CA2472689A1

CA2472689A1 - Circuit and method for driving an array of light emitting pixels

Info

Publication number: CA2472689A1
Application number: CA002472689A
Authority: CA
Inventors: Arokia Nathan; Yuriy Vygranenko; Shahin Jafarabadiashtiani
Original assignee: Ignis Innovation Inc
Priority date: 2004-06-29
Filing date: 2004-06-29
Publication date: 2005-12-29
Also published as: TW200601221A

Abstract

A technique for driving a column of pixels that include light emitting elements is presented. The technique incorporates feedback data provided from feedback data sources connected to the data line and to feedback line of the array, pixel driving circuit with feedback path. The technique can also include block of the reference elements for input signal corrections.

Description

Circuit and Method for Driving an Array of Light Emitting Pixels FIELD OF THE INVENTION
The present invention generally relates to a light emitting device display techniques, and more particularly, to a technique for driving light emitting elements that uses external feedback and compensation.
BACKGROUND OF THE INVENTION
Recently active-matrix organic light-emitting diode (OLED) displays have become more attractive due to advantages over conventional liquid crystal flat displays.
These advantages include the ability to fabricate OLED displays at a relatively low cost and high efficiency. Further the displays do not require backlighting and provide a wide viewing angle.
An active-matrix organic light-emitting diode display (AMOLED) compromises an array of rows and columns of pixels, each having an OLED and some active devices such as thin film transistors. Since OLEDs are current driven devices the pixel circuit of an AMOLED should be capable of providing an accurate and constant drive current to achieve a consistent and uniform luminance.
As disclosed in U.S. Patent. NO. 5,748,160, a simple pixel circuit comprises two thin film transistors (TFTs) and an OLED. In this circuit, the OLED is connected to the drain terminal of a driving TFT and a gate terminal of the driving TFT is connected to a column line through a switching TFT. A storage capacitor connected between the gate terminal of the driving TFT and ground is used to maintain the voltage at the gate terminal of the driving TFT when the pixel circuit is disconnected from column line. For this circuit the current through OLED strongly depends on the characteristic parameters of the driving TFT. Since the characteristic parameters of TFT, particularly, the threshold voltage under bias stress, vary by time, and such changes may differ from pixel to pixel, the induced image distortion may be unacceptably high.
One of the methods that have been employed to make the current driving circuit less sensitive to the shift in the threshold voltage is programming the pixel with current instead of voltage. In this method, the OLED current is less dependent on the voltage-current characteristics of driving transistors. Some references show implementations of current programmed pixel circuits for OLEDs (Yi HE et al., "Current Source a-Si:H Thin-Film Transistor Circuit for Active Matrix Organic Light-Emitting Displays", IEEE Electro Device Letters, Vol. 21, No. 12, p590-592, December 2000). A drawback of the current programming method is that the programming current of low level may not be able to charge the column line in a line time due to the large line capacitance.
Another method to make the drive current less sensitive to transistor parameters is using current feedback. U.S. patent application 20020101172A1 provides a driving system with current feedback. An external current comparator compares the pixel current with a reference current and generates an appropriate signal to control the pixel current. One drawback of the disclosed method is that the control signal is a current, which can limit the programming speed. Another drawback of the method is that the anode and cathode electrode of each OLED have to be patterned, which creates reliability concerns in the currently used OLED fabrication process.
Luminance feedback is another method that has been used to stabilize OLED
luminance. As described in U.S. patent application 20030151569 feedback readout circuits responsive to the feedback signal representing the light output of the OLED
can be used to provide brightness control. A drawback of the disclosed method is that every pixel requires a photo-sensor that is optically coupled to the OLED. This results in integration issues. Another drawback is that the low level of the feedback signal generated by a photo-sensor may lead to the poor signal-noise ratio, thereby narrowing the dynamic range of the system.

