Patent Publication Number: US-7916102-B2

Title: Pixel and organic light emitting display device including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2009-0016733, filed on Feb. 27, 2009 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 of an organic light emitting display device, and an organic light emitting display device including the same. 
     2. Discussion of Related Art 
     Recently, various flat panel display devices having reduced weight and volume when compared to cathode ray tubes have been developed. Flat panel display devices include liquid crystal display devices, field emission display devices, plasma display panels, and organic light emitting display devices, among others. 
     Among these, the organic light emitting display device displays an image using organic light emitting diodes that generate light by the recombination of electrons and holes. Such an organic light emitting display device is driven with low power consumption and rapid response times. 
     Generally, the organic light emitting display device represents gray levels by controlling the amount of current flowing to the organic light emitting diodes using a driving transistor included in each of pixels. In this case, an image having non-uniform brightness may be displayed due to variations in threshold voltages of the driving transistors included in the pixels. 
     SUMMARY OF THE INVENTION 
     Therefore, exemplary embodiments of the present invention provide a pixel that can compensate for the threshold voltage of a driving transistor, and an organic light emitting display device including the same. 
     According to an exemplary embodiment of the present invention, there is provided a pixel including: an organic light emitting diode coupled between a first power supply and a second power supply, the first power supply having a higher voltage than the second power supply; a first transistor coupled between the organic light emitting diode and the second power supply, and for controlling a driving current that flows from the first power supply to the second power supply via the organic light emitting diode; a second transistor coupled to a first node to which a gate electrode of the first transistor is coupled, and for bringing the first node to a first voltage in accordance with a first scan signal; a first capacitor coupled between the first node and a second node; a third transistor coupled between the second node and a data line, and for supplying a data signal from the data line to the second node in accordance with the first scan signal; a fourth transistor coupled between the first node and the second node, and for coupling the first node to the second node in accordance with a second scan signal; a fifth transistor coupled between the first transistor and the second power supply, and for coupling the first transistor to the second power supply in accordance with an emission control signal; and a second capacitor coupled between the second node and a third node between the first transistor and the fifth transistor. 
     According to another exemplary embodiment of the present invention, there is provided an organic light emitting display device, including a display region including a plurality of pixels, wherein each of the plurality of pixels includes: an organic light emitting diode coupled between a first power supply and a second power supply, the first power supply having a higher voltage than the second power supply; a first transistor coupled between the organic light emitting diode and the second power supply, and for controlling a driving current that flows from the first power supply to the second power supply via the organic light emitting diode; a second transistor coupled to a first node to which a gate electrode of the first transistor is coupled, and for bringing the first node to a first voltage in accordance with a first scan signal; a first capacitor coupled between the first node and a second node; a third transistor coupled between the second node and a data line, and for supplying a data signal from the data line to the second node in accordance with the first scan signal; a fourth transistor coupled between the first node and the second node, and for coupling the first node to the second node in accordance with a second scan signal; a fifth transistor coupled between the first transistor and the second power supply, and for coupling the first transistor to the second power supply in accordance with an emission control signal; and a second capacitor coupled between the second node and a third node between the first transistor and the fifth transistor. 
     With the pixel and the organic light emitting display device including the same according to exemplary embodiments of the present invention, the pixel circuit includes relatively fewer transistors, compensates for the threshold voltage of the driving transistor, while improving the image quality and the power consumption of the organic light emitting display device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention. 
         FIG. 1  is a schematic block diagram of an organic light emitting display device according to an embodiment of the present invention; 
         FIG. 2  is a circuit diagram showing an embodiment of a pixel of  FIG. 1 ; 
         FIG. 3  is a waveform view showing waveforms of input signals input to the pixel of  FIG. 2 ; 
         FIG. 4  is a circuit diagram showing another embodiment of the pixel of  FIG. 1 ; 
         FIG. 5  is a circuit diagram showing another embodiment of the pixel of  FIG. 1 ; and 
         FIG. 6  is a circuit diagram showing another embodiment of the pixel of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, certain exemplary embodiments according to the present invention will be described with reference to the accompanying drawings. Here, when a first element is described as being coupled to a second element, the first element may be directly coupled to the second element, or may be indirectly coupled to the second element via one or more additional elements. Further, some of the elements that are not essential to the complete understanding of the invention are omitted for clarity. Also, like reference numerals refer to like elements throughout. 
