Abstract:
It is known to compensate for threshold voltage variation of driving transistors in pixel circuits that drive light emission devices such as current driven organic light emission devices. However, programming and initialization of such pixel circuits can be slow and require a plurality of control or signal lines. The present invention provides a pixel circuit comprising an n-channel transistor for diode-connecting the driver transistor and a means for reducing the number of signal and control lines.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates, in general, to a pixel circuit of a type employed in a display system using a current driven organic or other light-emission device as a light source. 
   2. Description of the Prior Art 
   Display systems commonly comprise an array of pixel circuits having an organic light-emitting device (OLED) as a light source and a driving circuit for driving the OLED in accordance with a received data signal. The OLED consists of a light-emitting polymer (LEP) layer sandwiched between an anode layer and a cathode layer. Electrically, the OLED operates as a diode whilst optically, the OLED emits light when forward biased with the brightness of the emitted light increasing as the forward bias current increases. By integrating the driving circuits of individual pixel circuits in the array using low-temperature polysilicon Thin Film Transistor (TFT) technology, it is possible to control the brightness of each individual OLED in order to provide a still or a moving image on the display. 
   Since an OLED is a current driven device, if the pixel circuit receives a voltage signal, a driver transistor or the like is required to supply an appropriate level of current to the OLED in response to the received voltage signal. An example of a known voltage driven pixel circuit for an active matrix OLED display is illustrated in  FIG. 1 . Referring to  FIG. 1 , a pixel circuit  10  comprises a first p-channel TFT T 1  and a second p-channel TFT T 2  per pixel. The first TFT T 1  is a switch for addressing the pixel circuit  10  and comprises a terminal coupled to a first supply line  12  for receiving a voltage data signal VData. The first TFT T 1  also comprises a gate terminal coupled to a second supply line  14  for receiving a supply voltage VSEL, and a terminal coupled to a gate terminal of the second TFT T 2 . The second TFT T 2  comprises a terminal coupled to a third supply line  16  for receiving a supply voltage VDD, and a terminal coupled to an anode terminal of an OLED  18 , a cathode terminal of the OLED  18  being coupled to ground. The second TFT T 2  is an analogue driver TFT for converting the voltage data signal VData into a current signal that in turn drives the OLED  18  at a designated brightness. 
   Display systems employing an array of voltage driven pixel circuits as illustrated in  FIG. 1  can experience non-uniformity problems in their displayed images even though individual driving TFTs in the array are supplied with an identical voltage data signal and supply voltage. The non-uniformity arises due to a spatial variation in the threshold voltage of individual driving TFTs within the array of pixel circuits that form the display. Each OLED is therefore driven at a different brightness corresponding to the difference in threshold voltage between the driving TFTs. One approach to solving the non-uniformity problem has been disclosed by S. M. Choi, et al. in “A self-compensated voltage programming pixel structure for active-matrix organic light emitting diodes”, International Display Workshop 2003, p535-538. A pixel circuit embodiment as disclosed by Choi et al., is illustrated in  FIG. 2 . 
   Referring to  FIG. 2 , a pixel circuit  20  for compensating voltage threshold variations of individual driving TFTs comprises six TFTs M 1 , M 2 , M 3 , M 4 , M 5  and M 6 , one capacitor C 1  and two horizontal control lines, scan[n−1] and scan[n]. M 2 , M 3 , M 4 , M 5  and M 6  are switching TFTs, and M 1  is an analogue driver TFT for providing a current that in turn drives an OLED  22  at a designated brightness during a time period of one frame. 
   In operation, the fourth TFT M 4  provides a current path to establish a gate terminal voltage of the driver TFT M 1  at a predetermined value. The capacitor C 1  is a storage capacitor and stores the gate terminal voltage of the driver TFT M 1 . Since the pixel circuit  20  requires two row line time to complete data programming operation, the scan[n] (present row scan) and the scan[n−1] (previous row scan) signals are applied to program the pixel circuit  20 . 
   During the previous row scan, when the scan[n−1] signal is logic low, a gate terminal voltage of the driver TFT M 1  is charged to a voltage VI in a step referred to as initialisation. Next and during the present row scan, when the scan[n] signal is logic low, TFT M 2  and TFT M 3  are turned on so that the voltage data signal data[m] is programmed to a gate node of the driver TFT M 1  through diode connected driver TFT M 1 . At this time, the programmed voltage at the gate node of the driver TFT M 1  is automatically reduced to a value data signal voltage data[m] less a threshold voltage V TH  of the driver TFT M 1 . During initialisation and programming TFTs M 5  and M 6  are turned off. 
   Following the previous and present row scans, TFT M 5  and TFT M 6  are turned on by an em[n] signal to establish a current path from VDD to ground so that current can flow through the driver TFT M 1  and drive the OLED  22 . The driver TFT M 1  therefore moderates the current independently of the voltage threshold V TH . 
   Although the above pixel circuit  20  provides a means for compensating voltage threshold variations of individual driving TFTs, there is a need to increase the speed at which a pixel circuit can be programmed because an increase in programming speed is necessary in order that display systems can perform adequately when supplied with high bandwidth data or when employed in large size displays. Furthermore, there is a need for smaller display systems featuring lower power consumption in order to prolong the life of the power supply and expand the functionality of the system. 
