Abstract:
A pixel for displaying an image with uniform brightness is provided. The pixel includes an organic light emitting diode (OLED) that is driven by a pixel circuit. The pixel circuit is coupled to a data line, two scan lines, and an emission control line of a display device. The pixel is provided with power from external power supply sources and an initialization voltage source. The pixel circuit includes transistors and a storage capacitor that maintains a voltage at a gate of a driving transistor masking any variation between the threshold voltages of the driving transistors used in various pixels. An alternative embodiment, modifies a leakage path from the gate of the driving transistor to the initialization voltage source. Substantial impact of the leakage is shifted from the gate to drain of the driving transistor. As a result, a substantially uniform brightness is maintained in each pixel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2005-0107199, filed on Nov. 9, 2005, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a pixel and an organic light emitting display device using the same, and more particularly, to a pixel for displaying an image with uniform brightness and an organic light emitting display device using the same. 
     2. Discussion of Related Art 
       FIG. 1  is a circuit diagram illustrating a pixel of a conventional organic light emitting display device. The pixel  4  of the conventional organic light emitting display device includes a pixel circuit  2  coupled to an organic light emitting diode (OLED), a data line Dm, and a scan line Sn. The pixel circuit  2  controls the OLED. A first power source ELVDD and a second power source ELVSS are coupled to the pixel  4 . 
     An anode electrode of the OLED is coupled to the pixel circuit  2  and a cathode electrode of the OLED is coupled to the second power source ELVSS. The OLED generates light with brightness corresponding to the current supplied by the pixel circuit  2 . 
     The pixel circuit  2  controls the amount of current supplied to the OLED in response to a data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn. In order to perform this operation, the pixel circuit  2  includes a first transistor M 1 , a second transistor M 2 , and a storage capacitor Cst. The second transistor M 2  is coupled between the first power source ELVDD and the OLED. The first transistor M 1  is coupled to the second transistor M 2 , the data line Dm, and the scan line Sn. The storage capacitor Cst is coupled between a gate electrode and a first electrode of the second transistor M 2 . 
     A gate electrode of the first transistor M 1  is coupled to the scan line Sn and a first terminal of the first transistor M 1  is coupled to the data line Dm. A second electrode of the first transistor M 1  is coupled to one terminal of the storage capacitor Cst. One of the electrodes of each of the first and second transistors M 1 , M 2  is set as a source electrode and the other electrode is set as a drain electrode. For example, when the first electrode is set as the source electrode, the second electrode is set as the drain electrode. When the scan signal is supplied by the scan line Sn, the first transistor M 1  is turned on to supply the data signal supplied by the data line Dm to the storage capacitor Cst. As a result, a voltage corresponding to the data signal is charged in the storage capacitor Cst. 
     The gate electrode of the second transistor M 2  is coupled to one terminal of the storage capacitor Cst and the first electrode of the second transistor M 2  is coupled to the other terminal of the storage capacitor Cst and the first power source ELVDD. The second electrode of the second transistor M 2  is coupled to the anode electrode of the OLED. The second transistor M 2  controls the amount of current that flows from the first power source ELVDD to the OLED to correspond to the voltage value stored in the storage capacitor Cst. The OLED generates light with the brightness corresponding to the amount of current supplied by the second transistor M 2 . 
     However, according to the above-described conventional pixel  4 , it may not be possible to display an image with uniform brightness. To be specific, the threshold voltages of the second transistors M 2  included in different pixels  4  vary due to deviations introduced during the fabrication processes. When the threshold voltages of the second transistors M 2  are not uniform, although data signals corresponding to the same gray level are supplied to a number of pixels  4 , light components with different brightness are generated by the OLEDs of each pixel  4 . The difference in brightness is due to the difference between the threshold voltages of the second transistors M 2  of each pixel. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a pixel for displaying an image with uniform brightness and a light emitting display device using the same. 