2 SUMMARY OF INVENTION
The invention provides several driving circuits that can be used for driving a column of the light emitting devices and are suitable for use in Active Matrix Organic Light Emitting Diode (AMOLED) displays having a feedback control-system architecture.
According to an embodiment of the invention each pixel in the column is connected to the feedback-type control unit via signal line and feedback lines, and receives a scanning clock signal via select line connection terminal. The driving current through the fight emitting element can be accurately controlled by an external control unit through the use of feedback information in the form of a voltage or current level from the feedback part of pixel circuit.
The column control unit may be connected to the block of reference elements formed on the display substrate in order to correct an error in the output current level caused by inaccuracy of the pixel components or temperature drift. The block of reference elements may also include a photo-sensor optically coupled to the light emitting element in order to provide a luminance feedback compensation for brightness variations induced by instability of organic material or temperature changes.
Advantages Therefore, advantages of this invention include the ability to provide a stable current to the light emitting diode over time, thereby maintaining image quality.
Moreover, the combination of the external current feedback for pixel programming and luminance feedback for data signal preprocessing provides brightness control and compensation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an example of the configuration of a display device with feedback control-system architecture according to an embodiment of the invention.
FIG. 2 is a block diagram of pixel architecture according to an embodiment of the invention.
FIG. 3 is a circuit diagram of a pixel circuit and column control unit and its corresponding waveforms according to an embodiment of the invention.
FIG. 4 is a circuit diagram of a modification of the embodiment of FIG. 3.

3 FIG. 5 is a schematic of a pixel circuit for a common cathode OLED
configuration according to an embodiment of the invention.
FIG. 6 is a circuit diagram of a column control unit and a pixel circuit having p-channel type transistors according to an embodiment of the invention.
FIG. 7 is a circuit diagram of a column control unit and a pixel circuit with a p-channel type transistor switch and its corresponding waveforms according to an embodiment of the invention.
FIG. 8 is a circuit diagram of a column control unit and a pixel circuit having p-channel and n-channel type transistors according to an embodiment of the invention.
FIG. 9 is a circuit diagram of a column control unit and a pixel circuit with a current mirror as current driving circuit according to an embodiment of the invention.
FIG. 10 is a circuit diagram of a modification of the embodiment of FIG. 9.
FIG. 11 is a circuit diagram of a modification of the embodiment of FIG. 3.
FIG. 12 is a circuit diagram of a pixel circuit, column control unit and a reference cell with implemented luminance feedback according to an embodiment of the invention.
FIG. 13 is a circuit diagram of a pixel circuit and column control unit with a reference diode according to an embodiment of the invention.
FIG. 14 is a circuit diagram of a pixel circuit, column control unit with a reference OLED according to an embodiment of the invention.
FIG. 15 is a circuit diagram of a modification of the embodiment of FIG. 14.
FIG. 16 is a circuit diagram of a pixel circuit and column control unit according to an embodiment of the invention.
FIG. 17 is a circuit diagram of a modification of the embodiment of FIG. 16.
FIG. 18 is a circuit diagram of another embodiment of a pixel circuit and column control unit along with drive waveforms according to an embodiment of the invention.
FIG. 19 is a circuit diagram of a column control unit and a pixel circuit with a current mirror as current driving circuit according to an embodiment of the invention.
Fig 20 is a circuit diagram of a pixel circuit according to an embodiment of the invention.
Fig 21 is a circuit diagram of a pixel circuit according to an embodiment of the invention.
Fig 22 is a circuit diagram of a pixel circuit according to an embodiment of the invention.

