Abstract:
A laser diode driving circuit capable of providing a suitable output current to a laser diode is disclosed. The laser diode driving circuit includes a current supply, a first pseudo laser diode, a second pseudo laser diode, and an output current mirror circuit. An input current provided by the current supply is output through the output current mirror circuit to the laser diode. By making the characteristic of each of the first and second pseudo laser diodes substantially equal to the characteristic of the laser diode, the non-linearity between the input current and the output current is compensated.

Description:
FIELD  
       [0001]     The present invention relates to a laser diode driving circuit, and more particularly, to a laser diode driving circuit capable of providing a suitable output current to a laser diode.  
       BACKGROUND  
       [0002]     A laser diode is used as a light source for various apparatus, including an image forming apparatus, an optical disc apparatus, and a communication apparatus, for example. The laser diode is controlled by a current output from a laser diode driving circuit  100  of  FIG. 1 , for example.  
         [0003]     The background circuit  100  of  FIG. 1 , which drives a laser diode LD, includes a current supply  101 , a first current mirror circuit  104 , a second current mirror circuit  107 , and a switch  108 . As shown in  FIG. 1 , the first current mirror circuit  104  includes a pair of N-channel metal oxide semiconductor field effect transistors (“NMOS transistors”)  102  and  103 . The second current mirror circuit  107  includes a pair of P-channel metal oxide semiconductor field effect transistors (“PMOS transistors”)  105  and  106 .  
         [0004]     An input current ia, provided by the current supply  101 , is input to the first and second current mirror circuits  104  and  107 , and output as an output current iLD to drive the laser diode LD.  
         [0005]     Ideally, the output current iLD is proportional to the input current ia. However, in most cases, the input current ia and the output current iLD are not proportional to each other, depending on various factors including the channel length modulation effect, fluctuations in voltage, and variations in characteristic of the laser diode LD, etc. Accordingly, referring to  FIG. 2 , the characteristic of the background circuit  100 , defined by the ratio between the output current iLD and the input current ia, is not always linear as indicated by an observed characteristic curve Lc, as opposed to an expected characteristic curve Lt.  
       SUMMARY OF THE INVENTION  
       [0006]     Exemplary embodiments of the present disclosure include a laser diode driving circuit capable of providing a suitable output current to a laser diode.  
         [0007]     In one exemplary embodiment, the laser driving circuit includes a current supply, a first pseudo laser diode, a second pseudo laser diode, and an output current mirror circuit. The current supply inputs an input current. The first pseudo laser diode receives a first current substantially equal or proportional to the input current. The second pseudo laser diode receives a second current substantially equal to the first current. The output current mirror circuit generates an output current substantially equal or proportional to at least one of the first current and the second current, and provides the output current to the laser diode. In this exemplary embodiment, to achieve the linearity between the input current and the output current, the characteristic of each of the first pseudo laser diode and the second pseudo laser diode is made substantially equal to the characteristic of the laser diode.  
