Abstract:
The OLED voltage of a selected pixel is extracted from the pixel produced when the pixel is programmed so that the pixel current is a function of the OLED voltage. One method for extracting the OLED voltage is to first program the pixel in a way that the current is not a function of OLED voltage, and then in a way that the current is a function of OLED voltage. During the latter stage, the programming voltage is changed so that the pixel current is the same as the pixel current when the pixel was programmed in a way that the current was not a function of OLED voltage. The difference in the two programming voltages is then used to extract the OLED voltage.

Description:
FIELD OF THE INVENTION 
       [0001]    The present disclosure generally relates to circuits for use in displays, and methods of driving, calibrating, and programming displays, particularly displays such as active matrix organic light emitting diode displays. 
       BACKGROUND 
       [0002]    Displays can be created from an array of light emitting devices each controlled by individual circuits (i.e., pixel circuits) having transistors for selectively controlling the circuits to be programmed with display information and to emit light according to the display information. Thin film transistors (“TFTs”) fabricated on a substrate can be incorporated into such displays. TFTs tend to demonstrate non-uniform behavior across display panels and over time as the displays age. Compensation techniques can be applied to such displays to achieve image uniformity across the displays and to account for degradation in the displays as the displays age. 
         [0003]    Some schemes for providing compensation to displays to account for variations across the display panel and over time utilize monitoring systems to measure time dependent parameters associated with the aging (i.e., degradation) of the pixel circuits. The measured information can then be used to inform subsequent programming of the pixel circuits so as to ensure that any measured degradation is accounted for by adjustments made to the programming. Such monitored pixel circuits may require the use of additional transistors and/or lines to selectively couple the pixel circuits to the monitoring systems and provide for reading out information. The incorporation of additional transistors and/or lines may undesirably decrease pixel-pitch (i.e., “pixel density”). 
       SUMMARY 
       [0004]    In accordance with one embodiment, the OLED voltage of a selected pixel is extracted from the pixel produced when the pixel is programmed so that the pixel current is a function of the OLED voltage. One method for extracting the OLED voltage is to first program the pixel in a way that the current is not a function of OLED voltage, and then in a way that the current is a function of OLED voltage. During the latter stage, the programming voltage is changed so that the pixel current is the same as the pixel current when the pixel was programmed in a way that the current was not a function of OLED voltage. The difference in the two programming voltages is then used to extract the OLED voltage. 
         [0005]    Another method for extracting the OLED voltage is to measure the difference between the current of the pixel when it is programmed with a fixed voltage in both methods (being affected by OLED voltage and not being affected by OLED voltage). This measured difference and the current-voltage characteristics of the pixel are then used to extract the OLED voltage. 
         [0006]    A further method for extracting the shift in the OLED voltage is to program the pixel for a given current at time zero (before usage) in a way that the pixel current is a function of OLED voltage, and save the programming voltage. To extract the OLED voltage shift after some usage time, the pixel is programmed for the given current as was done at time zero. To get the same current as time zero, the programming voltage needs to change. The difference in the two programming voltages is then used to extract the shift in the OLED voltage. Here one needs to remove the effect of TFT aging from the second programming voltage first; this is done by programming the pixel without OLED effect for a given current at time zero and after usage. The difference in the programming voltages in this case is the TFT aging, which is subtracted from the calculated different in the aforementioned case. 
         [0007]    In one implementation, the current effective voltage V OLED  of a light-emitting device in a selected pixel is determined by supplying a programming voltage to the drive transistor in the selected pixel to supply a first current to the light-emitting device (the first current being independent of the effective voltage V OLED  of the light-emitting device), measuring the first current, supplying a second programming voltage to the drive transistor in the selected pixel to supply a second current to the light-emitting device, the second current being a function of the current effective voltage V OLED  of the light-emitting device, measuring the second current and comparing the first and second current measurements, adjusting the second programming voltage to make the second current substantially the same as the first current, and extracting the value of the current effective voltage V OLED  of the light-emitting device from the difference between the first and second programming voltages. 
         [0008]    In another implementation, the current effective voltage V OLED  of a light-emitting device in a selected pixel is determined by supplying a first programming voltage to the drive transistor in the selected pixel to supply a first current to the light-emitting device in the selected pixel (the first current being independent of the effective voltage V OLED  of the light-emitting device), measuring the first current, supplying a second programming voltage to the drive transistor in the selected pixel to supply a second current to the light-emitting device in the selected pixel (the second current being a function of the current effective voltage V OLED  of the light-emitting device), measuring the second current, and extracting the value of the current effective voltage V OLED  of the light-emitting device from the difference between the first and second current measurements. 
