Abstract:
A display apparatus includes a light-emitting device, a first scan line for transferring a first signal to select the light-emitting device, a data line for transferring a data current signal to drive the light-emitting device, a first transistor having a gate coupled to the first scan line for selecting the light-emitting device according to the first signal, and a current mirror electrically connected to the light-emitting device for transferring a driving current signal to drive the light-emitting device. The current mirror includes a second transistor having a gate coupled to the data line and one of the source and the drain of the first transistor for receiving the data current signal. In addition, the current mirror further includes a third transistor having a gate coupled to the other of the source and the drain of the first transistor for transferring the driving current signal.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The invention relates to a display apparatus and its pixel structure, and more particularly, to a current-driven organic light emitting diode (OLED) display apparatus and its pixel structure. 
   2. Description of the Prior Art 
   Referring to  FIG. 1 , which is a diagram of a conventional pixel  10  of a voltage-driven OLED display apparatus. As shown in  FIG. 1 , the pixel  10  comprises a scan line SL, a data line DL, a thin-film transistor (TFT) M 1 , a thin-film transistor M 2 , a capacitor C, and an organic light emitting diode (OLED). The gate of the TFT M 1  is connected to the scan line SL, the drain of TFT M 1  is connected to the data line DL, and the source of the TFT M 1  is connected to the gate of the TFT M 2  and the capacitor C. The drain of the TFT M 2  is connected to the organic light emitting diode (OLED), and the source of the TFT M 2  is connected to the capacitor and a voltage source Vdd. Furthermore, the organic light emitting diode (OLED) is connected to another voltage source Vss. 
   In addition, the operation of the pixel  10  is illustrated as follows. First of all, an external gate driver (not shown) drives the scan line SL and supplies a predetermined voltage to the scan line, the predetermined voltage is transferred to the gate of the TFT M 1  through the scan line SL, and the TFT M 1  is utilized as a switch. Therefore, the TFT M 1  is turned on. In addition, the voltage information carried by the data line DL can be transferred to the gate of the TFT M 2  and the capacitor C through the TFT M 1 . Please note that the voltage information carried by the data line DL is set by the external data driver (not shown) according to the display data (for example, a gray value of the pixel  10 ) to be displayed of the pixel  10 . 
   And then, because the above-mentioned voltage information is utilized to control the gate voltage of the TFT M 2 , the TFT M 2  can determine the current I, which passes through the TFT M 2 , according to the voltage information. On the other hand, because the luminace of the organic light emitting diode (OLED) is directly proportional to the current I, the organic light emitting diode (OLED) generates a corresponding amount of light according to the current I, and the pixel  10  is driven. 
   As shown in  FIG. 1 , the capacitor C is utilized to store the above-mentioned voltage information. When the voltage information passes through the TFT M 1 , the voltage information is not only utilized as the gate voltage of the TFT M 2  for turning on the TFT M 2 , but also affects the charges stored in the capacitor C. Therefore, when the capacitor C stores enough charges for maintaining the voltage level corresponding to the above-mentioned voltage information, the gate driver and the data driver can stop driving the pixel  10 . And then the capacitor C can be utilized to continuously drive the TFT M 2  to make the TFT M 2  output the current I for a predetermined time interval. Furthermore, because the capacitor C is utilized to drive the TFT M 2 , noise from data line DL no longer affects the TFT M 2 . Therefore, this can make the organic light emitting diode (OLED) stably generate light. In other words, the pixel  10  can stably output a wanted gray value. 
   However, inaccuracies in manufacturing the TFT M 2  (for example, an inaccurate doping concentration or an inaccurate distance between the gate and the substrate) may occur. This may cause an inaccuracy of the threshold voltage of the TFT M 2  or an inaccuracy of the mobility of the TFT M 2 . These inaccuracies may directly affect the current I. Therefore, even if the same voltage information is utilized, currents I of different pixels are still different. In other words, this makes different pixels having the same voltage information display with different luminance values. 
   SUMMARY OF INVENTION 
   The present invention has been made in view of the above-mentioned problems, and has an object of providing a current-driven OLED display apparatus and its pixel structure. 
   According to an exemplary embodiment of the present invention, a pixel structure is disclosed. The pixel comprises: a light-emitting device; a first scan line for transferring a first signal; a data line for transferring a data current signal; a first transistor having a gate coupled to the first scan line; and a current mirror electrically connected to the light-emitting device. The current mirror comprises: a second transistor having a gate connected to the data line and one of the source and the drain of the first transistor; and a third transistor having a gate coupled to the other of the source and the drain of the first transistor. 