         [0008]     For example, the characteristic of each of the first and second pseudo laser diodes may be determined based on at least one of a forward voltage and a resistance of the laser diode. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     A more complete appreciation of the disclosure and many of the attendant advantages thereof will be understood more readily by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0010]      FIG. 1  is a schematic diagram illustrating a circuit configuration of a background laser diode driving circuit;  
         [0011]      FIG. 2  is a graph showing the relationship between an input current and an output current of the circuit of  FIG. 1 ;  
         [0012]      FIG. 3  is a schematic diagram illustrating a circuit configuration of a laser diode driving circuit according to a preferred embodiment;  
         [0013]      FIG. 4  is a graph showing the relationship between a current and a voltage of a laser diode of  FIG. 3 ;  
         [0014]      FIG. 5  is a schematic diagram illustrating a circuit configuration of any one of the first and second pseudo laser diodes of  FIG. 3 , according to a preferred embodiment;  
         [0015]      FIG. 6  is a schematic diagram illustrating a circuit configuration of a voltage supply of  FIG. 5 ;  
         [0016]      FIG. 7  is a graph showing the relationship between a voltage and a current of any one of the first and second pseudo laser diodes of  FIG. 3 , according to a preferred embodiment;  
         [0017]      FIG. 8  is a schematic diagram illustrating a circuit configuration of any one of the first and second pseudo laser diodes of  FIG. 3 , according to another preferred embodiment;  
         [0018]      FIG. 9  is a graph showing the relationship between a voltage and a current of any one of the first and second pseudo laser diodes of  FIG. 3 , according to another preferred embodiment;  
         [0019]      FIG. 10  is a schematic diagram illustrating a circuit configuration of any one of the first and second pseudo laser diodes of  FIG. 3 , according to another preferred embodiment;  
         [0020]      FIG. 11  is a schematic diagram illustrating a circuit configuration of any one of the first and second pseudo laser diodes of  FIG. 3 , according to another preferred embodiment;  
         [0021]      FIG. 12  is a schematic diagram illustrating a circuit configuration of any one of the first and second pseudo laser diodes of  FIG. 3 , according to another preferred embodiment;  
         [0022]      FIG. 13  is a schematic diagram illustrating a circuit configuration of any one of the first and second pseudo laser diodes of  FIG. 3 , according to another preferred embodiment;  
         [0023]      FIG. 14  is a schematic diagram illustrating a circuit configuration of any one of the first and second pseudo laser diodes of  FIG. 3 , according to another preferred embodiment;  
         [0024]      FIG. 15  is a schematic diagram illustrating a circuit configuration of any one of the first and second pseudo laser diodes of  FIG. 3 , according to another preferred embodiment;  
         [0025]      FIG. 16  is a schematic diagram illustrating a circuit configuration of a laser diode driving circuit according to another preferred embodiment;  
         [0026]      FIG. 17  is a schematic diagram illustrating a circuit configuration of a laser diode driving circuit according to another preferred embodiment;  
         [0027]      FIG. 18  is a schematic diagram illustrating the fifth current mirror circuit of  FIG. 17 , according to a preferred embodiment;  
         [0028]      FIG. 19  is a schematic diagram illustrating a part of the fifth current mirror circuit of  FIG. 17 , according to another preferred embodiment;  
         [0029]      FIG. 20  is a schematic diagram illustrating the fifth current mirror circuit of  FIG. 17 , according to another preferred embodiment; and  
         [0030]      FIG. 21  is a schematic diagram illustrating the fifth current mirror circuit of  FIG. 17 , according to another preferred embodiment. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0031]     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 3 , a description is made of a laser diode driving circuit  1  according to a preferred embodiment.  
         [0032]     The driving circuit  1 , capable of driving a laser diode LD, includes a first pseudo laser diode LD 1 , a second pseudo laser diode LD 2 , a current supply  2 , a first current mirror circuit  3 , a second current mirror circuit  4 , a third current mirror circuit  5 , a fourth current mirror circuit  6 , an NMOS transistor N 5 , an NMOS transistor N 6 , an amplifier AMP, a first switch SWa, and a second switch SWb.  
         [0033]     As shown in  FIG. 3 , the current supply  2  and an NMOS transistor N 1  of the fourth current mirror circuit  6  are serially connected between a positive supply voltage Vdd and a negative supply voltage Vss. The current supply  2  outputs an input current i 1  according to a control signal SC received from the outside.  
         [0034]     The fourth current mirror circuit  6  includes parallel-connected NMOS transistors N 2  to N 4  in addition to the NMOS transistor N 1 . The NMOS transistor N 2  provides a current i 2  to the first current mirror circuit  3 . The NMOS transistor N 3  provides a current i 4  to the second current mirror circuit  4 . The NMOS transistor N 4  provides a current i 6  to the third current mirror circuits  5 . The gate and the drain of the NMOS transistor N 1  are connected to each other. The gate of the NMOS transistor N 1  is further connected to each of the gates of the NMOS transistors N 2 , N 3 , and N 4 . Each of the sources of the NMOS transistors N 1 , N 2 , N 3 , and N 4  is connected to the negative supply voltage Vss.  