   SUMMARY OF THE INVENTION 
   According to an aspect of the present invention, there is provided a pixel circuit comprising:
         a first transistor and a capacitor connected in series between a power supply line and a reference line, a gate terminal of the first transistor arranged to receive a first control signal;   a driving transistor and a light emitting device connected in series between the power supply line and a further line, the driving transistor having a gate terminal connected to a first node, which is between the first transistor and the capacitor, and a first terminal for receiving a data signal; and   a second transistor arranged to diode-connect the driving transistor in response to a second control signal received at a gate terminal of the second transistor, whereby the data signal can be passed through the driving transistor when diode-connected and held at the first node, the second transistor being an n-channel type transistor.       

   Preferably, a third transistor is connected in series between the power supply line and the driving transistor and a fourth transistor is connected in series between the light emitting device and the driving transistor, wherein one terminal of the second transistor is coupled to a second terminal of the driving transistor at a second node between the driving transistor and the third transistor. 
   Preferably, the third and fourth transistors are p-channel type transistors and their gate terminals are arranged to receive the second control signal. More preferably, a fifth transistor is connected between a data signal line and a third node between the driving transistor and the fourth transistor. The fifth transistor may be of an n-channel type transistor and comprise a gate terminal to receive the second control signal. 
   Preferably, a sixth transistor is coupled in series between the fourth transistor and the light emitting device, the sixth transistor being of the opposite channel type to the first transistor and having a gate terminal to receive the first control signal. 
   Preferably, a seventh transistor is coupled in series between the gate terminal of the driving transistor and the first node and an eighth transistor is coupled between the power supply line and a fourth node between one terminal of the seventh transistor and the gate terminal of the driving transistor, wherein the eighth transistor is of the same channel type as the first transistor and the seventh transistor is of the opposite channel type to the first transistor, the gate terminals of the seventh and eighth transistors being arranged to receive the first control signal. 
   The pixel circuit may further comprise a ninth transistor coupled between the first node and the terminal of the second transistor that is connected to the gate terminal of the driving transistor and a tenth transistor coupled between the first node and the other terminal of the second transistor that is connected to a second terminal of the driving transistor, wherein the ninth transistor is a p-channel type transistor and the tenth transistor is an n-channel type transistor and the gate terminals of the ninth and tenth transistors are arranged to receive the first and second control signals respectively. 
   According to another aspect of the present invention, there is provided a pixel circuit for driving a current driven element, comprising:
         a first transistor of which a conduction state corresponds to a current level of a driving current that is supplied to the current driven element, the first transistor having a first gate terminal, a first terminal, and a second terminal;   a second transistor having a second gate terminal; and   a third transistor arranged to control electrical connection between the first gate terminal and one of the first terminal and the second terminal, the third transistor having a third gate terminal,   the first terminal arranged to receive a data signal through the second transistor, the data signal determining the conduction state of the first transistor, and   a conduction type of the first transistor being different from a conduction type of the second transistor.       

   According to another aspect of the present invention, there is provided a pixel circuit for driving a current driven element, comprising:
         a first transistor of which a conduction state corresponds to a current level of a driving current that is supplied to the current driven element, the first transistor having a first gate terminal, a first terminal, and a second terminal;   a second transistor having a second gate terminal; and   a third transistor arranged to control electrical connection between the first gate terminal and one of the first terminal and the second terminal, the third transistor having a third gate terminal;   the first terminal arranged to receive a data signal through the second transistor, the data signal determining the conduction state of the first transistor, and   a conduction type of the first transistor being different from a conduction type of the third transistor.       

   Preferably, a fourth transistor having a fourth gate terminal is coupled in series between the current driven element and the first transistor. More preferably, a conduction type of the fourth transistor is different from a conduction type of the second transistor. 
   Preferably, a fifth transistor having a fifth gate terminal is coupled in series between the first transistor and a power supply line from which the driving current is supplied to the current driven element through the first transistor. 
   A conduction type of the fourth transistor may be the same as a conduction type of the fifth transistor. The conduction type of the first transistor may be of a p-channel type. Preferably, the fourth gate terminal, the second gate terminal and the third gate terminal are connected to one signal line. Preferably, the fifth gate terminal, the second gate terminal and the third gate terminal are connected to one signal line. Preferably, a sixth transistor is coupled in series between the fourth transistor and the current driven element. 
   Preferably, the first gate is connected to a power supply line through a capacitor. More preferably, a seventh transistor is connected between the first gate and the first capacitor. 
   Preferably, an eighth transistor is connected directly between the power supply line and the first gate. 
   Preferably, a ninth transistor is connected between the capacitor and the second terminal. 
   According to another aspect of the present invention, there is provided a display apparatus comprising a plurality of pixel circuits as described above. Preferably, the display apparatus is formed with at least a first signal line, a second signal line, a third signal line and a data signal line in a matrix, the first control signal line providing a first control signal for a first pixel circuit and the second control signal line providing a second control signal for the first pixel circuit; wherein a first control signal for a second pixel circuit is the second control signal for the first pixel circuit provided by the second control line, and the third control line provides a second control signal for the second pixel circuit. 
   According to another aspect of the present invention, there is provided a method of driving a pixel circuit comprising:
         applying a first control signal to switch on a first transistor connected between a power supply line and a reference line and in series with a first capacitor;   applying a second control signal to switch on a second transistor to diode-connect a driving transistor, the second transistor being an n-channel transistor and the driving transistor being connected in series to a light emitting device between the power supply line and a further line, a gate terminal of the driving transistor being connected to a first node between the first transistor and the first capacitor and a first terminal of the driving transistor arranged to receive a data signal;   applying the first control signal to switch off the first transistor;   applying the data signal to the first terminal of the driving transistor;   applying the second control signal to switch off the second transistor.       