   Furthermore, a pixel structure having an electro-luminescent diode and a capacitor is disclosed. The pixel structure comprises: a voltage source; a first transistor having a gate coupled to a scan line; a second transistor having a gate coupled to a data line and one of the source and the drain of the first transistor; a third transistor having a gate coupled to the other of the source and the drain of the first transistor, one of the source and the drain of the third transistor being coupled to the electro-luminescent diode, and the other of the source and the drain of the third transistor being coupled to the voltage source; and a fourth transistor having a gate coupled to the scan line, one of the source and the drain of the fourth transistor being coupled to the data line; wherein the other of the source and the drain of the fourth transistor is coupled to one of the source and the drain of the second transistor, and the gate of the second transistor is coupled to the gate of the third transistor through the first transistor to form a current mirror. 
   The present invention pixel utilizes the current-driven theorem so that the present invention pixel has better display stability. Furthermore, the present invention pixel can stably display a wanted gray-value luminance. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a diagram of a conventional pixel of a voltage-driven OLED display apparatus. 
       FIG. 2  is a diagram of a pixel in a current-driven LED display apparatus of a first embodiment of the present invention. 
       FIG. 3  is a flow diagram of an operation of driving the pixel shown in  FIG. 2 . 
       FIG. 4  is a diagram of a pixel in  FIG. 2  of a second embodiment of the present invention. 
       FIG. 5  is a diagram of a pixel in  FIG. 2  of a third embodiment of the present invention. 
       FIG. 6  is a diagram of a pixel in  FIG. 2  of a fourth embodiment of the present invention. 
       FIG. 7  is a diagram of a pixel in  FIG. 2  of a fifth embodiment of the present invention. 
       FIG. 8  is a diagram of a pixel in  FIG. 2  of a sixth embodiment of the present invention. 
       FIG. 9  is a diagram of a pixel in  FIG. 2  of a seventh embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 2 , which is a diagram of a pixel  20  in a current-driven light emitting diode (LED) display apparatus of a first embodiment of the present invention. Please note that as an example, the LED described is an organic light-emitting diode. As shown in  FIG. 2 , the pixel  20  comprises a scan line SL, a data line DL, a capacitor C, a plurality of TFTs T 1 , T 2 , T 3 , and T 4 , and an organic light emitting diode (OLED). Please note that the devices having the same name as those described previously (for example, the scan line SL, the data line DL, the capacitor C, and the organic light emitting diode (OLED)) have the same functions and operations, and thus the description is not repeated here. As shown in  FIG. 2 , the TFTs T 2 , and T 3  are mainly utilized to form a current mirror. It is well-known that the current mirror can drive the current I to pass through the TFT T 3  corresponding to the current I 0 , wherein the ratio of the current I to the current I 0  is the current ratio of the current mirror. Furthermore, the TFTs T 1  and T 4  are utilized as two switches. Simply speaking, when the current mirror operates, the gates of the TFTs T 2  and T 3  have to be coupled to each other and the TFT T 2  has to be coupled to the data line DL through the TFT T 4 . In this embodiment, the gate of the TFT T 1  is coupled to the scan line SL, the source of the TFT T 1  is coupled to the gate of the TFT T 3  and the capacitor C, and the drain of the TFT T 1  is coupled to the gate of the TFT T 2  and the data line DL. Furthermore, the source of the TFT T 3  is coupled to a voltage source Vdd, and the drain of the TFT T 3  is coupled to the organic light emitting diode (OLED). In addition, the source of the TFT T 2  is coupled to the voltage source Vdd, and the drain of the TFT T 2  is coupled to the source of the TFT T 4 . The gate of the TFT T 4  is coupled to the scan line SL, and the drain of the TFT T 4  is coupled to the data line DL. Furthermore, the capacitor C is connected to the voltage source, and the organic light emitting diode (OLED) is connected to another voltage source Vss. 