     Hereinafter, exemplary embodiments of the present invention will be described in more detail with reference to the accompanying drawings. 
       FIG. 1  is a schematic block diagram of an organic light emitting display device according to an embodiment of the present invention. 
     Referring to  FIG. 1 , the organic light emitting display device according to the embodiment of the present invention includes a timing controller  10 , a scan driver  20 , a data driver  30 , and a display region  40 . 
     The timing controller  10  generates a scan driving control signal SCS and a data driving control signal DCS corresponding to synchronization signals supplied from the outside. The scan driving control signal SCS generated from the timing controller  10  is supplied to the scan driver, and the data driving control signal DCS is supplied to the data driver  30 . Also, the timing controller  10  supplies the data Data supplied from the outside to the data driver  30 . 
     The scan driver  20  generates scan signals and emission control signals corresponding to the scan driving control signals SCS supplied from the timing controller  10 , and supplies them to scan lines S 1  to Sn+1 and emission control lines E 1  to En. When the scan signals are supplied sequentially to the scan lines S 1  to Sn+1, pixels  50  are selected sequentially row by row. When the emission control signals are supplied to the emission control lines E 1  to En, emission of the pixels  50  is controlled. 
     In the embodiment of the present invention, the scan driver  20  sequentially supplies the scan signals through the scan lines S 1  to Sn+1 arranged in respective horizontal lines, and supplies the emission control signal having a first voltage level (for example, a high level) to an i th  (i is a natural number) emission control line Ei during the initial portion (e.g., a first portion) of the period when an i th  scan signal (first scan signal) is supplied to an i th  scan line Si and the period after the supply of an (i+1) th  scan signal (second scan signal) to an (i+1) th  scan line (Si+1) is completed. The emission control signal having a second voltage level (for example, a low level) is supplied to the i th  emission control line Ei during the remaining portion of the period when the i th  scan signal is supplied and the period when the (i+1) th  scan signal is supplied. 
     The data driver  30  generates data signals corresponding to the data driving control signals DCS and the data Data supplied from the timing controller  10 , and supplies them to data lines D 1  to Dm. In particular, when the pixels  50  on the i th  horizontal line are selected by the i th  scan signal, the data driver  30  supplies the data signals for the selected pixels to the corresponding data lines D 1  to Dm. 
     The display region  40  includes areas or regions where the scan lines S 1  to Sn+1, the emission control lines E 1  to En, and the data lines D 1  to Dm cross one another, and includes the plurality of pixels each including an organic light emitting diode. 
     The respective pixels  50  are coupled to the scan lines S 1  to Sn+1, the emission control lines E 1  to En, and the data lines D 1  to Dm, respectively horizontally or vertically, and receive the scan signals, the emission control signals, and the data signals therefrom, respectively. Such pixels emit light at brightnesses corresponding to the data signals. 
     Meanwhile, the pixels  50  are driven by receiving driving power, such as a high potential power ELVDD (hereinafter, referred to as a first power supply) and a low potential power ELVSS (hereinafter, referred to as a second power supply) from a power supply unit. Also, the pixels  50  may further additionally receive a reference power Vref, or other power sources, according to the circuit arrangement. 
     In the described embodiment of the present invention, each of the pixels  50  includes a plurality of N-type transistors that are coupled between the cathode electrode of the organic light emitting diode and the second power supply ELVSS, and supply current, adjusted corresponding to the threshold voltage of a corresponding driving transistor, to the organic light emitting diode. 
     When the pixel circuit includes the plurality of N-type transistors coupled between the cathode electrode of the organic light emitting diode and the second power supply ELVSS as described above, the first power supply ELVDD may be supplied to the display region, and the second power supply ELVSS may be supplied to the pixels  50  via power supply lines PL. 
     More detailed explanation on the arrangement and the operation of the pixels  50  as above will be described below. 
       FIG. 2  is a circuit diagram showing an embodiment of the pixel of  FIG. 1 . For convenience, a pixel positioned on the i th  horizontal line and coupled to the m th  data line Dm will be described. 
     Referring to  FIG. 2 , the pixel  50  according to the embodiment includes an organic light emitting diode OLED that generates light having brightness corresponding to driving current and a pixel circuit  52  that controls the driving current that flows to the organic light emitting diode OLED. 
     The organic light emitting diode OLED is coupled between the first power supply ELVDD and the second power supply ELVSS. The organic light emitting diode OLED as described above emits light at a brightness corresponding to a driving current controlled by the pixel circuit  52 . 
     To this end, the anode electrode of the organic light emitting diode OLED is coupled to the first power supply ELVDD, and the cathode electrode thereof is coupled to the second power supply ELVSS via the pixel circuit  52 . 
     The pixel circuit  52  is coupled between the organic light emitting diode OLED and the second power supply ELVSS. The pixel circuit controls the driving current, corresponding to a data signal, flowing to the organic light emitting diode OLED during the emission period of the pixel  50 . 
