Abstract:
An organic light emitting diode (OLED) pixel circuit includes a first switch, a second switch, an energy storage element and an OLED. The first switch is controlled by a scan signal to transmit a data signal or not. The second switch provides a current to turn on the OLED. The energy storage element keeps a cross voltage between a gate and a source of the second switch and corresponding to the data signal so as to control the current generated by the second switch and thus to control a luminance of the OLED.

Description:
[0001]    This application claims the benefit of Taiwan application Serial No. 95140485, filed Nov. 1, 2006, the subject matter of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates in general to an organic light emitting diode (OLED) pixel, circuit and more particularly to an OLED pixel circuit including two transistor switches and one capacitor. 
         [0004]    2. Description of the Related Art 
         [0005]    Recently, the technology for controlling an OLED pixel using two transistor switches and one capacitor has been fully developed, and the associated circuit designs also have been proposed. However, those circuit designs still have some drawbacks. 
         [0006]      FIG. 1  (Prior Art) shows a conventional OLED pixel circuit. Agate of a switch  103  receives a scan signal SC 1  to control whether to transmit a data signal DT 1  to a switch  105 . When the switch  103  is turned on, a capacitor  104  is charged. A cross voltage of the capacitor  104  is a cross voltage between a gate and a source of the switch  105 . When the cross voltage of the capacitor  104  is higher than a threshold voltage of the switch  105 , the switch  105  is turned on and a current corresponding to the cross voltage is generated so that an OLED  106  is turned on to emit light. When the switch  103  is turned off, the cross voltage provided by the capacitor  104  is kept unchanged. Thus, the switch  105  is still kept on so that the OLED  106  is still kept on. 
         [0007]    The conventional design mentioned hereinabove restricts the structure of the organic diode pixel circuit to the same pattern. However, the display structure of the organic diode pixel still cannot satisfy various requirements in different circuit designs, which are needed to obtain the better electrical effect in the manufacturing processes thereof. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention is directed to an organic light emitting diode (OLED) pixel circuit using two transistors and one capacitor to drive and control a pixel. 
         [0009]    According to a first aspect of the present invention, an OLED pixel circuit is provided. The OLED pixel circuit includes a first switch, a second switch, a capacitor and an OLED. The first switch has its first terminal connected to a first node, its second terminal for receiving a data signal, and its control terminal for receiving a scan signal. The second switch has its first terminal connected to a second node, its second terminal connected to a third node, and its control terminal connected to the first node. The capacitor is connected between the first node and the second node. The OLED includes an anode connected to a fourth node, and a cathode connected to the second node. 
         [0010]    According to a second aspect of the present invention, another OLED pixel circuit is provided. The OLED pixel circuit includes a first switch, a second switch, a capacitor and an OLED. The first switch has its first terminal connected to a first node, its second terminal for receiving a data signal, and its control terminal for receiving a scan signal. The second switch has its first terminal connected to a second node, its second terminal connected to a third node, and its control terminal connected to the first node. The capacitor is connected between the first node and a fourth node. The OLED includes an anode connected to the fourth node and a cathode connected to the second node. 
         [0011]    According to a third aspect of the present invention, still another OLED pixel circuit is provided. The OLED pixel circuit includes a first switch, a second switch, a capacitor and an OLED. The first switch has its first terminal connected to a first node, its second terminal for receiving a data signal, and its control terminal for receiving a scan signal. The second switch has its first terminal connected to a second node, its second terminal connected to a third node, and its control terminal connected to the first node. The capacitor is connected between the first node and the third node. The OLED includes an anode connected to the third node, and a cathode connected to a fourth node. 
         [0012]    According to a fourth aspect of the present invention, yet still another OLED pixel circuit is provided. The OLED pixel circuit includes a first switch, a second switch, a capacitor and an OLED. The first switch has its first terminal connected to a first node, its second terminal for receiving a data signal, and its control terminal for receiving a scan signal. The second switch has its first terminal connected to a second node, its second terminal connected to a third node, and its control terminal connected to the first node. The capacitor is connected between the first node and a fourth node. The OLED includes an anode connected to the third node, and a cathode connected to the fourth node. 