         [0035]     The first current mirror circuit  3  includes PMOS transistors P 1  to P 4 , and provides a current i 3  to the first pseudo laser diode LD 1 . The PMOS transistors Pi and P 3  are serially connected between the positive supply voltage Vdd and the gate of the NMOS transistor N 2 . The PMOS transistors P 2  and P 4  are serially connected between the positive supply voltage Vdd and the anode of the first pseudo laser diode LD 1 . The PMOS transistor P 2  has the gate connected to the gate of the PMOS transistor P 1 , and the drain connected to the connecting point of the PMOS transistors P 1  and P 2 . The PMOS transistor P 3  has the gate connected to the gate of the PMOS transistor P 4 , and the drain connected to the connecting point of the PMOS transistors P 3  and P 4 .  
         [0036]     The second current mirror circuit  4  includes PMOS transistors P 5  and P 6 , and provides a current i 5  to the second pseudo laser diode LD 2 . The sources of the PMOS transistors P 5  and P 6  are each connected to the positive supply voltage Vdd. The gates of the PMOS transistors P 5  and P 6  are connected to each other. The drain of the PMOS transistor P 5  is connected to the connecting point of the PMOS transistors P 5  and P 6 . The second switch SWb and the NMOS transistor N 3  are serially connected between the drain of the PMOS transistor P 5  and the negative supply voltage Vss. The second pseudo laser diode LD 2  is connected between the drain of the PMOS transistor P 6  and the negative supply voltage Vss.  
         [0037]     The third current mirror circuit  5  includes PMOS transistors P 7  and P 8 , and provides an output current iLD to the laser diode LD. The sources of the PMOS transistors P 7  and P 8  are each connected to the positive supply voltage Vdd. The gates of the PMOS transistors P 7  and P 8  are connected to each other. The drain of the PMOS transistor P 7  is connected to the connecting point of the PMOS transistors P 7  and P 8 . The first switch SWa and the NMOS transistor N 4  are serially connected between the drain of the PMOS transistor P 7  and the negative supply voltage Vss. The laser diode LD is connected between the drain of the PMOS transistor P 8  and the negative supply voltage Vss.  
         [0038]     The amplifier AMP controls an operation of the NOMS transistors N 5  and N 6 . The positive input terminal of the amplifier AMP is connected to the connecting point at which the drain of the PMOS transistor P 4  and the anode of the first pseudo laser diode LD 1  are connected. The negative input terminal of the amplifier AMP is connected to the connecting point at which the drain of the PMOS transistor P 6  and the anode of the second pseudo laser diode LD 2  are connected. The output terminal of the amplifier AMP is connected to the gates of the NMOS transistors N 5  and N 6 , respectively. The NMOS transistor N 5  is connected in parallel to the NMOS transistor N 3 . The NMOS transistor N 6  is connected in parallel to the NMOS transistor N 4 .  
         [0039]     The first switch SWa controls on or off of the laser diode LD. The second switch SWb, which is regularly turned on, adjusts the impedance Z 1  of the first switch SWa, when the first switch SWa is turned on. Preferably, the impedance Z 1  is adjusted to be smaller than the impedance Z 2  of the second switch SWb.  
         [0040]     The second current mirror circuit  4  preferably has a transistor element size smaller than that of the third current mirror circuit  5 . Accordingly, the current consumption M 2   i  of the circuit  4  is less than the current consumption M 3   i  of the circuit  5 .  
