Patent Publication Number: US-7907852-B2

Title: Optical transmitter circuit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical transmitter circuit, for use in the field of optical communications, including a circuit capable of driving a light emitting element at a high speed. 
     2. Description of the Background Art 
     A commonly known type of a driving circuit for driving, at a high speed, a light emitting element (e.g., an LED) whose response speed is relatively slow employs a peaking technique. With the peaking technique, an instantaneous current (hereinafter referred to as a “peaking current”) is given to a light emitting element so as to force the light emitting element to respond at a high speed.  FIG. 20  shows an exemplary configuration of a common conventional light emitting element driving circuit using a peaking technique.  FIG. 21  shows waveform diagrams illustrating an operation of the conventional light emitting element driving circuit shown in  FIG. 20 . 
     The conventional light emitting element driving circuit shown in  FIG. 20  includes a light emitting element  101 , a peaking current generating section  102 , and a light emitting element driving section  103 . A digital signal S (the waveform (a) of  FIG. 21 ) is inputted to the light emitting element driving section  103 . The peaking current generating section  102  generates a spire-shaped peaking current P (the waveform (b) of  FIG. 21 ) at the rising and falling edges of the digital signal S. The light emitting element driving section  103  receives the digital signal S and the peaking current P, and outputs a driving current D (the waveform (c) of  FIG. 21 ) whose waveform is obtained by combining together an amplitude current according to the amplitude of the digital signal S and the peaking current P. The light emitting element  101  receives the driving current D, and outputs an optical signal (the waveform (d) of  FIG. 21 ) whose waveform substantially matches that of the digital signal S. This is how it is possible to realize a high speed response of the light emitting element  101 . 
     However, the response speed that can be realized with the conventional light emitting element driving circuit described above is on the order of Mbps at best. Realizing a response speed on the order of 100 Mbps or more requires the use of a very large peaking current P, which causes clipping at the falling edge in the light emitting element driving section  103 . Therefore, the light emitting element  101  cannot be operated at a high speed. A possible solution to this problem is to increase the DC current through the light emitting element driving section  103  so as to prevent the clipping at the falling edge. However, the solution has problems such as an increase in the power consumption, and deterioration in the extinction ratio, which is the ratio between the high level and the low level of the digital signal. In worst cases, the guaranteed range may be exceeded, and the light emitting element  101  may break down. 
     A technique for solving such a problem is proposed in a patent document (Japanese Laid-Open Patent Publication No. 2002-64433, FIG. 1), etc.  FIG. 22  shows an exemplary configuration of the conventional light emitting element driving circuit disclosed in this patent document.  FIG. 23  shows waveform diagrams illustrating an operation of the conventional light emitting element driving circuit shown in  FIG. 22 . 
     As compared with the conventional light emitting element driving circuit shown in  FIG. 20 , the conventional light emitting element driving circuit shown in  FIG. 22  further includes a discharge circuit  104  for pulling a portion of the driving current D flowing into the light emitting element  101 . The peaking current generating section  102  generates a large peaking current P (the waveform (b) of  FIG. 23 ). The light emitting element driving section  103  receives the digital signal S (the waveform (a) of  FIG. 23 ) and the peaking current, and outputs the driving current D (the waveform (c) of  FIG. 23 ) whose waveform is obtained by combining together an amplitude current of the digital signal S and the peaking current P, i.e., whose waveform is such that the amount of clipping at the falling edge is reduced as much as possible. The light emitting element  101  receives a current D′ (the waveform (d) of  FIG. 23 ), which is the remainder after pulling a current from the driving current D by the discharge circuit  104 , and outputs an optical signal (the waveform (e) of  FIG. 23 ). With this configuration, it is possible to reduce the DC current flowing through the light emitting element  101  to improve the extinction ratio, and the light emitting element  101  can be operated with an amount of current within the guaranteed range. 
     However, with the conventional light emitting element driving circuit shown in  FIG. 22 , it is necessary to increase the driving current D of the light emitting element  101  by the amount of current to be pulled by the discharge circuit  104 , thereby increasing the power consumption of the circuit. 
     It is necessary to increase the driving current D so as to prevent clipping, and it is necessary to provide a very large current, which makes the circuit scale impractically large. 
     Moreover, instances of clipping include those occurring at a transistor of the light emitting element driving section  103  and those occurring at the light emitting element  101 . With the conventional light emitting element driving circuit, it is possible to improve those occurring at the light emitting element  101  but not those occurring at a transistor. Therefore, the falling edge in the waveform of the optical output of the light emitting element  101  becomes deteriorated, as shown in the waveform (e) of  FIG. 23 . 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide an optical transmitter circuit which is capable of driving a light emitting element at a high speed with a desirable extinction ratio and a low power consumption, and without waveform deterioration of the optical output. 
