Abstract:
A compensated pixel driver circuit for an organic electroluminescent device, wherein the circuit comprises a unity gain buffer which is preferably implemented as an operational amplifier. The circuit provides a unity gain sample and hold function, thereby compensating the current supply to the electroluminescent element by providing a self adjusting load.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an organic electroluminescent device and particularly to a compensated pixel driver circuit thereof. 
   2. Description of Related Art 
   An organic electro-luminescent device (OELD) consists of a light emitting polymer (LEP) layer sandwiched between an anode layer and a cathode layer. Electrically, this device operates like a diode. Optically, it emits light when forward biased and the intensity of the emission increases with the forward bias current. It is possible to construct a display panel wide a matrix of OELDs fabricated on a transparent substrate and with one of the electrode layers being transparent. One can also integrate the driving circuit on the same panel by using low temperature polysilicon thin film transistor (TFT) technology. 
   In a basic analog driving scheme for an active mat OELD display, a minimum of two transistors are required per pixel (FIG.  1 ): T 1  is for addressing the pixel and T 2  is for converting the data voltage signal into current which drives the OELD at a designated brightness. The data signal is stored by the storage capacitor C storage  when the pixel is not addressed. Although p-channel TFTs are shown in the figures, the same principle can also be applied for a circuit with n-channel TFTs. 
   There are problems associated with TFT analog circuits and OELDs do not act like perfect diodes. The LEP material does, however, have relatively uniform characteristics. Due to the nature of the TFT fabrication technique, spatial variation of the TFT characteristics exists over the entire panel. One of the most important considerations in a TFT analog circuit is the variation of threshold voltage, ΔV T , from device to device. The effect of such variation in an OELD display, exacerbated by the non perfect diode behaviour, is the non-uniform pixel brightness over the display panel, which seriously affects the image quality. Therefore, a built-in compensation circuit is required. 
   A simple threshold voltage variation compensation, current driven, circuit has been proposed. The current driven circuit, also known as the current programmed threshold voltage compensation circuit is illustrated in FIG.  2 A. In this circuit, T 1  is for addressing the pixel. T 2  operates as an analog current control to provide the driving current. T 3  connects between the drain and gate of T 2  and toggles T 2  to be either a diode or in saturation. T 4  acts a switch. Either T 1  or T 4  can be ON at any one time. Initially, T 1  and T 3  are OFF, and T 4  is ON. When T 4  is OFF, T 1  and T 3  are ON, and a current of known value is allowed to flow into the OLED, through T 2 . This is the programming stage because the threshold voltage of T 2  is measured with T 2  operating as a diode (with T 3  turned ON) while the programming current is allowed to flow through T 1 , through T 2  and into the OELD. T 3  shorts the drain and gate of T 2  and turns T 2  in to a diode. The detected threshold voltage of T 2  is stored by the capacitor C 1  connected between the gate and source terminals of T 2  when T 3  and T 1  are switched OFF. Then T 4  is turned ON, the current is now provided by V DD . If the slope of the output characteristics were flat, the reproduced current would be the same as the programmed current for any threshold voltage of T 2  detected. By turning ON T 4 , the drain-source voltage of T 2  is pulled up, so a flat output characteristic will keep the reproduced current the same as the programmed current. Note that ΔV T2  shown in  FIG. 2A  is imaginary, not real. 
   A constant currant is provided, in theory, during the active programming stage, which is t 3  to t 4  in the timing diagram shown in FIG.  2 A. The reproduction stage starts at t 6  and ends at t 1  of the next cycle. 
   In practice, there is always a slope in the output characteristics, so the reproduced current is not the same as the programmed current. This issue limits the device channel length of the polysilicon TFTs because of the increase of the short channel effect in polysilicon TFTs when the device channel length gets smaller. Simulations show that the variation between the reproduced current and programmed current is unacceptable for L=4 μm and below. This limitation on the design of transistor T 2  is a very serious practical problem, especially when small data currents are used. It is therefore important to find a technique that will provide good compensation in short channel devices. 