         [0009]    In a modified implementation, the current effective voltage V OLED  of a light-emitting device in a selected pixel is determined by supplying a first programming voltage to the drive transistor in the selected pixel to supply a predetermined current to the light-emitting device at a first time (the first current being a function of the effective voltage V OLED  of the light-emitting device), supplying a second programming voltage to the drive transistor in the selected pixel to supply the predetermined current to the light-emitting device at a second time following substantial usage of the display, and extracting the value of the current effective voltage V OLED  of the light-emitting device from the difference between the first and second programming voltages. 
         [0010]    In another modified implementation, the current effective voltage V OLED  of a light-emitting device in a selected pixel is determined by supplying a predetermined programming voltage to the drive transistor in the selected pixel to supply a first current to the light-emitting device (the first current being independent of the effective voltage V OLED  of the light-emitting device), measuring the first current, supplying the predetermined programming voltage to the drive transistor in the selected pixel to supply a second current to the light-emitting device (the second current being a function of the current effective voltage V OLED  of the light-emitting device), measuring the second current, and extracting the value of the current effective voltage V OLED  of the light-emitting device from the difference between the first and second currents and current-voltage characteristics of the selected pixel. 
         [0011]    In a preferred implementation, a system is provided for controlling an array of pixels in a display in which each pixel includes a light-emitting device. Each pixel includes a pixel circuit that comprises the light-emitting device, which emits light when supplied with a voltage V OLED ; a drive transistor for driving current through the light-emitting device according to a driving voltage across the drive transistor during an emission cycle, the drive transistor having a gate, a source and a drain and characterized by a threshold voltage; and a storage capacitor coupled across the source and gate of the drive transistor for providing the driving voltage to the drive transistor. A supply voltage source is coupled to the drive transistor for supplying current to the light-emitting device via the drive transistor, the current being controlled by the driving voltage. A monitor line is coupled to a read transistor that controls the coupling of the monitor line to a first node that is common to the source side of the storage capacitor, the source of the drive transistor, and the light-emitting device. A data line is coupled to a switching transistor that controls the coupling of the data line to a second node that is common to the gate side of the storage capacitor and the gate of the drive transistor. A controller coupled to the data and monitor lines and to the switching and read transistors is adapted to:
       (1) during a first cycle, turn on the switching and read transistors while delivering a voltage Vb to the monitor line and a voltage Vd 1  to the data line, to supply the first node with a voltage that is independent of the voltage across the light-emitting device,   (2) during a second cycle, turn on the read transistor and turn off the switching transistor while delivering a voltage Vref to the monitor line, and read a first sample of the drive current at the first node via the read transistor and the monitor line,   (3) during a third cycle, turn off the read transistor and turn on the switching transistor while delivering a voltage Vd 2  to the data line, so that the voltage at the second node is a function of V OLED , and   (4) during a fourth cycle, turn on said read transistor and turn off said switching transistor while delivering a voltage Vref to said monitor line, and read a second sample the drive current at said first node via said read transistor and said monitor line. The first and second samples of the drive current are compared and, if they are different, the first through fourth cycles are repeated using an adjusted value of at least one of the voltages Vd 1  and Vd 2 , until the first and second samples are substantially the same.       
 
         [0016]    The foregoing and additional aspects and embodiments of the present invention will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments and/or aspects, which is made with reference to the drawings, a brief description of which is provided next. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings. 
           [0018]      FIG. 1  is a block diagram of an exemplary configuration of a system for driving an OLED display while monitoring the degradation of the individual pixels and providing compensation therefor. 
           [0019]      FIG. 2A  is a circuit diagram of an exemplary pixel circuit configuration. 
           [0020]      FIG. 2B  is a timing diagram of first exemplary operation cycles for the pixel shown in  FIG. 2A . 
           [0021]      FIG. 2C  is a timing diagram of second exemplary operation cycles for the pixel shown in  FIG. 2A . 
           [0022]      FIG. 3  is a circuit diagram of another exemplary pixel circuit configuration. 
           [0023]      FIG. 4  is a block diagram of a modified configuration of a system for driving an OLED display using a shared readout circuit, while monitoring the degradation of the individual pixels and providing compensation therefor. 