         [0013]    The invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  (Prior Art) is a circuit diagram showing a pixel circuit according to the prior art. 
           [0015]      FIG. 2A  is a circuit diagram showing an OLED pixel circuit according to a first embodiment of the invention. 
           [0016]      FIG. 2B  is another circuit diagram showing the OLED pixel circuit according to the first embodiment of the invention. 
           [0017]      FIG. 3A  is a circuit diagram showing an OLED pixel circuit according to a second embodiment of the invention. 
           [0018]      FIG. 3B  is another circuit diagram showing the OLED pixel circuit according to the second embodiment of the invention. 
           [0019]      FIG. 4A  is a circuit diagram showing an OLED pixel circuit according to a third embodiment of the invention. 
           [0020]      FIG. 4B  is another circuit diagram showing the OLED pixel circuit according to the third embodiment of the invention. 
           [0021]      FIG. 5A  is a circuit diagram showing an OLED pixel circuit according to a fourth embodiment of the invention. 
           [0022]      FIG. 5B  is another circuit diagram showing the OLED pixel circuit according to the fourth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The invention provides four OLED pixel circuits, each of which uses two transistor switches and one capacitor to control a luminance of an OLED pixel so as to provide various arrangements of elements in the organic pixel circuit. 
         [0024]      FIG. 2A  is a circuit diagram showing an OLED pixel circuit according to a first embodiment of the invention. Referring to  FIG. 2A , the device circuit of the pixel circuit includes a first switch  203 , a second switch  205 , a capacitor  204  and an OLED  206 . Each of the first switch  203  and the second switch  205  illustrated in the example of  FIG. 2A  is an N-type metal oxide semiconductor (NMOS) transistor, and may be various transistors in practice. 
         [0025]    Referring to  FIG. 2A , the first switch  203  includes a first terminal (drain), a second terminal (source) and a control terminal (gate), and the second switch  205  includes a first terminal (drain), a second terminal (source) and a control terminal (gate). The OLED  206  includes an anode and a cathode. The second terminal of the first switch  203  receives a data signal DT 2 , and the control terminal of the first switch  203  receives a scan signal SC 2 . The first terminal of the second switch  205  is coupled to the cathode of the OLED  206 , the second terminal of the second switch  205  is grounded, and the control terminal of the second switch  205  is coupled to the first terminal of the first switch  203 . The capacitor  204  is connected between the control terminal and the first terminal of the second switch  205 . The anode of the OLED  206  is coupled to a positive voltage source VDD. The first switch  203  controls whether to transmit the data signal DT 2  to the second switch  205 . When the first switch  203  is turned on, the capacitor  204  is charged to enable the capacitor  204  to generate a cross voltage, such that a cross voltage is generated between the gate and the source of the second switch  205 . When the cross voltage between the gate and the source of the second switch  205  is higher than a threshold voltage of the second switch  205 , the second switch  205  is turned on and generates a drain current corresponding to the cross voltage so that the OLED  206  is turned on to emit light. When the first switch  203  is turned off, the cross voltage provided by the capacitor  204  is kept unchanged. So, the cross voltage between the gate and the source of the second switch  205  is also kept unchanged, the second switch  205  is still kept on, and the same drain current is kept so that the OLED  206  is still kept on. 
         [0026]    In the illustrated example of  FIG. 2A , the anode of the OLED  206  is coupled to the positive voltage source VDD, and the second terminal of the second switch  205  is grounded. In practice, however, the anode of the OLED  206  may also be grounded, and the second terminal of the second switch  205  may be coupled to a negative voltage source −VDD.  FIG. 2B  is another circuit diagram showing the OLED pixel circuit according to the first embodiment of the invention. As shown in the device circuit diagram of the pixel circuit in  FIG. 2B , the anode of the OLED  206  is grounded, and the second terminal of the second switch  205  is coupled to a negative voltage. The other circuit connections and operation principles of  FIG. 2B  are the same as those of  FIG. 2A , so detailed descriptions thereof will be omitted. 