         [0041]     Further, the transistor sizes of the circuits  4  and  5  are preferably set such that the ratio between the current i 5  and the current iLD is substantially equal to the ratio between the current consumption M 2   i  and the current consumption M 3   i . In such a case, the ratio between the anode-current/anode-voltage characteristic (hereinafter, simply referred to as the “characteristic”) of the second pseudo laser diode LD 2  and the characteristic of the laser diode LD is substantially equal to the ratio between the current consumption M 2   i  and the current consumption M 3   i , as indicated by the equation: (i 5 /VLD 2 )/(iLD/VLD)=M 2   i /M 3   i.    
         [0042]     Furthermore, the ratio between the impedance Z 1  of the first switch SWa and the impedance Z 2  of the second switch SWb is, preferably, substantially equal to the inverse of the ratio between the current consumption M 2   i  and the current consumption M 3   i , as indicated by the equation: Z 2 /Z 1 =M 3   i /M 2   i.    
         [0043]     As shown in  FIG. 3 , the first current mirror circuit  3  includes two current mirror circuits stacked vertically, to compensate the channel length modulation effect. However, any kind of circuits, capable of compensating the channel length modulation effect, may be used, including a cascode current mirror circuit or a Wilson current mirror circuit, for example.  
         [0044]     As described above, the current i 1 , supplied by the current supply  2 , is input to the drain of the NMOS transistor N 1  as a drain current. The drain current is input to the first current mirror circuit  3  through the NMOS transistor N 2  as the current i 2 . The current i 2  is further supplied to the first pseudo laser diode LD 1  as the current i 3 .  
         [0045]     Similarly, the drain current is further input to the second current mirror circuit  4  through the NMOS transistor N 3 , as the current i 4 . The current i 4  is further supplied to the second pseudo laser diode LD 2  as the current i 5 .  
         [0046]     The NMOS transistor N 2  has an element size larger than that of the NMOS transistor N 3 . Accordingly, the current i 2  (i.e. the drain current of the NMOS transistor N 2 ) is larger than the current i 4  (i.e. the drain current of the NMOS transistor N 3 ). As a result, the current i 3  (i.e. the anode current of the first pseudo laser diode LD 1 ) becomes larger than the current i 5 . Assuming that the characteristics of the first and second pseudo laser diodes LD 1  and LD 2  are substantially equal to each other, the anode voltage VLD 1  of the first pseudo laser diode LD 1  becomes larger than the anode voltage VLD 2  of the second pseudo laser diode LD 2 .  
         [0047]     As shown in  FIG. 3 , the anode voltage VLD 1  is input to the positive input terminal of the amplifier AMP. The anode voltage VLD 2  is input to the negative input terminal of the amplifier AMP. The amplifier AMP, which controls the gate voltage of the NMOS transistor N 5 , can control the amount of the current i 4 , and thus, the amount of the current i 5 . In this way, the anode voltage VLD 1  and the anode voltage VLD 2  are made substantially equal to each other.  
         [0048]     The amplifier AMP, which controls the gate voltage of the NMOS transistor N 6 , may also control the amount of the current i 6 , and thus, the amount of the current iLD. Preferably, the current i 6  is set such that the ratio in drain current between the NMOS transistor N 5  and the NMOS transistor N 6  is substantially equal to the ratio between the current consumption M 2   i  and the current consumption M 3   i . In this way, the current iLD becomes substantially proportional to the current i 5 .  
         [0049]     Since the current i 5  is substantially equal to the current i 3 , and the current i 3  is substantially proportional to the current i 1 , the output current iLD is substantially proportional to the input current i 1 .  
         [0050]     In this exemplary case, each of the characteristics of the first and second pseudo laser diodes LD 1  and LD 2  is substantially equal to the characteristic of the laser diode LD. Referring to  FIG. 4 , the characteristic of the laser diode LD may be determined based on a forward voltage VF and a resistance RLD (indicated as the slope V/i). The forward voltage VF or the resistance RLD varies for each laser diode, depending on manufacturing conditions of the laser diode or environmental factors affecting the laser diode, for example.  
