Abstract:
A current mirror circuit is disclosed that is capable of supplying a desired second current regardless of whether the threshold voltages of the current mirror circuit&#39;s transistors are the same or different. The current mirror circuit includes a first transistor whose first terminal is electrically connected to a voltage source and whose gate terminal and second terminal are electrically connected to each other. A second transistor has a first terminal electrically connected to the voltage source and a gate terminal electrically connected to the gate terminal of the first transistor. A compensator that compensates for different threshold voltages of each of the first transistor and the second transistor is also included. Because differences in the threshold voltages of the transistors connected to the current mirror are compensated for, embodiments of the invention are able to generate a desired second current that can be used to power a driver or similar device. Additionally, the invention&#39;s current mirror circuit may function as a bias unit so that a driving circuit can be stably driven.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of Korean Patent Application Nos. 2004-95982, filed on Nov. 22, 2004 and 2004-96377, filed on Nov. 23, 2004, in the Korean Intellectual Property Office, each disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to current mirror circuits generally and to methods of driving such circuits, and more particularly to a current mirror circuit capable of supplying a desired current that corresponds to a first current regardless of the threshold voltages of one or more transistors included in a driving circuit that contains the current mirror circuit. The invention further relates to a method of driving the driving circuit that contains a current mirror circuit constructed in accordance with the principles of the invention. 
     2. Discussion of Related Art 
     A current mirror circuit is a circuit in which a value of an output current is determined by a value of an input current. The current mirror circuit is used for various circuits. 
       FIG. 1  illustrates a conventional current mirror circuit that includes a first transistor M 1  and a second transistor M 2 . Each transistor M 1  and M 2  includes a first terminal, a second terminal, and a gate terminal. 
     The first terminal of the transistor M 1  is electrically connected to a voltage source VDD. The second terminal and the gate terminal of the transistor M 1  are electrically connected to each other. That is, electric current flows through the transistor M 1  so that the transistor M 1  not only serves as a diode, but also supplies to its second terminal a current I 1  that corresponds to the voltage source VDD. The first terminal may be set as one of a source terminal and a drain terminal and the second terminal may be set to be different from the first terminal. For example, when the first terminal is set as the source terminal, the second terminal is set as the drain terminal. 
     The transistor M 2  is electrically connected to the transistor M 1  to form a current mirror circuit. Therefore, the gate terminal of the transistor M 2  is electrically connected to the gate terminal of the transistor M 1 , and the first terminal of the transistor M 2  is electrically connected to the voltage source VDD. Assuming that a ratio of a channel width and a channel length (W/L) of the transistor M 1  approximately equals a ratio of a channel width and a channel length (W/L) of the transistor M 2 , a current I 2  flowing from the transistor M 2  is preferably set to equal the current I 1  that flows from the transistor M 1 . That is, the current mirror circuit controls the first current I 1  to thereby control the value of the second current I 2  that is supplied to one or more external components. 
     The conventional current mirror circuit may have several applications. For example, the conventional current mirror circuit may be electrically connected to a driver  2  and used as a bias unit  1  as illustrated in  FIG. 2 . In operation, the bias unit  1 , which includes the current mirror circuit of  FIG. 1 , supplies the second current I 2  (that is, bias current) to the driver  2  so that the driver  2  can perform desired operations. In turn, the driver  2  performs a predetermined operation when the second current I 2  is supplied. By way of example, the driver  2  may function as either an amplifier or a buffer. 
     In the conventional current mirror circuit, the difference between a threshold voltage of the transistor M 1  and a threshold voltage of the transistor M 2  creates a difference in current outputted from each transistor. Therefore, it is not possible to generate a desired current outputted from the transistor M 2  that matches the current outputted from the transistor M 1 . A threshold voltage may be defined as a gate voltage at or below which the transistor remains turned off (and no current flows) and above which the transistor turns on (and current flows). 
     Otherwise identical transistors M 1  and M 2  may have different threshold voltages due to deviations caused by manufacturing or other processes. For example, the threshold voltage of the transistor M 1  may be set as 0.7V and the threshold voltage of the transistor M 2  may be set as 0.3V. In such a case, the first current I 1  that flows via the transistor M 1  and the second current I 2  that flows via the transistor M 2  will have different values, each of which corresponds to one of the different threshold voltages. Due to the different threshold voltages, it is not possible to generate a desired second current I 2 . This is particularly problematic when the current mirror circuit is used for a part sensitive to current such as a pixel of a light emitting display, since any deviation in current generated by the different threshold voltages of the transistors M 1  and M 2  causes serious problems. 
