JP2008146091A - Pixel circuit and driving method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide the driving method of a pixel circuit to which a compensating function for threshold voltage variation of a transistor is added. <P>SOLUTION: When a sampling transistor 115 is selected through a scanning line WSL 101, the pixel circuit 101 performs operation for sampling an input signal Vsig from a signal line DTL 101 and holding it in a holding capacitor C111, operation to make a drive transistor 111 current-drive an electrooptical element 117 according to the signal voltage held in the holding capacitor C111, and correcting operation for detecting the threshold voltage of the drive transistor 111 prior to the current driving of the electrooptical element 117 and holding the detected voltage in the holding capacitor C111. The correcting operation includes first operation for applying a predetermined voltage turning on the drive transistor 111 to between the gate and source of the drive transistor 111. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a pixel circuit that current-drives a load element arranged for each pixel of an active matrix display device and a driving method thereof. In particular, the present invention relates to a pixel circuit in which the amount of current flowing to a load element is controlled by an insulated gate field effect transistor provided in each pixel circuit, and a driving method thereof. Furthermore, the present invention relates to a pixel circuit having an electro-optical element whose luminance is controlled by a current value such as an organic EL as a load element, and a driving method thereof.

  In an image display device such as a liquid crystal display, an image is displayed by arranging a large number of liquid crystal pixels in a matrix and controlling the transmission intensity or reflection intensity of incident light for each pixel in accordance with image information to be displayed. This also applies to an organic EL display using an organic EL element as a pixel, but unlike a liquid crystal pixel, the organic EL element is a self-luminous element. Therefore, the organic EL display has advantages such as higher image visibility than the liquid crystal display, no backlight, and a high response speed. Further, the luminance level (gradation) of each light emitting element can be controlled by the value of the current flowing therethrough, and is greatly different from a liquid crystal display or the like in that it is a so-called current control type.

In the organic EL display, similarly to the liquid crystal display, there are a simple matrix method and an active matrix method as driving methods. Although the former has a simple structure, there is a problem that it is difficult to realize a large-sized and high-definition display. Therefore, the active matrix method is actively developed at present. In this method, a current flowing through a light emitting element in each pixel circuit is controlled by an active element (generally a thin film transistor or TFT) provided in the pixel circuit.
JP 2003-255856 A JP 2003-271095 A

  FIG. 8 is a block diagram showing a configuration of a general organic EL display device. The display device 100 includes a pixel array unit 102 in which pixel circuits (PXLC) 101 are arranged in an m × n matrix, a horizontal selector (HSEL) 103, a write scanner (WSCN) 104, a drive scanner (DSCN) 105, a horizontal The signal lines DTL101 to DTL10n selected by the selector 103 and supplied with signals according to the luminance information, the scanning lines WSL101 to WSL10m selectively driven by the write scanner 104, and the scanning lines DSL101 to DSL10m selectively driven by the drive scanner 105 are displayed. Have.

  FIG. 9 is a circuit diagram showing a configuration example of the pixel circuit shown in FIG. As shown in the figure, the pixel circuit 101 is basically composed of a p-channel thin film field effect transistor (hereinafter referred to as TFT). That is, the pixel circuit 101 includes a drive TFT 111, a switching TFT 112, a sampling TFT 115, an organic EL element 117, and a storage capacitor C111. The pixel circuit 101 having such a configuration is arranged at an intersection between the signal line DTL101 and the scanning lines WSL101 and DSL101. The signal line DTL101 is connected to the drain of the sampling TFT 115, the scanning line WSL101 is connected to the gate of the sampling TFT 115, and the other scanning line DSL101 is connected to the gate of the switching TFT 112.

  The drive TFT 111, the switching TFT 112, and the organic EL element 117 are connected in series between the power supply potential Vcc and the ground potential GND. That is, the source of the drive transistor 111 is connected to the power supply potential Vcc, while the cathode of the organic EL element (light emitting element) 117 is connected to the ground potential GND. In general, the organic EL element 117 is represented by a diode symbol because of its rectifying property. On the other hand, the sampling TFT 115 and the storage capacitor C111 are connected to the gate of the drive TFT111. The gate-source voltage of the drive TFT 111 is represented by Vgs.

  The operation of the pixel circuit 101 is as follows. First, when the scanning line WSL101 is selected (low level here) and a signal is applied to the signal line DTL101, the sampling TFT 115 is turned on and the signal is written into the holding capacitor C111. The signal potential written in the storage capacitor C111 becomes the gate potential of the drive transistor 111. Subsequently, when the scanning line WSL101 is in a non-selected state (here, high level), the signal line DTL101 and the drive TFT 111 are electrically disconnected, but the gate potential Vgs of the drive TFT 111 is stably held by the holding capacitor C111. . Subsequently, when another scanning line DSL101 is selected (here, at a low level), the switching TFT 112 becomes conductive, and a drive current flows through the TFT 111, TFT 112, and the light emitting element 117 from the power supply potential Vcc toward the ground potential GND. When the DSL 101 is in a non-selected state, the switching transistor 112 is turned off and the driving current does not flow. The switching TFT 112 is inserted to control the light emission time of the light emitting element 117.