     One embodiment provides a pixel comprising an organic light emitting diode (OLED) that is driven by a first transistor. A second transistor has a first electrode that is coupled to a data line and a gate coupled to a first scan line. The second transistor is to be turned on when a first scan signal is supplied to the first scan line. A storage capacitor has a first terminal is coupled to a second electrode of the second transistor. The first transistor is coupled to a second terminal of the storage capacitor to supply current corresponding to a value of the voltage applied to the second terminal of the storage capacitor from a first power source to a second power source through the OLED. A third transistor is coupled between the second terminal of the storage capacitor and the second electrode of the first transistor and is turned on when the first scan signal is being supplied. A fourth transistor is coupled between the second electrode of the first transistor and an initialization power source and is turned on when a second scan signal is being supplied to a second scan line. A fifth transistor is coupled between the first terminal of the storage capacitor and the initialization power source and is turned on while an emission control signal is not being supplied to an emission control line. The transistors may be of different conductivity types. The voltages of the first and second scan signal and the emission control signal vary depending on the conductivity type of the transistors used in the pixel. 
     Another embodiment provides an organic light emitting display device including a scan driving part supplying first scan signals to first scan lines, supplying second scan signals to second scan lines, and supplying emission control signals to emission control lines, a data driving part supplying data signals to data lines, and a display region including a pixel or a plurality of pixels coupled to the first scan lines, the second scan lines, and the data lines. Each of the pixels includes an OLED that is driven by a first transistor. A second transistor is coupled to a data line and a first scan line and is turned on when a first scan signal is supplied to the first scan line. A storage capacitor having a first terminal is coupled to a second electrode of the second transistor. The first transistor is coupled to a second terminal of the storage capacitor and supplies a current from a first power source to a second power source through the OLED. The current provided by the first transistor corresponds to a value of a voltage applied to the second terminal of the storage capacitor. A third transistor is coupled between the second terminal of the storage capacitor and the second electrode of the first transistor and is turned on when the first scan signal is being supplied. A fourth transistor is coupled between the second electrode of the first transistor and an initialization power source and is turned on when a second scan signal is being supplied to a second scan line. A fifth transistor is coupled between the first terminal of the storage capacitor and the initialization power source and is turned on while an emission control signal is not being supplied to an emission control line. In this embodiment, also, the transistors used may be of different conductivity types. Therefore, scan and emission control signals of appropriate voltage are applied to turn on or turn off each transistor based on its conductivity type. 
     In an organic light emitting display device including a plurality of pixels, the first scan signal, the second scan signal, and the emission control signal may be each applied in a sequential manner to their respective scan lines or to the emission control lines. In another embodiment, the first scan signal and the second scan signal may be two successive scan signals being applied to two adjacent scan lines as a part of a sequential application of the scan signal to the scan lines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic circuit diagram illustrating a conventional pixel. 
         FIG. 2  schematically illustrates an organic light emitting display device according to a first embodiment of the present invention. 
         FIG. 3  is a schematic circuit diagram illustrating a first embodiment of a pixel according to the present invention. 
         FIG. 4  schematically illustrates waveforms for describing a method of driving the pixel of  FIG. 3 . 
         FIG. 5  schematically illustrates an organic light emitting display device according to a second embodiment of the present invention. 
         FIG. 6  is a schematic circuit diagram illustrating a second embodiment of a pixel according to the present invention. 
         FIG. 7  schematically illustrates waveforms for describing a method of driving the pixel of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  schematically illustrates an organic light emitting display device according to a first embodiment of the present invention. 
     The organic light emitting display device according to the first embodiment of the present invention includes a scan driving part  110  for driving scan lines S 1  to Sn and emission control lines E 1  to En, a data driving part  120  for driving data lines D 1  to Dm, a display region  130  including pixels  140  formed in the regions partitioned by the scan lines S 1  to Sn and the data lines D 1  to Dm, and a timing controller  150  for controlling the scan driving part  110  and the data driving part  120 . 
     The timing controller  150  receives data Data and synchronizing signals (not shown) from outside of the display device. The timing controller  150  generates data driving control signals DCS and scan driving control signals SCS corresponding to the synchronizing signals supplied from outside. The data driving control signals DCS generated by the timing controller  150  are supplied to the data driving part  120  and the scan driving control signals SCS generated by the timing controller  150  are supplied to the scan driving part  110 . The timing controller  150  supplies the data Data supplied from the outside to the data driving part  120 . 
     The scan driving part  110  receives the scan driving control signals SCS from the timing controller  150 . The scan driving part  110  that has received the scan driving control signals SCS, generates scan signals to be supplied to the scan lines S 1  to Sn. Also, in response to the scan driving control signals SCS, the scan driving part  110  generates emission control signals to be supplied to the emission control lines E 1  to En. The scan signals may be generated in a sequential manner. The width of the emission control signals is equal to or larger than the width of the scan signals. 