     SUMMARY OF THE INVENTION 
     Accordingly, embodiments of the invention provide an improved current mirror circuit capable of supplying current regardless of the values of one or more particular threshold voltage(s). Embodiments of the invention also provide a driving circuit using the improved current mirror circuit, and a method of driving the driving circuit. 
     The foregoing and/or other aspects of the present invention are achieved by providing a current mirror circuit that includes a first transistor whose first terminal is electrically connected to a voltage source and whose gate terminal and second terminal are electrically connected to each other. The circuit further includes a second transistor whose first terminal is electrically connected to the voltage source and whose gate terminal is electrically connected to the gate terminal of the first transistor. The circuit also includes a compensator that compensates for the threshold voltages of each of the first and second transistors. 
     The threshold voltage of the first transistor may be stored in the first capacitor and the threshold voltage of the second transistor may be stored in the second capacitor when the first control signal is supplied. 
     Another embodiment of the invention provides a driving circuit that includes a bias unit for supplying bias current and a driver coupled with the bias unit and driven when the bias current is supplied. The bias unit includes a first transistor having a first terminal electrically connected to a voltage source and having a gate terminal and a second terminal that are electrically connected to each other. The bias unit may further include a second transistor having a first terminal electrically connected to the voltage source and having a gate terminal electrically connected to the gate terminal of the first transistor. The bias unit may further comprise a compensator for compensating the threshold voltages of each of the first and the second transistor. 
     Another embodiment of the invention provides a method of driving a current mirror circuit. In one embodiment, the method includes the step of compensating for the threshold voltages of each of the first and second transistors, which are connected to form a current mirror. The method further includes the steps of supplying a first current via the first transistor through which electric current flows so that the first transistor serves as a diode, and supplying a second current via a second transistor such that the value of the second current corresponds to the value of the first current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings. 
         FIG. 1  is a circuit diagram illustrating a conventional current mirror circuit. 
         FIG. 2  illustrates a driving circuit using the conventional current mirror circuit. 
         FIG. 3  is a circuit diagram illustrating a current mirror circuit according to an embodiment of the present invention. 
         FIG. 4  depicts exemplary waveforms that describe control signals supplied to the current mirror circuit illustrated in  FIG. 3 . 
         FIG. 5  illustrates that a control signal generator may be added to the current mirror circuit illustrated in  FIG. 3 . 
         FIG. 6  illustrates a current mirror circuit according to another embodiment of the invention. 
         FIG. 7  illustrates a first embodiment of a driving circuit having a current mirror circuit constructed in accordance with the principles of the invention. 
         FIG. 8  is a circuit diagram illustrating an example of the driver illustrated in  FIG. 7 . 
         FIG. 9  illustrates control signals supplied to the driver illustrated in  FIG. 8 . 
         FIG. 10  illustrates a second embodiment of a driving circuit having a current mirror circuit constructed in accordance with the principles of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, preferable embodiments according to the invention will be described with reference to the accompanying drawings, that is,  FIGS. 3 ,  4 ,  5 ,  6 ,  7 ,  8 ,  9 , and  10 . 
     Referring to  FIG. 3 , a current mirror circuit constructed according to one the embodiment of the invention may include transistors M 1  and M 2  that are electrically connected to each other. The current mirror circuit may also include a compensator  10  for compensating for the threshold voltages of each of the transistors M 1  and M 2 . Each of the transistors M 1  and M 2  include a first terminal, a second terminal, and a gate terminal. 
     The first terminal of the first transistor M 1  may electrically connect to a voltage source VDD, and the second terminal and the gate terminal of the transistor M 1  may electrically connect to each other. In operation, electric current flows through the transistor M 1  so that the transistor M 1  serves as a diode, thereby causing the transistor M 1  to supply a predetermined current to its second terminal. 
     The current mirror circuit may further be formed by electrically connecting the gate terminal of the transistor M 2  to the gate terminal of the transistor M 1 . Additionally, the first terminal of the transistor M 2  may be electrically connected to the voltage source VDD. 