   Referring to  FIG. 3 , which is a flow diagram of an operation of driving the pixel  20  shown in  FIG. 2 . In the following illustration, taking the current-driven LED for an example with the LED being an OLED, the operation of driving the pixel  20  comprises following steps: 
   Step  100 : Start; 
   Step  102 : The scan line SL transfers a signal to the gates of the TFTs T 1  and T 4  for turning on the TFTs T 1  and T 4 ; 
   Step  104 : The gate of the TFT T 2  establishes a voltage V pixel  according to the data current signal I 0  outputted by the data line DL; 
   Step  106 : The current mirror generates the current signal I according to the data current signal I 0 ; 
   Step  108 : The capacitor C stores the voltage V pixel ; 
   Step  110 : The current I drives the organic light emitting diode (OLED) to generate a corresponding intensity of light; 
   Step  112 : The scan line SL stops transferring the signal so that the TFTs T 1  and T 4  are no longer turned on; 
   Step  114 : The TFT T 3  utilizes the voltage V pixel  stored in the capacitor C to generate the current signal I in order to maintain the intensity of light generated by the organic light emitting diode (OLED); and 
   Step  116 : The operation of driving the pixel  20  completes. 
   At first, in a write stage, the scan line SL transfers a signal to the gates of the TFTs T 1  and T 4  to turn on the TFTs T 1  and T 4  (step  102 ). Therefore, the TFT T 4  can be regarded as being conductive. The data current signal I 0  of the data line DL can pass through the TFT T 2 . Therefore, the gate of the TFT T 2  generates a corresponding V pixel  according to the data current signal I 0  (step  104 ). Furthermore, because the TFT T 1  can also be regarded as being conductive, the voltage V pixel  is transferred to the capacitor C and the TFT T 3 . 
   And then, because of the characteristic of the current mirror, the current mirror generates a current signal I according to the data current signal I 0 , wherein the ratio of the current signal I to the data current signal I 0  is the current ratio (generally speaking, the current ratio is substantially equal to (W/L) T2 : (W/L) T3 , wherein the W/L is a ratio of the width to the length of the channel of the TFT) (step  106 ). Furthermore, the capacitor C maintains the above-mentioned voltage V pixel  so that the voltage difference between two terminals of the capacitor C is Vdd−V pixel  (step  108 ). At the same time, the current signal I passe through the organic light emitting diode (OLED) so that the organic light emitting diode (OLED) generates a corresponding intensity of light (step  110 ). After step  110 , the write stage completes. 
   And then, the reproducing stage starts. At this time, the scan line SL stops transferring the signal to turn off the TFTs T 1  and T 4  (step  112 ). Therefore, the TFTs T 1  and T 4  can be regarded as being non-conductive. As mentioned above, the capacitor C maintains the voltage difference as Vdd−V pixel . Furthermore, the capacitor C cannot discharge after the TFT T 1  is turned off. Therefore, the gate of the TFT T 3  can maintain the voltage V pixel , and the TFT T 3  can generate a stable current signal I because of the voltage V pixel . The organic light emitting diode (OLED) can generate stable light corresponding to the current I (step  114 ). Here, the driving operation of the pixel  20  completes (step  116 ). 
   Please note that in  FIG. 2 , the pixel  20  comprises 4 P-type TFTs. In fact, N-type TFTs can be utilized, also. This is also consistent with the original intention of the present invention. Referring to  FIG. 4 ,  FIG. 5 , and  FIG. 6 .  FIG. 4  are a diagram of a pixel shown in  FIG. 2  of a second embodiment of the present invention. In contrast to the first embodiment shown in  FIG. 2 , in the embodiment shown in  FIG. 4 , the TFTs T 1  and T 4 , which are utilized as switches, are implemented by N-type TFTs. Here, the operation and function of the N-type and P-type TFT are well-known, and thus omitted. 
     FIG. 5  is a diagram of a pixel  20  shown in  FIG. 2  of a third embodiment of the present invention.  FIG. 6  is a diagram of a pixel  20  shown in  FIG. 2  of a fourth embodiment of the present invention. As shown in  FIG. 5 , the pixel  20  utilizes a N-type TFT to be the current mirror. And the operation steps are illustrated as follows: 
   First, in the above-mentioned write stage, the scan line SL transfers a signal to the gates of the TFTs T 1  and T 4  to turn on the TFTs T 1  and T 4 , and TFT T 4  can be regarded as being conductive. Therefore, the data current signal I 0  of the data line DL can pass through the TFT T 2 , and the gate of the TFT T 2  generates a corresponding voltage V pixel  according to the data current signal I 0 . Furthermore, because the TFT T 1  can be regarded as being conductive, the voltage V pixel  is transferred to the capacitor C and the TFT T 3 . 