       
    
    
       [0024]    While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0025]      FIG. 1  is a diagram of an exemplary display system  50 . The display system  50  includes an address driver  8 , a data driver  4 , a controller  2 , a memory storage  6 , and display panel  20 . The display panel  20  includes an array of pixels  10  arranged in rows and columns. Each of the pixels  10  is individually programmable to emit light with individually programmable luminance values. The controller  2  receives digital data indicative of information to be displayed on the display panel  20 . The controller  2  sends signals  32  to the data driver  4  and scheduling signals  34  to the address driver  8  to drive the pixels  10  in the display panel  20  to display the information indicated. The plurality of pixels  10  associated with the display panel  20  thus comprise a display array (“display screen”) adapted to dynamically display information according to the input digital data received by the controller  2 . The display screen can display, for example, video information from a stream of video data received by the controller  2 . The supply voltage  14  can provide a constant power voltage or can be an adjustable voltage supply that is controlled by signals from the controller  2 . The display system  50  can also incorporate features from a current source or sink (not shown) to provide biasing currents to the pixels  10  in the display panel  20  to thereby decrease programming time for the pixels  10 . 
         [0026]    For illustrative purposes, the display system  50  in  FIG. 1  is illustrated with only four pixels  10  in the display panel  20 . It is understood that the display system  50  can be implemented with a display screen that includes an array of similar pixels, such as the pixels  10 , and that the display screen is not limited to a particular number of rows and columns of pixels. For example, the display system  50  can be implemented with a display screen with a number of rows and columns of pixels commonly available in displays for mobile devices, monitor-based devices, and/or projection-devices. 
         [0027]    The pixel  10  is operated by a driving circuit (“pixel circuit”) that generally includes a driving transistor and a light emitting device. Hereinafter the pixel  10  may refer to the pixel circuit. The light emitting device can optionally be an organic light emitting diode, but implementations of the present disclosure apply to pixel circuits having other electroluminescence devices, including current-driven light emitting devices. The driving transistor in the pixel  10  can optionally be an n-type or p-type amorphous silicon thin-film transistor, but implementations of the present disclosure are not limited to pixel circuits having a particular polarity of transistor or only to pixel circuits having thin-film transistors. The pixel circuit  10  can also include a storage capacitor for storing programming information and allowing the pixel circuit  10  to drive the light emitting device after being addressed. Thus, the display panel  20  can be an active matrix display array. 
         [0028]    As illustrated in  FIG. 1 , the pixel  10  illustrated as the top-left pixel in the display panel  20  is coupled to a select line  24   i , a supply line  26   i , a data line  22   j , and a monitor line  28   j . A read line may also be included for controlling connections to the monitor line. In one implementation, the supply voltage  14  can also provide a second supply line to the pixel  10 . For example, each pixel can be coupled to a first supply line  26  charged with Vdd and a second supply line  27  coupled with Vss, and the pixel circuits  10  can be situated between the first and second supply lines to facilitate driving current between the two supply lines during an emission phase of the pixel circuit. The top-left pixel  10  in the display panel  20  can correspond a pixel in the display panel in a “ith” row and “jth” column of the display panel  20 . Similarly, the top-right pixel  10  in the display panel  20  represents a “jth” row and “mth” column; the bottom-left pixel  10  represents an “nth” row and “jth” column; and the bottom-right pixel  10  represents an “nth” row and “mth” column. Each of the pixels  10  is coupled to appropriate select lines (e.g., the select lines  24   i  and  24   n ), supply lines (e.g., the supply lines  26   i  and  26   n ), data lines (e.g., the data lines  22   j  and  22   m ), and monitor lines (e.g., the monitor lines  28   j  and  28   m ). It is noted that aspects of the present disclosure apply to pixels having additional connections, such as connections to additional select lines, and to pixels having fewer connections, such as pixels lacking a connection to a monitoring line. 