     In use, the compensator  10  compensates for the (different) threshold voltages of each of the transistors M 1  and M 2  so that a desired current output via the transistor M 2  can be supplied regardless of any deviation in the threshold voltages of each of the transistors M 1  and M 2 . Thus, the compensator  10  includes a third transistor M 3 , a fourth transistor M 4 , a first capacitor C 1 , and a second capacitor C 2 , which may be arranged and configured as described below. 
     The capacitor C 1 , electrically connected between the gate terminal of the transistor M 1  and the gate terminal of the transistor M 2 , stores a voltage that corresponds to the threshold voltage of the transistor M 1 . 
     The capacitor C 2 , electrically connected between the capacitor C 1  and the gate terminal of the transistor M 2 , stores a voltage that corresponds to the threshold voltage of the transistor M 2 . 
     The transistor M 3 , electrically connected between the voltage source VDD and a first node N 1  that is a common node of the first and second capacitors C 1  and C 2  turns on when a first control signal CS 1  is supplied. When actuated, the transistor M 3  supplies the voltage of the voltage source VDD to the node N 1 . 
     The transistor M 4 , electrically connected between the gate terminal and the second terminal of the transistor M 2 , turns on when the control signal CS 1  is supplied so that electric current flows to the transistor M 2  and so that the transistor M 2  serves as a diode. 
     Additionally, the current mirror circuit may also include a fifth transistor M 5  connected to the second terminal of the transistor M 1  and a sixth transistor M 6  connected to the second terminal of the transistor M 2 . 
     The transistor M 5  supplies current I 1  from the transistor M 1  to the second terminal of the transistor M 5  when a second control signal CS 2  is supplied. The transistor M 6  supplies current I 2  from the transistor M 2  to the second terminal of the transistor M 6  when the control signal CS 2  is supplied. In other words, when the control signal CS 2  turns on the fifth and sixth transistors M 5  and M 6 , then a predetermined current I 1  flows via the first transistor M 1  and a predetermined current I 2  flows via the transistor M 2 . The value of the current I 2  may correspond to the value of the current I 1 . 
       FIG. 4  illustrates control signals supplied to the current mirror circuit illustrated in  FIG. 3 . Referring to  FIG. 4 , the first control signal CS 1  and the control signal CS 2  are sequentially supplied to the current mirror circuit. Thus, in a first period where the control signal CS 1  is supplied, the threshold voltages of each of the transistors M 1  and M 2  are compensated for. In a second period where the control signal CS 2  is supplied, a desired current is supplied via the transistor M 6  to one or more external components. 
     An exemplary operation of a driving circuit of the invention will be described in detail with reference to  FIGS. 3 and 4 . First, the control signal CS 1  may be supplied from a control source that is coupled with the circuit. When the control signal CS 1  is supplied, the third and fourth transistors M 3  and M 4  turn on. 
     When the transistor M 3  turns on, the voltage of the node N 1  increases to the voltage of the voltage source VDD. When the transistor M 4  turns on, electric current flows through the transistor M 2  so that the transistor M 2  serves as a diode. Then, the voltage of the gate terminal of the transistor M 2  has a value equal to what may be obtained by subtracting the threshold voltage V th  of the transistor M 2  from the voltage of the voltage source VDD. In this case, the threshold voltage V th  of the transistor M 2  corresponds to the difference between the voltage of the node N 1  and the voltage of the gate terminal of the transistor M 2 . The threshold voltage V th  is stored in the capacitor C 2 . 
     Since current flows through the transistor M 1  so that the transistor M 1  serves as a diode, the voltage of the gate terminal of the transistor M 1  has a value equal to what may be obtained by subtracting the threshold voltage V th  of the transistor M 1  from the voltage V th  of the voltage source VDD during the period when the control signal CS 1  is supplied. Thus, the threshold voltage V th  of the transistor M 1  is stored in the capacitor C 1 . The threshold voltage V th  corresponds to the difference between the voltage of the node N 1  and the voltage of the gate terminal of the transistor M 1 . 
     That is, according to the present invention, the threshold voltage V th  of the transistor M 1  may be stored in the capacitor C 1  and the threshold voltage V th  of the transistor M 2  may be stored in the capacitor C 2  in the period when the control signal CS 1  is supplied. 