  The current flowing through the TFT 111 and the light emitting element 117 has a value corresponding to the gate-source voltage Vgs of the TFT 111, and the light emitting element 117 continues to emit light with a luminance corresponding to the current value. The operation of selecting the scanning line WSL101 and transmitting the signal given to the signal line DTL101 to the inside of the pixel circuit 101 as described above is hereinafter referred to as “writing”. As described above, once a signal is written, the light emitting element 117 continues to emit light at a constant luminance until the next rewriting.

  As described above, in the pixel circuit 101, the value of the current flowing through the EL light emitting element 117 is controlled by changing the gate application voltage of the TFT 111 serving as the drive transistor in accordance with the input signal. At this time, the source of the p-channel type drive transistor 111 is connected to the power supply potential Vcc, and the TFT 111 always operates in the saturation region. Therefore, the drive transistor 111 is a constant current source having a value represented by the following formula (1).

Ids = (1/2) · μ · (W / L) · Cox · (Vgs−Vth) ² (1)
Here, Ids represents a current flowing between the drain and source of a transistor operating in the saturation region. Further, μ represents mobility, W represents channel width, L represents channel length, Cox represents gate capacitance, and Vth represents a threshold voltage of the transistor. As apparent from the equation (1), in the saturation region, the drain current Ids of the transistor is controlled by the gate-source voltage Vgs. Since the drive transistor 111 shown in FIG. 9 holds Vgs constant, the drive transistor 111 operates as a constant current source, and the light emitting element 117 can emit light with constant luminance.

  FIG. 10 is a graph showing a change with time of current-voltage (IV) characteristics of the organic EL element. In the graph, the curve indicated by the solid line indicates the characteristic in the initial state, and the curve indicated by the broken line indicates the characteristic after change with time. Generally, the IV characteristic of an organic EL element deteriorates over time as shown in the graph. On the other hand, in the pixel circuit shown in FIG. 9, since the drive transistor is driven at a constant current, the constant current Ids continues to flow through the organic EL element, and the IV characteristic of the organic EL element deteriorates. The light emission luminance does not deteriorate with time.

  The pixel circuit shown in FIG. 9 is configured by a p-channel TFT. However, if the pixel circuit can be configured by an n-channel TFT, a conventional amorphous silicon (a-Si) process can be used for TFT fabrication. It becomes possible. As a result, the cost of the TFT substrate can be reduced, and development is expected.

  FIG. 11 is a circuit diagram showing a configuration in which the p-channel TFT of the pixel circuit shown in FIG. 9 is replaced with an n-channel TFT. As shown in the figure, the pixel circuit 101 includes n-channel TFTs 111, 112, and 115, a storage capacitor C111, and an organic EL element 117 that is a light emitting element. The TFT 111 is a drive transistor, the TFT 112 is a switching transistor, and the TFT 115 is a sampling transistor. In the figure, DTL 101 represents a signal line, and DSL 101 and WSL 101 represent scanning lines, respectively. In the pixel circuit 101, the drain side of the TFT 111 as a drive transistor is connected to the power supply potential Vcc, and the source is connected to the anode of the EL element 117, thereby forming a source follower circuit.

  FIG. 12 is a timing chart for explaining the operation of the pixel circuit shown in FIG. When a selection pulse is applied to the scanning line WSL101, the sampling transistor 115 is turned on, samples a signal from the signal line DTL101, and writes it to the storage capacitor C111. As a result, the gate potential of the drive transistor 111 is held at the sampled signal potential. This sampling operation is performed line-sequentially. That is, after a selection pulse is applied to the first scanning line WSL101, a selection pulse is subsequently applied to the second scanning line WSL102, and pixels for one row are selected every one horizontal period (1H). To go. Since the DSL 101 is selected simultaneously with the selection of the WSL 101, the switching transistor 112 is turned on. As a result, a drive current flows through the light emitting element via the drive transistor 111 and the switching transistor 112, so that light is emitted. In the middle of one field period (1f), the DSL 101 is in a non-selected state, and the switching transistor 112 is turned off. As a result, the light emission stops. The scanning line DSL101 controls the light emission time (duty) in one field period.

  Here, FIG. 13A is a graph showing operating points of the drive transistor 111 and the EL element 117 in the initial state. In the figure, the horizontal axis represents the drain-source voltage Vds of the drive transistor 111, and the vertical axis represents the drain-source current Ids. As illustrated, the source potential is determined by the operating point of the drive transistor 111 and the EL element 117, and the voltage value varies depending on the gate voltage. Since the drive transistor 111 is driven in the saturation region, the drive current Ids having the current value defined in the above-described equation (1) is supplied with respect to Vgs corresponding to the source voltage at the operating point.