         [0027]      FIG. 3A  is a circuit diagram showing an OLED pixel circuit according to a second embodiment of the invention. As shown  FIG. 3A , the device circuit of the pixel circuit includes a first switch  303 , a second switch  305 , a capacitor  304  and an OLED  306 . Each of the first switch  303  and the second switch  305  illustrated in  FIG. 3A  is an N-type metal oxide semiconductor (NMOS) transistor, and may be various transistors in practice. 
         [0028]    Referring to  FIG. 3A , the first switch  303  includes a first terminal (drain), a second terminal (source) and a control terminal (gate), and the second switch  305  includes a first terminal (drain), a second terminal (source) and a control terminal (gate). The OLED  306  includes an anode and a cathode. The second terminal of the first switch  303  receives a data signal DT 3 , and the control terminal of the first switch  303  receives a scan signal SC 3 . The first terminal of the second switch  305  is coupled to the cathode of the OLED  306 , the second terminal of the second switch  305  is grounded, and the control terminal of the second switch  305  is coupled to the first terminal of the first switch  303 . The capacitor  304  is connected between the control terminal of the second switch and the anode of the OLED. The anode of the OLED  306  is coupled to a positive voltage source VDD. 
         [0029]    The first switch  303  controls whether to transmit the data signal DT 3  to the second switch  305 . When the first switch  303  is turned on, the capacitor  304  is charged to make the capacitor  304  generate a cross voltage. The cross voltage of the capacitor  304  makes a cross voltage be generated between the gate and the source of the second switch  305 . When the cross voltage between the gate and the source of the second switch  305  is higher than a threshold voltage of the second switch  305 , the second switch  305  is turned on and generates a drain current corresponding to the cross voltage so that the OLED  306  is turned on to emit light. When the first switch  303  is turned off, the cross voltage provided by the capacitor  304  is kept unchanged. So, the cross voltage between the gate and the source of the second switch  305  is also kept unchanged, the second switch  305  is still kept on, and the same drain current is kept so that the OLED  306  is still kept on. 
         [0030]    In the illustrated example of  FIG. 3A , the anode of the OLED  306  is coupled to the positive voltage source VDD, and the second terminal of the second switch  305  is grounded. In practice, however, the anode of the OLED  306  may also be grounded, and the second terminal of the second switch  305  may be coupled to a negative voltage source −VDD.  FIG. 3B  is another circuit diagram showing the OLED pixel circuit according to the second embodiment of the invention. As shown in the device circuit diagram of the pixel circuit in  FIG. 3B , the anode of the OLED  306  is grounded, and the second terminal of the second switch  305  is coupled to a negative voltage. The other circuit connections and operation principles of  FIG. 3B  are the same as those of  FIG. 3A , so detailed descriptions thereof will be omitted. 
         [0031]      FIG. 4A  is a circuit diagram showing an OLED pixel circuit according to a third embodiment of the invention. Referring to  FIG. 4A , the device circuit of this pixel circuit includes a first switch  403 , a second switch  405 , a capacitor  404  and an OLED  406 . Each of the first switch  403  and the second switch  405  illustrated in  FIG. 4A  is an N-type metal oxide semiconductor (NMOS) transistor, and may be various transistors in practice. 
         [0032]    Referring to  FIG. 4A , the first switch  403  includes a first terminal (drain), a second terminal (source) and a control terminal (gate), and the second switch  405  includes a first terminal (drain), a second terminal (source) and a control terminal (gate). The OLED  406  includes an anode and a cathode. The second terminal of the first switch  403  receives a data signal DT 4 , and the control terminal of the first switch  403  receives a scan signal SC 4 . The first terminal of the second switch  405  is coupled to a positive voltage source VDD, the second terminal of the second switch  405  is coupled to the anode of the OLED  406 , and the control terminal of the second switch  405  is coupled to the first terminal of the first switch  403 . The capacitor  404  is connected between the control terminal and the second terminal of the second switch. The cathode of the OLED  406  is grounded. 
