Abstract:
It is an object of the present invention to provide a driving circuit which can drive a light emitting element in spite of a reduced power supply voltage and which is thus suitable for size reduction. The present invention provides a driving circuit which supplies a bias current to a light emitting element and which carries out modulation on the basis of voltage driving, the driving circuit comprising a boosting circuit that increases a power supply voltage Vcc, a resistance element connected between an output of the boosting circuit and an anode terminal of the light emitting element, and a capacitive element connected to the anode terminal of the light emitting element to supply a modulation signal to the light emitting element for voltage driving.

Description:
[0001]    This application claims priority from Japanese Patent Application Nos. 2002-133243 and 2002-133244 both filed May 8, 2002, which are incorporated hereinto by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a driving circuit, and more specifically, to a driving circuit used to drive a light emitting element in an optical transmitter that requires a reduced power supply voltage.  
           [0004]    2. Description of the Related Art  
           [0005]    An optical transmission system using an optical fiber as a transmission line is used for various applications. An optical transmitter in the optical transmission system comprises a light emitting element, a driving circuit that drives the light emitting element, and an encoding circuit that converts an input data signal into an output signal suitable for transmission to input the driving circuit. In recent years, applications for short-distance transmission system have required a transmission speed of 1 Gbps or more and thus an optical transmitter suitable for an increased transmission speed. On the other hand, a reduced power supply voltage improves reduction of current consumption and heat generation.  
           [0006]    The driving circuit driving the light emitting element is based on one of two known methods: a current driving method and a voltage driving method. FIG. 1 shows a configuration of a conventional driving circuit based on the voltage driving method. The driving circuit is composed of a light emitting element  11 , a variable resistor R 11  that connects an anode terminal of the light emitting element  11  and a power supply (Vcc), and a coupling capacitor C 11 . A bias current through the light emitting element  11  is determined by the power supply voltage Vcc, a forward voltage at the light emitting element  11 , and resistance of the variable resistor R 11 . An input data signal provides a voltage swing to the light emitting element  11  via the coupling capacitor C 11 .  
           [0007]    The voltage driving method enables the light emitting element to be directly driven by the output from the encoding circuit. Thus, the voltage driving method is more suitable for reducing the size of the optical transmitter than the current driving method, which requires a voltage-current converting circuit.  
           [0008]    However, with the reduced power supply voltage Vcc, the conventional voltage-driving-based driving circuit requires the resistance of the variable resistor R 11  to be set at a small value in order to obtain a bias current. On the other hand, when the resistance of the variable resistor R 11  is reduced, load impedance estimated from the output of the encoding circuit equivalently decreases. This makes it difficult to directly drive the light emitting element  11  on the basis of an output from the encoding circuit or the like.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention is provided in view of these problems. It is an object of the present invention to provide a driving circuit which can drive a light emitting element in spite of a reduced power supply voltage and which is thus suitable for size reduction.  
           [0010]    To accomplish this object, the present invention provides a driving circuit which supplies a bias current to a light emitting element and which carries out modulation on the basis of voltage driving, the driving circuit comprising a boosting circuit that increases a power supply voltage, a resistance element connected between an output of the boosting circuit and an anode terminal of the light emitting element, and a capacitive element connected to the anode terminal of the light emitting element to supply a modulation signal to the light emitting element for voltage driving.  
           [0011]    With this configuration, the resistance of the resistance element can be increased by using the boosting circuit to increase the power supply voltage. Consequently, the light emitting element can be driven in spite of a reduced power supply voltage.  
           [0012]    Further, the resistance element may be a variable resistor, and the driving circuit may further comprise a light receiving element that outputs a current according to an optical output from the light emitting element and a control circuit that can vary the resistance of the resistance element according to the output current from the light emitting element.  
           [0013]    Furthermore, the variable resistor may be a digital potentiometer, the control circuit including an analog-digital (A/D) converter can control the digital potentiometer on the basis of a digital signal output from the A/D converter to vary the resistance.  
           [0014]    Moreover, the driving circuit may comprise a light receiving element that outputs a current according to an optical output from the light emitting element, and a control circuit that can vary an output voltage from the boosting circuit according to the output current from the light receiving element.  