         [0029]    With reference to the top-left pixel  10  shown in the display panel  20 , the select line  24   i  is provided by the address driver  8 , and can be utilized to enable, for example, a programming operation of the pixel  10  by activating a switch or transistor to allow the data line  22   j  to program the pixel  10 . The data line  22   j  conveys programming information from the data driver  4  to the pixel  10 . For example, the data line  22   j  can be utilized to apply a programming voltage or a programming current to the pixel  10  in order to program the pixel  10  to emit a desired amount of luminance. The programming voltage (or programming current) supplied by the data driver  4  via the data line  22   j  is a voltage (or current) appropriate to cause the pixel  10  to emit light with a desired amount of luminance according to the digital data received by the controller  2 . The programming voltage (or programming current) can be applied to the pixel  10  during a programming operation of the pixel  10  so as to charge a storage device within the pixel  10 , such as a storage capacitor, thereby enabling the pixel  10  to emit light with the desired amount of luminance during an emission operation following the programming operation. For example, the storage device in the pixel  10  can be charged during a programming operation to apply a voltage to one or more of a gate or a source terminal of the driving transistor during the emission operation, thereby causing the driving transistor to convey the driving current through the light emitting device according to the voltage stored on the storage device. 
         [0030]    Generally, in the pixel  10 , the driving current that is conveyed through the light emitting device by the driving transistor during the emission operation of the pixel  10  is a current that is supplied by the first supply line  26   i  and is drained to a second supply line  27   i . The first supply line  26   i  and the second supply line  27   i  are coupled to the voltage supply  14 . The first supply line  26   i  can provide a positive supply voltage (e.g., the voltage commonly referred to in circuit design as “Vdd”) and the second supply line  27   i  can provide a negative supply voltage (e.g., the voltage commonly referred to in circuit design as “Vss”). Implementations of the present disclosure can be realized where one or the other of the supply lines (e.g., the supply line  27   i ) is fixed at a ground voltage or at another reference voltage. 
         [0031]    The display system  50  also includes a monitoring system  12 . With reference again to the top left pixel  10  in the display panel  20 , the monitor line  28   j  connects the pixel  10  to the monitoring system  12 . The monitoring system  12  can be integrated with the data driver  4 , or can be a separate stand-alone system. In particular, the monitoring system  12  can optionally be implemented by monitoring the current and/or voltage of the data line  22   j  during a monitoring operation of the pixel  10 , and the monitor line  28   j  can be entirely omitted. Additionally, the display system  50  can be implemented without the monitoring system  12  or the monitor line  28   j . The monitor line  28   j  allows the monitoring system  12  to measure a current or voltage associated with the pixel  10  and thereby extract information indicative of a degradation of the pixel  10 . For example, the monitoring system  12  can extract, via the monitor line  28   j , a current flowing through the driving transistor within the pixel  10  and thereby determine, based on the measured current and based on the voltages applied to the driving transistor during the measurement, a threshold voltage of the driving transistor or a shift thereof. 
         [0032]    The monitoring system  12  can also extract an operating voltage of the light emitting device (e.g., a voltage drop across the light emitting device while the light emitting device is operating to emit light). The monitoring system  12  can then communicate signals  32  to the controller  2  and/or the memory  6  to allow the display system  50  to store the extracted degradation information in the memory  6 . During subsequent programming and/or emission operations of the pixel  10 , the degradation information is retrieved from the memory  6  by the controller  2  via memory signals  36 , and the controller  2  then compensates for the extracted degradation information in subsequent programming and/or emission operations of the pixel  10 . For example, once the degradation information is extracted, the programming information conveyed to the pixel  10  via the data line  22   j  can be appropriately adjusted during a subsequent programming operation of the pixel  10  such that the pixel  10  emits light with a desired amount of luminance that is independent of the degradation of the pixel  10 . In an example, an increase in the threshold voltage of the driving transistor within the pixel  10  can be compensated for by appropriately increasing the programming voltage applied to the pixel  10 . 
         [0033]      FIG. 2A  is a circuit diagram of an exemplary driving circuit for a pixel  110 . The driving circuit shown in  FIG. 2A  is utilized to calibrate, program and drive the pixel  110  and includes a drive transistor  112  for conveying a driving current through an organic light emitting diode (“OLED”)  114 . The OLED  114  emits light according to the current passing through the OLED  114 , and can be replaced by any current-driven light emitting device. The OLED  114  has an inherent capacitance C OLED . The pixel  110  can be utilized in the display panel  20  of the display system  50  described in connection with  FIG. 1 . 