     The present invention is directed to an optical transmitter circuit for driving a light emitting element according to a received digital signal. In order to achieve the object set forth above, the optical transmitter circuit of the present invention includes first and second peaking current generating sections, and first and second light emitting element driving sections. The first peaking current generating section generates a first peaking current in synchronism with a rising edge of the digital signal. The second peaking current generating section generates a second peaking current in synchronism with a falling edge of the digital signal. The first light emitting element driving section produces a first driving current obtained by combining together a signal amplitude current according to an amplitude of the digital signal and the first peaking current. The second light emitting element driving section produces a second driving current obtained by combining together the signal amplitude current according to the amplitude of the digital signal and second peaking current. The first and second light emitting element driving sections drive the light emitting element by using a current obtained by subtracting the first driving current from the second driving current. 
     Preferably, the first light emitting element driving section adjusts the signal amplitude current so that the driving current does not have a peaking current in synchronism with the falling edge of the digital signal, and the second light emitting element driving section adjusts the signal amplitude current so that the driving current does not have a peaking current in synchronism with the rising edge of the digital signal. 
     The optical transmitter circuit may further include a DC current supply section for supplying a DC current to the light emitting element, or a third light emitting element driving section for supplying an amplitude current according to the amplitude of the digital signal directly to the light emitting element. In such a configuration, the first light emitting element driving section may output only the peaking current in synchronism with the rising edge to the light emitting element, and the second light emitting element driving section may output only the peaking current in synchronism with the falling edge to the light emitting element. In such a case, only the rising edge of the digital signal may be compensated for while omitting the second peaking current generating section and the second light emitting element driving section. 
     Typically, each of the first and second peaking current generating sections includes a first resistor and a second resistor connected in series with each other, and a capacitor connected in parallel to the first resistor. In the configuration, the first light emitting element driving section includes an NPN-type transistor, and the second light emitting element driving section includes a PNP-type transistor. The light emitting element may be provided within the optical transmitter circuit, and the light emitting element is preferably an LED. 
     In order to achieve the object set forth above, another optical transmitter circuit of the present invention includes a peaking current generating section, a light emitting element driving section, a signal analysis section, and a clipping section. The peaking current generating section generates a peaking current in synchronism with a rising edge and a falling edge of the digital signal. The light emitting element driving section produces a driving current obtained by combining together a signal amplitude current according to an amplitude of the digital signal and the peaking current, so as to drive the light emitting element by using the driving current. The signal analysis section analyzes the digital signal so as to set a control signal based on at least one of a pulse width and the amplitude of the digital signal. The clipping section clips the peaking current of the driving current according to the control signal set by the signal analysis section. 
     Preferably, the clipping section sets a ratio of a clipping current amount with respect to the peaking current amount to be less than or equal to a predetermined value. Preferably, the clipping section controls a bias current of the driving current produced by the light emitting element driving section. 
     Typically, the signal analysis section includes a pulse width detection section for detecting a pulse width of the digital signal, and a pulse width control section for setting a control signal according to the detected pulse width. Alternatively, the signal analysis section includes an amplitude detection section for detecting the amplitude of the digital signal, and an amplitude control section for setting a control signal according to the detected amplitude. 
     Alternatively, the signal analysis section may include the pulse width detection section, the pulse width control section, the amplitude detection section, the amplitude control section, and a process section for setting, as a control signal, a signal obtained by adding together the signal outputted from the pulse width control section and the signal outputted from the amplitude control section. With such a configuration, the signal analysis section may further include: a light receiving element for receiving an optical signal transmitted from a communication unit with which the optical transmitter circuit is communicating; an amplifier section for amplifying the signal received by the light receiving element; a signal detection section for detecting the amplitude of the signal amplified by the amplifier section; and an amplitude control section for controlling the amplitude of the digital signal inputted to the pulse width detection section based on a detection result of the signal detection section. Instead of using the amplitude control section, the peaking current generating section may be used to control an amount of peaking current to be generated based on a detection result of the signal detection section. 
     Specifically, the peaking current generating section includes a first resistor and a second resistor connected in series with each other, and a capacitor connected in parallel to the first resistor. Particularly, where the amount of peaking current to be generated is controlled based on the detection result of the signal detection section, it is preferred that the peaking current generating section includes a plurality of blocks, each block including a first resistor and a second resistor connected in series with each other, and a capacitor connected in parallel to the first resistor, and the blocks are switched from one to another based on a detection result of the signal detection section. 
     A predetermined value a 2 /a 1 , being a ratio of a clipping current amount a 2  with respect to a peaking current amount a 1 , is preferably determined as shown in Expression 6 set forth below in the description of preferred embodiments. Particularly, the clipping section preferably determines the predetermined value a 2 /a 1  so as to satisfy 0&lt;a 2 /a 1 ≦0.8 in a case where the light emitting element is driven at a transmission speed of about 500 Mbps. 