4 DETAILED DESCRIPTION
The present invention encompasses a technique for driving of column of pixels where each pixel comprises a light emitting element, in particular, an organic light emitting diode (OLED).
FIG.1 presents a display device having a feedback control-system architecture and an array of addressable pixels 11. The pixels 11 are controlled by a select line driver 12 and data driver 13. As shown in FIG. 1, a separate feedback control unit 14 is provided on each column line of the array. The feedback control unit 14 of a given column is connected to each pixel in the column via a signal line 15 and a feedback line 16. A block of reference elements 17, located on the display substrate, may also be provided. The block of the reference elements 17 includes some elements of the pixel circuit for input signal corrections and may also include a photo-sensor that is optically coupled to a light emitting element to implement a luminance feedback.
The structure of a given pixel 11, according to our embodiment of the invention is shown in FIG. 2. As shown in FIG.2, the pixel has an OLED 21, a current driving circuit 22, controlled by the stored voltage level using a storage capacitor 23, a feedback circuit 24, and switches S1 and S2. The switches S1 and S2 can be any suitable switching device, but are preferably an insulating gate type field effect transistor. The pixel 11 operates in writing and hold modes. In the writing mode, when select lines) are activated, the switches S1 and S2 are turned on, and the current driving circuit 22 receives the signal voltage from control unit 14, while the feedback circuit 24 feeds the feedback signal in the form of voltage or current level.
The driving current through the OLED 21 can thereby be accurately controlled through the use of negative feedback. In the hold mode, the switches S1 and S2 are turned off and the driving circuit 22 provides the driving current having a current level in accordance with the held voltage level to the storage capacitor, 23.
Although the exemplary embodiments of the present invention are described in conjunctions with OLEDs, it is also contemplated that the present invention could be used with other similar display elements such as a light emitting diode (LED).
FIG. 3A shows a pixel drive circuit according to an alternative embodiment and a circuit diagram of the column control unit 14 and controlling signals.

The pixel drive circuit comprises three transistors T1, T2 and T3, a resistor 32, a storage capacitor Cs and an OLED 31. The pixel drive circuit is connected to a select fine, a feedback line, and a signal line. A power supply node having a positive potential Vdd and common ground are also shown.
Transistors T1, T2 and T3 can be fabricated using amorphous silicon, poly silicon, appropriate organic semiconductors and NMOS or CMOS technologies. Thin film resistor 32 can be fabricated with any appropriate material and technology, which provides sufficient stability. For instance, in amorphous silicon technology the resistor 32 can be fabricated using N+ amorphous silicon or N+ microcrystalline silicon.
The drain terminal of driving transistor T2 is connected to the anode of OLED
31. The source terminal of transistor T2 is connected to resistor 32 and the gate terminal is connected to the signal line through transistor T1. Resistor 32 is connected between the source terminal of transistor T2 and the common ground.
Transistors T1 and T3 are driving switch and feedback switch transistors, respectively. The gate terminals of transistors T1 and T3 are connected to the select line. The source terminal of transistor T1 is connected to the signal line and the drain terminal is connected to the gate terminal of transistor T2. The source terminal of transistor T3 is connected to the feedback line and the drain terminal is connected to resistor 32. The cathode of OLED 31 is connected to the drain terminal of transistor T2. All OLEDs have a common anode electrode, connected to the voltage supply node (Vdd). Storage capacitor Cs is connected between the gate terminal of transistor T2 and common ground. It can be connected between gate and source terminals of transistor T2. In the latter case, capacitor Cs can be implemented by the gate-source capacitance of transistor T2.
The external controlling unit 33 in its simplest form is a high-gain, low offset difference amplifier with a negative feedback connection.
During the writing mode, the select signal goes high, turning on transistors T1 and T3. As a result, the driving transistor T2, along with the external difference amplifier 33 and resistor 32 make a circuit with negative feedback. The difference in the voltage level between an input signal voltage and a voltage drop across the resistor 32 is amplified by the difference amplifier 33, adjusting the potential on the gate of transistor T2. After the initial transients the output current stabilizes and in the case of a high-gain feedback loop the current passing through the OLED 31 is:
I Vinp ' (1) oLEn = R f.
During the hold mode, the select line goes low, so transistors T1 and T3 are turned off and the pixel is disconnected. Since the gate voltage of driving transistor T2 is stored in capacitor Cs, the drive current does not change during the hold mode.
In the configuration shown in FIG. 3A, the current of the pixel 31 depends on the absolute resistance of resistor 32, which is not desirable due to possible inherent inaccuracy and poor thermal stability of integrated resistors. FIG. 4 presents an architecture, according to another embodiment of the invention that addresses this problem by implementing a reference resistor 42 and an external data current source 41. The reference resistor 42 is made with the same material as integrated resistors and formed on the display substrate. This enhances the temperature stability of the circuit. The programmed level of the drive current for this circuit is:
Rr I DLED = linp ~f, , (2) where Rr is the resistance of the reference resistor 42, and Rf is the resistance of the feedback resistor 32. The above equation indicates a considerable improvement in the accuracy of the programming current because of insensitivity of the resistance ratio to the temperature variations.
A current pixel drive circuit according to another embodiment of the invention and a section of the column driver circuitry are shown in FIG 5. The circuit is similar to the circuit of FIG. 3A however, in the circuit of FIG. 5, the cathode of OLED 51 is common and is connected to a negative power supply potential Vss. As a result, the cathode of the OLEDs is not patterned.
The anode of OLED 51 is connected to the source terminal of transistor T2. The feedback resistor 32 is connected between the drain terminal of transistor T2 and ground node. The voltage level of the select line during the writing mode should be high enough to guarantee that transistor T1 is in an "on" state for the entire output current range. The feedback line in this configuration is connected to the non-inverting input of the difference amplifier 33 to provide a negative feedback.