         [0033]    The first switch  403  controls whether to transmit the data signal DT 4  to the second switch  405 . When the first switch  403  is turned on, the capacitor  404  is charged to make the capacitor  404  generate a cross voltage, such that a cross voltage be generated between the gate and the source of the second switch  405 . When the cross voltage between the gate and the source of the second switch  405  is higher than a threshold voltage of the second switch  405 , the second switch  405  is turned on and generates a drain current corresponding to the cross voltage so that the OLED  406  is turned on to emit light. When the first switch  403  is turned off, the cross voltage provided by the capacitor  404  is kept unchanged. So, the cross voltage between the gate and the source of the second switch  405  is also kept unchanged, the second switch  405  is still kept on, and the same drain current is kept so that the OLED  406  is still kept on. 
         [0034]    In the illustrated example of  FIG. 4A , the first terminal of the second switch  405  is coupled to the positive voltage source VDD, and the cathode of the OLED  406  is grounded. In practice, however, the first terminal of the second switch  405  may be grounded and the cathode of the OLED  406  may be coupled to the negative voltage source −VDD.  FIG. 4B  is another circuit diagram showing the OLED pixel circuit according to the third embodiment of the invention. As shown in the device circuit diagram of the pixel circuit in  FIG. 4B , the cathode of the OLED  406  is coupled to a negative voltage, and the first terminal of the second switch  405  is grounded. The other circuit connections and operation principles of  FIG. 4B  are the same as those of  FIG. 4A , so detailed descriptions thereof will be omitted. 
         [0035]      FIG. 5A  is a circuit diagram showing an OLED pixel circuit according to a fourth embodiment of the invention. Referring to  FIG. 5A , the device circuit of the pixel circuit includes a first switch  503 , a second switch  505 , a capacitor  504  and an OLED  506 . Each of the first switch  503  and the second switch  505  illustrated in  FIG. 5A  is an N-type metal oxide semiconductor (NMOS) transistor, and may be various transistors in practice. 
         [0036]    Referring to  FIG. 5A , the first switch  503  includes a first terminal (drain), a second terminal (source) and a control terminal (gate), and the second switch  505  includes a first terminal (drain), a second terminal (source) and a control terminal (gate). The OLED  506  includes an anode and a cathode. The second terminal of the first switch  503  receives a data signal DT 5 , and the control terminal of the first switch  503  receives a scan signal SC 5 . The first terminal of the second switch  505  is coupled to a positive voltage source VDD, the second terminal of the second switch  505  is coupled to the anode of the OLED  506 , and the control terminal of the second switch  505  is coupled to the first terminal of the first switch  503 . The capacitor  504  is connected between the control terminal of the second switch and the cathode of the OLED. The cathode of the OLED  506  is grounded. 
         [0037]    The first switch  503  controls whether to transmit the data signal DT 5  to the second switch  505 . When the first switch  503  is turned on, the capacitor  504  is charged to generate a cross voltage, such that a cross voltage is generated between the gate and the source of the second switch  505 . When the cross voltage between the gate and the source of the second switch  505  is higher than a threshold voltage of the second switch  505 , the second switch  505  is turned on and generates a drain current corresponding to the cross voltage so that OLED  506  is turned on to emit light. When the first switch  503  is turned off, the cross voltage provided by the capacitor  504  is kept unchanged. So, the cross voltage between the gate and the source of the second switch  505  is also kept unchanged, the second switch  505  is still kept on, and the same drain current is kept so that the OLED  506  is still kept on. 
         [0038]    In the illustrated example of  FIG. 5A , the first terminal of the second switch  505  is coupled to the positive voltage source VDD, and the cathode of the OLED  506  is grounded. In practice, the cathode of the OLED  506  is coupled to a negative voltage source −VDD, and the first terminal of the second switch  505  is grounded. 
         [0039]      FIG. 5B  is another circuit diagram showing the OLED pixel circuit according to the fourth embodiment of the invention. In the device circuit diagram of the pixel circuit, the cathode of the OLED  506  is coupled to a negative voltage, and the first terminal of the second switch  505  is grounded. The other circuit connections and operation principles of  FIG. 5B  are the same as those of  FIG. 5A , so detailed descriptions thereof will be omitted. 
         [0040]    In each of the circuits of four different OLED pixel circuits, two transistor switches and one capacitor are used to control the luminance of one OLED pixel. When the scan signal is enabled, a current corresponding to the data signal flows through a diode to control the luminance of the pixel. 
         [0041]    While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.