     The light emitting element may be provided within the optical transmitter circuit, and the light emitting element is preferably an LED. 
     According to the present invention, a rising-edge peaking current and a falling-edge peaking current are supplied separately to thereby prevent a deterioration of the optical signal waveform outputted from the light emitting element, whereby the light emitting element can be driven with an intended rising speed and an intended falling speed. Moreover, it is not necessary to increase the DC current for preventing the clipping of the falling-edge peaking current, whereby it is possible to reduce the power consumption and improve the extinction ratio. 
     Moreover, in the present invention, the ratio of the clipping current with respect to the instantaneous driving current is set to be less than or equal to a predetermined value, whereby it is possible to realize a high-speed response of a light emitting element and to reduce the power consumption. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram showing a general configuration of an optical transmitter circuit that is common between first to fourth embodiments of the present invention; 
         FIG. 2  shows waveform diagrams illustrating an operation of the optical transmitter circuit shown in  FIG. 1 ; 
         FIG. 3  shows, in detail, a configuration of an optical transmitter circuit according to the first embodiment of the present invention; 
         FIG. 4  shows, in detail, a configuration of an optical transmitter circuit according to the second embodiment of the present invention; 
         FIG. 5  shows, in detail, a configuration of an optical transmitter circuit according to the third embodiment of the present invention; 
         FIG. 6  shows, in detail, a configuration of an optical transmitter circuit according to the fourth embodiment of the present invention; 
         FIG. 7  shows, in detail, a configuration of an optical transmitter circuit according to an alternative embodiment of the present invention; 
         FIG. 8  is a waveform diagram illustrating an operation of an optical transmitter circuit according to fifth to ninth embodiments of the present invention; 
         FIG. 9  shows the relationship between a clipping current and a fall time of an optical transmitter circuit according to fifth to ninth embodiments of the present invention; 
         FIG. 10  shows, in detail, a configuration of an optical transmitter circuit according to the fifth embodiment of the present invention; 
         FIG. 11  is a detailed circuit diagram of a pulse width control section  11 ; 
         FIG. 12  shows, in detail, a configuration of an optical transmitter circuit according to the sixth embodiment of the present invention; 
         FIG. 13  is a detailed circuit diagram of an amplitude control section  13 ; 
         FIG. 14  shows, in detail, a configuration of an optical transmitter circuit according to the seventh embodiment of the present invention; 
         FIG. 15  shows, in detail, a configuration of an optical transmitter circuit according to the eighth embodiment of the present invention; 
         FIG. 16  is a detailed circuit diagram of an input signal control section  18 ; 
         FIG. 17  shows, in detail, a configuration of an optical transmitter circuit according to the ninth embodiment of the present invention; 
         FIG. 18  is a detailed circuit diagram of a peaking current generating section  19 ; 
         FIG. 19  is another detailed circuit diagram of a clipping section  8 ; 
         FIG. 20  is a functional block diagram showing a general configuration of a conventional light emitting element driving circuit; 
         FIG. 21  shows waveform diagrams illustrating an operation of the light emitting element driving circuit shown in  FIG. 20 ; 
         FIG. 22  is a functional block diagram showing a general configuration of another conventional light emitting element driving circuit; and 
         FIG. 23  shows waveform diagrams illustrating an operation of the light emitting element driving circuit shown in  FIG. 22 . 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  is a functional block diagram showing a general configuration of an optical transmitter circuit that is common between first to fourth embodiments of the present invention. Referring to  FIG. 1 , the optical transmitter circuit includes a first peaking current generating section  1 , a first light emitting element driving section  2 , a second peaking current generating section  3 , a second light emitting element driving section  4 , and a light emitting element  5 . The light emitting element  5  may be, for example, a light emitting diode (LED), a laser diode (LD), a superluminescent diode (SLD), or a vertical cavity surface-emitting laser (VCSEL). In the following embodiments, it is assumed that the light emitting element  5  is included in the optical transmitter circuit. However, the light emitting element  5  may be separated from the rest of the circuit in other embodiments. 
     The operation of the optical transmitter circuit having such a configuration will now be described in detail with reference to the waveform diagrams of  FIG. 2 . 
     The first light emitting element driving section  2  and the second light emitting element driving section  4  each receive a digital signal S (the waveform (a) of  FIG. 2 ). The first peaking current generating section  1  generates a spire-shaped peaking current P 1  (the waveform (b) of  FIG. 2 ) that is in synchronism with the transitions of the digital signal S, being positive at the rising edge and negative at the falling edge. The second peaking current generating section  3  generates a spire-shaped peaking current P 2  (the waveform (c) of  FIG. 2 ) that is in synchronism with the transitions of the digital signal S, being negative at the rising edge and positive at the falling edge. 
     The first light emitting element driving section  2  receives the digital signal S and the peaking current P 1 , and produces a driving current D 1  (the waveform (d) of  FIG. 2 ) obtained by adding the peaking current P 1  to the amplitude current of the digital signal S. In this process, the first light emitting element driving section  2  adjusts the DC current so that the produced driving current D 1  has no peaking current around the falling edge. 