  However, the IV characteristic of the EL element deteriorates with time as described above. As shown in (B), the operating point changes due to the deterioration over time, and the source voltage of the transistor changes even when the same gate voltage is applied. As a result, the gate-source voltage Vgs of the drive transistor 111 changes, and the flowing current value fluctuates. At the same time, the value of current flowing through the EL element 117 also changes. Thus, when the IV characteristic of the EL element 117 changes, the pixel circuit having the source follower configuration shown in FIG. 11 has a problem that the light emission luminance of the organic EL element changes with time.

  In order to avoid the above problem, it may be possible to reverse the arrangement of the drive transistor 111 and the EL element 117. That is, a circuit configuration in which the source of the drive transistor 111 is connected to the ground potential GND, the drain is connected to the cathode of the EL element 117, and the anode of the EL element 117 is connected to the power supply potential Vcc is also conceivable. In this method, as in the pixel circuit having the p-channel TFT configuration shown in FIG. 9, the source potential is fixed, and the drive transistor 111 is driven as a constant current source, resulting in deterioration of the IV characteristics of the EL element. Changes in brightness can also be prevented. However, in this method, it is necessary to connect the drive transistor to the cathode side of the EL element, and this cathode connection requires the development of a new anode electrode and cathode electrode, which is considered to be very difficult with the current technology. Yes. As described above, an organic EL display using an n-channel transistor that does not change in luminance in the conventional method has not been put into practical use.

  In the active matrix organic EL display, in addition to fluctuations in the characteristics of the EL elements, the threshold voltage of the n-channel TFT constituting the pixel circuit also changes over time. As is clear from the above equation (1), when the threshold voltage Vth of the drive transistor fluctuates, the drain current Ids changes. Thereby, even if the same gate voltage Vgs is given, there is a problem that the light emission luminance changes due to the variation of Vth.

  SUMMARY OF THE INVENTION In view of the above-described problems of the conventional technology, an object of the present invention is to provide a pixel circuit that can stably drive a load element even when the threshold voltage of a transistor constituting the pixel circuit changes with time, and a driving method thereof. It is what.

  In order to achieve this purpose, the following measures were taken. That is, the present invention is arranged at a portion where the scanning line and the signal line intersect, and includes an electro-optic element, a storage capacitor, a sampling transistor, and a drive transistor, and the drive transistor includes a source and a gate between the source and the gate. A driving method of a pixel circuit in which a holding capacitor is connected and the sampling transistor is connected between the gate of the drive transistor and the signal line, and the signal is output when the sampling transistor is selected by a scanning line. An operation of sampling an input signal from a line and holding it in the holding capacitor; an operation in which the drive transistor drives the electro-optic element in accordance with a signal voltage held in the holding capacitor; and A correction operation for detecting a threshold voltage of the drive transistor prior to current driving and holding the detected voltage in the storage capacitor; Wherein the correction operation, characterized in that it comprises a first operation of applying a predetermined voltage to the drive transistor is turned on between the gate and source of the drive transistor.

  Further, the correction operation includes a second operation in which a current is supplied to the drive transistor that is on in the first operation, and the threshold voltage is detected by reducing the voltage between the gate and the source of the drive transistor. . The pixel circuit includes a switching transistor connected between a power supply and the drive transistor, and the switching transistor is in an ON state when the second operation is performed, and causes a current to flow through the drive transistor.

  Preferably, in the correction operation, a voltage is applied between the gate and the source of the drive transistor to turn the drive transistor on, while the electro-optic element is reverse-biased, and a current flows through the drive transistor. On the other hand, no current flows through the electro-optic element in the reverse bias state.

  According to another aspect of the present invention, there is provided a pixel circuit arranged at a portion where a scanning line and a signal line intersect with each other, and includes an electro-optic element, a storage capacitor, a sampling transistor, and a drive transistor, The holding capacitor is connected to the gate, the sampling transistor is connected between the gate of the drive transistor and the signal line, and when the sampling transistor is selected by a scanning line, an input signal is output from the signal line. Is sampled and held in the holding capacitor, and the drive transistor drives the electro-optic element in accordance with the signal voltage held in the holding capacitor, and prior to the current driving of the electro-optic element, the drive transistor Correction means for detecting a threshold voltage and holding the detected voltage in the holding capacitor, the correction means comprising the drive Transistor is a predetermined voltage which is turned and applying between the gate and source of the drive transistor.