   Preferably, the method further comprises applying the second control signal to a third transistor connected in series between the power supply and the driving transistor and to a fourth transistor connected in series between the light emitting device and the driving transistor to switch off the third and fourth transistors whilst the second transistor is switched on, and switch on the third and fourth transistors whilst the second transistor is switched off, wherein one terminal of the second transistor is coupled to one terminal of the driving transistor at a second node between the driving transistor and the third transistor. 
   Preferably, the third and fourth transistors are p-channel type transistors. Preferably, the method also comprises applying the second control signal to a fifth transistor connected between a data signal line and a third node between the driving transistor and the fourth transistor to switch on the fifth transistor whilst the second transistor is switched on and switch off the fifth transistor whilst the second transistor is switched off. 
   Preferably, the method further comprises applying the first control signal to a sixth transistor coupled in series between the fourth transistor and the light emitting device, to switch off the sixth transistor whilst the first transistor is switched on, the sixth transistor being of the opposite channel type to the first transistor. 
   Preferably, the method also includes applying the first control signal to a seventh transistor coupled in series between the gate terminal of the driving transistor and the first node and to an eighth transistor coupled between the power supply line and a fourth node between one terminal of the seventh transistor and the gate terminal of the driving transistor, wherein the eighth transistor is of the same channel type as the first transistor and the seventh transistor is of the opposite channel type to the first transistor, to switch off the seventh transistor and to switch on the eighth transistor whilst the first transistor is switched on. 
   Preferably, the method further comprises applying the first control signal to a ninth transistor connected between the first node and the terminal of the second transistor that is connected to the gate terminal of the driving transistor and applying the second control signal to a tenth transistor coupled between the first node and the other terminal of the second transistor that is connected to a second terminal of the driving transistor, wherein the ninth transistor is a p-channel type transistor and the tenth transistor is an n-channel type transistor, to switch off the ninth transistor when the first transistor is switched on and to switch on the tenth transistor when the second transistor is switched on. 
   The reference line may be a data signal line, or, wherein the first transistor is connected in series between the fifth transistor and the capacitor, the data signal line is the reference line, the method further comprising:
         after applying the first control signal to switch on the first transistor and before applying the first control signal to switch off the first transistor, applying a pre-charge signal on the data signal line, the pre-charge signal having a value lower than the data signal.       

   According to another aspect of the present invention, there is provided a method of driving a pixel circuit that includes a first transistor having a first gate terminal, a first terminal and a second terminal, a second transistor having a second gate terminal, a third transistor that has a third gate terminal and that controls electrical connection between the first gate terminal and the second terminal, a fourth terminal that controls electrical connection between a current driven element and the first transistor, and a fifth terminal that controls electrical connection between the second terminal and a predetermined voltage, the method comprising:
         producing a first state of the pixel circuit in which the second terminal being set to a predetermined voltage by turning on the fifth transistor;   producing a second state of the pixel circuit in which the first terminal is electrically connected to the second terminal through the third transistor at least a part of a first period during which the first terminal receives a data signal through the second transistor; and   producing a third state of the pixel circuit in which a driving current of which a current level corresponds to a conduction state set through the second state is supplied to a current driven element through the first transistor and the fourth transistor,   the second terminal being electrically disconnected from the predetermined voltage in the second state,   the first terminal being electrically disconnected from the current driven element in the second state, and   one control signal being supplied to the second gate terminal, the third terminal, the fourth terminal, and the fifth terminal in common.       

   When in use, the time taken for initialisation and programming of the pixel circuit according to the present invention is reduced thereby providing a more efficient, faster and more versatile display system than in the prior art. The third signal em[n] used in the prior art is no longer required since the arrangement of the pixel circuit permits signals em[n] and scan[n] to be replaced by a single control signal. In a preferred embodiment, a reference signal supply line is no longer required thereby providing a more compact display system. The number of control lines can also be reduced thereby also providing a more compact and efficient display system than is known from the prior art. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described by way of further example only and with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic diagram of a prior art voltage driven pixel circuit for an active matrix OLED display; 
       FIG. 2  is a schematic diagram of a prior art self-compensated voltage programming pixel structure for an active-matrix OLED display; 
       FIG. 3  is a schematic diagram illustrating two ways to diode connect a transistor; 
       FIG. 4  is a schematic diagram of a pixel circuit according to a first embodiment of the present invention; 
       FIG. 5  is a schematic diagram illustrating a section of the pixel circuit of  FIG. 4  at a steady state voltage; 
       FIG. 6  is a schematic diagram of a pixel circuit according to a second embodiment of the present invention; 
       FIG. 7  is a schematic diagram of a pixel circuit according to a third embodiment of the present invention; 
       FIG. 8  is a schematic diagram of a pixel circuit according to a fourth embodiment of the present invention; 
       FIG. 9  is a schematic diagram of a pixel circuit according to a fifth embodiment of the present invention; 
       FIG. 10  is a schematic diagram of general driving waveforms for the pixel circuits as illustrated in  FIGS. 4 ,  6 ,  7 ,  8  and  9 ; 
       FIG. 11  is a schematic diagram of general driving waveforms for the pixel circuits as illustrated in  FIGS. 6 ,  7 ,  8  and  9 ; 
       FIG. 12  is a schematic diagram of architecture for the pixel circuits as illustrated in  FIGS. 4 ,  6 ,  7  and  8 ; 
       FIG. 13  is a schematic diagram of architecture for the pixel circuits as illustrated in  FIG. 9 ; 
       FIG. 14  is a schematic diagram of a simulation of a voltage at the node newdg for the pixel circuit as illustrated in  FIG. 4 ; 
       FIG. 15  is a schematic diagram of a simulation of an output current for varying values of ΔV T ; 
       FIG. 16  is a schematic diagram of a simulation of an output current for different input voltages and for varying values of ΔV T ; 
       FIG. 17  is a schematic view of a mobile telephone incorporating a display system according to the present invention; 
       FIG. 18  is a schematic view of a mobile personal computer incorporating a display system according to the present invention; and 
       FIG. 19  is a schematic view of a digital camera incorporating a display system according to the present invention. 