     The second light emitting element driving section  4  receives the digital signal S and the peaking current P 2 , and produces a driving current D 2  (the waveform (e) of  FIG. 2 ) obtained by adding the peaking current P 2  to the amplitude current of the digital signal S. In this process, the second light emitting element driving section  4  adjusts the DC current so that the produced driving current D 2  has no peaking current around the rising edge. 
     Outputted to the light emitting element  5  is a driving current D 3  (the waveform (f) of  FIG. 2 ) obtained by subtracting by a subtractor the driving current D 1  produced in the first light emitting element driving section  2  from the driving current D 2  produced in the second light emitting element driving section  4 . The driving current D 3  has a peaking current for the falling edge and another peaking current for the rising edge, and the amplitude current there is above the zero level. By inputting the driving current D 3  to the light emitting element  5 , there is obtained an optical signal (the waveform (g) of  FIG. 2 ) with no deterioration in the amplitude waveform. Thus, the rising-edge peaking current and the falling-edge peaking current are produced by separate circuits, whereby it is possible to easily avoid the influence of clipping and thus to realize a high-speed response of the light emitting element  5 . 
     Detailed configurations of the optical transmitter circuits of the present invention will now be described. 
     FIRST EMBODIMENT 
       FIG. 3  shows, in detail, a configuration of an optical transmitter circuit according to the first embodiment of the present invention. In the optical transmitter circuit of the first embodiment, the functional blocks shown in  FIG. 1  are each configured as follows. 
     The first light emitting element driving section  2  includes a transistor Q 2 , resistors R 3  and R 4 , and a capacitor C 2 . The transistor Q 2  may be an NPN-type bipolar transistor, an N-channel field effect transistor, or the like. The base of the transistor Q 2  is connected to a power supply VCC via the resistor R 3  and grounded via the resistor R 4 , and receives the digital signal S via the capacitor C 2 . The collector of the transistor Q 2  is connected to the light emitting element  5 . The resistor R 3 , the resistor R 4  and the capacitor C 2  are used for adjusting the DC current through the first light emitting element driving section  2 . 
     The second light emitting element driving section  4  includes a transistor Q 1 , resistors R 1  and R 2 , and a capacitor C 1 . The transistor Q 1  may be a PNP-type bipolar transistor, a P-channel field effect transistor, or the like. The base of the transistor Q 1  is connected to the power supply VCC via the resistor R 1  and grounded via the resistor R 2 , and receives the digital signal S via the capacitor C 1 . The collector of the transistor Q 1  is connected to the light emitting element  5 . The resistor R 1 , the resistor R 2  and the capacitor C 1  are used for adjusting the DC current through the second light emitting element driving section  4 . 
     The first peaking current generating section  1  includes resistors R 7  and R 8  and a capacitor C 4 . The resistor R 7  and the resistor R 8  are connected in series with each other, and are inserted between the emitter of the transistor Q 2  of the first light emitting element driving section  2  and the ground. The capacitor C 4  is connected in parallel to the resistor R 7 . 
     The second peaking current generating section  3  includes resistors R 5  and R 6  and a capacitor C 3 . The resistor R 5  and the resistor R 6  are connected in series with each other, and are inserted between the power supply VCC and the emitter of the transistor Q 1  of the second light emitting element driving section  4 . The capacitor C 3  is connected in parallel to the resistor R 5 . 
     When the digital signal S transitions from high to low, the base voltage of the transistor Q 2  decreases and the emitter voltage of the transistor Q 2  accordingly becomes equal to the ground level, whereby the first light emitting element driving section  2  is turned OFF, and the second light emitting element driving section  4  is turned ON. Thus, the driving current D 2 , which is obtained by combining together the falling-edge peaking current outputted from the second light emitting element driving section  4  and the amplitude current adjusted by the resistors R 1  and R 2  of the second light emitting element driving section  4 , is supplied from the emitter of the transistor Q 1  to the light emitting element  5 . 
     When the digital signal S transitions from low to high, the base voltage of the transistor Q 2  increases, whereby the first light emitting element driving section  2  is turned ON, and the second light emitting element driving section  4  is brought to a state where it conducts therethrough only the bias voltage of the light emitting element  5  (=the driving current D 2 ). Thus, the driving current D 1 , which is obtained by combining together the rising-edge peaking current outputted from the first light emitting element driving section  2  and the amplitude current adjusted by the resistors R 3  and R 4  of the first light emitting element driving section  2 , flows from the collector of the transistor Q 2  toward the emitter thereof. Since the driving current D 1  in the opposite direction flows through the light emitting element  5 , the falling-edge peaking current and the amplitude current are supplied to the light emitting element  5 . 
     As described above, with the optical transmitter circuit according to the first embodiment of the present invention, a rising-edge peaking current and a falling-edge peaking current are supplied separately to thereby prevent the deterioration of the optical signal waveform outputted from the light emitting element  5 . Thus, the light emitting element  5  can be driven with an intended rising speed and an intended falling speed. 
     SECOND EMBODIMENT 
       FIG. 4  shows, in detail, a configuration of an optical transmitter circuit according to the second embodiment of the present invention. The optical transmitter circuit of the second embodiment differs from the optical transmitter circuit of the first embodiment in the configuration of the second light emitting element driving section  4 . 