     The width of a signal may refer to the duration of a pulse of the signal. Some signals may have pulses that correspond to a voltage level below a reference level and other signals may have pulses corresponding to a voltage level above the reference level. For example, some signals may have positive pulses and other signals may have negative pulses. If the signals are being applied to gates of transistors for controlling the transistors, then negative pulses turn on PMOS transistors and positive pulses turn on NMOS transistors. Alternatively, if a signal includes positive pulses, then the positive pulses of the signal may be used to turn off a PMOS transistor. 
     The data driving part  120  receives the data driving control signals DCS from the timing controller  150 . The data driving part  120  that has received the data driving control signals DCS generates data signals to be supplied to the data lines D 1  to Dm in synchronization with the scan signals. 
     The display region  130  receives power from a first power source ELVDD and a second power source ELVSS and supplies the power to the pixels  140 . The pixels  140  that have received power from the first power source ELVDD and the second power source ELVSS generate light components corresponding to the data signals. The emission times, or duration of emission, of the pixels  140  are controlled by the emission control signals. 
       FIG. 3  is a schematic circuit diagram illustrating a first embodiment pixel according to the present invention. The first embodiment pixel  140  may be included in the display device of the first embodiment of the present invention that is shown  FIG. 2 . For convenience sake, a pixel  140  coupled to an mth data line Dm, an nth scan line Sn, an (n− 1 )th scan line Sn− 1 , and an nth emission control line En is illustrated in  FIG. 3 . 
     The pixel  140  includes a pixel circuit  142  that is coupled to the OLED, and also to the data line Dm, the scan lines Sn− 1  and Sn, and the emission control line En to control the amount of current supplied to the OLED. 
     An anode electrode of the OLED is coupled to the pixel circuit  142  and a cathode electrode of the OLED is coupled to the second power source ELVSS. The voltage value of the second power source ELVSS is set to be smaller than the voltage value of the first power source ELVDD. The OLED generates light with brightness corresponding to the amount of current supplied by the pixel circuit  142 . 
     The pixel circuit  142  controls the amount of current supplied to the OLED in response to the data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn. The pixel circuit  142  includes first to sixth transistors M 11 , M 12 , M 13 , M 14 , M 15 , M 16  and a storage capacitor C 1 st. 
     A first electrode of the second transistor M 12  is coupled to the data line Dm and a second electrode of the second transistor M 12  is coupled to a first node N 11 . A gate electrode of the second transistor M 12  is coupled to the nth scan line Sn. When the scan signal is supplied to the nth scan line Sn, the second transistor M 12  is turned on to supply the data signal supplied from the data line Dm to the first node N 11 . 
     A first electrode of the first transistor M 11  is coupled to the first node N 11  and a second electrode of the first transistor M 11  is coupled to a first electrode of the sixth transistor M 16 . A gate electrode of the first transistor M 11  is coupled to the storage capacitor C 1 st. The first transistor M 11  supplies the current corresponding to the voltage charged in the storage capacitor C 1 st to the OLED. 
     A first electrode of the third transistor M 13  is coupled to the second electrode of the first transistor M 11  and a second electrode of the third transistor M 13  is coupled to the gate electrode of the first transistor M 11 . A gate electrode of the third transistor M 13  is coupled to the nth scan line Sn. When the scan signal is supplied to the nth scan line Sn, the third transistor M 13  is turned on, the first transistor M 11  serves as a diode, and current flow is established through the first transistor M 11 . 
     A gate electrode of the fourth transistor M 14  is coupled to the (n− 1 )th scan line Sn− 1  and a first electrode of the fourth transistor M 14  is coupled to one terminal of the storage capacitor C 1 st and the gate electrode of the first transistor M 1 . A second electrode of the fourth transistor M 14  is coupled to an initialization power source Vint. When the scan signal is supplied to the (n− 1 )th scan line Sn− 1 , the fourth transistor M 14  is turned on to change the voltages of the terminal of the storage capacitor C 1 st coupled to the fourth transistor M 14  and the gate electrode of the first transistor M 11  to the voltage of the initialization power source Vint. 