  According to the present invention, the pixel circuit can stably drive the electro-optic element by detecting the threshold voltage of the drive transistor and compensating for the change with time in a circuit.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. For convenience of explanation, a pixel circuit having a characteristic variation compensation function (bootstrap function) of a light emitting element that is a load element will be described first, followed by a pixel circuit having a drive transistor threshold voltage variation compensation function added, Finally, a pixel circuit having these compensation functions and having a reduced number of circuit elements will be described. FIG. 1 is a block diagram illustrating a configuration of a display device including a pixel circuit having a bootstrap function that is a compensation function for characteristic variation of a light-emitting element that is an electro-optical element. This pixel circuit configuration is the same as that described in Japanese Patent Application No. 2003-146758 (filed on May 23, 2003), which is a prior application of the same applicant.

  As shown in FIG. 1, the display device 100 includes a pixel array unit 102 in which pixel circuits (PXLC) 101 are arranged in a matrix, a horizontal selector (HSEL) 103, a write scanner (WSCN) 104, and a drive scanner (DSCN) 105. The signal lines DTL101 to DT110n selected by the horizontal selector 103 and supplied with video signals according to the luminance information, the scanning lines WSL101 to WSL10m selectively driven by the write scanner 104, and the scanning line DSL101 selectively driven by the drive scanner 105 ~ DSL 10m. Note that FIG. 1 shows a specific configuration of one pixel circuit for simplification of the drawing.

  As shown in FIG. 1, the pixel circuit 101 includes n-channel TFTs 111 to 115, a capacitor C111, a light emitting element 117 composed of an organic EL element (OLED: electro-optical element), and nodes ND111 and ND112. In FIG. 1, DTL 101 indicates a signal line, WSL 101 indicates a scanning line, and DSL 101 indicates another scanning line. Among these components, the TFT 111 constitutes a driving field effect transistor, the sampling TFT 115 constitutes a first switch, the TFT 114 constitutes a second switch, and the capacitor C111 constitutes a storage capacitor element. Yes.

  In the pixel circuit 101, a light emitting element (OLED) 117 is connected between the source of the TFT 111 and the ground potential GND. Specifically, the anode of the light emitting element 117 is connected to the source of the TFT 111, and the cathode side is connected to the ground potential GND. A node ND 111 is configured by a connection point between the anode of the light emitting element 117 and the source of the TFT 111. The source of the TFT 111 is connected to the drain of the TFT 114 and the first electrode of the capacitor C111, and the gate of the TFT 111 is connected to the node ND112. The source of the TFT ll4 is connected to a fixed potential (the ground potential GND in this embodiment), and the gate of the TFT 114 is connected to the scanning line DSL101. The second electrode of the capacitor C111 is connected to the node ND112. The source and drain of the sampling TFT 115 are connected to the signal line DTL101 and the node ND112, respectively. The gate of the TFT 115 is connected to the scanning line WSL101.

  Thus, in the pixel circuit 101 according to the present embodiment, the capacitor C111 is connected between the gate and the source of the TFT 111 as the drive transistor, and the source potential of the TFT 111 is connected to the fixed potential via the TFT 114 as the switch transistor. It is configured.

  Next, the operation of the above configuration will be described with reference to FIGS. 2A to 2F and FIGS. 3A to 3F, focusing on the operation of the pixel circuit. 3A shows the scanning signal ws [1] applied to the first row scanning line WSL101 of the pixel array, and FIG. 3B shows the scanning signal WSL102 applied to the second row scanning line WSL102 of the pixel array. 3C shows the scanning signal ws [2] to be applied, FIG. 3C shows the driving signal ds [1] applied to the scanning line DSL101 in the first row of the pixel array, and FIG. 3D shows the second signal of the pixel array. FIG. 3E shows the gate potential Vg (node ND112) of the TFT 111, and FIG. 3F shows the source potential Vs of the TFT 111 (node ND111). Respectively.

  First, when the normal EL light emitting element 117 is in the light emitting state, as shown in FIGS. 3A to 3D, scanning signals ws [1], ws from the light scanner 104 to the scanning lines WSL101, WSL102,. [2],... Are selectively set to a low level, and drive signals ds [1], ds [2],... To the scanning lines DSL101, DSL102,. Set to As a result, in the pixel circuit 101, as shown in FIG. 2A, the TFT 115 and the TFT 114 are held in an off state.

  Next, during the non-light emitting period of the EL light emitting element 117, as shown in FIGS. 3A to 3D, the scanning signals ws [1], ws [] from the light scanner 104 to the scanning lines WSL101, WSL102,. 2],... Are held at a low level, and the drive signals ds [1], ds [2],... To the scanning lines DSL101, DSL102,. . As a result, in the pixel circuit 101, as shown in FIG. 2B, the TFT 114 is turned on while the TFT 115 is kept off. At this time, a current flows through the TFT 114, and as shown in FIG. 3F, the source potential Vs of the TFT 111 drops to the ground potential GND. Therefore, the voltage applied to the EL light emitting element 117 is also 0 V, and the EL light emitting element 117 does not emit light.