     The second light emitting element driving section  4  includes the transistor Q 1 , the resistors R 1  and R 2 , and capacitors C 1  and C 5 . The resistor R 1 , the resistor R 2  and the capacitor C 1  are used for adjusting the DC current through the second light emitting element driving section  4 . The capacitor C 5  is inserted between the collector of the transistor Q 1  and the light emitting element  5 . With such a configuration, the driving current D 2  outputted from the second light emitting element driving section  4  when the digital signal S is inputted is an AC component current obtained by cutting the DC component off the current shown in the waveform (e) of  FIG. 2 , i.e., only the falling-edge peaking current. The driving current D 2  made only of an AC component and the driving current D 1  outputted from the first light emitting element driving section  2  are supplied to the light emitting element  5 . 
     As described above, with the optical transmitter circuit according to the second embodiment of the present invention, a DC component is cut off from the output of the second light emitting element driving section  4 , whereby it is possible to obtain a greater amount of peaking current than that required for the amplitude current. Therefore, it is possible to drive the light emitting element  5  at a higher speed. 
     THIRD EMBODIMENT 
       FIG. 5  shows, in detail, a configuration of an optical transmitter circuit according to the third embodiment of the present invention. The optical transmitter circuit of the third embodiment differs from the optical transmitter circuit of the first embodiment in the configuration of the first light emitting element driving section  2 . 
     The first light emitting element driving section  2  includes the transistor Q 2 , the resistors R 3 , R 4  and R 9 , and capacitors C 2  and C 6 . The collector of the transistor Q 2  is connected to the power supply VCC via a resistor R 9 , and is connected to the light emitting element  5  via the capacitor C 6 . With such a configuration, the driving current D 1  outputted from the first light emitting element driving section  2  when the digital signal S is inputted is an AC component current obtained by cutting the DC component off the current shown in the waveform (d) of  FIG. 2 , i.e., only the rising-edge peaking current. The driving current D 1  made only of an AC component and the driving current D 2  outputted from the second light emitting element driving section  4  are supplied to the light emitting element  5 . 
     As described above, with the optical transmitter circuit according to the third embodiment of the present invention, a DC component is cut off from the output of the first light emitting element driving section  2 , whereby it is possible to obtain a greater amount of peaking current than that required for the amplitude current. Therefore, it is possible to drive the light emitting element  5  at a higher speed. 
     Fourth Embodiment 
       FIG. 6  shows, in detail, a configuration of an optical transmitter circuit according to the fourth embodiment of the present invention. The optical transmitter circuit of the fourth embodiment differs from the optical transmitter circuit of the first embodiment in the configuration of the first light emitting element driving section  2  and the second light emitting element driving section  4 . 
     As in the third embodiment, the first light emitting element driving section  2  includes the transistor Q 2 , the resistors R 3 , R 4  and R 9 , and the capacitors C 2  and C 6 . 
     The second light emitting element driving section  4  includes a circuit block A and a circuit block B. The circuit block A includes the transistor Q 1 , the resistors R 1  and R 2 , and the capacitors C 1  and C 5 , as in the second embodiment. The circuit block B includes a transistor Q 3 , resistors R 10  and R 11 , and a capacitor C 7 . The transistor Q 3  may be a PNP-type bipolar transistor, a P-channel field effect transistor, or the like. 
     Only the rising-edge peaking current is outputted, as described above in the third embodiment, from the first light emitting element driving section  2 . Only the falling-edge peaking current is outputted, as described above in the second embodiment, from the circuit block A of the second light emitting element driving section  4 , and an amplitude current obtained by adjusting the DC current of the digital signal S is outputted from the circuit block B (=the third light emitting element driving section). 
     As described above, with the optical transmitter circuit according to the fourth embodiment of the present invention, the peaking currents for compensating for the rising speed and the falling speed and the amplitude current are supplied to the light emitting element  5 , whereby it is possible to more easily drive the light emitting element  5  with an intended rising speed and an intended falling speed. 
     A DC current supply section  6  may be further included, as shown in  FIG. 7 , in the circuit described above in the first to fourth embodiments, for adjusting the amount of DC current to be supplied to the light emitting element  5 . While a current mirror circuit is used in  FIG. 7  as an example of the DC current supply section  6 , it may be any other suitable configuration capable of supplying a DC current. 
     FIFTH EMBODIMENT 
     As described above, it is ideal to use the driving current D 3  shown in the waveform (f) of  FIG. 2  for driving the light emitting element  5 . However, a research by the present inventors has revealed that the optical transmitter circuit can be used in practice even if the falling-edge peaking current is reduced to some extent. 
     Fifth to ninth embodiments of the present invention are directed to optical transmitter circuits in which the falling-edge peaking current is reduced to thereby reduce the power consumption. 
     First, the degree to which the falling-edge peaking current can be reduced will be discussed below. Mathematical expressions used in the following discussion are based on the configuration of the peaking current generating section  3 , and it is understood that the expressions vary for different circuit configurations. 
     In the driving current waveform of  FIG. 8 , the absolute value IPH of the rising-edge peak current and the absolute value IPL of the falling-edge peak current can be expressed as shown in Expressions 1 and 2, respectively, where IH is the high-level current of the digital signal S, IL is the low-level current thereof, R 5  and R 6  are resistance values of the resistors R 5  and R 6  of the second peaking current generating section  3 . B 1  is a constant.
 