     To this end, the pixel circuit  52  includes first to fifth transistors M 1  to M 5  that are N-type transistors, and first and second capacitors C 1  and C 2 . In other embodiments, some or all of the transistors may instead be P-type transistors. Those skilled in the art will recognize that the pixel circuit would be accordingly changed if P-type transistors are implemented. 
     The first transistor M 1 , which is a driving transistor, is coupled between the organic light emitting diode OLED and the second power supply ELVSS, wherein a gate electrode of the first transistor M 1  is coupled to a first node N 1 . The first transistor M 1  controls the amount of current that flows to the second power supply ELVSS from the first power supply ELVDD via the organic light emitting diode OLED, corresponding to the voltage difference between the gate electrode and a source electrode of the first transistor M 1 , during the emission period. 
     The second transistor M 2  is coupled between the first node N 1  and a reference power supply Vref, wherein a gate electrode of the second transistor M 2  is coupled to a scan line Si (hereinafter, referred to as a first scan line). When a first scan signal (e.g., high level) is supplied to the first scan line Si, the second transistor M 2  brings the first node N 1  to a first voltage. In the present embodiment, the first voltage is set to the voltage of the reference power Vref. 
     The third transistor M 3  is coupled between the data line Dm and a second node N 2 , wherein a gate electrode of the third transistor M 3  is coupled to the first scan line Si. When the first scan signal is supplied to the first scan line Si, the third transistor M 3  supplies the data signal to the second node N 2 . 
     The fourth transistor M 4  is coupled between the first node N 1  and the second node N 2 , wherein a gate electrode of the fourth transistor M 4  is coupled to a next scan line Si+1 (hereinafter, referred to as a second scan line). When a second scan signal (e.g., high level) is supplied to the second scan line Si+1, the fourth transistor M 4  couples the first node N 1  to the second node N 2 . 
     The fifth transistor M 5  is coupled between the first transistor M 1  and the second power supply ELVSS, wherein a gate electrode of the fifth transistor M 5  is coupled to the emission control line Ei. The fifth transistor M 5  couples the first transistor M 1  to the second power supply ELVSS corresponding to the emission control signal supplied to the emission control line Ei. 
     The first capacitor C 1  is coupled between the first node N 1  and the second node N 2 , and the second capacitor C 2  is coupled between the second node N 2  and a third node N 3  that is a node between the first and fifth transistors M 1  and M 5 . 
       FIG. 3  is a waveform view showing waveforms of input signals input to the pixel of  FIG. 2 . 
     Referring to  FIG. 3 , the first scan signal and the second scan signal are supplied sequentially through the first scan line Si and the second scan line Si+1. Here, the first scan signal is the i th  scan signal that is supplied to the i th  scan line Si arranged on an i th  horizontal line coupled to the pixel. The second scan signal is the (i+1) th  scan signal that is supplied following the first scan signal through the (i+1) th  scan line. 
     The emission control signal supplied to the emission control line Ei maintains a first voltage level (e.g., a high level) during an initial portion t 1  (a first period) of the period when the first scan signal is supplied, maintains a second voltage level (e.g., a low level) during a remaining portion t 2  (a second period) of the period when the first scan signal is supplied and the period t 3  (a third period) when the second signal is supplied, and maintains the first voltage level during the emission period t 4  (a fourth period) after the supply of the second scan signal is completed or stopped. 
     Here, the first voltage level is set to a voltage level where the corresponding transistor is turned on, for example, a high level. The second voltage level is set to a voltage level where the corresponding transistor is turned off, for example, a low level. 
     Hereinafter, the method of driving the pixel of  FIG. 2  will be described in detail by applying the waveforms of the input signals of  FIG. 3  to the pixel of  FIG. 2 . 
     First, during the first period, the second and third transistors M 2  and M 3  are turned on by the first scan signal, and the fifth transistor M 5  is kept on by the emission control signal having a high level. 
     When the second transistor M 2  is turned on, the voltage of the reference power Vref is supplied to the first node N 1 . At this time, the voltage of the reference power Vref is set at a voltage that can turn on the first transistor M 1 , that is, a voltage that is higher than the voltage of the second power supply ELVSS by at least the threshold voltage of the first transistor M 1 . Also, the voltage of the reference power Vref may be set at a voltage that is below or lower than the voltage of the first power supply ELVDD. 
     The fifth transistor M 5  also maintains the turn-on state during the first period t 1  as above, such that the voltage of the source electrode of the first transistor M 1  is initialized to a low voltage by the second power ELVSS. 
     Meanwhile, when the third transistor M 3  is turned on, a voltage of the data signal is transferred to the second node N 2 . 
     Thereafter, during the second period t 2 , the first scan signal maintains the high level, and the voltage level of the emission control signal is dropped to a low level, such that the fifth transistor M 5  is turned off. 
     The first transistor M 1  maintains the turn-on state during the initial period of the second period t 2  as above, but is turned off when a voltage difference between the gate electrode and the source electrode thereof becomes the threshold voltage. 
     In other words, during the second period t 2 , the voltage of the third node N 3  is charged with the voltage obtained by subtracting the threshold voltage of the first transistor M 1  from the voltage of the reference power Vref, as shown in the following equation 1.
 