  Next, during the non-light emission period of the EL light emitting element 117, as shown in FIGS. 3A to 3D, the drive scanner 105 drives the drive signals ds [1], ds [to the scanning lines DSL101, DSL102,. The scanning signals ws [1], ws [2],... From the write scanner 104 to the scanning lines WSL101, WSL102,. Is done. As a result, in the pixel circuit 101, as shown in FIG. 2C, the TFT 115 is turned on while the TFT 114 is kept on. As a result, the input signal (Vin) propagated to the signal line DTL101 by the horizontal selector 103 is written into the capacitor C111 as a storage capacitor. At this time, as shown in FIG. 3 (F), the source potential Vs of the TFT 111 as the drive transistor is at the ground potential level (GND level). Therefore, as shown in FIGS. The potential difference between the gate and the source becomes equal to the voltage Vin of the input signal.

  Thereafter, during the non-light emitting period of the EL light emitting element 117, as shown in FIGS. 3A to 3D, drive signals ds [1], ds [2] to the scanning lines DSL101, DSL102,. The scanning signals ws [1], ws [2],... From the write scanner 104 to the scanning lines WSL101, WSL102,... Are selectively set to the low level while being held at the high level. The As a result, in the pixel circuit 101, as shown in FIG. 2D, the TFT 115 is turned off, and writing of the input signal to the capacitor C111 as a storage capacitor is completed.

  Thereafter, as shown in FIGS. 3A to 3D, the scanning signals ws [1], ws [2],... From the light scanner 104 to the scanning lines WSL101, WSL102,. The drive scanner 105 selectively sets the drive signals ds [1], ds [2],... To the scanning lines DSL101, DSL102,. As a result, in the pixel circuit 101, the TFT 114 is turned off as illustrated in FIG. When the TFT 114 is turned off, as shown in FIG. 3F, the source potential Vs of the TFT 111 as a drive transistor rises, and a current also flows through the EL light emitting element 117.

  Although the source potential Vs of the TFT 111 fluctuates, there is a capacitance between the gate and source of the TFT 111, so that the gate-source potential is always Vin as shown in FIGS. It is kept. At this time, since the TFT 111 as the drive transistor is driven in the saturation region, the current value Ids flowing through the TFT 111 is determined by Vin which is the gate-source voltage of the TFT 111. This current Ids also flows in the EL light emitting element 117 in the same manner, and the EL light emitting element 117 emits light. Since the equivalent circuit of the EL light emitting element 117 is as shown in FIG. 2F, at this time, the potential of the node ND111 rises to the gate potential through which the current Ids flows in the EL light emitting element 117. As the potential rises, the potential of the node ND112 similarly rises through the capacitor 111 (retention capacitor). As a result, the gate / source potential of the TFT 111 is kept at Vin as described above.

  Here, in general, the EL characteristics of the EL light emitting element deteriorate as the light emission time becomes longer. Therefore, even if the drive transistor passes the same current value, the potential applied to the EL light emitting element changes, and the potential of the node ND111 decreases. However, in this circuit, since the potential of the node ND111 decreases while the gate-source potential of the drive transistor is kept constant, the current flowing through the drive transistor (TFT 111) does not change. Therefore, the current flowing through the EL light emitting element does not change, and a current corresponding to the input voltage Vin continues to flow even if the IV characteristics of the EL light emitting element deteriorate.

  As described above, according to this embodiment, the source of the TFT 111 as the drive transistor is connected to the anode of the light emitting element 117, the drain is connected to the power supply potential Vcc, and the capacitor C111 is connected between the gate and source of the TFT 111. In addition, since the source potential of the TFT 111 is connected to the fixed potential via the TFT 114 as a switch transistor, the following effects can be obtained. That is, even if the IV characteristic of the EL light emitting element changes with time, a source follower output without luminance deterioration can be performed. A source follower circuit of an n-channel transistor becomes possible, and the n-channel transistor can be used as a drive element of an EL light-emitting element while using the current anode / cathode electrodes. In addition, the transistor of the pixel circuit can be configured with only the n channel, and the a-Si process can be used in the TFT formation. Thereby, the cost of the TFT substrate can be reduced.

  FIG. 4 shows a pixel circuit configuration in which a threshold voltage canceling function is further added to the pixel circuit having the bootstrap function shown in FIG. This pixel circuit is the same as that described in Japanese Patent Application No. 2003-159646 (filed on June 4, 2003), which is a prior application of the same applicant. In order to facilitate understanding, portions corresponding to those of the pixel circuit shown in FIG. The pixel circuit of FIG. 4 is basically obtained by adding a threshold voltage cancel circuit to the pixel circuit of FIG. However, the gate of the switching transistor 114 included in the bootstrap circuit is connected to the scanning line WSL101 instead of the scanning line DSL101 to simplify the circuit. Basically, the switching transistor 114 included in the bootstrap circuit may be controlled to open and close in accordance with the sampling of the video signal, and thus such simplification is possible. Of course, a dedicated scanning line DSL101 may be connected to the gate of the switching transistor 114 as in the example of FIG.