   

   DETAILED DESCRIPTION 
   Throughout the following description like reference numerals shall be used to identify like parts. 
   Referring to  FIG. 3 , a driver transistor  74  having pins  1 ,  2 ,  3  can be diode-connected in two ways although in either configuration of a diode-connected transistor, a gate terminal is always connected to a drain terminal. Pins  1  and  2  can be connected thereby forming a cathode terminal with pin  3  forming an anode terminal. Alternatively, pins  2  and  3  can be connected thereby forming a cathode terminal with pin  1  forming an anode terminal. 
   As noted above, similar TFTs have varying threshold voltages even when they are manufactured at the same time and by the same process. All TFTs in an array can be considered to have a common nominal threshold voltage V T . In addition, individual TFTs can be considered to have different threshold voltage variations ΔV T . Thus, the actual threshold voltage for each TFT is V T +ΔV T , with ΔV T  varying between TFTs. 
   In the present invention, driver transistors have the property that the threshold voltage V T +ΔV T  is the same irrespective of the direction in which current flows—in other words, which terminal is set as the source and which terminal is set as the drain. 
   Driver transistors that are symmetrical between the source and the drain terminal and which have not been stressed have this property. In symmetrical transistors, the source and drain terminal are equally doped and are symmetrical with respect to the gate terminal. Such transistors are commonly self-aligned. For a symmetrical driver transistor  74  with a nominal threshold voltage V T  and a threshold voltage variation ΔV T , the observed threshold voltage of the driver transistor  74  when diode connected remains V T +ΔV T  and is independent of the way the driver transistor  74  is diode connected. 
   Referring to  FIG. 4 , a pixel circuit  50  according to a first embodiment of the present invention comprises a first rail  52  having a first node  54  coupled to a first terminal of a first capacitor  56 . A second terminal of the first capacitor  56  is coupled to a second node  58  (referred to as newdg) which is coupled to a source terminal of a first n-channel transistor  60  and a third node  62 . The first n-channel transistor  60  comprises a gate terminal and also a drain terminal that is coupled to a second rail  64 . 
   The first rail  52  comprises a fourth node  66  coupled to a source terminal of a first p-channel transistor  68  comprising a gate terminal coupled to a fifth node  70  and a drain terminal coupled to a sixth node  72  (referred to as int). The sixth node  72  int is coupled to a first terminal of the driver transistor  74  comprising a gate terminal and a third terminal. The driver transistor  74  is a second p-channel transistor. As best seen with reference to  FIG. 3  and also described in detail later with reference to  FIG. 5 , the first terminal and the third terminal of the driver transistor  74  can interchange as a source and a drain terminal depending upon whether the driver transistor  74  is diode-connected. The third terminal of the driver transistor  74  is coupled to a seventh node  76  (referred to as ipn) and the gate terminal is coupled to the third node  62 . 
   The sixth node  72  int is also coupled to a source terminal of a second n-channel transistor  78  comprising a gate terminal coupled to an eighth node  80  and a drain terminal coupled to the third node  62 . The eighth node  80  is coupled to an ninth node  82  which is coupled to a gate terminal of a third n-channel transistor  84  and to a gate terminal of a third p-channel transistor  86 . A drain terminal of the third n-channel transistor  84  is coupled to the seventh node  76  ipn and a source terminal is coupled to a third rail  88 . A source terminal of the third p-channel transistor  86  is coupled to the seventh node  76  ipn and a drain terminal is coupled to an anode terminal of an OLED  96  comprising a cathode terminal coupled to the fourth rail  94 . A second capacitor  92  is also included in the pixel circuit  50  to represent an associated parasitic capacitance of the OLED  96 . 
   With reference to the description above and throughout the following description, a reference to a node in the pixel circuit  50  is descriptive only. As an example, nodes  70 ,  80 , and  82  of  FIG. 4  can, alternatively, be illustrated as one connection. 
   In operation, a voltage V DD  for example of 5V is applied across the pixel circuit  50  to drive the OLED  96 , although other voltages can be used. As discussed above with reference to  FIG. 3 , the driver transistor  74  has a nominal threshold voltage V T  and a threshold voltage variation ΔV T . The observed threshold voltage of the driver transistor  74  when diode connected is therefore V T +ΔV T . The threshold voltage variation ΔV T  is represented in  FIG. 4  and those following by a variable voltage source connected in series with the gate terminal of the driver transistor  74 . The first n-channel transistor  60 , second n-channel transistor  78  and third n-channel transistor  84  together with the first p-channel transistor  68  and third p-channel transistor  86  operate as switches under the control of a first signal φ 1  and a second signal φ 2  whilst the second p-channel transistor is the driver transistor  74  for supplying a controlled level of current to the OLED  96 . 