 V ( N 3)= V ref− V th( M 1)  [Equation 1]
 
     In equation 1, V(N 3 ) represents the voltage at the third node N 3 , Vref represents the voltage of the reference power supply, and Vth(M 1 ) represents the threshold voltage of the first transistor M 1 . 
     At this time, the second node N 2  is charged with the voltage of the data signal so that the second capacitor C 2  is charged with the voltage as shown in the following equation 2.
 
 V ( C 2)= V data−( V ref− V th( M 1))= V data− V ref+ V th( M 1)  [Equation 2]
 
     In equation 2, V(C 2 ) represents the voltage stored in the second capacitor C 2 , and Vdata represents the voltage of the data signal. 
     Thereafter, during the third period t 3 , the fourth transistor M 4  is turned on by the second scan signal having a high level. 
     If the fourth transistor M 4  is turned on, the first node N 1  is coupled to the second node N 2  so that the voltage stored in the first capacitor C 1  is set to 0 and the voltage between the gate electrode and source electrode of the first transistor M 1  is set to the voltage charged in the second capacitor C 2  as shown in the following equation 3.
 
 V gs( M 1)= V data− V ref+ V th( M 1)  [Equation 3]
 
     In equation 3, Vgs (M 1 ) represents the voltage between the gate electrode and source electrode of the first transistor M 1 . 
     Thereafter, during the fourth period t 4 , the voltage level of the emission control signal is raised to the high level, such that the fifth transistor M 5  is turned on. 
     Then, the current path of the driving current that flows to the second power supply ELVSS from the first power supply ELVDD via the organic light emitting diode OLED and the first and fifth transistors M 1  and M 5  is formed. 
     At this time, the driving current that flows through the organic light emitting diode OLED is determined by the voltage Vgs (M 1 ) between the gate electrode and source electrode of the first transistor M 1 , as shown in the following equation 4.
 