           [0015]    This boosting circuit includes a pulse wide modulation (PWM) circuit that can change the output voltage. The control circuit including an A/D converter can control the PWM circuit on the basis of a digital signal output from the A/D converter to vary the output voltage.  
           [0016]    When a digital potentiometer is provided which feeds back the output voltage to the PWM circuit that can vary the output voltage, the control circuit including the A/D converter can control the digital potentiometer on the basis of a digital signal output from the A/D converter to vary the output voltage.  
           [0017]    Furthermore, the present invention provides a driving circuit which supplies a bias current to a light emitting element and which carries out modulation on the basis of voltage driving, the driving circuit comprising a boosting circuit that increases a power supply voltage, a first resistance element connected between an output of the boosting circuit and an anode terminal of the light emitting element, a first capacitive element connected to the anode terminal of the light emitting element to supply a modulation signal with a positive phase to-the light emitting element for voltage driving, a second capacitive element connected to a cathode terminal of the light emitting element to supply a modulation signal with a negative phase to the light emitting element for voltage driving, and a second resistance element connected between the cathode terminal of the light emitting element and a ground.  
           [0018]    With this configuration, the resistance of the first resistance element can be increased by using the boosting circuit to increase the power supply voltage. Consequently, the light emitting element can be driven in spite of a reduced power supply voltage.  
           [0019]    Further, with this configuration, the modulation signal with the positive phase and the modulation signal with the negative phase are inputted to provide a voltage swing to the light emitting element. As a result, a double voltage swing can be provided compared to the case that a signal with a single phase is inputted to the light emitting element. Therefore, the light emitting element can be directly driven using an output from an encoding circuit or the like. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a circuit diagram showing a configuration of a conventional driving circuit based on a voltage driving method;  
         [0021]    [0021]FIG. 2 is a diagram showing a configuration of a driving circuit according to a first embodiment of the present invention;  
         [0022]    [0022]FIG. 3 is a circuit diagram showing a DC-DC converter in the driving circuit according to the first embodiment;  
         [0023]    [0023]FIG. 4 is a diagram showing a configuration of a driving circuit according to a second embodiment of the present invention;  
         [0024]    [0024]FIG. 5 is a diagram showing a configuration of a driving circuit according to a third embodiment of the present invention;  
         [0025]    [0025]FIG. 6 is a diagram showing a configuration of a driving circuit according to a fourth embodiment of the present invention;  
         [0026]    [0026]FIG. 7 is a diagram showing a configuration of a driving circuit according to a fifth embodiment of the present invention; and  
         [0027]    [0027]FIG. 8 is a diagram showing a configuration of a driving circuit according to a sixth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0028]    Embodiments of the present invention will be described below in detail with reference to the drawings.  
         [0029]    (First Embodiment)  
         [0030]    [0030]FIG. 2 shows a configuration of a driving circuit according to a first embodiment of the present invention. The driving circuit is comprised of a light emitting element  21 , a resistor R 21  connected to an anode terminal of the light emitting element  21 , a DC-DC converter  22  connecting the resistor R 21  and a power supply (Vcc), and a coupling capacitor C 21 .  
         [0031]    The light emitting elements include, but are not limited to, light emitting diodes and laser diodes. The DC-DC converter  22  is a boosting circuit which increases the power supply voltage Vcc and which supplies the increased power supply voltage Vcc to the resistor R 21  and the light emitting element  21 . A bias current through the light emitting element  21  is determined by the increased voltage, a forward voltage at the light emitting element  21 , and resistance of the resistor R 21 . An inputted data signal is supplied to the light emitting element  21  via the coupling capacitor C 21  as a modulation signal to provide a voltage swing to the element  21 .  
         [0032]    [0032]FIG. 3 shows a circuit configuration of the DC-DC converter. In the DC-DC converter  22 , a Pulse Width Modulation (PWM) control circuit  31  performs such control as turns on and off a transistor  32  to the DC power supply voltage Vcc into an AC voltage of several dozen kHz. Then, a diode D 31  and a capacitor C 31  smooth the voltage to convert it back into a DC voltage and then output the resultant voltage. The PWM control circuit  31  use resistors R 31  and R 32  to feed back its output to perform such control as makes the output voltage constant. In this manner, an arbitrary output voltage can be generated by using the PWM control circuit  31  to control pulse width, frequency, or the like.  