     A first electrode of the fifth transistor M 15  is coupled to the first power source ELVDD and a second electrode of the fifth transistor M 15  is coupled to the first node N 11 . A gate electrode of the fifth transistor M 15  is coupled to the emission control line En. When the emission control signal is not being supplied by the emission control line En, the fifth transistor M 15  is turned on to electrically connect the first power source ELVDD and the first node N 11  to each other. 
     The first electrode of the sixth transistor M 16  is coupled to the second electrode of the first transistor M 11  and a second electrode of the sixth transistor M 16  is coupled to the anode electrode of the OLED. A gate electrode of the sixth transistor M 16  is coupled to the emission control line En. When the emission control signal is not being supplied, the sixth transistor M 16  is turned on to supply the current supplied by the first transistor M 11  to the OLED. 
     The operation of the pixel  140  will be described in detail with reference to waveforms of  FIG. 4 .  FIG. 4  shows the waveforms of the signals applied to the (n− 1 )th scan line Sn− 1 , the nth scan line Sn, and the nth emission control line En. First, a scan signal is supplied to the (n− 1 )th scan line Sn− 1  so that the fourth transistor M 14  is turned on. When the fourth transistor M 14  is turned on, the voltage of the initialization power source Vint is supplied to one terminal of the storage capacitor C 1 st and the gate terminal of the first transistor M 11 , that are both coupled to the first electrode of the fourth transistor M 14 . That is, when the fourth transistor M 14  is turned on, the voltages of one terminal of the storage capacitor C 1 st and the gate terminal of the first transistor M 11  are initialized to the voltage of the initialization power source Vint. For the exemplary embodiment shown in  FIG. 3 , the voltage value of the initialization power source Vint is set to be smaller than the voltage value of the data signal. 
     Then, the scan signal is supplied to the nth scan line Sn. When the scan signal is supplied to the nth scan line Sn, the second and third transistors M 12 , M 13  are turned on. When the third transistor M 13  is turned on, current flows through the first transistor M 11  so that the first transistor M 11  serves as a diode. When the second transistor M 12  is turned on, the data signal supplied to the data line Dm is supplied to the first node N 11  through the second transistor M 12 . At this time, because the voltage at the gate of the first transistor M 11  is initialized to the voltage of the initialization power source Vint and because the voltage of Vint is set to be lower than the voltage of the data signal supplied to the first node N 11 , the first transistor M 11  is turned on. 
     When the first transistor M 11  is turned on, the data signal applied to the first node N 11  is supplied to the terminal of the storage capacitor C 1 st, that is coupled to the gate of the first transistor M 11 , through the first and third transistors M 11 , M 13 . The data signal is supplied to the storage capacitor C 1 st through the first transistor M 11  which serves as a diode and through which current flows. Therefore, the voltage corresponding to the data signal and a threshold voltage of the first transistor M 11  is charged in the storage capacitor C 1 st. 
     After the voltage corresponding to the data signal and the threshold voltage of the first transistor M 11  is charged in the storage capacitor C 1 st, supply of the emission control signal is stopped so that the fifth and sixth transistors M 15 , M 16  are turned on. When the fifth and sixth transistors M 15 , M 16  are turned on, a current path from the first power source ELVDD to the OLED is formed. In this case, the first transistor M 11  controls the amount of current that flows from the first power source ELVDD to the OLED to correspond to the voltage charged in the storage capacitor C 1 st. 
     As described above, the voltage corresponding to the data signal and the threshold voltage of the first transistor M 11  is charged in the storage capacitor C 1 st included in the pixel  140 . The voltages charged in the storage capacitors C 1 st of different pixels  140  may be different because threshold voltages of the first transistors M 11  used in each pixel may be different from one another. However, the threshold voltage is included in the voltage charging the capacitor. As a result, it is possible to control the amount of current that flows to the OLED regardless of the threshold voltage of the first transistor M 11 . Therefore, various pixels  140  according to the first embodiment of the present invention can display an image with substantially uniform brightness regardless of the threshold voltages of the first transistors M 11  used in each of the pixels  140 . 
     However, in the pixel  140  according to the first embodiment of the present invention, undesired leakage current may originate from the gate terminal of the first transistor M 11 . To be specific, when the fourth transistor M 14  is off, the voltage of the gate electrode of the first transistor M 11  is different from the voltage of the initialization power source Vint. As described above, when the voltage of the gate electrode of the first transistor M 11  is different from the voltage of the initialization power source Vint, although the fourth transistor M 14  is turned off, a leakage current is generated that changes the voltage of the gate electrode of the first transistor M 11 . That is, in the pixel  140  illustrated in  FIG. 3 , the voltage of the gate electrode of the first transistor M 11  is changed by the leakage current through the fourth transistor M 14  so that an image with desired brightness is not displayed. 