   And then, because of the characteristic of the current mirror, the current mirror generates a current signal I according to the data current signal I 0 , wherein the ratio of the current signal I to the data current signal I 0  is the current ratio. Furthermore, the capacitor C maintains the above-mentioned voltage V pixel  to keep the voltage difference between the two terminals of the capacitor C at a predetermined value. Simultaneously, the current signal I can pass through the organic light emitting diode (OLED) so that the organic light emitting diode (OLED) generates a corresponding intensity of light. Here, the write stage completes. 
   And then, the reproducing stage starts. At this time, the scan line SL stops transferring the signal to turn off the TFTs T 1  and T 4 , and the TFTs T 1  and T 4  can be regarded as being non-conductive. Because the capacitor C maintains the voltage difference between the two terminals of the capacitor C and the capacitor C cannot discharge because the TFT T 1  is turned off, the capacitor C can maintain the voltage difference between the gate and the source of the TFT T 3 . Therefore, the TFT T 3  can maintain the current signal I so that the organic light emitting diode (OLED) can maintain the generated light. Here, the driving operation of the pixel  20  completes. 
   Referring to  FIG. 6 . As shown in  FIG. 6 , all TFTs of the pixel  20  are N-type TFTs. In contrast to the pixel  20  shown in  FIG. 5 , the pixel  20  shown in  FIG. 6  only comprises two N-type TFTs T 1  and T 4  as switches. Here, the operation and the functions of the N-type and P-type TFTs are well-known. In addition, other operations of the pixel  20  shown in  FIG. 6  are similar to the pixel  20  shown in  FIG. 5 , and are thus omitted here. 
   Furthermore, Referring to  FIG. 7 , which is a diagram of a pixel  20  shown in  FIG. 2  of a fifth embodiment of the present invention. As shown in  FIG. 7 , the connection of the capacitor C is not limited to being connected between the voltage source Vdd and the gate of the TFT T 3 . In this embodiment, the capacitor C is coupled between the gate of the TFT T 3  and another voltage source Vss. Therefore, the capacitor C maintains the voltage difference between the two terminals of the capacitor C as V pixel −Vss. That is, the capacitor C also achieves the purpose of maintaining the gate voltage of the TFT T 3  as the voltage V pixel . Referring to  FIG. 8 , which is a diagram of a pixel  20  shown in  FIG. 2  of a sixth embodiment of the present invention. In this embodiment, the position of the organic light emitting diode (OLED) changes. That is, the organic light emitting diode (OLED) is coupled between the voltage source Vdd and the TFT T 3 . Because the current signal I passes through the TFT T 3  (from the voltage source Vdd to the voltage source Vss), as long as the organic light emitting diode (OLED) is placed in the path of the current signal I, the current signal can drive the organic light emitting diode (OLED) to generate wanted light. 
   Referring to  FIG. 9  in conjunction with  FIG. 2 .  FIG. 9  is a diagram of a pixel  20  shown in  FIG. 2  of a seventh embodiment of the present invention. The difference between the first embodiment shown in  FIG. 2  and the seventh embodiment shown in  FIG. 9  is the number of scan lines. In this embodiment, the TFTs T 1  and T 4  are controlled by two scan lines SL 1  and SL 2 , respectively. This can reduce the feed-through effect on the voltage V pixel  of the capacitor C. The feed-through effect is caused because the TFTs T 1  and T 4  switch. Therefore, two scan lines SL 1  and SL 2  are utilized in this embodiment. In other words, when the TFT T 4  has not been turned on yet, the scan line SL 1  can first transfer the signal to turn on the TFT T 1 . And when the TFT T 1  has not been turned off, the scan line SL 2  can first transfer the signal to turn off the TFT T 4 . 
   Please note that in the pixel  20  of the present invention, the gate of the TFT T 2  is electrically connected to the data line DL. Therefore, in the above-mentioned write stage, this structure can help the pixel quickly write the gate voltage of the TFT T 2 . That is, when the scan line SL turns on the TFTs T 1  and T 4 , the wanted gate voltage V pixel  of the TFT T 2  can be quickly established. Therefore, the present invention pixel  20  has better response speed. 
   In addition, in contrast to the prior art, the present invention pixel utilizes the current-driven theorem so that the present invention pixel has better display stability. Furthermore, the present invention pixel can stably display a wanted gray-value luminance. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.