     Then, the control signal CS 2  is supplied so that the fifth and sixth transistors M 5  and M 6  turn on. When the transistor M 5  turns on, the first current I 1  flows from the voltage source VDD via the first and fifth transistors M 1  and M 5 . 
     When the transistor M 6  turns on, the second current I 2 , which may have a value that corresponds to a value of the first current I 1 , is supplied to one or more external components. When a ratio of a channel width and a channel length (W/L) of the transistor M 1  equals a ratio of a channel width and a channel length (W/L) of the transistor M 2 , the first current I 1  and the second current I 2  are set as the same current. That is, according to the invention, it is possible to generate a desired second current I 2  whose value corresponds to the value of the first current I 1  regardless of the threshold voltages of each of the transistors M 1  and M 2 . That is, according to the invention, since the threshold voltages of each of the transistors M 1  and M 2  are stored in the first and second capacitors C 1  and C 2 , respectively, during the period when the control signal CS 1  is supplied, it is possible to generate a desired current I 2  whether or not the value of the threshold voltage of the transistor M 1  is the same or different than the value of the threshold voltage of the transistor M 2 . 
     Additionally, as illustrated in  FIG. 5 , the current mirror circuit of the invention may further include a control signal generator  20  for generating the first and second control signals CS 1  and CS 2 . The control signal generator  20  may sequentially generate the first control signal CS 1  and the second and control signal CS 2  that are illustrated in  FIG. 4 . 
       FIG. 6  illustrates a current mirror circuit constructed according to another embodiment of the invention. In  FIG. 6 , description of the same structure as the structure of  FIG. 3  will be omitted. 
     Referring to  FIG. 6 , the current mirror circuit depicted therein further includes a current controller  30 , which was not shown in  FIG. 3 . Thus, the structure and operation of the current mirror circuit are identical to the structure and operation of the current mirror circuit illustrated in  FIG. 3  except that the current mirror circuit of  FIG. 6  further includes the current controller  30 . 
     The current controller  30  is electrically connected to the transistor M 5 . The current controller  30  controls the amount of current supplied from the transistor M 5  to the current controller  30 . When the amount of current is controlled by the current controller  30 , the amount of current I 2  supplied to one or more external components via the transistor M 6  is also controlled. That is, the current controller  30  according to the present invention controls the current I 1  that flows via the transistor M 5  to control the amount of current I 2  output via the transistor M 6 . 
       FIG. 7  illustrates a first embodiment of a driving circuit that includes the current mirror circuit according to the invention. Referring to  FIG. 7 , a driving circuit according to one embodiment of the invention includes a current mirror circuit (a bias unit) and a driver  40 . The driver  40  receives the second current I 2  from the current mirror circuit to be driven. The driver  40  may be an amplifier or a buffer to be driven when the second current I 2  is supplied. Because the current mirror circuit supplies a desired second current I 2  regardless of the threshold voltages of each of the transistors M 1  and M 2 , the driver  40  can be stably driven. 
       FIG. 8  is a circuit diagram illustrating an example of the driver illustrated in  FIG. 7 . Referring to  FIG. 8 , the driver  40  includes transistors M 21  and M 22  that receive the second current I 2 , a transistor M 23  connected between the transistor M 21  and a ground potential GND, and a transistor M 24  connected between the transistor M 22  and the ground potential GND. The 24 th  transistor M 24  is further connected to the transistor M 23  to form (another) current mirror circuit. 
     In use, transistor M 21  supplies part of the second current I 2  to the transistor M 23 . Because the first terminal of the transistor M 23  is electrically connected to the gate terminal of transistor M 23 , the voltage applied to the first terminal of the transistor M 23  corresponds to the voltage supplied to the gate terminal thereof. Here, the gate terminal of the transistor M 21  receives voltage from the second terminal (that is, an 11 th  node N 11 ) of the transistor M 22 . The transistor M 22  supplies the remaining current of the second current I 2  to the transistor M 24 , when a voltage is supplied to the gate terminal of the transistor M 22 . 
     Current flows through the transistor M 23  so that the transistor M 23  serves as a diode and so that the current supplied from the transistor M 21  is supplied to the ground potential GND. The transistor M 24  is electrically connected to the transistor M 23  by a current mirror such that the transistor M 24  supplies the same value of current the transistor M 23  to the ground potential GND. 
