor  31 ; the variation of the average level of the output Vout depending on the change of the average of the input photocurrent Iin may be effectively suppressed. Moreover, the fourth transistor connected in parallel to the third transistor  41 , and the fourth transistor  42  receives the fixed bias Vbias same as the first transistor  21 , the operating conditions of the third transistor  41  may be equal to the conditions of the second transistor  31 . 
       FIG. 4  shows variation of the collector currents, I C1  and I C2 , of respective transistors,  21  and  31 . The solid line corresponds to the collector current I C1  of the first transistor  21 , while, the chain line corresponds to the collector current I C2  of the second transistor  31 . When the photocurrent Iin exceeds a value of 0.4 mA, the second transistor  31  in the current shunt  30  begins to flow the collector current I C2 , and the decrease of the collector current I C1  in the common base circuit becomes precipitous. 
     When the photocurrent varies between 0 and 1 mA, the collector current I c1  of the first transistor  21  varies between 4.4 and 1.6 mA, while, the other collector current I c2  of the second transistor  31  varies between substantially 0 and 1.8 mA. 
       FIG. 5  shows the variation of the collector current I C1  of the first transistor  21 , that I C3  of the third transistor  41 , and the current I L  flowing in the road resistor  22 . The third collector current I C3  flowing in the third transistor  41  in the current supplement  40  behaves as those of the collector current I C2  of the second transistor  31  in the current shunt  30 ; but the current I L  flowing in the load resistor  22  shows substantially no change because the third transistor  41  in the current supplement  40  may compensate the decrease of the current shunt by the current shunt  30 . 
       FIG. 6  shows eye diagrams of the output Vout of the pre-amplifier  13  according to the embodiment of the preset invention as varying the input photocurrent Iin from 100 μA to 2000 μA. As shown in those eye diagrams in  FIG. 6 , even when the magnitude of the photocurrent Iin increases, the cross point CP of the eye diagram does not shift toward a higher level side and stays substantially in constant at around 50%. Thus, the preamplifier  13  with the common base input  20 , the current shunt  30 , and the current supplement  40  may effectively suppress the distortion appeared in the output Vout thereof. 
       FIG. 7  shows the amplitude of the output Vout in a peak-to-peak unit as varying the input photocurrent Iin. The output amplitude shows a linear dependence on the input photocurrent Iin until about 0.4 mA; but saturates in a region exceeding 0.4 mA. Thus, the pre-amplifier  13  of the present embodiment may effectively suppress the output saturation thereof without shift of the cross point, which may recover the waveform of the input optical signal. 
     In the foregoing detailed description, the amplifier, the current-to-voltage converter and the optical receiver of the present invention have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. 
     For instance, the embodiments described above concentrate on the active device made of a bipolar transistor; however, the embodiments may replace the bipolar transistor with a field effect transistor (FET). The detector may be not restricted to implement the integrator for detecting the average of the output Vout; other circuits able to decide the average of the output Vout may be applicable. Moreover, the embodiment described above outputs the voltage signal Vout, that is, the amplifier with the common base circuit operates as the current-to-voltage converter; however, the amplifier may have the configuration of the current amplifier to output the collector current of the first transistor. Thus, the present specification and figures are accordingly to be regarded as illustrative rather than restrictive.