 I   OLED =β( V gs( M 1)− V th( M 1)) 2 =β(( V data− V ref+ V th( M 1))− V th( M 1)) 2 =β( V data− V ref) 2   [Equation 4]
 
     In equation 4, I OLED  represents the driving current that flows through the organic light emitting diode OLED, and β represents a constant value related to a process transconductance parameter μ n C ox  and an aspect ratio W/L of the first transistor M 1 . 
     Referring to equation 4, the driving current that flows through the organic light emitting diode OLED is determined by the difference between the voltage of the data signal and the voltage of the reference power Vref. Here, the voltage of the reference power Vref is a fixed voltage such that the driving current is determined to be uniform for the data signal corresponding to each brightness level, irrespective of variations in the threshold voltages of the first transistors M 1 . 
     With the pixel  50  as described above, relatively fewer transistors are utilized to compensate for the threshold voltage of the driving transistor (first transistor M 1 ), for displaying a uniform image irrespective of the variations of the threshold voltage of the driving transistor and to improve image quality. Also, the pixel  50  determines the driving current corresponding to the voltage of the reference power Vref rather than the voltage of the first or second power supply ELVDD or ELVSS, such that the image quality may not be non-uniform due to a voltage drop from the first and second power supplies ELVDD and ELVSS across the display device. 
     Also, with the pixel  50  as described above, the source electrode of the first transistor M 1  can be effectively initialized by controlling the timing of the emission control signal. While a voltage that can offset the threshold voltage of the first transistor M 1  is stored between the gate electrode and the source electrode of the first transistor M 1 , the emission of the pixel  50  can be controlled by, for example, preventing or reducing the emission of the pixel  50  and/or controlling the duration of the emission period, etc. Therefore, power consumption can be improved and furthermore, a blurring phenomenon where a screen is blurred can be prevented or reduced. 
       FIG. 4  is a circuit diagram showing another embodiment of a pixel of  FIG. 1 . When explaining  FIG. 4 , the same features as features described in the embodiment of  FIG. 2  will be provided with the same reference numerals, and a detailed description thereof will be omitted. 
     Referring to  FIG. 4 , a pixel  50 ′ according to the present embodiment is coupled between the reference power supply Vref and the cathode electrode of the organic light emitting diode OLED, and further includes a sixth transistor M 6  whose gate electrode is coupled to the first scan line Si. Here, the sixth transistor M 6  may be implemented using the same type transistor as that in the first to fifth transistors M 1  to M 5 , for example, an N-type transistor. 
     When the first scan signal is supplied, the sixth transistor M 6  transfers the voltage form the reference power supply Vref (e.g., a first voltage) to the cathode electrode of the organic light emitting diode OLED. 
     The pixel  50 ′ according to the present embodiment, which further includes the sixth transistor M 6 , prevents or reduces overcurrent from abruptly flowing to the organic light emitting diode OLED during the period when the first scan signal is supplied (in particular, the first period t 1  in  FIG. 3 ). The pixel  50 ′ may control the emission of the organic light emitting diode OLED by controlling the voltage of the reference power Vref. 
     In particular, if the difference between the voltage of the reference power Vref and the voltage of the first power ELVDD is set below the threshold voltage of the organic light emitting diode OLED, where the voltage of the reference power Vref falls within the voltage range set in the embodiment of  FIG. 2 , the emission of the organic light emitting diode OLED can be effectively prevented or minimized during the period when the first scan signal is supplied. 
       FIG. 5  is a circuit diagram showing another embodiment of a pixel of  FIG. 1 . When explaining  FIG. 5 , the same features as features described in the embodiment of  FIG. 2  will be provided with the same reference numerals, and a detailed description thereof will be omitted. 
     Referring to  FIG. 5 , a second transistor M 2 ″ of a pixel  50 ″ is coupled between the cathode electrode of the organic light emitting diode OLED and the first node N 1 . 
     In other words, in the present embodiment, one electrode of the second transistor M 2 ″ is coupled to the first node N 1  and a gate electrode of the second transistor M 2 ″ is coupled to the first scan line Si in the same manner as described with respect to the embodiment of  FIG. 2 , but the other electrode of the second transistor M 2 ″ is coupled to the cathode electrode of the organic light emitting diode OLED rather than a reference power supply Vref. 
     When the first scan signal is supplied to the first scan line Si, the second transistor M 2 ″ as described above transfers a first voltage to the first node N 1 . Here, current flows via the organic light emitting diode OLED during the first period t 1  of  FIG. 3 , so that the first voltage is set to have a value corresponding to subtracting the threshold voltage of the organic light emitting diode OLED from the voltage of the first power ELVDD. 
     Hereinafter, the method of driving the pixel of  FIG. 5  will be described by applying the waveforms of the input signals of  FIG. 3  to the pixel of  FIG. 5 . 
     First, during the first period t 1 , the second and third transistors M 2 ″ and M 3  are turned on by the first scan signal having a high level, and the fifth transistor M 5  is kept on by the emission control signal having a high level. 
     When the fifth transistor M 5  is on, there occurs an abrupt flow of current from the first power supply ELVDD to the second power supply ELVSS, and the voltage of the source electrode of the first transistor M 1  is initialized to a low voltage by the second power supply ELVSS. 
     When the second transistor M 2 ″ is turned on, the voltage of the cathode electrode of the organic light emitting diode OLED is transferred to the first node N 1 . In other words, in the present embodiment, the first voltage transferred to the first node N 1  by the second transistor M 2 ″ is set to ELVDD−Vto, corresponding to subtracting the threshold voltage of the organic light emitting diode OLED (hereinafter, Vto) from the voltage of the first power ELVDD. 
     Meanwhile, when the third transistor M 3  is turned on, the voltage of the data signal is transferred to the second node N 2 . 
     Thereafter, during the second period t 2 , the first scan signal maintains a high level and the voltage level of the emission control signal is dropped to a low level, such that the fifth transistor M 5  is turned off. 
     During the initial portion of the second period t 2 , as above, the first transistor M 1  maintains a turn-on state and then is turned off when a voltage difference between the gate electrode and the source electrode becomes the threshold voltage. 
     In other words, during the second period t 2 , the voltage of the third node N 3  is brought to a voltage obtained by subtracting the threshold voltage Vth(M 1 ) of the first transistor M 1  from the voltage ELVDD−Vto from the first node N 1 , as shown in the following equation 5.
 