     The driver  40  further includes a transistor M 25  provided between an input terminal and the transistor M 22 . The transistor M 25  may be controlled by a third control signal CS 3 . The driver  40  may also include a transistor electrically connected between the input terminal and the gate terminal of the transistor M 21  to be controlled by a fourth control signal CS 4 . The driver  40  may further include a transistor M 27  electrically connected between the transistor M 26  and the gate terminal of the transistor M 21 . The transistor M 27  may be controlled by the control signal CS 3 . The driver may also include a transistor M 28  electrically connected between the node N 11  and an output terminal. The transistor M 28  may be controlled by the control signal CS 4 . Finally, the driver  40  may include a capacitor C electrically connected between the transistor M 25  and the transistor M 26 . 
     When the control signal CS 3  is supplied, the transistor M 25  supplies to the gate terminal of the transistor M 22  and to one side of the capacitor C, a predetermined voltage provided via the input terminal. When the control signal CS 3  is supplied, the transistor M 27  electrically connects the gate terminal of the transistor M 21  and the other side of the capacitor C to each other. When the control signal CS 4  is supplied, the transistor M 26  supplies a predetermined voltage provided from the input terminal to the other side of the capacitor C. Additionally, the transistor M 28  supplies the voltage applied to the node N 11  to the output terminal. 
     Operation of the first embodiment of the driving circuit of the invention will be described in detail with reference to  FIGS. 8 and 9 . First, the control signal CS 3  is supplied from a control source. When the control signal CS 3  is supplied, the transistors M 25  and M 27  turn on. When the transistor M 25  turns on, the input voltage supplied from the input terminal flows to the gate terminal of the transistor M 22 . In turn, the transistor M 22  supplies, to the node N 11 , a current that corresponds to an input voltage applied to the gate terminal of the transistor M 22 . At this time, a predetermined voltage is applied to the node N 11  and the voltage is supplied to the gate terminal of the transistor M 21 . Then, the transistor M 21  supplies, to the transistor M 23 , a current that corresponds to an input voltage supplied to the gate terminal of the transistor M 21 . 
     For example, when the voltage of the gate terminal of the transistor M 22  is lower than the voltage of the gate terminal of the transistor M 21 , the current that flows from the transistor M 21  is set to be higher than the current that flows from the transistor M 22 . That is, the transistor M 21  can supply a current that amounts to about ⅔ of the second current I 2  to the transistor M 23 , and the transistor M 22  can supply the current that amounts to about ⅓ of the second current I 2  to the transistor M 24 . 
     At this time, the transistor M 23  supplies the current that amounts to about ⅔ of the second current I 2  to the ground potential GND. The transistor M 24 , electrically connected to the transistor M 23  by a current mirror circuit, also supplies the current that amounts to about ⅔ of the second current I 2  to the ground potential GND. Therefore, the node N 11  receives both the current that amounts to about ⅓ of the second current I 2  from the transistor M 22  and the current that amounts to about ⅓ of the second current I 2  from the gate terminal of the transistor M 21 . As a result, the voltage of the node N 11  increases. Actually, the voltage of the node N 11  increases until the value of the current that flows from the transistor M 21  equals the value of the current that flows from the transistor M 22 . That is, the voltage of the node N 11  increases to equal the input voltage supplied to the gate terminal of the transistor M 21 . 
     On the other hand, when the transistor M 27  turns on in response to the control signal CS 3 , the voltage of the node N 11  is supplied to the other side of the capacitor C. Then, the capacitor C stores a voltage having a value that corresponds to the difference between the input voltage and the voltage applied to the node N 11 . 
     Then, the control signal CS 4  is supplied. When the control signal CS 4  is supplied, the transistors M 26  and M 28  turn on. When the transistor M 26  turns on, the input voltage is supplied to the other side of the capacitor C. Then, a voltage that corresponds to the sum of the voltage stored in the capacitor C and the input voltage is supplied to the gate terminal of the transistor M 22 . 
     Then, a predetermined voltage is applied to the node N 11  and the (predetermined) voltage is supplied to the gate terminal of the transistor M 21  and the output terminal. By way of example, the node N 11  may have a voltage equal to the input voltage, and this voltage may be supplied to the output terminal so that the driver  40  is driven as a buffer. 
     As described above, the driver  40  included in the driving circuit of the invention can be used in various circuits. For example, the driving circuit of the invention can be used for a flat panel display such as a liquid crystal display and a light emitting display. For example, the driving circuit of the present invention can be used for either a buffer or an amplifier that is included in a light emitting display. 