Fig.'s 6, 7A and 8 illustrate pixel drive circuit, according to other aspects of the invention wherein p-channel MOS transistors are used.
In the pixel circuit shown in FIG.6, all of the transistors are p-channel transistors.
Here the anode of the OLED 51 is connected to the drain terminal of the transistor T2 and the common cathode electrode of the OLED 51 is connected to the negative power supply potential Vss.
FIG. 7A shows a pixel circuit, where the feedback switch use transistor T3 is p-channel transistor. The circuit is similar to the circuit of FIG. 5, however the implementation of the PMOS transistor requires an additional select line. FIG.

shows corresponding waveforms for select line A and select line B. The advantage of this circuit over the circuit of FIG. 5 is the lower voltage swing for the select lines that is required.
FIG. 8 shows a pixel circuit in which, the transistors T2 and T1 are p-channel transistors and the transistor T3 is an n-channel transistor. As an embodiment of FIG.
7 this circuit also has two select tines marked as A and B having reduced voltage swing.
FIG.'s. 9 and 10 show configurations of the pixel circuits according to alternative embodiments of the invention. In these pixel circuits, the current driving circuitry is a current mirror having of transistors T3 and T4. The current level of the signal current and the current level of the drive current are proportional. In the circuit of FIG. 9, all transistors are n-channel transistors and in the circuit of FIG. 10 all transistors are p-channel transistors.
The feedback resistor 32 is connected between the drain terminal of transistor and common ground. The gate terminals of the transistors T3 and T4 are connected to each other. In the circuit of FIG. 9, the cathode electrode of OLED 31 is connected to the drain terminal of transistor T4, the anode is common and the transistor T4 is the driving transistor and is connected to OLED 31. In the circuit of FIG. 10, the cathode of the OLED 51 is common and the anode is connected to the drain terminal of the transistor T4. The transistors T2 and T3 are driving and feedback switches, respectively.