  The threshold voltage cancel circuit basically includes a drive transistor 111, a switching transistor 112, an additional switching transistor 113, and a storage capacitor C111. In addition to these, the pixel circuit includes a coupling capacitor C112 and a switching transistor. The source / drain of the added switching transistor 113 is connected between the gate and drain of the drive transistor 111. The drain of the switching transistor 116 is connected to the drain of the sampling transistor 115, and the source is supplied with the offset voltage Vofs. The coupling capacitor C112 is interposed between the node ND114 on the sampling transistor 115 side and the node ND112 on the drive transistor side. A scanning line AZL 101 for canceling a threshold voltage (Vth) is connected to the gates of the switching transistors 113 and 116.

  FIG. 5 is a timing chart for explaining the operation of the pixel circuit shown in FIG. This pixel circuit sequentially performs Vth correction, signal writing, and bootstrap operation during one field (1f). Vth correction and signal writing are performed during the non-light emission period of 1f, and the bootstrap operation is performed at the beginning of the light emission period. First, in the Vth correction period, the scanning line AZL101 rises to a high level while the scanning line DSL101 is at a high level. As a result, the switching transistors 112 and 113 are simultaneously turned on, so that a current flows and the potential of the node ND112 connected to the gate of the drive transistor 111 rises. Thereafter, the DSL 101 falls to a low level and enters a non-light emitting state. As a result, the charge accumulated in the node ND112 is discharged through the switching transistor 113, and the potential of the ND112 gradually decreases. When the potential difference between the node ND112 and the node ND111 becomes Vth, no current flows through the drive transistor 111. As is apparent from the figure, the potential difference between ND112 and ND111 corresponds to Vgs, and Ids becomes 0 when Vgs = Vth from equation (1). As a result, the potential difference Vth between ND112 and ND111 is held in the holding capacitor C111.

  Subsequently, the scanning line WSL101 becomes high level for 1H and the sampling transistor 115 becomes conductive, and signal writing is performed. That is, the video signal Vsig supplied to the DTL 101 is sampled by the sampling transistor 115 and written to the holding capacitor C111 via the coupling capacitor C112. As a result, the holding potential Vin of the holding capacitor C111 is the sum of the previously written Vth and Vsig. However, the input gain of Vsig is not 100%, and there is some loss.

  Thereafter, the DSL 101 rises to a high level and starts light emission, and a bootstrap operation is performed. As a result, the signal potential Vin applied to the gate of the drive transistor 111 rises by ΔV according to the ID characteristic of the light emitting element 117. In this manner, the pixel circuit of FIG. 4 adds Vth and ΔV in addition to the net signal component applied to the gate of the drive transistor 111. Even if Vth and ΔV change, the influence can always be canceled, so that the light emitting element 117 can be driven stably.

  FIG. 6 is a circuit diagram showing an embodiment in which the number of elements is saved as compared with the pixel circuit shown in FIG. 4 in the pixel circuit according to the present invention. As shown in the figure, the pixel circuit 101 is arranged at a portion where the scanning line and the signal line intersect, and can be applied to an active matrix display device. While the signal line is one of the DTL 101, the scanning line is a total of four lines of WSL101, DSL101, AZL101a, and AZL101b. The pixel circuit 101 includes five N-channel thin film transistors including an electro-optic element 117, one holding capacitor C111, a sampling transistor 115, a drive transistor 111, a switching transistor 112, a first detection transistor 114, and a second detection transistor 113. It consists of and. As described above, the pixel circuit 101 includes one storage capacitor and five transistors, and has a configuration in which one capacitive element is fewer and one transistor is fewer than the pixel circuit illustrated in FIG. ing. Since the number of constituent elements is small, the yield is improved and the cost can be reduced.

  The storage capacitor C111 has one terminal connected to the source of the drive transistor 111 and the other terminal connected to the gate of the drive transistor 111. In the figure, the gate of the drive transistor 111 is represented by a node ND112, and the source of the drive transistor 111 is represented by a node ND111. Accordingly, the storage capacitor C111 is connected between the node ND111 and the node ND112. The electro-optical element 117 is made of an organic EL element having a diode structure, for example, and includes an anode and a cathode. The anode of the organic EL element 117 is connected to the source (node ND111) of the drive transistor 111, and the cathode is connected to a predetermined cathode potential Vcath. The organic EL element 117 includes a capacitive component between the anode and the cathode, and this is represented by Cp.

