Patent Publication Number: US-8531360-B2

Title: Pixel circuit

Description:
RELATED APPLICATIONS 
     The present application is a divisional of U.S. application Ser. No. 11/692,258, filed on Mar. 28, 2007, the disclosure of which is hereby incorporated by reference herein in its entirely. 
    
    
     BACKGROUND 
     1. Field of Invention 
     The present invention relates to a pixel circuit, and more particularly relates to an AMOLED voltage type compensation pixel circuit. 
     2. Description of Related Art 
       FIG. 1  shows an organic light emitting diode pixel circuit of the prior art. The pixel circuit is a voltage type compensation pixel circuit. The pixel circuit has an organic light emitting diode  180 , a first transistor  170 , a driving transistor  130 , a capacitor  150 , and a second transistor  110 . The first transistor  170  has a source/drain  176  coupled to the light emitting diode  180 , wherein the first transistor  170  is controlled by a first scan signal (SCAN 1 ). The driving transistor  130  has source/drains  132  and  136 . The source/drain  132  couples to a power source terminal  140  through the transistor  160 , and the source/drain  136  couples to a source/drain  172  of the first transistor  170 . The capacitor  150  couples a gate  134  of the driving transistor  130  to the power source terminal  140 . When a second scan signal (SCAN 2 ) is asserted, the second transistor  110  respectively couples the source/drain  172  of the first transistor  170  to the capacitor  150 , and couples the gate  134  and the source/drain  136  of the driving transistor  130  together. 
     The pixel circuit also has a third transistor  190  controlled by the second scan signal to couple a data line  120  and the source/drain  132  of the driving transistor  130 . 
     The drawback of the conventional pixel circuit is that it has five transistors (transistors  110 ,  130 ,  160 ,  170  and  190 ). These transistors reduce the aperture ratio of the pixel circuit. 
     SUMMARY 
     According to one embodiment of the present invention, the pixel circuit has an organic light emitting diode, a driving transistor, a capacitor and a first switch. The organic light emitting diode has a first end coupled to a first power source terminal. The driving transistor has a source and a drain respectively coupled to a second power source terminal and a second end of the light emitting diode. The capacitor couples a gate of the driving transistor to a reference voltage terminal. The first switch couples the second end of the light emitting diode to the capacitor, and couples the gate and the drain of the driving transistor together when a first scan signal is asserted. 
     According to another embodiment of the present invention, the pixel circuit operates during a precharge stage, a programming stage, and a display stage sequentially. The pixel circuit has an organic light emitting diode, a driving transistor, a capacitor, and a first switch. The organic light emitting diode has a first end coupled to a first power source terminal. The driving transistor has a source and a drain respectively coupled to a second power source terminal and a second end of the light emitting diode. The capacitor couples a gate of the driving transistor to a reference voltage terminal. The first switch is controlled by a first scan signal to coupe/decouple the second end of the organic light emitting diode to/from the gate of the driving transistor. The first scan signal is asserted during the precharge and programming stages, and the first scan signal is deasserted during the display stage. 