       FIG. 10  illustrates a second embodiment of a driving circuit having the current mirror circuit of the invention. Referring to  FIG. 10 , the driving circuit according to the second embodiment of the invention includes a bias unit  50  and a driver  60 . 
     The bias unit  50  may include a current mirror circuit and an output unit  54  connected to the current mirror circuit. Here, when the two second currents I 2  are supplied to the driver  60 , one output unit  54  is included in the bias unit  50 . When three second currents I 2  are supplied to the driver  60 , the two output units  54  are included in the bias unit  50 . 
     The output unit  54  includes a seventh transistor M 7  connected to the transistor M 1  by a current mirror circuit, a compensator  56  for compensating for the threshold voltage of the transistor M 7 , and a ninth transistor M 9 . The transistor M 7  may electrically connect to the transistor M 1  to form a current mirror circuit. In such a configuration, the gate terminal of the transistor M 7  may electrically connect to the gate terminal of the transistor M 1 . Additionally, the first terminal of the transistor M 7  may electrically connect to the voltage source VDD. 
     The transistor M 9  may electrically connect between the second terminal of the transistor M 7  and the driver  60 . The transistor M 9  turns on when the control signal CS 2  is supplied and electrically connects the second terminal of the transistor M 7  with the driver  60 . 
     The compensator  56  compensates for the threshold voltage of the transistor M 7 , and thus includes an eighth transistor M 8 , a tenth transistor M 10 , an eleventh transistor M 11  and a third capacitor C 3 . 
     The capacitor C 3  may electrically connect between the gate terminal of the transistor M 7  and the node N 1 . The capacitor C 3  stores a voltage that corresponds to the threshold voltage of the transistor M 7 . 
     The transistor M 8  may be electrically connected between the gate terminal and the second terminal of the transistor M 7 . The transistor M 8  turns on when the control signal CS 1  is supplied so that electric current flows through the transistor M 7  and so that the transistor M 7  also serves as a diode. 
     The transistor M 10  may electrically connect between one side of the capacitor C 3  and the node N 1 , that is, between the second node N 2  and the voltage source VDD. The transistor M 10  turns on when the control signal CS 1  is supplied from a control source to supply voltage from the voltage source VDD to the node N 2 . 
     The transistor M 11  may electrically connect between the node N 1  and the node N 2 . The transistor M 11  electrically connects the node N 1  and the node N 2  to each other when the control signal CS 2  is supplied. 
     Operations of the second embodiment of the driving circuit of the invention will be described in detail with reference to  FIGS. 4 and 10 . First, the control signal CS 1  is supplied to turn on the eighth and tenth transistors M 8  and M 10 . When the transistor M 10  turns on, the voltage of the node N 2  rises to the voltage of the voltage source VDD. When the transistor M 8  turns on, electric current flows through the transistor M 7  so that the transistor M 7  serves as diode. When electric current flows through the transistor M 7  so that the transistor M 7  serves as a diode, the voltage of the gate terminal of the transistor M 7  has a value equal to what may be obtained by subtracting the threshold voltage V th  of the transistor M 7  from the voltage of the voltage source VDD. At this time, a voltage that corresponds to the threshold voltage V th  of the transistor M 7  is stored in the capacitor C 3 . 
     Then, the control signal CS 2  is supplied to turn on the ninth and eleventh transistors M 9  and M 11 . The first current I 1  flows via the transistor M 1  in the period when the control signal CS 2  is supplied. When the transistor M 9  turns on, it electrically connects the driver  60  and the transistor M 7  to each other. When the transistor M 11  turns on, it electrically connects the first and second nodes N 1  and N 2  to each other. Since the transistor M 7  is electrically connected to the transistor M 1  by a current mirror, the transistor M 7  also supplies the second current I 2  to the driver  60  to drive the driver  60 . The value of the current I 2  may correspond to the value of the current I 1 . 
     According to an embodiment of the invention, the tenth and eleventh transistors M 10  and M 11  included in the compensator  56  may be removed. If the transistors M 10  and M 11  are removed, the voltage corresponding to the threshold voltage of the transistor M 7  is stably stored in the capacitor C 3 . 
     Although several embodiments of the invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made to the exemplary embodiments described herein without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.