  The first detection transistor 114 has a source connected to the first ground potential Vss1, a drain connected to the source (node ND111) of the drive transistor 111, and a gate connected to the scanning line AZL101a. The second detection transistor 113 has a source connected to the second ground potential Vss2, a drain connected to the gate (node ND112) of the drive transistor 111, and a gate connected to the scanning line AZL101b.

  Sampling transistor 115 has its source connected to signal line DTL101, its drain connected to the gate (node ND112) of drive transistor 111, and its gate connected to scan line WSL101. The switching transistor 112 has a drain connected to the power supply potential Vcc, a source connected to the drain of the drive transistor 111, and a gate connected to the scanning line DSL101. The scanning lines AZL 101a, AZL 101b, and DSL 101 are arranged in parallel with the scanning line WSL101, and are scanned line-sequentially at appropriate timing by the peripheral scanner.

  The sampling transistor 115 operates when selected by the scanning line WSL101, samples the input signal Vsig from the signal line DTL101, and holds it in the storage capacitor C111 via the node ND112. The drive transistor 111 current-drives the electro-optic element 117 according to the signal potential Vin held in the holding capacitor C111. The switching transistor 112 becomes conductive when selected by the scanning line DSL101 and supplies a current to the drive transistor 111 from the power supply potential Vcc. The first detection transistor 114 and the second detection transistor 113 operate when selected by the scanning lines AZL 101a and AZL 101b, respectively, detect the threshold voltage Vth of the drive transistor 111 prior to current driving of the electro-optic element 117, and influence the effect beforehand. In order to cancel, the detected potential is held in the holding capacitor C111.

  As a condition for ensuring the normal operation of the pixel circuit 101, the first ground potential Vss1 is set lower than the level obtained by subtracting the threshold voltage Vth of the drive transistor from the second ground potential Vss2. That is, Vss1 <Vss2-Vth. The level obtained by adding the threshold voltage VthEL of the organic EL element 117 to the cathode potential Vcath is set higher than the level obtained by subtracting the threshold voltage Vth of the drive transistor 111 from the second ground potential Vss2. Expressed as an equation, Vcath + VthEL> Vss2-Vth. Preferably, the level of the second ground potential Vss2 is set in the vicinity of the lowest level of the input signal Vsig supplied from the signal line DTL101.

  The operation of the pixel circuit shown in FIG. 6 will be described in detail with reference to the timing chart of FIG. The timing chart shown in the figure shows that one field (1F) starts at timing T1 and one field ends at timing T6. At timing T0 before entering the field, the scanning lines WSL101, AZL101a, and AZL101b are at a low level, while the scanning line DSL101 is at a high level. Accordingly, the switching transistor 112 is in the on state, while the sampling transistor 115 and the pair of detection transistors 113 and 114 are in the off state. At this time, the drive transistor 111 causes a drive current to flow in accordance with the signal potential appearing at the node ND112, thereby causing the organic EL element 117 to emit light. At this time, the source potential of the drive transistor 111 (the potential of the node ND111) is held at a predetermined operating point. The timing chart of FIG. 7 shows the potential of the node ND112 and the potential of the node ND111, and represents the change of the gate potential and the source potential of the drive transistor 111, respectively.

  At timing T1, both the scanning lines AZL101a and AZL101b rise from the low level to the high level. As a result, both the first detection transistor 114 and the second detection transistor 113 are switched from the off state to the on state. As a result, the node ND112 rapidly decreases to Vss2, and the node ND111 also rapidly decreases to the first ground potential Vss1. At this time, since Vss1 <Vss2−Vth is set, the drive transistor 111 is kept on, and the drain current Ids flows. At this time, since the relationship of Vcath + Vth (EL)> Vss2−Vth is established, the organic EL element 117 is in a reverse bias state and no current flows. Accordingly, the non-light emitting state is entered. The drain current Ids of the drive transistor 111 flows to the Vss1 side through the first detection transistor 114 in the on state.

  Subsequently, at timing T2, since the scanning line AZL 101a returns from the high level to the low level, the first detection transistor 114 changes from the on state to the off state. As a result, since the current path of the drain current Ids flowing through the drive transistor 111 is interrupted, the potential of the node ND111 gradually increases. When the potential difference between the node ND111 and the node ND112 is exactly Vth, the drive transistor 111 is turned off and no drain current flows. The potential difference Vth appearing between the node ND111 and the node ND112 is held in the holding capacitor C111. In this manner, the first and second detection transistors 114 and 113 operate when selected at appropriate timings by the scanning lines AZL101a and AZL101b, respectively, detect the threshold voltage Vth of the drive transistor 111, and store this in the storage capacitor C111. Hold.