         [0051]     In order to respond to variations in the characteristics of the laser diode LD, each of the first and second pseudo laser diodes LD 1  and LD 2  (collectively, referred to as the “pseudo laser diode”) may have a configuration illustrated in  FIG. 5 , according to a preferred embodiment.  
         [0052]     The pseudo laser diode of  FIG. 5  includes a resistor circuit RA and a voltage supply VS. One terminal of the resistor circuit RA is connected to the positive electrode of the voltage supply VS, while the other terminal of the resistor circuit RA functions as the anode of the pseudo laser diode. The negative electrode of the voltage supply VS functions as the cathode of the pseudo laser diode.  
         [0053]     The voltage supply VS may change an amount of supply voltage, corresponding to the characteristic of the laser diode LD. Referring to  FIG. 6 , the voltage supply VS includes an NMOS transistor  15 , an amplifier  16 , and a D/A (digital/analog) converter DAC. The positive input terminal of the amplifier  16  is connected to the D/A converter DAC. The output terminal of the amplifier  16  is connected to the gate of the NMOS transistor  15 . The drain of the NMOS transistor  15  functions as the positive electrode of the voltage supply VS, while the source of the NMOS transistor  15  functions as the negative electrode of the voltage supply VS. The voltage of the voltage supply VS may be changed by changing the output voltage of the D/A converter DAC.  
         [0054]     If the voltage of the voltage supply VS can be freely changed in the above-described manner, the forward voltage of the pseudo laser diode can be changed accordingly, for example, from VF 0  to VF 1  as illustrated in  FIG. 7 , depending on the forward voltage VF of the laser diode LD shown in  FIG. 4 .  
         [0055]     In another example, the resistor circuit RA may be changed to have a different amount of resistance, corresponding to the resistance RLD of the laser diode LD.  
         [0056]     In order to change resistance, the pseudo laser diode may have a configuration illustrated in  FIG. 8 , for example. The pseudo laser diode of  FIG. 8  includes a plurality of resistors R 0  to Rn with n being an integer greater than 1, a plurality of switches SW 0  to SWn, and the voltage supply VS. The terminals of the resistors RØ to Rn are connected to the voltage supply VS, while the other terminals of the resistors R to Rn are connected to the terminals of the corresponding switches SW 0  to SWn. The other terminals of the switches SW 0  to SWn function as the anode of the pseudo laser diode. The amount of resistance may be controlled by turning on or off at least one of the switches SW 1  to SWn.  
         [0057]     The resistance values of the resistors RØ to Rn may be equal to one another, or they may be different from one another. For example, when the resistance values of the resistors RØ to Rn are previously determined such that the resistance value ratios for the resistors R 0  to Rn are 1:1:½ . . . ½ n−1 , a wide range of resistance values may be obtained for the pseudo laser diode by controlling the switches SW 0  to SWn. Referring to  FIG. 9 , when the switch SW 0  is turned on, the resistance defined by the line L 0  is obtained. When the switches SW 0  and SW 1  are turned on, the resistance defined by the line L 1  is obtained. When the switches SW 0  to SW 2  are turned on, the resistance defined by the line L 2  is obtained. When the switches SW 0  to SWn are turned on, the resistance defined by the line Ln is obtained.  
         [0058]     Referring to  FIG. 8 , the other terminals of the switches SW 0  to SWn are connected at the anode side. However, they may be arranged at the cathode side as illustrated in  FIG. 10 .  
         [0059]     Further, any one of the switches SW 0  to SWn may be implemented as a MOS transistor, as illustrated in  FIG. 11 . In addition, the switches SW 0  to SWn may be controlled by a programmable register  25 , as illustrated in  FIG. 11 . Referring to  FIG. 11 , the gates of the NMOS transistors N 0  to Nn are connected to the register  25 . In order to turn on one or more of the transistors N 0  to Nn, the register  25  is previously programmed to send corresponding one or more of high level signals G 0  to Gn to the corresponding one or more of the gates.  