 V ( N 3)=(ELVDD− V to)− V th( M 1)  [Equation 5]
 
     At this time, the second node N 2  is charged with a voltage of the data signal so that the second capacitor C 2  is charged with the voltage as shown in the following equation 6.
 
 V ( C 2)= V data−((ELVDD− V to)− V th( M 1))= V data−ELVDD+ V to+ V th( M 1)  [Equation 6]
 
     Thereafter, during the third period t 3 , the fourth transistor M 4  is turned on by the second scan signal having a high level. 
     If the fourth transistor M 4  is turned on, the first node N 1  is coupled to the second node N 2  so that the voltage stored in the first capacitor C 1  is set to 0, and the voltage between the gate electrode and source electrode of the first transistor M 1  is set to the voltage charged in the second capacitor C 2  as shown in the following equation 7.
 
 V gs( M 1)= V data−ELVDD+ V to+ V th( M 1)  [Equation 7]
 
     Thereafter, during the fourth period t 4 , the voltage level of the emission control signal is raised to the high level, such that the fifth transistor M 5  is turned on. 
     Then, the current path of the driving current that flows to the second power supply ELVSS from the first power supply ELVDD via the organic light emitting diode OLED and the first and fifth transistors M 1  and M 5  is formed. 
     At this time, the driving current that flows to the organic light emitting diode OLED is determined by the voltage Vgs(M 1 ) between the gate electrode and source electrode of the first transistor M 1 , as shown in the following equation 8.
 