   The pixel circuit  50  has three stages of operation: a pre-charge stage, a self-adjustment stage and an output stage. 
   In the pre-charge stage, the first signal φ 1  is logic 1 and is applied to the gate terminal of the second n-channel transistor  78 , the third n-channel transistor  84 , the first p-channel transistor  68  and the third p-channel transistor  86 . The second n-channel transistor  78  and the third n-channel transistor are therefore switched on whilst the first p-channel transistor  68  and the third p-channel transistor  86  are switched off. Also in the pre-charge stage, the second signal φ 2  is logic 1 and is applied to the gate terminal of the first n-channel transistor  60  thereby switching on the first n-channel transistor  60 . The driver transistor  74  is therefore diode-connected using the second n-channel transistor  78 , isolated from the V DD  to ground path by the switching off of the first p-channel transistor  68  and the second node  58  newdg is earthed through the switching on of the first n-channel transistor  60 . 
   The third rail  88  is at a voltage V DAT  that in the pre-charge stage of the present embodiment is, for example, 0V although other voltages can be used. Consequently, the second node  58 , newdg, is pre-charged to a voltage Vnewdg equal to that of the second rail  64  such as ground (0V) and the pixel circuit  50  can be represented by the pixel circuit  50  illustrated in  FIG. 5(   a ). As such, the voltage across the first capacitor  56  which is given by V DD −Vnewdg=5V. 
   The second node  58  newdg and the sixth node  72  int are connected through the second n-channel transistor  78  and the voltage across the second node  58  Vnewdg equals the voltage across the sixth node  72  Vint. The supply rail  88  that supplies the voltage V DAT  is connected to the seventh node  76  ipn through the third n-channel transistor  84  and the voltage across the seventh node  76  Vipn equals V DAT . As such, the second node  58  newdg is the cathode terminal and the seventh node  76  ipn is the anode terminal of the diode-connected driver transistor  74 . 
   In the self-adjustment stage, and more particularly during data transfer of the self-adjustment stage, the first signal φ 1  remains logic 1 applied to the gate terminal of the second n-channel transistor  78 , the third n-channel transistor  84 , the first p-channel transistor  68  and the third p-channel transistor  86 . The second n-channel transistor  78  and the third n-channel transistor remain switched on whilst the first p-channel transistor  68  and the third p-channel transistor  86  remain switched off. 
   The second signal φ 2  becomes logic 0 applied to the gate terminal of the first n-channel transistor  60  thereby switching off the first n-channel transistor  60  causing the second node, newdg to no longer be earthed. 
   Voltage V DAT  now pulses to a required value of V DAT  for driving the OLED  96 , for example 3V. Preferably, the commencement of the pulse to the required value of V DAT  occurs simultaneously or later than the switching off of the first n-channel transistor  60 . 
   Since the second node  58 , newdg, is pre-charged to ground (0V) and is less than V DAT  (3V), the diode-connected driver transistor  74  is forward-biased and current, I, flows to the first capacitor  56  to discharge the first capacitor  56  until a steady state is reached. 
   At steady state, Vnewdg=V DAT −(V T +ΔV T ). The voltage across the first capacitor  56  is therefore: V DD −Vnewdg=V DD −(V DAT −(V T +ΔV T )). If a value of 1.1V is provided for the nominal threshold voltage V T , the voltage across the first capacitor  56  at steady state equals 3.1V+ΔV T . The time taken for steady state to be reached is primarily dependent upon the RC time constant generated between the first capacitor  56  and the impedance of the second n-channel transistor  78  that enables the driving transistor  74  to be diode-connected. Although less significant, the resistance of the driver transistor  74  and the third n-channel transistor  84  also contribute to the time taken for steady state to be reached. 
   The effective voltage of the gate terminal, Vdg=Vnewdg+ΔV T . Therefore, when steady state is reached, the effective voltage of the gate terminal Vdg can be written as Vdg=V DAT −V T ,=1.9V which is independent of any threshold variation ΔV T . 
   In the output stage, the first signal φ 1  is logic 0 and is applied to the gate terminal of the second n-channel transistor  78 , the third n-channel transistor  84 , the first p-channel transistor  68  and the third p-channel transistor  86 . The second n-channel transistor  78  and the third n-channel transistor are therefore switched off whilst the first p-channel transistor  68  and the third p-channel transistor  86  are switched on. In the output stage, the second signal φ 2  remains logic 0. 
   As best shown in  FIG. 5(   b ), in the output stage, the driver transistor  74  is no longer diode-connected between the first terminal and the gate terminal and therefore acts as a constant current source for the OLED  96 . The amplitude of the current passed to the OLED  96  by the driver transistor  74  is dependent on the value of V DAT  (more specifically, the value that V DAT  pulses to in the self-adjustment stage) and not the threshold variation ΔV T . Therefore, all pixel circuits  50  in an array forming a display are driven to the same brightness for the same value of V DAT . 