         [0033]    According to the present embodiment, the DC-DC converter  22  can increase the power supply voltage Vcc and thus the resistance of the resistor R 21 . Consequently, the light emitting element can be driven in spite of a reduced power supply voltage.  
         [0034]    (Second Embodiment)  
         [0035]    [0035]FIG. 4 shows a configuration of a driving circuit according to a second embodiment of the present invention. The driving circuit is composed of the light emitting element  21 , a variable resistor R 22  connected to the anode terminal of the light emitting element  21 , a DC-DC converter  22  connecting the variable resistor R 22  and a power supply (Vcc), and the coupling capacitor C 21 . The driving circuit further comprises a light receiving element  23  that monitors a portion of an optical output from the light emitting element  21  to output a current corresponding to the optical output, and an A/D converter  24  that converts an output current from the light receiving element  23  into a digital signal.  
         [0036]    With this configuration, the A/D converter  24  varies the resistance of the variable resistor R 22  according to the output current from the light receiving element  23 . This allows the A/D converter  24  to control the bias current through the light emitting element  21  to maintain a constant optical output from the light emitting element  21 . The variable resistor R 22  uses a digital potentiometer that can be controlled using a digital signal.  
         [0037]    The present embodiment uses the A/D converter and the digital potentiometer. However, the optical output from the light emitting element  21  may be controlled to be constant by using an automatic power control (APC) circuit. The APC circuit is composed of a current voltage converting circuit that converts an output current from the light receiving element into a corresponding voltage signal and a combinatorial circuit that can vary the resistance on the basis of the voltage signal.  
         [0038]    (Third Embodiment)  
         [0039]    [0039]FIG. 5 shows a configuration of a driving circuit according to a third embodiment of the present invention. The driving circuit is composed of the light emitting element  21 , the resistor R 21  connected to the anode terminal of the light emitting element  21 , the DC-DC converter  22  connecting the resistor R 21  and the power supply (Vcc), and the coupling capacitor C 21 . The driving circuit further comprises the light receiving element  23  that monitors a portion of an optical output from the light emitting element  21  to output a current corresponding to the optical output, and the A/D converter  24  that converts an output current from the light receiving element  23  into a digital signal.  
         [0040]    With this configuration, the A/D converter  24  controls the PWM control circuit  31  of the DC-DC converter  22  according to the output current from the light receiving element  23 . Thus, the A/D converter  24  sets different values for voltages supplied to the resistor R 21  and the light emitting element  21  to control the bias current through the light emitting element  32 . The optical output from the light emitting element  21  is therefore kept constant. Alternatively, the resistor R 31  or R 32  may be a digital potentiometer so that the resistance of the resistor R 31  or R 32  can be varied depending on an output from the A/D converter  24 . Thus, the A/D converter  24  sets different values for voltages supplied to the resistor R 21  and the light emitting element  21  to control the bias current through the light emitting element  21 .  
         [0041]    (Fourth Embodiment)  
         [0042]    [0042]FIG. 6 shows a configuration of a driving circuit according to a fourth embodiment of the present invention. The driving circuit comprises the light emitting element  21 , the resistor R 21  connected to the anode terminal of the light emitting element  21 , the DC-DC converter  22  connecting the resistor R 21  and the power supply (Vcc), and the resistor R 22  connecting the cathode terminal of the light emitting terminal  21  and the ground. The driving circuit further has coupling capacitors C 21  and C 22  that provide a positive-phase signal input and a negative-phase signal input from the encoding circuit of the optical transmitter or the like, to the light emitting element  21  as a voltage swing. The DC-DC converter  22  increases the power supply voltage Vcc and supplies the increased power supply voltage Vcc to the light emitting element  21  and the resistors R 21  and R 22 . The bias current is determined by the increased voltage, the forward voltage at the light emitting element  21 , and the resistances of the resistors R 21  and R 22 .  