     It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  shows an organic light emitting diode pixel circuit of the prior art; 
         FIG. 2A  shows an organic light emitting diode pixel circuit according to an embodiment of the invention; 
         FIG. 2B  shows the waveform diagrams of the signals of the embodiment shown in  FIG. 2A ; 
         FIG. 2C  shows the organic light emitting diode pixel circuit during a precharge stage according to the embodiment of the invention; 
         FIG. 2D  shows the organic light emitting diode pixel circuit during a programming stage according to the embodiment of the invention; 
         FIG. 2E  shows the organic light emitting diode pixel circuit during a display stage according to the embodiment of the invention; 
         FIG. 3A  shows an organic light emitting diode pixel circuit according to another embodiment of the invention; 
         FIG. 3B  shows the waveform diagrams of the signals of the embodiment shown in  FIG. 3A ; 
         FIG. 4A  shows an organic light emitting diode pixel circuit according to another embodiment of the invention; and 
         FIG. 4B  shows the waveform diagrams of the signals of the embodiment shown in  FIG. 4A . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
       FIG. 2A  shows an organic light emitting diode pixel circuit according to an embodiment of the invention. The pixel circuit is a voltage type compensation pixel circuit with PMOS transistors. The pixel circuit has an organic light emitting diode  210 , a driving transistor  230 , a capacitor  250  and a first switch  270 . The organic light emitting diode  210  has a first end  212  coupled to a first power source terminal  220 . The driving transistor  230  has a source  232  and a drain  236  respectively coupled to a second power source terminal  240  and a second end  216  of the light emitting diode  210 . The capacitor  250  couples a gate  234  of the driving transistor  230  to a reference voltage terminal  260 . The first switch  270  couples the second end  216  of the light emitting diode  210  to the capacitor  250 , and couples the gate  234  and the drain  236  of the driving transistor  230  together when a first scan signal (SCAN) is asserted. 
     The pixel circuit has a second switch  280  controlled by the first scan signal (SCAN) to couple the source  232  of the driving transistor  230  to a data line  299 . Therefore, when the first scan signal is asserted, the data signals from the data line  299  are transmitted to the pixel circuit. 
       FIG. 2B  shows the waveform diagrams of the signals of the embodiment shown in  FIG. 2A . The pixel circuit is a voltage compensation type pixel circuit. The first scan signal (SCAN) turns on the first switch  270  and the second switch  280  during a precharge and a programming stages, and turns off the first switch  270  and the second switch  280  during the display stage. 
     The second power source terminal  240  (VDDX) floats (HIZ, high impedance) during the precharge and programming stages (i.e. when the first scan signal, SCAN, is asserted) and has a high voltage (VDD) to supply power to the organic light emitting diode  210  during the display stage. 
     The reference voltage terminal  260  provides a first reference voltage (VREF 1 ) when the pixel circuit is in the precharge stage, provides a second reference voltage (VREF 2 ) when the pixel circuit is in the programming stage, and provides a third reference voltage (VREF 3 ) when the pixel circuit is in the display stage. The driving transistor  230  is a PMOS transistor, thus the second reference voltage is not higher than (lower than or equal to) the first reference voltage. Therefore, the lower voltage, second reference voltage, makes writing the data signals (VDATA) into the pixel circuit easy in the programming stage. Moreover, the low second reference voltage also enables the pixel circuit to be driven by low voltage data signals. Thus, the pixel circuit can operate with low power consumption. 
     Otherwise, the first power source terminal  220  provides a ground voltage when the pixel circuit is in the precharge stage, makes the first end  212  of the organic light emitting diode  210  high impedance (HIZ) when the pixel circuit is in the programming stage, and provides the ground voltage when the pixel circuit is in the display stage. Therefore, the high impedance at the first end  212  of the organic light emitting diode  210  also improves the pixel circuit&#39;s performance of the programming stage. 
     The first switch  270 , the second switch  210  and the third switch  290  can be implemented by transistors. In this embodiment shown in the  FIG. 2A , the switches  270 ,  210  and  290  are PMOS transistors. If the switches  270 ,  210  and  290  are configured by NMOS transistors, the control signals have to be inversed. 
     Compared with the prior art in  FIG. 1 , there are only three transistors (switches  270 ,  280 , and the driving transistor  230 ) in this embodiment. Therefore, the aperture ratio of each pixel circuit is increased thereby. 
       FIG. 2C ,  FIG. 2D  and  FIG. 2E  respectively show the organic light emitting diode pixel circuit during the precharge, programming and display stages according to the embodiment of the invention. The pixel circuit operates during the precharge stage, the programming stage, and the display stage sequentially. Refer to the  FIG. 2A  at the same time, the pixel circuit has an organic light emitting diode  210 , a driving transistor  230 , a capacitor  250 , and a first switch  270 . The organic light emitting diode  210  has a first end  212  coupled to a first power source terminal  220 . The driving transistor  230  has a source  232  and a drain  236  respectively coupled to a second power source terminal  240  and a second end  216  of the light emitting diode  210 . The capacitor  250  couples a gate  234  of the driving transistor  230  to a reference voltage terminal  260 . The first switch  270  controlled by a first scan signal to coupe/decouple the second end  216  of the organic light emitting diode  210  to/from the gate  234  of the driving transistor  230 . 