   Exemplary driving waveforms for the pixel circuit  50  as illustrated in  FIG. 4  are illustrated in  FIG. 10 . Referring to  FIG. 10(   a ), the first signal φ 1  and the second signal φ 2  are both logic 1 indicating the commencement of the pre-charge stage in order to set the second node  58  newdg to a voltage equal to ground as described above. As the second signal φ 2  drops to logic 0, the self-adjustment stage commences and V DAT  pulses to a value of e.g., 3V. Since, the second node  58 , newdg, is pre-charged to a voltage equal to that of ground and is less than V DAT  (3V), the diode-connected driver transistor  74  is forward-biased and current, I, flows to the first capacitor  56  to discharge the first capacitor  56  until a steady state is reached. On reaching a steady state, the first signal φ 1  becomes logic 0 and the output stage commences so as to drive the OLED  96  independently of threshold variation ΔV T . As should be appreciated by a person skilled in the art, the driving waveforms illustrated in  FIGS. 10(   b ) to ( d ) are also equally applicable for use with the pixel circuit  50  described above. 
   In common with the arrangements discussed below, the arrangement shown in  FIG. 4  has the advantages that the time taken for initialisation and programming of the pixel circuit is significantly reduced compared with prior art arrangements, thereby providing a more efficient, faster and more versatile display system. Moreover, the size of an individual pixel circuit is reduced in the present invention, thereby providing a more compact and efficient display with an improved aperture ratio. 
   In an alternative embodiment to the pixel circuit  50  of  FIG. 4 , the first n-channel transistor  60  is coupled to a supply line V SS  instead of the second rail  64 . The cathode terminal of the OLED  96  can also or instead be coupled to the supply line V SS  rather than to the fourth rail  94 . 
   Referring to  FIG. 6 , the pixel circuit  50  of  FIG. 4  according to a second embodiment of the present invention comprises an additional fourth p-channel transistor  98  comprising a source terminal coupled to the drain terminal of the third p-channel transistor  86  and a drain terminal coupled to the anode terminal of the OLED  96 . 
   In operation, in the pre-charge stage, the second signal φ 2  is applied to a gate terminal of the fourth p-channel transistor  98 . The first n-channel transistor  60  is switched on and the fourth p-channel transistor  98  is switched off thereby isolating the OLED  96  during the pre-charge stage even if the first signal φ 1  is logic 0 when the second signal φ 2  is logic 1. The second embodiment therefore allows different driving waveforms to be used as described below with reference to  FIGS. 11(   a ) and  11 ( b ). 
   Referring to  FIGS. 11(   a ) and ( b ), the second signal φ 2  is logic 1 prior to the first signal φ 1  becoming logic 1. If these driving waveforms were to be used in the circuit of  FIG. 4 , then when the second signal φ 2  is logic 1 node newdg  58  is earthed and the gate voltage of the p-type driving transistor is earthed as well. Thus, the driving transistor  74  may be briefly switched on before the first signal φ 1  is logic 1 and transistors  68  and  86  are switched off. At that time, the OLED  96  would be briefly driven to the maximum brightness. However, in the pixel circuit of  FIG. 6  this does not matter since switch  98  is switched off when switch  60  is switched on and the OLED  96  is isolated, as discussed above. 
   Referring to  FIG. 7 , the pixel circuit  50  of  FIG. 4  according to a third embodiment of the present invention comprises an additional fifth p-channel transistor  102  and an additional fourth n-channel transistor  104 . The fourth n-channel transistor  104  comprises a source terminal coupled to the first rail  52  and a drain terminal coupled to a node  108  referred to as newdg 2 . The node newdg 2  is coupled to the third node  62 —that is, node newdg 2  and the third node  62  are technically the same—and to a first terminal of the fifth p-channel transistor  102 . The fifth p-channel transistor  102  comprises a second terminal coupled to the second node  58  (newdg). 
   In operation, in the pre-charge stage, the second signal φ 2  is applied to a gate terminal of the fourth n-channel transistor  104  and a gate terminal of the fifth p-channel transistor  102 . When the second signal φ 2  is logic 1 and the first n-channel transistor  60  is switched on, the fifth p-channel transistor  102  is switched off and the fourth n-channel transistor  104  is switched on thereby ensuring that the driver transistor  74  is also off in order to isolate the OLED  96 . 
   Driving waveforms described above and below with reference to  FIGS. 11(   a ) and  11 ( b ) can also be used with the pixel circuit  50  shown in  FIG. 7 . More specifically, in  FIG. 7  node newdg 2   108  is held at V DD  all the time that node newdg  58  is earthed, so the gate voltage of the driving transistor equals V DD  and the driving transistor is not switched on. Accordingly, there is no need for transistor  98  provided in  FIG. 6 . 
   In an alternative to the arrangement shown in  FIG. 7 , transistor  104  can be changed from an n-channel transistor to a p-channel transistor and transistor  102  can be changed from a p-channel transistor to an n-channel transistor. This is beneficial for drawing current from the power supply V DD . However, with the gates of both of the thus altered transistors connected to the second signal φ 2 , the two transistors act as an inverter. If only this change were to be made, the resultant inverter would output the inverted second signal φ 2  bar at node newdg 2 . Thus, at the same time φ 2  is high so that transistor  60  is switched on and node newdg is earthed, the inverter formed by transistors  104 ,  102  would output the inverted φ 2 bar (in other words a low) at newdg 2 . In that circumstance, the p-type driving transistor would be switched on and the OLED would emit before φ 1  goes high and before the driving transistor is diode connected. 
   To counter this, a further inverter is added between the second signal line and the inverter formed by altered transistors  104 ,  102 . Accordingly, the signal input to the inverter formed by altered transistors  104 ,  102  is φ 2  bar. Thus, at the same time φ 2  is high so that transistor  60  is switched on and node newdg is earthed, the inverter formed by transistors  104 ,  102  has φ 2  bar as an input and outputs the φ 2  (in other words a high) at newdg 2 . Consequently, the p-type driving transistor is switched off so the OLED  96  does not emit before φ 1  goes high and before the driving transistor is diode connected. 