         [0034]    The driving circuit for the pixel  110  also includes a storage capacitor  116  and a switching transistor  118 . The pixel  110  is coupled to a select line SEL, a voltage supply line Vdd, a data line Vdata, and a monitor line MON. The driving transistor  112  draws a current from the voltage supply line Vdd according to a gate-source voltage (Vgs) across the gate and source terminals of the drive transistor  112 . For example, in a saturation mode of the drive transistor  112 , the current passing through the drive transistor  112  can be given by Ids=β(Vgs−Vt) 2 , where β is a parameter that depends on device characteristics of the drive transistor  112 , Ids is the current from the drain terminal to the source terminal of the drive transistor  112 , and Vt is the threshold voltage of the drive transistor  112 . 
         [0035]    In the pixel  110 , the storage capacitor  116  is coupled across the gate and source terminals of the drive transistor  112 . The storage capacitor  116  has a first terminal, which is referred to for convenience as a gate-side terminal, and a second terminal, which is referred to for convenience as a source-side terminal. The gate-side terminal of the storage capacitor  116  is electrically coupled to the gate terminal of the drive transistor  112 . The source-side terminal  116   s  of the storage capacitor  116  is electrically coupled to the source terminal of the drive transistor  112 . Thus, the gate-source voltage Vgs of the drive transistor  112  is also the voltage charged on the storage capacitor  116 . As will be explained further below, the storage capacitor  116  can thereby maintain a driving voltage across the drive transistor  112  during an emission phase of the pixel  110 . 
         [0036]    The drain terminal of the drive transistor  112  is connected to the voltage supply line Vdd, and the source terminal of the drive transistor  112  is connected to (1) the anode terminal of the OLED  114  and (2) a monitor line MON via a read transistor  119 . A cathode terminal of the OLED  114  can be connected to ground or can optionally be connected to a second voltage supply line, such as the supply line Vss shown in  FIG. 1 . Thus, the OLED  114  is connected in series with the current path of the drive transistor  112 . The OLED  114  emits light according to the magnitude of the current passing through the OLED  114 , once a voltage drop across the anode and cathode terminals of the OLED achieves an operating voltage (V OLED ) of the OLED  114 . That is, when the difference between the voltage on the anode terminal and the voltage on the cathode terminal is greater than the operating voltage V OLED , the OLED  114  turns on and emits light. When the anode-to-cathode voltage is less than V OLED , current does not pass through the OLED  114 . 
         [0037]    The switching transistor  118  is operated according to the select line SEL (e.g., when the voltage on the select line SEL is at a high level, the switching transistor  118  is turned on, and when the voltage SEL is at a low level, the switching transistor is turned off). When turned on, the switching transistor  118  electrically couples node A (the gate terminal of the driving transistor  112  and the gate-side terminal of the storage capacitor  116 ) to the data line Vdata. 
         [0038]    The read transistor  119  is operated according to the read line RD (e.g., when the voltage on the read line RD is at a high level, the read transistor  119  is turned on, and when the voltage RD is at a low level, the read transistor  119  is turned off). When turned on, the read transistor  119  electrically couples node B (the source terminal of the driving transistor  112 , the source-side terminal of the storage capacitor  116 , and the anode of the OLED  114 ) to the monitor line MON. 
         [0039]      FIG. 2B  is a timing diagram of exemplary operation cycles for the pixel  110  shown in  FIG. 2A . During a first cycle  150 , both the SEL line and the RD line are high, so the corresponding transistors  118  and  119  are turned on. The switching transistor  118  applies a voltage Vd 1 , which is at a level sufficient to turn on the drive transistor  112 , from the data line Vdata to node A. The read transistor  119  applies a monitor-line voltage Vb, which is at a level that turns the OLED  114  off, from the monitor line MON to node B. As a result, the gate-source voltage Vgs is independent of V OLED  (Vd 1 −Vb−Vds 3 , where Vds 3  is the voltage drop across the read transistor  119 ). The SEL and RD lines go low at the end of the cycle  150 , turning off the transistors  118  and  119 . 
         [0040]    During the second cycle  154 , the SEL line is low to turn off the switching transistor  118 , and the drive transistor  112  is turned on by the charge on the capacitor  116  at node A. The voltage on the read line RD goes high to turn on the read transistor  119  and thereby permit a first sample of the drive transistor current to be taken via the monitor line MON, while the OLED  114  is off. The voltage on the monitor line MON is Vref, which may be at the same level as the voltage Vb in the previous cycle. 