During writing mode, the transistors T2 and T3 are in an "on° state, thus the transistor T1 along with feedback resistor 32 and external control unit (the difference amplifier 33) form the feedback loop. The transistor T4 does not directly take part in the feedback loop, but since the transistors T4 and T1 have same gate-source voltage, the current of the transistor T4 is proportional to the current of the transistorT1. The ratio of current through transistors T4 to T1 is determined by the aspect ratios of these transistors. In these circuits, the feedback resistor 32 and the OLED
31, of FIG. 9 and 51 of FIG. 10, are not in the same current path thus a greater lifetime is expected.
Several methods have been used to reduce the charge injection and clock feed-through effects in integrated circuits. As the simplest approach, a dummy transistor that is driven by the inverse signal of the select line connected to the gate of driving transistor can reduce both charge injection and clock feed-through errors caused by the driving switch. The drain and source terminals of the dummy transistor are connected the gate of the driving transistor. FIG. 11 shows an example of such modification for the embodiment of FIG. 3. The width of dummy transistor T4 is half of the width of driving transistor T2. It will be apparent to one skilled in the art that the width of the dummy transistor T4 can be any appropriate size. The dummy transistor approach can be used in this invention.
FIG.12 is a schematic circuit diagram of another embodiment of a pixel circuit, column control unit and a reference cell according to the present invention.
Here, the implemented luminance feedback improves the linearity of the video signal -light output characteristics, and also provides a compensation for brightness instability caused by instability of the organic material, ageing, temperature changes, or other environment stresses. The compensating circuit with luminance feedback includes a resistor R1, a difference amplifier 121, and a NMOS transistor 122, which are parts of the control unit, and the elements of the reference cell 123 including an OLED
124, and photodiode 125. The photodiode 125 is optically coupled to the reference OLED
124 to form a feedback current signal in response to emitted light. The circuit is balanced when the input current passing through the resistor R1 is equal to a feedback current generated by the photodiode 125. The current flowing through OLED 124 via transistor 122 and resistor 42 is an input signal for next stage of the device, which is the same as the embodiment of FIG.4.