 IPH=B 1×( R 5 /R 6)×( IH - IL )+ IH   Exp. 1
 
 IPL=−B 1×( R 5 /R 6)×( IH - IL )+ IL   Exp. 2
 
     Hence, the peaking current amount al shown in  FIG. 8  can be expressed as shown in Expression 3 below.
 
 a 1 =IPH - IH=IL - IPL =B 1×( R 5 /R 6)×( IH - IL )  Exp. 3
 
     The operation when clipping occurs as shown in  FIG. 8  will now be discussed. As the bias current Ib supplied to the base of the transistor Q 1  is decreased, the falling-edge peak current IPL decreases below the zero level. In practice, however, clipping occurs in the light emitting element  5 , and the peak current (=IPL′) is at the zero level. Thus, with regard to the falling-edge peak current IPL where clipping occurs, it is considered that a current equal to the falling-edge peak current with no clipping is flowing below the zero level. Therefore, the clipping current a 2  can be expressed by using the absolute value IPL of the falling-edge peak current as shown in Expression 4 below.
 
 a 2=− IPL   Exp. 4
 
     For example, where the light emitting element  5  is driven with a transmission speed of 500 Mbps and a pulse current amplitude (=IH-IL) of 14.4 mApp, the bias current Ib required for the condition (a 2 /a 1 =0) under which clipping does not occur is experimentally 139.5 mA, and B 1 ×(R 5 /R 6 )=9.15. The rising-edge peak current IPH and the falling-edge peak current IPL are 278.4 mA and 0 mA based on Expressions 1 and 2, respectively. When the bias current Ib is gradually decreased from the condition under which clipping does not occur, clipping occurs, and the output waveform of the light emitting element  5  becomes deteriorated. Then, the value of a 2 /a 1  at which the fall time tf is 1 ns (equivalent to a transmission speed of 500 Mbps) is determined to be about 0.8. Under the condition where the a 2 /a 1 =0.8, the bias current Ib is 36.6 mA, whereby the power consumption can be reduced by about 75% compared with a case where clipping does not occur. Other values are as follows: IPH=175.5 mA, IPL′=0 mA (IPL=−102.9 mA), a 1 =131.7 mA, and a 2 =102.9 mA. 
       FIG. 9  shows experimental results for the fall time tf when a 2 /a 1  (i.e., the ratio between the peaking current amount al and the clipping current amount a 2 ) was varied with the pulse current amplitude (=IH-IL) and the bias current Ib being parameters. The greater the fall time tf is, the lower the response speed is. Where the transmission speed is 500 Mbps with the fall time tf being 1 ns, a high response speed of 500 Mbps can be realized by setting a 2 /a 1  to satisfy Expression 5 below. 
     
       
         
           
             
               
                 
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       FIG. 9  can be represented by Expression 6 below. With regard to the time tf, the fall time of the pulse determined by the peaking current (the first term) is dominant when a 2 /a 1  is small, and the fall time determined by clipping current outputted from the light emitting element driving section  4  (the second term) is dominant when a 2 /a 1  is large. In Expression 6, A 1 , A 2 , N 1  and N 2  are constants. The time constant τ 1  in the first term is determined by the transient response of the peak current set by the resistors R 5  and R 6  and the capacitor C 3  of the second peaking current generating section  3 , and the second time constant τ 2  is determined by the transient response of the transistor Q 1  and the light emitting element  5  of the second light emitting element driving section  4 . Thus, a 2 /a 1  can be set according to the transmission speed. 
     
       
         
           
             
               
                 