   Referring to  FIG. 8 , a fourth embodiment of the present invention comprises the pixel circuit  50  of  FIG. 7  with the fourth n-channel transistor  104  in an alternative configuration. The fourth n-channel transistor  104  comprises a terminal coupled to the sixth node  72  int and a terminal coupled to the second node newdg. The fourth n-channel transistor  104  comprises a gate terminal coupled to the eighth node  80  for receiving the first signal φ 1 . 
   In operation and when the first signal φ 1  is logic 1 during the pre-charge stage and the self-adjustment stage, the fourth n-channel transistor  104  is switched on in order to improve the conductive path between the seventh node ipn and the second node newdg. 
   Referring to  FIG. 9 , the pixel circuit  50  of  FIG. 4  according to a fifth embodiment of the present invention comprises a terminal of the first n-channel transistor  60  coupled to the seventh node ipn instead of being coupled to the second rail  64 . Therefore, the driver transistor  74  is coupled to a terminal of the third p-channel transistor  86  and a terminal of the third n-channel transistor  84 . 
   In operation, the voltage V DAT  provides a pre-charge stage voltage to the second node newdg through the first n-channel transistor  60  and the third n-channel resistor  84 . Therefore the second rail  64  is no longer needed as ground (0V) nor as replaced by a supply line V SS . During the pre-charge stage, the voltage V DAT  must be less than the voltage that V DAT  pulses to in the self-adjustment stage so that the driver transistor  74  can behave as a forward-biased diode-connected transistor. 
   Exemplary driving waveforms for the pixel circuit  50  as illustrated in  FIG. 9  are illustrated in  FIG. 11(   b ). In the pre-charge stage, when the first signal φ 1  is logic 0 and the second signal φ 2  becomes logic 1, node newdg initially discharges through the first n-channel transistor  60 , the third p-channel transistor  86  and the OLED  96  to ground. The first signal φ 1  becomes logic 1 and V DAT  increases to a value V DAT  low. As such, the driver transistor  74  becomes diode connected and the node newdg is initialised to the voltage V DAT  low through the third n-channel transistor  84  and the first n-channel transistor  60 , the driver transistor  74  and the second n-channel transistor  78 . 
   As the second signal φ 2  drops to logic 0, and in the self-adjustment stage, V DAT  low increases to a value V DAT  high. As such, the node newdg increases to a value V DAT  high−(V T +ΔV T ) through the third n-channel transistor  84 , the driver transistor  74  and the second n-channel transistor  78 . 
   At the output stage, the first signal φ 1  is logic 0 and the driver transistor  74  is no longer diode-connected between the first terminal and the gate terminal. The driver transistor  74  therefore acts as a constant current source for the OLED  96  through the first p-channel transistor  68 , the driver transistor  74  and the third p-channel transistor  86 . The amplitude of the current passed to the OLED  96  by the driver transistor  74  is dependent on the value of V DAT  (more specifically, the value of V DAT  high in the self-adjustment stage) and not the threshold variation ΔV T . Therefore, all pixel circuits  50  in an array forming a display are driven to the same brightness. 
   In a further alternative, the transistor  98  shown in  FIG. 6  can also be included in each of the arrangements shown in  FIGS. 7 to 9 . Thus, in each case the pixel circuit includes p-channel transistor  98  coupled in series between transistor  86  and the OLED  96 . The control signal φ 2  is applied to the gate of p-channel transistor  98  so that p-channel transistor  98  is switched off whilst n-channel transistor  60  is switched on. 
   Referring to  FIG. 12 , an architecture for the pixel circuit  50  as illustrated in  FIGS. 4 ,  6 ,  7 , and  8  is shown in an array  150  forming a display system. The array  150  is driven by any one of the exemplary waveforms of  FIG. 10  or  FIG. 11(   a ). Each pixel circuit  50  of the array  150  comprises a ground line Gnd, which can be replaced by a supply line V SS  as discussed above. The architecture also comprises two separate horizontal control lines to supply the first and second supply signals φ 1  and φ 2 . 
   Referring to  FIG. 13 , an architecture for the pixel circuit  50  as illustrated in  FIG. 9  is shown in an array  200  forming a display system. By employing a waveform as illustrated in  FIG. 11(   d ) in the case of the pixel circuit  50  as illustrated in  FIG. 9  a reduction in the number of horizontal control lines is demonstrated when compared to the architecture of  FIG. 12 . 
   The reduction in the number of horizontal control lines is realised since the control line SEL, 2  (referred to as a control signal V SELn+1  in  FIGS. 11(   c ) and ( d )) provides both the first control signal φ 1  and the second control signal φ 2  for adjacent pixel circuits  50 . 
   Of course, the architecture shown in  FIG. 12 , in which two signal lines are provided for each row of pixels, could be adjusted so that the capacitor in each pixel circuit discharges to a data line VDAT instead of to ground Gnd, similar to  FIG. 13 . By employing a waveform as illustrated in  FIG. 11(   c ) in the case of the pixel circuit  50  as illustrated in  FIGS. 6 ,  7  and  8  a reduction in the number of horizontal lines would be demonstrated when compared to the architecture of  FIG. 12 . 