       FIG. 5  illustrates an organic light emitting display device according to a second embodiment of the present invention. 
     The organic light emitting display device according to the second embodiment of the present invention includes a scan driving part  210 , a data driving part  220 , a display region  230 , and a timing controller  250 . The scan driving part  210  drives first scan lines S 11  to S 1   n , second scan lines S 21  to S 2   n , and emission control lines E 1  to En. The data driving part  220  drives data lines D 1  to Dm. The display region  230  includes pixels  240  formed in regions partitioned by the first scan lines S 11  to S 1   n , the second scan lines S 21  to S 2   n , and the data lines D 1  to Dm. The timing controller  250  controls the scan driving part  210  and the data driving part  220 . 
     The timing controller  250  generates data driving control signals DCS and scan driving control signals SCS in response to synchronizing signals supplied from the outside of the display device. The data driving control signals DCS generated by the timing controller  250  are supplied to the data driving part  220  and the scan driving control signals SCS generated by the timing controller  250  are supplied to the scan driving part  210 . The timing controller  250  supplies data Data supplied from the outside to the data driving part  220 . 
     The scan driving part  210  receives the scan driving control signals SCS from the timing controller  250 . The scan driving part  210  that has received the scan driving control signals SCS supplies a first scan signal to the first scan lines S 11  to S 1   n  and supplies a second scan signal to the second scan lines S 21  to S 2   n . The first scan signals may be supplied to the first scan lines S 11  to S 1   n  in a sequential manner. Similarly, the second scan signals may be supplied to the second scan lines S 21  to S 2   n  in a sequential manner. The first and second scan signals supplied to the same pixel  240  are supplied at substantially the same point in time and a width or duration of the first scan signal is set to be larger than a width of the second scan signal. Thus, the first scan signal lasts longer than the second scan signal. The scan driving part  210  generates emission control signals in response to the scan driving control signals SCS and supplies the generated emission control signals to the emission control lines E 1  to En. The emission control signals are supplied to overlap the first scan signals. Further, the width or duration of the emission control signal is set to be larger than the width of the first scan signal. 
     The data driving part  220  receives the data driving control signals DCS from the timing controller  250 . The data driving part  220 , that has received the data driving control signals DCS, generates data signals and supplies the generated data signals to the data lines D 1  to Dm in synchronization with the first and second scan signals. 
     The display region  230  receives power from a first power source ELVDD, a second power source ELVSS and an initialization power source Vint located outside the display region  230 . The display region  230  supplies the power from the first power source ELVDD, the second power source ELVSS, and the initialization power source Vint to the pixels  240 . The pixels  240  that have received power from the first power source ELVDD, the second power source ELVSS, and the initialization power source Vint, generate light components corresponding to the data signals. The emission times, including the time of commencing the emission and the duration of emission, of the pixels  240  are controlled by the emission control signals. 
       FIG. 6  is a circuit diagram illustrating a second embodiment of a pixel  240  according of the present invention. The second embodiment pixel  240  may be included in the display device of the second embodiment of the present invention shown in  FIG. 5 . For convenience sake, a pixel coupled to an mth data line Dm, an nth first scan line S 1   n , an nth second scan line S 2   n , and an nth emission control line En is illustrated in  FIG. 6 . 
     The pixel  240  according to the second embodiment of the present invention includes a pixel circuit  242  coupled to an OLED, the data line Dm, the first and second scan lines S 1   n , S 2   n , and the emission control line En to control the amount of current supplied to the OLED. 
     The anode electrode of the OLED is coupled to the pixel circuit  242  and the cathode electrode of the OLED is coupled to the second power source ELVSS. The voltage value of the second power source ELVSS is set to be smaller than the voltage value of the first power source ELVDD. The OLED generates light with brightness corresponding to the amount of current supplied by the pixel circuit  242 . 