         [0060]     Alternatively, any one of the switches SW 0  to SWn may be implemented in other ways, including as a fuse, as long as the resistance of the pseudo laser diode can be controlled.  
         [0061]     In order to respond to variations in the characteristics of the laser diode LD, the pseudo laser diode may have a configuration illustrated in  FIG. 12 , according to another preferred embodiment.  
         [0062]     The pseudo laser diode of  FIG. 12  includes a plurality of switches SW 0  to SWn connected in parallel to one another, and a plurality of NMOS transistors N 0  to Nn connected in parallel to one another. The terminals of the switches SW 0  to SWn are connected, respectively, to the NMOS transistors N 0  to Nn. The other terminals of the switches SW 0  to SWn are connected to one another at the anode of the pseudo laser diode. The characteristic of the pseudo laser diode may be controlled by turning on at least one of the switches SW 0  to SWn.  
         [0063]     In this exemplary case, any one of the switches SW 0  to SWn may be implemented as an NMOS transistor, as illustrated in  FIG. 13 . Further, the switches SW 0  to SWn may be controlled by the programmable register  25 , as illustrated in  FIG. 13 . Referring to  FIG. 13 , the gates of the NMOS transistors SN 0  to SNn are connected to the register  25 . In order to turn on one or more of the transistors SN 0  to SNn, the register  25  is programmed to send corresponding one or more of high level signals G 0  to Gn to the corresponding one or more of the gates of the transistors SN 0  to SNn.  
         [0064]     Further, any one of the switches SW 0  to SWn of  FIG. 12  may be implemented as a transmission gate, as illustrated in  FIG. 14 . Referring to  FIG. 14 , the pseudo laser diode includes a plurality of NMOS transistors N 0  to Nn and Ng 0  to Ngn, a plurality of transmission gates TG 0  to TGn, and a plurality of inverters INV 0  to INVn. The drains of the NMOS transistors N 0  to Nn are connected to one another at the anode side of the pseudo laser diode. The sources of the NMOS transistors N 0  to Nn are connected to one another at the cathode side. The corresponding one of the transmission gates TG 0  to TGn is connected between the gate and the drain of each of the NMOS transistors N 0  to Nn.  
         [0065]     When any one of the transmission gates TG 0  to TGn is turned on, the gate and the drain of the corresponding one of the NMOS transistors N 0  to Nn is connected to each other to function as a diode.  
         [0066]     When any one of the transmission gates TG 0  to TGn is turned off, and the corresponding one of the NMOS transistors Ng 0  to Ngn is turned on, the corresponding one of the NMOS transistors N 0  to Nn is connected to the negative voltage supply Vss, such as the ground. Accordingly, the corresponding one of the NMOS transistors N 0  to Nn becomes isolated. In this way, the characteristic of the pseudo laser diode may be controlled, depending on the characteristic of the laser diode LD.  
         [0067]     When the forward voltage VF of the laser diode LD is larger than a predetermined value, another set of NMOS transistors N 10  to N 1 n may be introduced, as illustrated in  FIG. 15 . In this exemplary case, instead of controlling the NMOS transistors N 0  to Nn, the NMOS transistors N 10  to N 1 n may be controlled to obtain a desired characteristic.  
         [0068]     Referring now to  FIG. 16 , a laser diode driving circuit  21  is explained according to another preferred embodiment.  
         [0069]     The driving circuit  21 , capable of driving the laser diode LD, includes the current supply  2 , the first pseudo laser diode LD 1 , the second pseudo laser diode LD 2 , the second current mirror circuit  4 , the third current mirror circuit  5 , the amplifier AMP, the NMOS transistor N 5 , the NMOS transistor N 6 , the first switch SWa, and the second switch SWb. As shown in  FIG. 16 , the circuit  21  has a configuration less complex than the configuration of the circuit  1  of  FIG. 3 .  