   Similarly, the architecture shown in  FIG. 13 , in which signal lines are shared between adjacent rows of pixels, could be adjusted so that the capacitor in each pixel circuit discharges to ground Gnd instead of to a data line VDAT, similar to  FIG. 12 . By employing a waveform as illustrated in  FIG. 11(   b ) in the case of the pixel circuit  50  as illustrated in  FIG. 9  a reduction in the number of horizontal control lines would be demonstrated when compared to the architecture of  FIG. 12 . 
   Of course, the arrays in  FIGS. 12 and 13  are also applicable to all suitable alternatives of the pixel circuits of the present invention, whether or not described above. 
   It is noted that in each of  FIGS. 11(   a ) to ( d ) the first and second control signals φ 1  and φ 2  are overlapping. That is, φ 1  is high for a part of the time that φ 2  is high and φ 2  is high for a part of the time that φ 1  is high. However, φ 1  is also high for a part of the time that φ 2  is low and φ 2  is also high for a part of the time that φ 1  is low. This possibility of using overlapping control signals, which is hitherto unknown, allows increased scanning speeds and consequently improves the quality of displayed moving images. 
   Referring to  FIG. 14 , a simulation of the voltage Vnewdg at the second node  58  for the pixel circuit  50  as illustrated in  FIG. 4  is shown graphically against time in microseconds. In the pre-charge stage (labeled as PRESET in  FIG. 12 ) the voltage Vnewdg drops substantially to ground (0V). In the self-adjustment stage (labeled as PROGRAM) in  FIG. 12  the voltage Vnewdg climbs to a value V DAT −(V T +ΔV T ) as V DAT  pulses to a voltage for driving the OLED  96 . In the output stage (referred to as LOCK DOWN) in  FIG. 12 , the voltage Vnewdg is maintained by the first capacitor  56  until the process is repeated. As can be readily appreciated from  FIG. 12 , the voltage Vnewdg varies with respect to varying values of ΔV T . 
   From  FIG. 14  it can be seen that the pre-charge and self-adjustment stages can be completed in a matter of only a few microseconds. This is approximately two orders of magnitude (or 100 times) faster than that achieved in the prior art. In addition, lower voltages can be used. Accordingly, the present invention provides improved display quality and reduced power consumption. Moreover, a pixel circuit and a display device according to the present invention are smaller and more compact than those of the prior art. 
   Referring to  FIG. 15 , a simulation of an output current (IOLED) for driving the OLED  96  is plotted against varying values of ΔV T . As such,  FIG. 15  demonstrates that the output current IOLED is the same, irrespective of ΔV T , so the pixel circuits forming an array can be driven to the same brightness despite varying values of ΔV T . 
     FIG. 16 , illustrates a similar effect. In  FIG. 16(   a ), the output current IOLED is plotted graphically against time in microseconds for varying values of input voltages, V DD , which result in varying amplitudes of output current IOLED, and varying values of ΔV T , which do not affect output IOLED.  FIG. 16(   b ) shows variation of IOLED with variation in V DAT , for different ΔV T . The output current IOLED is substantially equal, irrespective of ΔV T , and therefore output currents IOLED for respective values of ΔV T  are superimposed. The pixel circuits forming an array can therefore be driven to the same brightness despite varying values of ΔV T . 
   A display system  1000  using the pixel circuit  50  as described above is advantageous for use in small, mobile electronic products such as mobile phones, personal digital assistants (PDA), computers, CD players, DVD players and the like—although it is not limited thereto. 
   Several terminal devices in which the display system  1000  can be embedded will now be described. 
   An example in which the display system  1000  is applied to a portable or mobile phone will be described.  FIG. 17  is an isometric view illustrating the configuration of the portable phone. In the drawing, the portable phone  1200  is provided with a plurality of operation keys  1202 , an earpiece  1204 , a mouthpiece  1206 , and the display system  1000  in the form of a display panel. The mouthpiece  1206  or earpiece  1204  may be used for outputting speech. 
   An example in which the display system  1000  according to one of the above embodiments is applied to a mobile personal computer will now be described. 
     FIG. 18  is an isometric view illustrating the configuration of this personal computer. In the drawing, the personal computer  1100  is provided with a body  1104  including a keyboard  1102  and the display system  1000  in the form of a display panel. 
   Next, a digital still camera using the display system  1000  will be described.  FIG. 19  is an isometric view illustrating the configuration of the digital still camera and the connection to external devices in brief. 
   Typical cameras sensitise films based on optical images from objects, whereas the digital still camera  1300  generates imaging signals from the optical image of an object by photoelectric conversion using, for example, a charge coupled device (CCD). The digital still camera  1300  is provided with the display system  1000  in the form of a display panel at the back face of a case  1302  to perform display based on the imaging signals from the CCD. Thus, the display system  1000  functions as a finder for displaying the object. A photo acceptance unit  1304  including optical lenses and the CCD is provided at the front side (behind in the drawing) of the case  1302 . The display system  1000  may be embodied in the digital still camera. 
   Further examples of terminal devices, other than the portable phone shown in  FIG. 17 , the personal computer shown in  FIG. 18 , and the digital still camera shown in  FIG. 19 , include a personal digital assistant (PDA), television sets, view-finder-type and monitoring-type video tape recorders, car navigation systems, pagers, electronic notebooks, portable calculators, word processors, workstations, TV telephones, point-of-sales system (POS) terminals, and devices provided with touch panels. Of course, the display system of the present invention can be applied to any of these terminal devices. 
   The aforegoing description has been given by way of example only and a person skilled in the art will appreciate that modifications can be made without departing from the scope of the present invention.