Fig.13 is a schematic diagram of an alternative embodiment of the embodiment of Fig. 4. In this embodiment, diodes D1 and D2 are used in place of feedback resistor R1 and reference resistor R2 of FIG. 3, respectively. Since the circuit functionality with reasonably low error in the programmed current level requires a good match between the reference diode and pixel diodes, the used technology must be efficient for fabrication of the diode array with reproducible forward current-voltage characteristics.
A schematic diagram of a circuit according to another embodiment of the invention is shown in FIG. 14. Two variations are presented: (a) common cathode and (b) common anode OLED array configurations. In the writing mode, the input current from an external current data source 41 creates a voltage drop across a reference OLED 141. A difference amplifier 33 in negative feedback connection is designed to hold the same voltage level on a pixel OLED 142. During the hold mode, the current with a programmed current level flows through both the transistor T2 and the OLED
142 due to the voltage stored across the capacitor Cs.
FIG.15 is a schematic diagram of an alternative of the embodiment of FIG. 14, wherein the control unit and reference cell 151 are modified to implement the luminance feedback. The compensating circuit includes two resistors R1 and R2, a difference amplifier 153 and a photodiode 152, which is optically coupled to the reference OLED 141. The circuit is balanced at a certain level of the light signal emitted by OLED 141, when the feedback current generated by the photodiode 152 is the same level as the input current level through resistor R1. The programmed brightness of the pixel OLED 142 is expected to be the same as the brightness of the reference OLED 141 due to implemented voltage feedback.
FIG. 16 shows a circuit diagram of another embodiment of a pixel circuit and column control unit, where the feedback signal is in form of the current level. The pixel circuit contains an OLED 31, four transistors, T1-T4, and a storage capacitor Cs. The control unit contains a difference amplifier 33 and a resistor R1.
In particular, the source terminals of the transistors T1 and T3 are connected to the select line A. The drain terminal of the transistor T1 is connected to the signal line and the source terminal is connected to the gate terminal of the transistor T2 and to the storage capacitor Cs. The source terminal of the transistor T3 is connected to the feedback line, and the drain terminal is connected to the drain of the transistor T4, the source terminal of the transistor T2 as well as to the storage capacitor Cs. The source terminal of the transistor T4 is grounded and the gate terminal is connected to the select line B. The drain terminal of the transistor T2 is connected to the cathode electrode of OLED 31.
During the writing mode, the select line A is held at a logic-high potential and both the transistors T1 and T3 are turned on, while the select line B is held at a logic-low potential and the transistor T4 is turned off. The input signal, which is in the form of a negative voltage level, increases a voltage level on the output terminal of the difference amplifier 33, which is applied through the signal line and the transistor T1 to the gate of the transistor T2 inducing the conductive state of transistor T2. The drive current flows through the pixel OLED 31, transistor T2, and transistor T3 and resistor R1. During the hold mode, the select line A is held at a logic-low potential and both transistors T1 and T3 are turned off, the select line B is held at a logic-high potential and transistor T4 is turned on, transistor T2 has certain impedance according to a potential stored on capacitor Cs and the current of programmed level flows through the pixel OLED 31, transistor T2, transistor T4 to the ground node.
The circuit diagram in FIG. 17 is a modification of the embodiment of Fig. 16.
The transistor T4 in FIG.16 is replaced by the diode D1 in FIG.17, the non-inverting input-terminal of the difference amplifier 33 is connected to -Vr terminal of the reference voltage source to hold the feedback line under negative potential for proper functionality of the switching circuit comprising the transistor T3 and the diode D1, and a voltage shifter is implemented to adjust the level of the input signal.
The voltage shifter comprises an operational amplifier 171 with resistors R2, R3 and R4.
Resistors R3 and R4 have the same resistance R3=R4=R to achieve the -Vr shift in the signal level. During the writing mode, the transistor T3 is turned on and diode D1 is under reverse bias condition since the potential difference between source and drain terminals of the transistor T3 is less than Vr in the operating range of drive current, and the drive current flows through the OLED 31 via the transistor T3 and the resistor R1. The programmed current level is Ytnn . R
Rl ~ R2 ~ (3) During the hold mode, the transistor T3 is turned off and the drive current of the programmed level flows through the OLED 31 via the transistor T2, the diode D1 to the ground node.
FIG. 18 shows a circuit diagram of another embodiment of a pixel circuit and column control unit with a feedback signal in form of the current level. Referring to FIG.18A
the pixel circuit contains an OLED 51, two transistors, T1-T2, two diodes, D1 and D2, and a storage capacitor Cs. The circuit does not have connection to the ground node, but the select line B is implemented for drive current sink. The control unit contains a difference amplifier 33 and a resistor R1. The drive waveforms for the select line A
and select line B are shown in FIG. 18B. During the writing mode, the select line A is held at high potential and transistor T1 is turned on enabling a signal path between the output terminal of the difference amplifier 33 and the gate of transistor T2. Since the potential of the select line B is negative, the diode D1 is reverse biased and the input voltage signal of positive polarity causes the current flow through resistor R1, diode D2, transistor T2 and OLED 51. During the hold mode, the potential of the select line B is positive, diode D2 is under reverse bias condition and the drive current flows via diode D1.
FIG.19 shows a circuit diagram of another embodiment of a pixel circuit and column control unit, where the current driving circuit comprises two transistors, T3 and T4, arranged into a current mirror structure. When the pixel circuit is selected, the gate of transistor T3 is connected to signal line via transistor T1 and difference amplifier 33 controls the current flowing via transistors T2 and T3, as well as current through T4 and OLED 31. When the pixel circuit is deselected, the transistor T4 has certain impedance according to a potential stored on capacitor Cs and the current of programmed level flows through the pixel OLED 31. The advantage of this circuit over other circuit with current feedback that the power dissipation is reduced because there is no element such as resistor, diode or transistor switch connected in series to the driving transistor. Additionally, adjusting the current mirror ratio can optimize the level of feedback current.
The circuit diagram in FIG. 20 is a modification of the embodiment of FIG. 19.
Here the drain of switch use transistor T2 is connected to the drain of transistor T4 in order to avoid a nonlinearity of the current mirror, which can occur due to an asymmetry in biasing transistorsT3 and T4. To minimize the on-state voltage across transistor T2, its channel width should be large than the channel width of transistor T3.
Thus, during the writing mode the transistors T3 and T4 will be under the equal biasing conditions yielding a linear amplification.
FIG.21 shows a circuit diagram of another embodiment of a pixel circuit and column control unit, with a feedback signal in form of the current IeveLThe current driving circuit comprises two transistors, T3 and T4, arranged into a current mirror structure.
The drain and gate terminals of transistor T3 are connected to the signal line via transistors T2 and T1 respectively.
When the pixel circuit is selected, the drain and gate terminals of transistor T3 are connected to the signal line via transistors T2 and T1 and difference amplifier 33 controls the current flowing via transistors T2 and T3, as well as current through T4 and OLED 31. The source terminal of transistor T3 is always connected to the inverting input terminal of the difference amplifier 33 via feedback line and is virtually grounded.
When the pixel is deselected, transistor T4 let a certain current flows through pixel OLED 31 according to a potential stored on capacitor Cs.
FIG.22 shows a circuit diagram of another embodiment of a pixel circuit and column control unit, with a feedback signal in form of the current leveLThe current driving circuit comprises two transistors, T3 and T4, arranged into a current mirror structure.
The drain and gate terminals of transistor T3 are connected to each other and to the signal line via transistors T1 and T2 respectively. When the pixel circuit is selected, the drain and gate terminals of transistor T3 are connected to the signal line via transistors T2 and T1 and difference amplifier 33 controls the current flowing via transistors T2 and T3, as well as current through T4 and OLED 31. The source terminal of transistor T3 is always connected to the inverting input terminal of the difference amplifier 33 via feedback line and is virtually grounded.
When the pixel is deselected, transistor T4 let a certain current flows through pixel OLED 31 according to a potential stored on capacitor Cs.
Although the invention is described herein with reference to particular embodiments, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. A pixel circuit for use in a display, the display comprising a plurality of pixels with each pixel having a select line, a signal line, a feedback line and the pixel circuit, the pixel circuit comprising:
a light emitting element;
a drive part for providing a drive current to the light emitting element, the drive part having a storage capacitor and a switch use transistor having a gate terminal connected to the select line, a first terminal connected to said signal line, and a second terminal; and a feedback circuit for generating a feedback signal representing the drive current provided to the light emitting element, the feedback signal being provided to the feedback line.