         [0070]     The amplifier AMP, which controls the gate voltage of the NMOS transistor N 5 , can adjust the current i 5 , to make the anode voltage VLD 1  and the anode voltage VLD 2  substantially equal to each other. Further, the amplifier, which controls the gate voltage of the NMOS transistor N 6 , can adjust the current i 6 , i.e. the current iLD, to be substantially proportional to the current i 5 . In this way, the current iLD is made substantially proportional to the current i 1 .  
         [0071]     Referring now to  FIG. 17 , a laser diode driving circuit  31  is explained according to another preferred embodiment.  
         [0072]     The driving circuit  31 , capable of driving the laser diode LD, includes the current supply  2 , the second current mirror circuit  4 , the third current mirror circuit  5 , the NMOS transistor N 5 , the NMOS transistor N 6 , the first switch SWa, the second switch SWb, a fifth current mirror circuit  32 . As shown in  FIG. 17 , the circuit  31  has a configuration less complex than the configuration of the circuit  21  of  FIG. 16 .  
         [0073]     The current supply  2  is connected between the positive supply voltage Vdd and the output terminal of the fifth current mirror circuit  32 . The connecting point between the current supply  2  and the fifth current mirror circuit  32  is further connected to the gates of the NMOS transistors N 5  and N 6 , respectively.  
         [0074]     The sources of the PMOS transistors P 5  and P 6  are each connected to the positive supply voltage Vdd. The PMOS transistor P 5  has the gate connected to the gate of the PMOS transistor P 6 , and the drain connected to the connecting point between the PMOS transistors P 5  and P 6 . The second switch SWb and the NMOS transistor N 5  are serially connected between the drain of the PMOS transistor P 5  and the negative supply voltage Vss.  
         [0075]     The sources of the PMOS transistors P 7  and P 8  are each connected to the positive supply voltage Vdd. The PMOS transistor P 7  has the gate connected to the gate of the PMOS transistor P 8 , and the drain connected to the connecting point between the PMOS transistors P 7  and P 8 . The first switch SWa and the NMOS transistor N 6  are serially connected between the drain of the PMOS transistor P 7  and the negative supply voltage Vss.  
         [0076]     Referring to  FIG. 18 , the fifth current mirror circuit  32  includes NMOS transistors N 0  to Nn, Nb 0  to Nbn, SN 0  to SNn, and SNb 0  to SNbn, and a register  25 . Each one of the NMOS transistors SNb 0  to SNbn and the corresponding one of the NMOS transistors Nb 0  to Nb 1  are serially connected to each other between the current supply  2  and the negative supply voltage Vss. Each one of the NMOS transistors SN 0  to SNn and the corresponding one of the NMOS transistors N 0  to Nn are serially connected to each other between the drain of the PMOS transistor P 6  and the negative supply voltage Vss.  
         [0077]     Each of the gates of the NMOS transistors SN 0  to SNn is connected to the corresponding one of the gates of the NMOS transistors SNb 0  to SNbn. Each of the connecting points of the NMOS transistors SN 0  to SNn and the NMOS transistors SNb 0  to SNbn is further connected to the register  25 . Each of the gates of the NMOS transistors N 0  to Nn is connected to the corresponding one of the gates of the NMOS transistors Nb 0  to Nbn. Each of the connecting points of the NMOS transistors N 0  to Nn and the NMOS transistors Nb 0  to Nbn is further connected to the corresponding one of the connecting points of the NMOS transistors N 0  to Nn and the NMOS transistors SN 0  to SNn.  
         [0078]     Each of the NMOS transistors Nb 0  to Nbn functions as the first pseudo laser diode LD 1  shown in any one of  FIGS. 3 and 16 . Each of the NMOS transistors N 0  to Nn functions as the second pseudo laser diode LD 2  shown in any one of  FIGS. 3 and 16 .  