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     The optical transmitter circuit according to the fifth embodiment of the present invention will now be described. 
       FIG. 10  shows a configuration of the optical transmitter circuit according to the fifth embodiment of the present invention. Referring to  FIG. 10 , the optical transmitter circuit of the fifth embodiment includes the peaking current generating section  3 , the light emitting element  5 , a light emitting element driving section  7 , a clipping section  8 , and a signal analysis section  9 . The signal analysis section  9  includes a pulse width detection section  10  and a pulse width control section  11 . The configurations of the peaking current generating section  3  and the light emitting element  5  are as described above in the first to fourth embodiments. The optical transmitter circuit of the fifth embodiment will now be described while focusing on the light emitting element driving section  7 , the clipping section  8 , the pulse width detection section  10  and the pulse width control section  11  whose configurations are different from the above embodiments. 
     The light emitting element driving section  7  includes the transistor Q 1 , a resistor R 16 , and the capacitors C 1  and C 8 . The transistor Q 1  may be a PNP-type bipolar transistor, P-channel field effect transistor, or the like. The base of the transistor Q 1  is grounded via the resistor R 16  and a capacitor C 8  connected in series with each other, and receives the digital signal S via the capacitor C 1 . The collector of the transistor Q 1  is connected to the light emitting element  5 . The DC voltage outputted from the clipping section  8  is applied to the connecting point between the resistor R 16  and the capacitor C 8 . 
     The pulse width detection section  10  outputs the received digital signal S to the light emitting element driving section  7 , detects the pulse width of the digital signal S, and outputs the detection result to the pulse width control section  11  as the detected pulse width. The pulse width control section  11  includes a comparing section  11   a , for example, as shown in  FIG. 11 , for comparing a predetermined reference pulse width with the detected pulse width to output a control signal based on the comparison result to the clipping section  8 . While an example of the pulse width detection section  10  may be a section for detecting the falling or rising edge of the pulse, any other suitable configuration may be employed. While the comparing section  11   a  is used as an example of the pulse width detection section  10 , there may be provided a memory section storing various control signals so that one of the control signals is read out from the memory section according to the detected pulse width. 
     The clipping section  8  includes resistors R 17  to R 19 , a variable resistor R 20 , and a transistor Q 6 . The transistor Q 6  may be a PNP-type bipolar transistor, a P-channel field effect transistor, or the like. The variable resistor R 20 , the resistor R 19 , and the resistor R 18  are connected in series with one another, and are inserted between the power supply VCC and GND. The connecting point between the resistor R 18  and the resistor R 19  is connected to the base of the transistor Q 6 . The emitter of the transistor Q 6  is connected to the power supply VCC via the resistor R 17 , and the DC voltage appearing at the emitter is outputted to the connecting point between the resistor R 16  and the capacitor C 8  of the light emitting element driving section  7 . The collector of the transistor Q 6  is grounded. The resistance value of the variable resistor R 20  varies according to the control signal outputted from the pulse width control section  11 . The variation of the resistance value is controlled so as to adjust the amount of clipping so that the ratio of the clipping current amount with respect to the peaking current amount is less than or equal to a predetermined value. 
     For example, where the detected pulse width is longer than the reference pulse width (i.e., a lower transmission speed), the bias current Ib of the clipping section  8  is decreased. Where the detected pulse width is shorter than the reference pulse width (i.e., a higher transmission speed), the bias current Ib of the clipping section  8  is increased. Thus, it is possible to supply, to the light emitting element  5 , a driving current with the clipping current amount a 2  being adjusted to a value according to the transmission speed of the digital signal S. 
     As described above, with the optical transmitter circuit according to the fifth embodiment of the present invention, it is possible to automatically adjust the amount of clipping so that the ratio of the clipping current amount with respect to the peaking current amount is less than or equal to a predetermined value, according to the transmission speed of the digital signal S, thus arriving at the minimum amount of clipping for the transmission speed. Thus, it is possible to realize a high response speed of the light emitting element  5  while reducing the power consumption. 
     SIXTH EMBODIMENT 
       FIG. 12  shows a configuration of the optical transmitter circuit according to the sixth embodiment of the present invention. Referring to  FIG. 12 , the optical transmitter circuit of the sixth embodiment includes the peaking current generating section  3 , the light emitting element  5 , the light emitting element driving section  7 , the clipping section  8 , and the signal analysis section  9 . The signal analysis section  9  includes an amplitude detection section  12  and an amplitude control section  13 . The sixth embodiment differs from the fifth embodiment in the configurations of the amplitude detection section  12  and the amplitude control section  13 . The optical transmitter circuit of the sixth embodiment will now be described while focusing on these configurations different from those of the first embodiment. 
     The amplitude detection section  12  outputs the received digital signal S to the light emitting element driving section  7 , detects the amplitude of the digital signal S, and outputs the detection result to the amplitude control section  13  as the detected amplitude. The amplitude control section  13  includes a comparing section  13   a , for example, as shown in  FIG. 13 , for comparing a predetermined reference amplitude with the detected amplitude to output a control signal based on the comparison result to the clipping section  8 . Instead of the comparing section  13   a , there may be provided a memory section storing various control signals so that one of the control signals is read out from the memory section according to the detected amplitude. The clipping section  8  varies the resistance value of the variable resistor R 20  shown in  FIG. 10  according to the control signal outputted from the amplitude control section  13  so as to adjust the amount of clipping so that the ratio of the clipping current amount with respect to the peaking current amount is less than or equal to a predetermined value. 
     For example, where the detected amplitude is larger than the reference amplitude, the peak current occurring in the peaking current generating section  3  becomes large, whereby the bias current Ib of the clipping section  8  is increased. Where the detected amplitude is smaller than the reference amplitude, the bias current Ib of the clipping section  8  is decreased. Thus, it is possible to supply, to the light emitting element  5 , a driving current with the clipping current amount a 2  being adjusted to a value according to the amplitude of the digital signal S. 
     As described above, with optical transmitter circuit according to the sixth embodiment of the present invention, it is possible to automatically adjust the amount of clipping so that the ratio of the clipping current amount with respect to the peaking current amount is less than or equal to a predetermined value, according to the amplitude of the digital signal S, thus arriving at the minimum amount of clipping required for the amplitude. Thus, it is possible to realize a high response speed of the light emitting element  5  while reducing the power consumption. 
     SEVENTH EMBODIMENT 
       FIG. 14  shows a configuration of the optical transmitter circuit according to the seventh embodiment of the present invention. Referring to  FIG. 14 , the optical transmitter circuit of the seventh embodiment includes the peaking current generating section  3 , the light emitting element  5 , the light emitting element driving section  7 , the clipping section  8 , and the signal analysis section  9 . The signal analysis section  9  includes the pulse width detection section  10 , the pulse width control section  11 , the amplitude detection section  12 , the amplitude control section  13 , and a process section  14 . The configuration of the seventh embodiment is obtained by combining the fifth embodiment with the sixth embodiment, with the process section  14  being the difference from the above embodiments. The optical transmitter circuit of the seventh embodiment will now be described while focusing on these configurations different from those of the above embodiments. 
     The process section  14  adds together a control signal outputted from the pulse width control section  11  and a control signal outputted from the amplitude control section  13 , and outputs the addition result to the clipping section  8  as the final control signal. Thus, it is possible to output a control signal according both to the transmission speed and to the amplitude of the digital signal S. 