         [0043]    The DC-DC converter  22  is configured as shown in FIG. 3. With this configuration, an input data signal provides a voltage swing to the light emitting element  21  via the coupling capacitors C 21  and C 22  as a modulation signal with a positive phase and a modulation signal with a negative phase. As a result, a double voltage swing can be provided compared to the case that a signal with a single phase is inputted to the light emitting element as in the case with the first embodiment shown in FIG. 2.  
         [0044]    According to the present embodiment, the DC-DC converter  22  can increase the power supply voltage Vcc and thus the resistance of the resistor R 21 . Consequently, the light emitting element can be driven in spite of a reduced power supply voltage. Further, since the resistor R 22  can be interposed into the circuit, the light emitting element can be driven using not only the positive-phase signal input but also the negative-phase signal input from the encoding circuit or the like.  
         [0045]    (Fifth Embodiment)  
         [0046]    [0046]FIG. 7 shows a configuration of a driving circuit according to a fifth embodiment of the present invention. The driving circuit comprises the light emitting element  21 , a variable resistor R 23  connected to the anode terminal of the light emitting element  21 , the DC-DC converter  22  connecting the variable resistor R 23  and the power supply (Vcc), and a variable resistor R 24  connecting the cathode terminal of the light emitting terminal  21  and the ground. The driving circuit further has the coupling capacitors C 21  and C 22  that provide a positive-phase signal input and a negative-phase signal input from the encoding circuit of the optical transmitter or the like, to the light emitting element  21  as a voltage swing. Furthermore, the driving circuit comprises the light receiving element  23  that monitors a portion of an optical output from the light emitting element  21  to output a current corresponding to the optical output, and the A/D converter  24  that converts an output current from the light receiving element  23  into a digital signal.  
         [0047]    With this configuration, the A/D converter  24  varies the resistances of the variable resistors R 23  and R 24  according to an output current from the light receiving element  23 . Thus, the A/D converter  24  controls the bias current through the light emitting element  21  to maintain a constant optical output from the light emitting element  21 . The variable resistors R 23  and R 24  are each composed of a digital potentiometer that can be controlled by a digital signal.  
         [0048]    The present embodiment uses the A/D converter and the digital potentiometer. However, the optical output from the light emitting element  21  may be controlled to be constant by using an APC circuit. The APC circuit is composed of a current voltage converting circuit that converts an output current from the light receiving element into a corresponding voltage signal and a combinatorial circuit that can vary the resistance on the basis of the voltage signal.  
         [0049]    (Sixth Embodiment)  
         [0050]    [0050]FIG. 8 shows a configuration of a driving circuit according to a sixth embodiment of the present invention. The driving circuit comprises the light emitting element  21 , the resistor R 21  connected to the anode terminal of the light emitting element  21 , the DC-DC converter  22  connecting the resistor R 21  and the power supply (Vcc), and the resistor R 22  connecting the cathode terminal of the light emitting terminal  21  and the ground. The driving circuit further has the coupling capacitors C 21  and C 22  that provide a positive-phase signal and a negative-phase signal from the encoding circuit of the optical transmitter or the like, to the light emitting element  21  as a voltage swing. Furthermore, the driving circuit comprises the light receiving element  23  that monitors a portion of an optical output from the light emitting element  21  to output a current corresponding to the optical output, and the A/D converter  24  that converts an output current from the light receiving element  23  into a digital signal.  
         [0051]    With this configuration, the A/D converter  24  controls the PWM control circuit  31  of the DC-DC converter  22  according to the output current from the light receiving element  23 . Thus, the A/D converter  24  sets different values for voltages supplied to the resistor R 21  and the light emitting element  21  to control the bias current through the light emitting element  32 . The optical output from the light emitting element  21  is therefore kept constant. Alternatively, the resistor R 31  or R 32  may be a digital potentiometer so that the resistance of the resistor R 31  or R 32  can be varied on the basis of an output from the A/D converter  24 . Thus, the A/D converter  24  sets different values for voltages supplied to the resistor R 21  and the light emitting element  21  to control the bias current through the light emitting element  21 .