   The driving waveforms used are shown in timing chart fashion in FIG.  2 B. The threshold voltage V T  shown at the bottom of  FIG. 2B  is that for transistor T 2 . As can be seen from  FIG. 2B , this threshold voltage has a range of −1V to +1V. Such a range is much larger than the variation ΔV T  across a practical OELD mate. 
   Typical variation between the reproduced current and programmed current supplied to the OELD is illustrated in FIG.  2 C.  FIG. 2C  illustrates three cycles of OELD current supply: one from 0 to 30 μs, one from 30 μs to 60 μs, and one from 60 μs to 90 μs. The first half of each of these cycles is the programming stage and the second half of the cycle is the reproduction stage. It is to be noted that the current output levels in the reproduction stage compared with those in the corresponding program stage are remarkably different firm each other. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the present invention there is provided a compensated pixel driver circuit for an organic electroluminescent device, wherein the circuit comprises a unity gain buffer. Preferably the unity gain buffer is implemented as an operational amplifier. 
   According to a second aspect of the present invention there is provided a method of compensating the current supply to an organic electroluminescent pixel comprising. The step of using a unity gain buffer to provide a self adjusting load. 
   According to a third aspect of the present invention there is provided an organic electroluminescent display device comprising one or more compensated pixel driver circuits according to the first aspect of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: 
       FIG. 1  shows a conventional OELD pixel driver circuit using two transistors, 
       FIG. 2  shows a current programmed OELD driver with threshold voltage compensation, 
       FIG. 3  shows a compensated pixel driver circuit according to an embodiment of the present invention, 
       FIG. 4  is a table of requirements for one specific example of an operational amplifier which can be used in the circuit of  FIG. 3 , 
       FIG. 5  is an example of a circuit for implementing the operational amplifier shown in  FIG. 3 , 
       FIG. 6  is a graph illustrating the unity-gain buffer characteristics of the compensating circuit of  FIG. 3 , 
       FIG. 7  is a graph illustrating the total required supply current, 
       FIG. 8  is a driving waveform timing diagram, and 
       FIG. 9  illustrates the current output to the OELD using the circuit of FIG.  3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A compensated pixel driver circuit according to an embodiment of the present invention is shown in FIG.  3 . Compared with the circuit of  FIG. 2 , there is added an operational amplifier OpAmp A, a capacitor C 2  and a transistor T 5 . As shown in  FIG. 3 , V out  of the OpAmp is connected to the inverting input V_ thereof. The OpAmp thus has unity gain. Capacitor C 2  ensures a sample ad hold function and transistor T 5  acts as a control switch to store the voltage on C 2 . In effect the circuit provides a self-adjusted load or voltage source (V DD ) and by thus holding the operative voltage constant the effect of the slope in the output characteristics can be avoided. In it&#39;s generic form, the OpAmp A is a unity gain buffer having it&#39;s input connected to the source-drain path of transistor T 5  and it&#39;s output connected to the source-drain path of transistor T 4 , the input being connected to ground via capacitor C 2 . 
   As shown in  FIG. 3 , a TFT operational amplifier configured as a sample and hold circuit is used to provide a variable voltage source so that the drain-source voltage of T 2  in the reproduction stage is the same as that during the programming stage. During the programming stage, the voltage at the source of T 2  is passed to the storage capacitor C 2  at the input of the unity-gain OpAmp. The output of the OpAmp faithfully reproduces the voltage and also provides the current to the OELD through T 2 . The driving waveform is the same as that for the circuit of FIG.  2 . 
   The program current path is from V DD2  through node V 4 , T 1 , T 2  and the OELD. The reproduction current path is from V DD1 , through the OpAmp, V out , T 4 , node V 4 , T 2  and the OELD. 
   In the circuit of  FIG. 3 , the voltage at point V 4  is substantially the same in the reproduction cycle to the voltage at that point in the programming cycle. Additionally, a very high Open-Loop Gain (OLG) is not required in contrast to usual TFT circuits. An advantage of the embodiment of the present invention shown in  FIG. 3  is that the current flow to the OELD during the reproduction cycle is less sensitive to the variation in the output V out  of the OpAmp than ΔV T2  detection of the same percentage error. Furthermore, the OpAmp design constraints are not stringent. 