     The first scan signal is asserted during the precharge ( FIG. 2C ) and programming ( FIG. 2D ) stages, and the first scan signal is de-asserted during the display stage ( FIG. 2E ). Therefore, the capacitor  250  is coupled to the light emitting diode  210  during the precharge and programming stages in the  FIG. 2C  and  FIG. 2D , and is decoupled from the light emitting diode  210  during the display stage in the  FIG. 2E . 
       FIG. 3A  shows an organic light emitting diode pixel circuit according to another embodiment of the invention. The pixel circuit is a voltage type compensation pixel circuit with NMOS transistors. The pixel circuit has an organic light emitting diode  310 , a driving transistor  330 , a capacitor  350  and a first switch  370 . The organic light emitting diode  310  has a first end  312  coupled to a first power source terminal  320 . The driving transistor  330  has a source  332  and a drain  336  respectively coupled to a second power source terminal  340  and a second end  316  of the light emitting diode  310 . The capacitor  350  couples a gate  334  of the driving transistor  330  to a reference voltage terminal  360 . The first switch  370  couples the second end  316  of the light emitting diode  310  to the capacitor  350 , and couples the gate  334  and the drain  336  of the driving transistor  330  together when a first scan signal (SCAN) is asserted. 
     The pixel circuit has a second switch  380  controlled by the first scan signal (SCAN) to couple the source  332  of the driving transistor  330  to a data line  399 . Therefore, when the first scan signal is asserted, the data signals from the data line  399  are transmitted to the pixel circuit. 
       FIG. 3B  shows the waveform diagrams of the signals of the embodiment shown in  FIG. 3A . Since the pixel circuit of  FIG. 2A  is implemented by PMOS transistors, and the pixel circuit of  FIG. 3A  is implemented by NMOS transistors, the waveform diagrams of  FIG. 2B  and  FIG. 3B  are opposite. The driving transistor  330  is a NMOS transistor, thus the second reference voltage (VREF 2 ) is not lower than (higher than or equal to) the first reference voltage (VREF 1 ). Therefore, the lower voltage, second reference voltage, makes writing the data signals (VDATA) into the pixel circuit easy in the programming stage. Moreover, the low second reference voltage also enable the pixel circuit to be driven by the data signals with low voltages. Thus, the pixel circuit can operate with low power consumption. 
       FIG. 4A  shows an organic light emitting diode pixel circuit according to another embodiment of the invention. This pixel circuit is implemented by PMOS transistors, and it also can be implemented by NMOS transistors. The difference between the embodiments of  FIG. 2A  and  FIG. 4A  is that the pixel circuit in  FIG. 4A  has a third switch  490  controlled by a second scan signal (SCANB) to couple the second power source terminal  240  to the reference voltage terminal  260 . 
       FIG. 4B  shows the waveform diagrams of the signals of the embodiment shown in  FIG. 4A . The first scan signal (SCAN) and the second scan signal (SCANB) are opposite. Therefore, the second power source terminal  240  and the reference voltage terminal  260  are disconnected when the second scan signal is deasserted at the precharge and programming stages. The third switch  490  is turned on to couple the reference voltage terminal  260  to the second power source terminal  240  when the pixel circuit operates in the display stage. Thus the voltages at the reference voltage terminal  260  and the second power source terminal  240  in the display stage are VDD. 
     By the description above, the embodiments of this invention with the voltage compensation function has fewer transistors than the conventional pixel circuit. Otherwise, the variable voltages at the reference voltage terminal make the pixel circuit operates more efficiently than the conventional pixel circuit, too. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.