     The pixel circuit  242  receives the data signal from the data line Dm when the scan signals are supplied to the first and second scan lines S 1   n  and S 2   n . The pixel circuit  242  controls the amount of current supplied to the OLED in response to the data signal. To provide a controlled current to the OLED, the pixel circuit  242  includes first to sixth transistors M 21 , M 22 , M 23 , M 24 , M 25 , M 26  and a storage capacitor C 2 st. 
     A first electrode of the second transistor M 22  is coupled to the data line Dm and a second electrode of the second transistor M 22  is coupled to a first node N 21 . A gate electrode of the second transistor M 22  is coupled to the first scan line S 1   n . The second transistor M 22  is turned on when the first scan signal is supplied to the first scan line S 1   n . When turned on, the second transistor M 22  supplies the data signal, that is supplied to the data line Dm, to the first node N 21 . 
     A first electrode of the first transistor M 21  is coupled to the first power source ELVDD and a second electrode of the first transistor M 21  is coupled to a first electrode of the sixth transistor M 26 . A gate electrode of the first transistor M 21  is coupled to a second node N 22 . The first transistor M 21  supplies the current corresponding to the voltage applied to the second node N 22  to the OLED. The current supplied by the first transistor M 21  to the OLED corresponds to and is controlled by the voltage at the second node N 22 . 
     A first electrode of the third transistor M 23  is coupled to the second electrode of the first transistor M 21  and a second electrode of the third transistor M 23  is coupled to the gate electrode of the first transistor M 21 . A gate electrode of the third transistor M 23  is coupled to the first scan line S 1   n . The third transistor M 23  is turned on when the first scan signal is supplied to the first scan line S 1   n . When the third transistor M 23  is turned on, the first transistor M 21  serves as a diode. 
     A first electrode of the fourth transistor M 24  is coupled to the second electrode of the first transistor M 21  and a second electrode of the fourth transistor M 24  is coupled to the initialization power source Vint. A gate electrode of the fourth transistor M 24  is coupled to the second scan line S 2   n . The fourth transistor M 24  is turned on when the second scan signal is supplied to the second scan line S 2   n.    
     A first electrode of the fifth transistor M 25  is coupled to the first node N 21  and a second electrode of the fifth transistor M 25  is coupled to the initialization power source Vint. A gate electrode of the fifth transistor M 25  is coupled to the emission control line En. In the exemplary embodiment shown, the fifth transistor M 25  is turned on when the emission control signal is not being supplied by the emission control line En. When turned on, the fifth transistor M 25  changes the voltage value of the first node N 21  to the voltage value of the initialization power source Vint. 
     The first electrode of the sixth transistor M 26  is coupled to the second electrode of the first transistor M 21  and a second electrode of the sixth transistor M 26  is coupled to the anode electrode of the OLED. A gate electrode of the sixth transistor M 26  is coupled to the emission control line En. In the exemplary embodiment shown, the sixth transistor M 26  is turned on when the emission control signal is not supplied. When turned on, the sixth transistor M 26  supplies the current supplied by the first transistor M 21  to the OLED. 
     The storage capacitor C 2 st is provided between the first node N 21  and the second node N 22  to be charged to a voltage established between these two nodes N 21 , N 22 . 
     The operations of the pixel  240  will be described in detail with reference to the waveforms of  FIG. 7 . Waveforms of  FIG. 7  include a second scan signal being applied to the second scan line S 2   n , a first scan signal being applied to the first scan line S 1   n , and an emission control signal being applied to the emission control line En. First, the emission control signal is supplied to the emission control line En during a first period T 1 . When the emission control signal is being supplied to the emission control line En, the fifth and sixth transistors M 25 , M 26  are turned off. 
     In the exemplary embodiments shown, the transistors are shown as PMOS transistors that are turned on by a negative gate to source voltage and turned off by a positive gate to source voltage. Also, in the exemplary embodiment shown, the emission control signal being supplied to the emission control line En is shown to be a positive signal. Accordingly, application of the positive signal to the emission control line turns off the PMOS transistors. In alternative embodiments, other types of transistors, for example NMOS transistors, may be used which are turned on and off by signals different from those shown. 