 I   OLED =β( V gs( M 1)− V th( M 1)) 2 =β(( V data−ELVDD+ V to+ V th( M 1))− V th( M 1)) 2 =β( V data−ELVDD+ V to) 2   [Equation 8]
 
     Referring to equation 8, the driving current that flows to the organic light emitting diode OLED is determined by the voltage of the data signal, the voltage of the first power supply ELVDD, and the threshold voltage of the organic light emitting diode. Here, the voltage of the first power supply ELVDD and the threshold voltage of the organic light emitting diode are fixed voltages such that the driving current is determined to be uniform for the data signal corresponding to each brightness level, irrespective of variations in the threshold voltages of the first transistors M 1 . 
     With the pixel  50 ″ as described above, relatively fewer transistors are utilized, and a separate reference power supply (Vref in the previously described embodiments) is not utilized for compensating for the threshold voltage of the driving transistor (first transistor M 1 ). Therefore, the pixel  50 ″ displays a uniform image, irrespective of variations in the threshold voltages of the driving transistors of the pixels, and thereby improving image quality. 
     Also, with the pixel  50 ″ as described above, the emission of the pixel  50 ″ is easily controlled in a similar manner as with the pixels  50  and  50 ′ in the previously described embodiments, thereby improving power consumption and further preventing or reducing the blurring phenomenon where the screen is blurred. 
     Meanwhile, the pixel  50 ″ according to the present embodiment can be usefully applied to a structure where the voltage drop from the first power supply ELVDD can be easily prevented or reduced. For example, an electrode plate that supplies the first power supply ELVDD can be formed of conductive material having low specific resistance, and a rear emitting structure can be designed relatively free from thickness restrictions, for example. 
       FIG. 6  is a circuit diagram showing another embodiment of a pixel of  FIG. 1 . When explaining  FIG. 6 , the same features as features described in the embodiment of  FIG. 5  will be provided with the same reference numerals and the detailed description thereof will be omitted. 
     Referring to  FIG. 6 , a pixel  50 ′″ is coupled between the anode electrode and the cathode electrode of the organic light emitting diode OLED, and further includes a sixth transistor M 6 ′″ whose gate electrode is coupled to the first scan line Si. Here, the sixth transistor M 6 ′″ may be implemented using the same type transistor as that in the first to fifth transistors M 1  to M 5 , for example, an N-type transistor. 
     When the first scan signal is supplied, the sixth transistor M 6 ′″ couples the anode electrode of the organic light emitting diode OLED to the cathode electrode thereof. In other words, while the first signal is supplied, the anode electrode and the cathode electrode of the organic light emitting diode OLED are at substantially a same voltage. 
     Therefore, in the present embodiment, while the first scan signal is supplied, the emission of the organic light emitting diode OLED can be effectively prevented or minimized. 
     Meanwhile, in the pixel  50 ′″ according to the present embodiment, the voltage of the first power ELVDD is transferred to the first node N 1  while the first scan signal is supplied, in contrast to the pixel  50 ″ in the embodiment of  FIG. 5 . In other words, in the present embodiment, a first voltage that is transferred to the first node N 1  by the sixth transistor M 6 ′″ is set as the voltage of the first power supply ELVDD. 
     Therefore, during the third period t 3  of  FIG. 3 , the voltage between the gate electrode and the source electrode of the first transistor M 1  of the pixel  50 ′″ according to the present embodiment is represented in the form where the threshold voltage Vto of the organic light emitting diode OLED is removed from equation 7, as shown in the following equation 9.
 
 V gs( M 1)= V data−ELVDD+ V th( M 1)  [Equation 9]
 
     Therefore, a driving current that flows to the organic light emitting diode OLED during the fourth period t 4  of  FIG. 3 , that is, during the emission period, is represented by the following equation 10.
 
 I   OLED =β( V gs( M 1)− V th( M 1)) 2 =β(( V data−ELVDD+ V th( M 1))− V th( M 1)) 2 =β( V data−ELVDD) 2   [Equation 10]
 
     The pixel  50 ′″ according to the embodiment as described above prevents or reduces emission of the organic light emitting diode OLED during the non-emission period, in addition to the other features provided by the pixel  50 ″ in the embodiment of  FIG. 5 . 
     While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiment, but is instead intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.