2. The pixel circuit according to claim 1, wherein the drive part further comprises a drive use transistor having a gate terminal connected to the second terminal of the switch use transistor, a first terminal connected to the light emitting element and a second terminal.

3. The pixel circuit according to claim 1, wherein the drive part further comprises a first transistor having a gate terminal, a first terminal and a second terminal, and a second drive use transistor having a gate terminal, a first terminal and a second terminal, the first transistor and the second drive use transistor arranged to form a current mirror structure, the gate terminals connected to the second terminal of the switch use transistor, the first terminal of the first transistor and the first terminal of the second drive use transistor connected to a power supply node, the second terminal of the second drive use transistor connected to the light emitting element.

4. The pixel circuit according to claim 3 wherein the feedback circuit further comprises a resistor connected between the second terminal of the first transistor and a ground node to provide the feedback signal in form of voltage level proportional to the current level of drive current and a feedback transistor connected between the second terminal of the first transistor and the feedback line and having a gate connected to the select line.

5. The pixel circuit according to claim 1, wherein the drive part further comprises a first transistor having a gate terminal, a first terminal and a second terminal and a second drive use transistor having a gate terminal, a first terminal and a second terminal, the first transistor and the second drive us transistor being arranged to form a current mirror structure, the gate terminals being connected to the second terminal of the switch use transistor, the second terminal of the first transistor being connected to said feedback line, the first terminal of the second drive use transistor is connected to said light emitting element, the second terminal of the drive use transistor is connected to ground.

6. The pixel circuit according to claim 5 wherein the feedback circuit further comprises a conductive path between the second terminal of the first transistor and the feedback line providing the feedback signal in form of a current level proportional to the drive current and a feedback transistor connected between the first terminal of the first transistor and the power supply node and having a gate terminal connected to the select line.

7. The pixel circuit according to claim 2, wherein the feedback circuit further comprises a resistor connected between the second terminal of said drive use transistor and a potential to provide the feedback signal in form of a voltage level proportional to the drive current, and a feedback transistor connected between the second terminal of the drive use transistor and the feedback line and having a gate connected to the select line.