         [0041]    During the third cycle  158 , the voltage on the select line SEL is high to turn on the switching transistor  118 , and the voltage on the read line RD is low to turn off the read transistor  119 . Thus, the gate of the drive transistor  112  is charged to the voltage Vd 2  of the data line Vdata, and the source of the drive transistor  112  is set to V OLED  by the OLED  114 . Consequently, the gate-source voltage Vgs of the drive transistor  112  is a function of V OLED  (Vgs=Vd 2 −V OLED ). 
         [0042]    During the fourth cycle  162 , the voltage on the select line SEL is low to turn off the switching transistor, and the drive transistor  112  is turned on by the charge on the capacitor  116  at node A. The voltage on the read line RD is high to turn on the read transistor  119 , and a second sample of the current of the drive transistor  112  is taken via the monitor line MON. 
         [0043]    If the first and second samples of the drive current are not the same, the voltage Vd 2  on the Vdata line is adjusted, the programming voltage Vd 2  is changed, and the sampling and adjustment operations are repeated until the second sample of the drive current is the same as the first sample. When the two samples of the drive current are the same, the two gate-source voltages should also be the same, which means that: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0044]    After some operation time (t), the change in V OLED  between time  0  and time t is ΔV OLED =V OLED (t)−V OLED ( 0 )=Vd 2 ( t )−Vd 2 ( 0 ). Thus, the difference between the two programming voltages Vd 2 ( t ) and Vd 2 ( 0 ) can be used to extract the OLED voltage. 
         [0045]      FIG. 2C  is a modified schematic timing diagram of another set of exemplary operation cycles for the pixel  110  shown in  FIG. 2A , for taking only a single reading of the drive current and comparing that value with a known reference value. For example, the reference value can be the desired value of the drive current derived by the controller to compensate for degradation of the drive transistor  112  as it ages. The OLED voltage V OLED  can be extracted by measuring the difference between the pixel currents when the pixel is programmed with fixed voltages in both methods (being affected by V OLED  and not being affected by V OLED ). This difference and the current-voltage characteristics of the pixel can then be used to extract V OLED . 
         [0046]    During the first cycle  200  of the exemplary timing diagram in  FIG. 2C , the select line SEL is high to turn on the switching transistor  118 , and the read line RD is low to turn off the read transistor  118 . The data line Vdata supplies a voltage Vd 2  to node A via the switching transistor  118 . During the second cycle  201 , SEL is low to turn off the switching transistor  118 , and RD is high to turn on the read transistor  119 . The monitor line MON supplies a voltage Vref to the node B via the read transistor  118 , while a reading of the value of the drive current is taken via the read transistor  119  and the monitor line MON. This read value is compared with the known reference value of the drive current and, if the read value and the reference value of the drive current are different, the cycles  200  and  201  are repeated using an adjusted value of the voltage Vd 2 . This process is repeated until the read value and the reference value of the drive current are substantially the same, and then the adjusted value of Vd 2  can be used to determine V OLED . 
         [0047]      FIG. 3  is a circuit diagram of two of the pixels  110   a  and  110   b  like those shown in  FIG. 2A  but modified to share a common monitor line MON, while still permitting independent measurement of the driving current and OLED voltage separately for each pixel. The two pixels  110   a  and  110   b  are in the same row but in different columns, and the two columns share the same monitor line MON. Only the pixel selected for measurement is programmed with valid voltages, while the other pixel is programmed to turn off the drive transistor  12  during the measurement cycle. Thus, the drive transistor of one pixel will have no effect on the current measurement in the other pixel. 
         [0048]      FIG. 4  illustrates a modified drive system that utilizes a readout circuit  300  that is shared by multiple columns of pixels while still permitting the measurement of the driving current and OLED voltage independently for each of the individual pixels  10 . Although only four columns are illustrated in  FIG. 4 , it will be understood that a typical display contains a much larger number of columns, and they can all use the same readout circuit. Alternatively, multiple readout circuits can be utilized, with each readout circuit still sharing multiple columns, so that the number of readout circuits is significantly less than the number of columns. Only the pixel selected for measurement at any given time is programmed with valid voltages, while all the other pixels sharing the same gate signals are programmed with voltages that cause the respective drive transistors to be off. Consequently, the drive transistors of the other pixels will have no effect on the current measurement being taken of the selected pixel. Also, when the driving current in the selected pixel is used to measure the OLED voltage, the measurement of the OLED voltage is also independent of the drive transistors of the other pixels. 
         [0049]    While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.