     As described above, with the optical transmitter circuit according to the seventh embodiment of the present invention, it is possible to automatically adjust the amount of clipping so that the ratio of the clipping current amount with respect to the peaking current amount is less than or equal to a predetermined value, according both to the transmission speed and to the amplitude of the digital signal S, thus arriving at the minimum amount of clipping required for the transmission speed and the amplitude. Thus, it is possible to realize a high response speed of the light emitting element  5  while reducing the power consumption. 
     The process of detecting the transmission speed of the digital signal S and the process of detecting the amplitude of the digital signal S may be switched around. While the process section  14  adds together the control signal from the pulse width control section  11  and the control signal from the amplitude control section  13 , and outputs the addition result as the final control signal in the example described above, there may be provided a memory section storing various final control signals so that one of the final control signals is read out from the memory section according to various control signals. 
     EIGHTH EMBODIMENT 
       FIG. 15  shows a configuration of the optical transmitter circuit according to the eighth embodiment of the present invention. Referring to  FIG. 15 , the optical transmitter circuit of the eighth embodiment includes the peaking current generating section  3 , the light emitting element  5 , the light emitting element driving section  7 , the clipping section  8 , and the signal analysis section  9 . The signal analysis section  9  includes the pulse width detection section  10 , the pulse width control section  11 , the amplitude detection section  12 , the amplitude control section  13 , the process section  14 , a light receiving element  15 , an amplifier section  16 , a signal detection section  17 , and an input signal control section  18 . The eighth embodiment differs from the seventh embodiment in the configurations of the light receiving element  15 , the amplifier section  16 , the signal detection section  17 , and the input signal control section  18 . The optical transmitter circuit of the eighth embodiment will now be described while focusing on these configurations different from those of the seventh embodiment. 
     The light receiving element  15  receives an optical signal from a communication unit (not shown) with which the optical transmitter circuit is communicating, and outputs an electrical signal according to the optical signal to the amplifier section  16 . An antenna may be provided instead of the light receiving element  15 , in which case the optical transmitter circuit receives a wireless signal from the communication unit. The amplifier section  16  amplifies the electrical signal from the light receiving element  15  with a predetermined gain. The signal detection section  17  detects the amplitude of the electrical signal amplified by the amplifier section  16 , and outputs the detection result to the input signal control section  18  as the detected signal. 
     The input signal control section  18  includes, for example, a comparing section  18   a  and a variable gain amplifier  18   b , as shown in  FIG. 16 . The comparing section  18   a  compares a predetermined reference signal with the detected signal to output a control signal based on the comparison result to the variable gain amplifier  18   b  as a gain control signal. The variable gain amplifier  18   b  controls the amplitude of the digital signal S according to the gain control signal. Any other suitable unit may be used instead of the variable gain amplifier  18   b , as long as the amplitude of the digital signal S can be controlled. The digital signal S whose amplitude is controlled is inputted to the pulse width detection section  10 . 
     For example, where the transmission distance is long and the optical signal received by the light receiving element  15  is small, the signal detection section  17  detects an amplitude smaller than the amplitude being the reference in the input signal control section  18 , whereby the input signal control section  18  performs a control operation such that the amplitude of the digital signal S is increased. Based on the result, the bias current Ib of the clipping section  8  is increased. Where the transmission distance is short and the optical signal received by the light receiving element  15  is large, a control operation opposite to the above operation is performed. 
     As described above, with the optical transmitter circuit according to the eighth embodiment of the present invention, it is possible to automatically adjust the amount of clipping so that the ratio of the clipping current amount with respect to the peaking current amount is less than or equal to a predetermined value, according both to the transmission speed and to the amplitude of the digital signal S based on the distance to the communication unit over which signals are transmitted, thus arriving at the minimum amount of clipping required for the transmission speed and the amplitude. Thus, it is possible to realize a high response speed of the light emitting element  5  while reducing the power consumption. 
     NINTH EMBODIMENT 
       FIG. 17  shows a configuration of the optical transmitter circuit according to the ninth embodiment of the present invention. Referring to  FIG. 17 , the optical transmitter circuit of the ninth embodiment includes the peaking current generating section  19 , the light emitting element  5 , the light emitting element driving section  7 , the clipping section  8 , and the signal analysis section  9 . The signal analysis section  9  includes the pulse width detection section  10 , the pulse width control section  11 , the amplitude detection section  12 , the amplitude control section  13 , the process section  14 , the light receiving element  15 , the amplifier section  16 , and the signal detection section  17 . The ninth embodiment differs from the eighth embodiment in the configuration of the peaking current generating section  19 . The optical transmitter circuit of the ninth embodiment will now be described while focusing on the configuration different from that of the eighth embodiment. 
     The signal detection section  17  detects the amplitude of the electrical signal amplified by the amplifier section  16 , and outputs the detection result to the peaking current generating section  19  as the detected signal. The peaking current generating section  19  includes a plurality of waveform peaking sections  19   a  of different values and a selector section  19   b , for example, as shown in  FIG. 18 . The selector section  19   b  selects one of the waveform peaking sections  19   a  of different values according to the detected signal. 
     As described above, with the optical transmitter circuit according to the ninth embodiment of the present invention, it is possible to automatically adjust the amount of clipping so that the ratio of the clipping current amount with respect to the peaking current amount is less than or equal to a predetermined value, according both to the transmission speed and to the amplitude of the digital signal S based on the distance to the communication unit over which signals are transmitted, thus arriving at the minimum amount of clipping required for the transmission speed and the amplitude. Thus, it is possible to realize a high response speed of the light emitting element  5  while reducing the power consumption. 
     It is understood that particular circuits of the first to ninth embodiments using resistors, capacitors and transistors are all illustrative, and each of them may be replaced by any other suitable circuit as long as the same function is provided. For example, in the clipping section  8 , the variable resistor R 20  may be replaced by a combination of a plurality of resistors of different resistance values and a selector switch, as shown in  FIG. 19 . If the input digital signal S is static, the resistance value of the variable resistor R 20  may be fixed without detecting the pulse width or the amplitude of the digital signal S. 
     In the first to fourth embodiments, the emitter of the transistor Q 2  of the first light emitting element driving section  2  is grounded via the first peaking current generating section  1 , and the emitter of the transistor Q 1  of the second light emitting element driving section  4  is connected to the power supply VCC via the second peaking current generating section  3 . Alternatively, the emitter of the transistor Q 2  may be connected to the power supply VCC, with the emitter of the transistor Q 1  being grounded. While the. power supply VCC and the ground level are used as the upper limit voltage and the lower limit voltage, respectively, in the embodiments above, the lower limit voltage may be a negative-voltage power supply. 
     In the first to fourth embodiments, a peaking current is generated by each of the first and second peaking current generating sections, and the rising speed and the falling speed are both compensated for. However, the present invention is advantageous over conventional configurations even when only the falling speed is compensated for by using the first peaking current generating section. 
     While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.