     In the embodiment shown, while the first scan signal is supplied during periods T 2  and T 3 , the second scan signal is supplied only during the period T 2 . In other words, the first and second scan signals of the second embodiment coincide partially in time during the period T 2 . After the fifth and sixth transistors M 25 , M 26  are turned off, the first scan signal is supplied to the first scan line S 1   n  and, at the same time, the second scan signal is supplied to the second scan line S 2   n . When the first scan signal is being supplied, the second and third transistors M 22 , M 23  are turned on. When the second scan signal is being supplied, the fourth transistor M 24  is turned on. When the second transistor M 22  is turned on, the data signal supplied to the data line Dm is supplied to the first node N 21 . When the third and fourth transistors M 23 , M 24  are turned on together, the voltage of the initialization power source Vint is supplied to the second node N 22 . In the exemplary embodiment shown, the voltage value of the initialization power source Vint is set to be smaller than the voltage value of the data signal. 
     Then, during a third period T 3 , supply of the second scan signal to the second scan line S 2   n  is stopped. As a result, the fourth transistor M 24  is turned off. At this time, because current flows through the third transistor M 21  so that the first transistor M 21  serves as a diode, the voltage value of the second node N 22  is obtained by subtracting the threshold voltage value of the first transistor M 21  from the voltage value of the first power source ELVDD. The storage capacitor C 2 st is charged to the voltage difference between the first node N 21  and the second node N 22 . 
     During a fourth period T 4 , supply of the first scan signal to the first scan line S 1   n  is stopped. Then, the second and third transistors M 22 , M 23  are turned off. 
     During a fifth period T 5 , supply of the emission control signal is stopped. Then, the fifth transistor M 25  and the sixth transistor M 26  are turned on. When the fifth transistor M 25  is turned on, the voltage value of the first node N 21  is reduced to the voltage value of the initialization power source Vint. That is, the voltage value of the first node N 21  is reduced from the voltage value of the data signal to the voltage value of the initialization power source Vint. In this case, because the third transistor M 23  is off and the second node N 22  is floating, the voltage value of the second node N 22  is reduced corresponding to the reduction in the voltage value of the first node N 21  in order to maintain the same voltage difference between the two nodes N 22 , N 21 . For example, when the voltage at the first node N 21  is reduced by the voltage value of the data signal, then the voltage value of the second node N 22  is also reduced by the voltage value of the data signal from its previous voltage value that was obtained by subtracting the threshold voltage value of the first transistor M 21  from the voltage value of the first power source ELVDD. 
     Then, the first transistor M 21  supplies current corresponding to the value of the voltage applied to the second node N 22  to the OLED through the sixth transistor M 26  during the fifth period T 5  so that light of controlled brightness is generated by the OLED. The first to fifth periods, T 1 , T 2 , T 3 , T 4 , T 5  are consecutive in the exemplary embodiment of  FIG. 7 . 
     In the pixel  240  according to the second embodiment of the present invention, the voltage value of the second node N 22  is initially set as the value obtained by subtracting the threshold voltage value of the first transistor M 21  from the voltage value of the first power source ELVDD. The voltage value of the second node N 22  is subsequently reduced from the initially set voltage value by the voltage value corresponding to the voltage value of the data signal. The second node N 22  is coupled to the gate of the first transistor M 21  and the voltage at the second node N 22  determines the amount of current supplied to the OLED by the first transistor M 21 . As a result, in the pixel  240  according to the second embodiment of the present invention, it is possible to control the amount of current that flows to the OLED regardless of the threshold voltage value of the first transistor M 21 . Therefore, the pixel  240  according to the second embodiment of the present invention can display an image with substantially uniform brightness regardless of the threshold voltage of the first transistor M 21 . 
     In the pixel  240  according to the second embodiment of the present invention, the fourth transistor M 24  that supplies the initialization power source Vint is coupled to the second electrode of the first transistor M 21 . Therefore, the leakage current through the fourth transistor M 24  is from the second electrode of the first transistor M 21 . As a result, leakage current does not flow from the second node N 22  that is the gate electrode of the first transistor M 21  to the initialization power source Vint so that it is possible to display an image with desired brightness. 
     As described above, in the pixel according to the embodiments of the present invention and the organic light emitting display device using the same, the amount of current that flows to the OLED is controlled regardless of the threshold voltage of the first transistor. Therefore, it is possible to display an image with uniform brightness. According to the present invention, because the fourth transistor for supplying the initialization power source is coupled to the second electrode of the first transistor, it is possible to reduce or prevent leakage current flowing from the gate electrode of the first transistor so that it is possible to display an image with desired brightness. 
     Although certain embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes might be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.