     FIG. 5  is a circuit diagram of one arrangement for implementing the OpAmp shown in FIG.  3 . The specific requirements for this circuit are shown in the table of FIG.  4 . Of particular note is the minimal off-set voltage. Typically is might be a few millivolts, in contrast to the variation of several volts which may typically arise in the conventional arrangement due to the slope of the output characteristics. The circuit of  FIG. 5  essentially consists of a differential pair circuit and a driver. The differential pair circuit comprises the toy two transistors connected to the V DD1  rail, the respective transistors having their gates providing the two input terminals of the OpAmp, and the transistor whose gate receives V bias1 . The output driver comprises a transistor receiving V bias2  at its gate and a transistor connected between the V DD1  rail and V out . 
   All of the transistors of the circuit of  FIG. 5  are TFTs having a channel length of 10 μm (in contrast to T 2 ). This channel length avoids the devices being stressed by the high value of V DD . The transistor connected between the V DD1  rail and V out  has a channel width of 100 μm in order to ensure sufficient current output. The area required to implement the circuit of  FIG. 5  can be reduced by varying the W/L absolute size ratio of the transistors, subject to a corresponding reduction in the maximum drive current. The space occupation value of 270 μm×70 μm given in the table of  FIG. 4  can, for example, be reduced to approximately 130 μm ×10 μm, subject to a reduction in the maximum drive current from 5 μA to 1.5 μA. However, in practice a maximum drive current of 1 μA might suffice (as indicated in FIG.  4 ). 
   In the specific example given, the current I DP  flowing through the differential pair circuit has a maximum value of 1 μA and the current I OB  flowing through the driver circuit has a maximum value of 5 μA. The additional current required by the presence of the OpAmp is thus minimal. 
     FIG. 6  is a graph illustrating the unity-gain buffer characteristics of the compensating circuit of FIG.  3 . As shown, the plot of V out  against V +  is the same for both the load and the no-load conditions. The load condition is 5 MΩ, which corresponds to a current of 1 μA through the OELD. 
   The total current supply required by the OpAmp of  FIG. 3 , in one specific example, is shown in FIG.  7 . The total current supply required is that required by the differential pair circuit (FIG.  5 ), that required by the OpAmp driver circuit ( FIG. 5 ) and that required to drive the OELD. Again load (5 MΩ) and no-load conditions are shown. 
   The driving waveforms used with one implementation of the circuit of  FIG. 3  are shown in timing chart fashion in FIG.  8 . Of course, the threshold voltage V T  shown at the bottom of  FIG. 8  is that for transistor T 2 . As can be seen from  FIG. 8 , this threshold voltage has a range of −1V to +1V. Such a range is much larger than the variation ΔV T  across a practical OELD matrix. Threshold variation ΔV T  in other transistors (T 1 , T 3 , T 4 , T 5 ) have little effect as they are used as switches and operate under voltage ranges greater than ΔV T . 
   The output current supplied to the OELD using the circuit of  FIG. 3  is illustrated in FIG.  9 .  FIG. 9  illustrates three cycles of OELD current supply one from 0 to 30 μs, one from 30 μs to 60 μs, and one from 60 μs to 90 μs. The first half of each of these cycles is, of course, the program stage and the second half of the cycle is the reproduction stage. In each cycle, five different program currents are illustrated (ie vertically—at 0.2, 0.4, 0.6, 0.8 and 1.0). It is to be noted that the current output levels in the reproduction stage compared with those in the corresponding program stage are remarkably close. The comparison is slightly less good for larger program currents, but is still relatively small. Moreover, the difference can be predicted (as shown in  FIG. 9 ) and can therefore be included in a gamma compensation (eg use 1.1 μA instead of 1 μA in the programming stage). 
   It will be apparent to persons skilled in the art that variations and modifications can be made to the arrangements described with respect to  FIGS. 3  to  9  without departing from the scope of the invention.