Abstract:
An OLED display device includes a current source and an OLED equivalent module. The current source includes a first controller, two biased sources, and two PMOS transistors. The current source outputs current to the OLED equivalent module to enable the OLED equivalent module emitting light.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an OLED display device, and more particularly, to an OLED display device with high output impedance. 
         [0003]    2. Description of the Prior Art 
         [0004]    Please refer to  FIG. 1 .  FIG. 1  is a circuit diagram illustrating a conventional OLED display device  100 . The OLED display device  100  comprises a current source  110  and an OLED equivalent module  120 . The current source  110  comprises a voltage source VH, a controller  111 , and a p-type transistor Q 1 . The OLED equivalent module  120  comprises 2 resistors R 1  and R 2 , an OLED D 1 , and an equivalent capacitor C 1 . The voltage source VH biases the p-type transistor Q 1  at the saturation region. The current source  110  transmits controlling voltage V 1  to the gate of the p-type transistor Q 1  to control the current I 1  by the controller  111 . Theoretically, the output resistance of the current source should be infinite, and thus, no matter what the load coupled to the current source is, stability and output abilities of the current source are not affected. In fact, the conventional current source  110  has an equivalent output resistor having a resistance of Rd. Thus, if the OLED equivalent module  120  overloads the current source  110 , that is, the equivalent load resistor of the OLED equivalent module  120  is not ignored compared with the resistor Rd, the current I 1  is affected and the size of the current I 1  is changed from the ideal status. Thus, the lumen is not even everywhere on the OLED display device. 
       SUMMARY OF THE INVENTION  
       [0005]    It is therefore a primary objective of the claimed invention to provide an OLED display device to solve the above-stated problems. 
         [0006]    The present invention provides an organic light emitting diode (OLED) display device with high output impedance, comprising a current source, comprising a first MOS transistor comprising a gate; a first node; and a second node; a second MOS transistor providing high impedance comprising a gate; a first node, coupled to the second node of the first MOS transistor; and a second node; a first controller coupled to the gate of the first MOS transistor, providing a controlling signal for controlling a current through the first MOS transistor; a first voltage source coupled to the gate of the second MOS transistor, providing a first voltage to the gate of the second MOS transistor; and a second voltage source coupled to the first node of the first MOS transistor, providing a second voltage to the first MOS transistor; and an OLED equivalent module coupled to the second node of the second MOS transistor, generating a light according to a current through the second MOS transistor. 
         [0007]    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 THE DRAWINGS  
         [0008]      FIG. 1  is a circuit diagram illustrating a conventional OLED display device. 
           [0009]      FIG. 2  is a circuit diagram illustrating the OLED display device of the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0010]    Please refer to  FIG. 2 .  FIG. 2  is a circuit diagram illustrating the OLED display device  200  of the present invention. The OLED display device  200  comprises a current source  210 , an OLED equivalent module  220 , a second controller  240 , and a switch  230 . The current source  210  comprises a first controller  211 , 2 voltage sources VH 2  and VH 3 , 2 p-type transistors Q 2  and Q 3 . The OLED equivalent module  220  comprises 2 resistors R 3  and R 4 , an OLED D 2 , and a capacitor C 2 . As shown in the current source  210  of  FIG. 2 , the output node of the first controller  211  is coupled to the gate of the p-type transistor Q 2 , the source of the p-type transistor Q 2  is coupled to the voltage source VH 2 , and the drain of the p-type transistor Q 2  is coupled to the p-type of the transistor Q 3 . The gate of the p-type transistor Q 3  is coupled to the voltage source VH 3 , the source of the p-type transistor Q 3  is coupled to the p-type transistor Q 2 , and the drain of the p-type transistor Q 3  is coupled to the OLED equivalent module  220 . In the OLED equivalent module  220 , the resistor R 3  is coupled between the source of the p-type transistor Q 3  and the positive node of the OLED D 2 , the resistor R 4  is coupled between the switch  230  and the negative node of the OLED D 2 , the capacitor C 2  is coupled between the resistors R 3  and R 4 , and the OLED D 2  is coupled between the resistors R 3  and R 4 . The output node of the second controller  240  is coupled to the controlling node of the switch  230 , and the switch  230  is coupled between the ground and the resistor R 4 . The voltage sources VH 2  and VH 3  bias the p-type transistors Q 2  and Q 3  at the saturation region. The current source  210  transmits the controlling voltage V 2  to the gate of the p-type transistor Q 2  by the first controller  211  for controlling the current I 2 . And the OLED equivalent module  220  generates light according the size of the current I 2 . The current source  210  of the OLED display device  200  has the equivalent output resistor which the resistance of is μRd and is much bigger than the output resistor of the current source of the conventional OLED display device. Thus, the current source of the present invention is able to be used for higher loads while the effect from the load is comparatively smaller. Consequently, the current source  210  provides a more stable current to the OLED display device  200  so that the OLED display device  200  has a better performance. 
         [0011]    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.