         [0079]     The register  25  is previously programmed to send a high level signal to at least one of the NMOS transistors SN 0  to SNn to turn on the corresponding one of the NMOS transistors N 0  to Nn. Similarly, the register  25  sends a high level signal to at least one of the NMOS transistors SNb 0  to SNbn to turn on the corresponding one of the NMOS transistors Nb 0  to Nbn.  
         [0080]     For example, when the register  25  sends a high level signal to the NMOS transistor SNk and the NMOS transistor SNbk, with k being an integer between 0 and n, the current i 1 , supplied by the current supply  2 , is input to the drain of the NMOS transistor Nbk as a drain current. The drain voltage of the NMOS transistor Nbk is output as the anode voltage VLD 1  as illustrated in  FIG. 17 . The voltage VLD 1  is further input to the gate of the NMOS transistor N 5 , which is connected to the second current mirror circuit  4 . As a result, the current i 4 , i.e. the current i 5 , is made substantially equal to the current i 1 .  
         [0081]     Still referring to  FIG. 17 , the voltage VLD 1  is further input to the gate of the NMOS transistor N 6 , which is connected to the third current mirror circuit  5 . Thus, the current i 6  becomes substantially proportional to the current i 4 . Since the circuit  4  and the circuit  5  are substantially similar in circuit configuration, the current iLD becomes substantially proportional to the current i 1 .  
         [0082]     The fifth current mirror circuit  32  may have a configuration other than the configuration of  FIG. 18 . For example, the fifth current mirror circuit  32  may include any kind of circuits, capable of compensating the channel length modulation effect. For example, a cascade current mirror circuit shown in  FIG. 19  may preferably be used. Alternatively, the channel length modulation effect may be suppressed by controlling the gate channel length of any one of the NMOS transistors of the circuit  32 .  
         [0083]     In another example, referring to  FIG. 20 , the fifth current mirror circuit  32  may include a plurality of NMOS transistors N 0  to Nn, Nb 0  to Nbn, and Ng 0  to Ngn, a plurality of transmission gates TG 0  to TGn, and a plurality of inverters INV 0  to INVn. The drains of the NMOS transistors Nb 0  to Nbn are connected to one another. The sources of the NMOS transistors Nb 0  to Nbn are connected to one another, and further to the negative supply voltage Vss. The drains of the NMOS transistors N 0  to Nn are connected to one another. The sources of the NMOS transistors N 0  to Nn are connected to one another, and further to the negative supply voltage Vss. The corresponding one of the transmission gates TG 0  to TGn is connected between the gate and the drain of each of the NMOS transistors N 0  to Nn.  
         [0084]     Further, when the forward voltage VF of the laser diode LD is larger than a predetermined value, another set of NMOS transistors N 10  to N 1 n may be introduced, as illustrated in  FIG. 21 , for example.  
         [0085]     Any one of the above-described laser diode driving circuits and other light source driving circuits according to the present disclosure may be incorporated in any kind of light emitting system. For example, a light emitting system may include a controller, which outputs a control signal. Based on the control signal, the light source driving circuit of the present disclosure generates an input current, and further generates an output current equal to or proportional to the input current.  
         [0086]     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and/or appended claims.  
         [0087]     Further, the invention of this disclosure and/or appended claims may be implemented using one or more conventional general purpose microprocessors and/or signal processors programmed according to the teachings of the present disclosure, as will be appreciated by those skilled in the relevant art(s). Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will also be apparent to those skilled in the relevant art(s). Alternatively, as described above, the invention of this disclosure and/or appended claims may be implemented by ASIC, prepared by interconnecting an appropriate network of conventional component circuits or by a combination thereof with one or more conventional general purpose microprocessors and/or signal processors programmed accordingly.  
         [0088]     This patent specification claims priority to Japanese patent application No. JPAP2004-058970 filed on Mar. 3, 2004, in the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.