8. The pixel circuit according to claim 2, wherein said feedback circuit further comprises a diode connected between the second terminal of said drive use transistor and a predetermined potential to provide the feedback signal in form of voltage level or current level and said feedback switch is an insulating gate type field effect transistor connected between the second terminal of said drive use transistor and said feedback line and having a gate connected to said select line.

9. The pixel circuit according to claim 2 further comprising a feedback transistor connected between the second terminal of the drive use transistor and the feedback line and having a gate connected to the first select line and the feedback circuit further comprises a switch transistor having a gate terminal connected to the second select line, a first terminal connected to the second terminal of the drive use transistor, a second terminal connected to a ground potential to provide the feedback signal in form of current level equal to the drive current.

10. The pixel circuit according to claim 2 further comprising a feedback switch, the feedback switch being a diode connected between the second terminal of the drive use transistor and the feedback line, and the feedback circuit comprises one switch use diode connected between the second terminal of the drive use transistor and a second select line to provide the feedback signal in form of current level equal to the drive current.

11. The pixel circuit according to claim 2, wherein the second terminal of the drive use transistor is connected to a power line, and the feedback circuit is a conductive path between the first terminal of the drive use transistor and the feedback switch to provide the feedback signal in form of a voltage level equal to the voltage drop across the light emitting element, and a feedback transistor connected between the first terminal of the drive use transistor and said feedback line and having a gate connected to the select line.

12. The pixel circuit according to claim 1, wherein said light emitting element is an organic light emitting diode.

13. The pixel circuits according to claim 5, wherein the transistors are insulating gate type field effect transistors that comprise n-channel and p-channel type transistors.

14. A display device, comprising:
a select line;
a signal line to which a signal in accordance with both brightness and feedback information is supplied;
a feedback line to which a feedback signal in accordance with current level of drive current is supplied;
a plurality of pixels forming an array of pixels, each pixel of the plurality formed on a substrate at an intersecting portion of said scanning line and said signal and feedback lines, each pixel comprising:
a light emitting element;
a current driving circuit having a storage capacitor and a switch use transistor; and a feedback circuit to provide feedback signals representing a current output of said current driving circuit;

a display column control circuit for receiving input signals, adjusting the input signals using a reference circuit formed on the substrate at each column, and modifying the input signals in response to the feedback signals from pixels in the column to produce a desired brightness level of said light emitting element in a selected pixel; and a selecting line drive circuit for successively activating selecting lines.

15. The display device claimed in claim 14, wherein said feedback circuit includes a resistor to provide the feedback signal in form of a voltage level proportional to the drive current, and said reference circuit includes a reference resistor made from the same material as said resistor.

16. The display device claimed in claim 14, wherein said reference circuit includes a photo-sensor optically coupled to a reference light emitting element for brightness control, and said display column control circuit includes a compensating part to compare a generated photocurrent level with an input current level and to adjust a current through said reference light emitting element to achieve a desired brightness level.

17.A method of driving a plurality of light emitting elements arranged in a column at a desired brightness, comprising the steps of:
selecting one pixel of a plurality of pixels in the column;
establishing the desired brightness of a reference light emitting element by adjusting a reference current flowing through the light emitting element in response to a photocurrent from a photo-sensor that is optically coupled with the reference light emitting element;
converting the reference current into a corresponding voltage level;
transmitting the voltage level to the selected pixel;
converting the voltage level into a drive current and generating a feedback signal representing a drive current level;
adjusting the voltage level in response to the feedback signal from the selected pixel to establish a drive current substantially equal to the reference current;
storing the adjusted voltage level; and driving the light emitting element with the drive current in accordance with the adjusted voltage level to produce the desired brightness level in the pixel.

18. The pixel circuit according to claim 5 wherein the feedback circuit further comprises a conductive path between the second terminal of the first transistor and the feedback line providing the feedback signal in form of a current level proportional to the drive current and a feedback transistor connected between the first terminal of the first transistor and the first terminal of the drive use transistor and having a gate terminal connected to the select line.