  Thereafter, at the timing T3, the scanning line AZL101b is switched from the high level to the low level, and the scanning line DSL101 is also switched from the high level to the low level at substantially the same time. As a result, the second detection transistor 113 and the switching transistor 112 are switched from the on state to the off state. In the timing chart, a period between timings T2 and T3 is referred to as a Vth correction period, and the detected threshold voltage Vth of the drive transistor 111 is held in the storage capacitor C111 as a correction potential.

Thereafter, at timing T4, the scanning line WSL101 rises from the low level to the high level. As a result, the sampling transistor 115 becomes conductive, and the input potential Vin is written into the storage capacitor C111. This input potential Vin is held in a form that is added to the threshold voltage Vth of the drive transistor. As a result, fluctuations in the threshold voltage Vth of the drive transistor 111 are always canceled, and Vth correction is performed. The input potential Vin written to the storage capacitor C111 is expressed by the following equation.
Vin = Cp / (Cs + Cp) × (Vsig−Vss2)
Here, Cs represents the capacitance value of the storage capacitor C111. On the other hand, Cp is a capacitance component of the organic EL element 117 as described above. In general, Cp is much larger than Cs. Therefore, Vin is approximately equal to Vsig−Vss2. At this time, if Vss2 is set near the black level of Vsig, as a result, Vin is substantially equal to Vsig.

  Thereafter, the scanning line WSL101 returns from the high level to the low level to finish sampling the input signal Vsig, but at the subsequent timing T5, the scanning line DSL101 rises from the low level to the high level, and the switching transistor DSL101 is turned on. As a result, a drive current is supplied from the power supply potential Vcc to the drive transistor 111, and the light emitting operation of the organic EL element 117 is started. Since a current flows through the organic EL element 117, a voltage drop occurs, and the potential of the node ND111 increases. In conjunction with this, the potential of the node ND112 also rises, so that the gate potential Vgs of the drive transistor 111 is always maintained at Vin + Vth regardless of the potential rise of the node ND111. As a result, the organic EL element 117 continues to emit light with a luminance corresponding to the input voltage Vin. When the scanning signals AZL 101a and AZL 101b rise again at the timing T6 at the end of one field, the light emission of the organic EL element 117 is stopped while entering the Vth correction period of the next field.

It is a block diagram which shows the reference example of a pixel circuit. FIG. 2 is a schematic diagram for explaining an operation of the pixel circuit shown in FIG. 1. 2 is a timing chart for explaining the operation of the pixel circuit shown in FIG. 1. It is a circuit diagram which shows the other reference example of a pixel circuit. 5 is a timing chart for explaining the operation of the pixel circuit shown in FIG. 4. It is a circuit diagram which shows the structure of the pixel circuit concerning this invention. 7 is a timing chart for explaining the operation of the pixel circuit shown in FIG. 6. It is a block diagram which shows an example of the conventional pixel circuit. It is a circuit diagram which shows an example of the conventional pixel circuit. It is a graph which shows the time-dependent change of the characteristic of an EL element. It is a circuit diagram which shows the other example of the conventional pixel circuit. 12 is a timing chart for explaining the operation of the pixel circuit shown in FIG. 11. It is a graph which shows the operating point of a drive transistor and an EL element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 ... Pixel circuit, 111 ... Drive transistor, 112 ... Switching transistor, 113 ... Second detection transistor, 114 ... First detection switching transistor, 115 ... Sampling transistor, 117 ... .Electro-optic element, C111 ... retention capacity

Claims (5)

The scanning line and the signal line are arranged at a crossing portion, and includes an electro-optic element, a storage capacitor, a sampling transistor, and a drive transistor, and the storage capacitor is connected between the source and gate of the drive transistor. A driving method of a pixel circuit in which the sampling transistor is connected between the gate of the drive transistor and the signal line,
When the sampling transistor is selected by a scanning line, an operation of sampling an input signal from the signal line and holding it in the holding capacitor;
An operation in which the drive transistor drives the electro-optic element in accordance with a signal voltage held in the holding capacitor;
A correction operation for detecting a threshold voltage of the drive transistor prior to current driving of the electro-optic element and holding the detected voltage in the holding capacitor,
The correction operation is as follows:
A pixel circuit driving method comprising: a first operation of applying a predetermined voltage at which the drive transistor is turned on between a gate and a source of the drive transistor. The correction operation is as follows:
2. A second operation of passing a current through the drive transistor that is in an on state in the first operation and detecting a threshold voltage by reducing a voltage between a gate and a source of the drive transistor. Item 8. A driving method of a pixel circuit according to Item 1. The pixel circuit includes a switching transistor connected between a power supply and the drive transistor,
3. The pixel circuit driving method according to claim 2, wherein the switching transistor is in an ON state when the second operation is performed, and a current flows through the drive transistor. The correction operation is as follows:
A voltage is applied between the gate and source of the drive transistor to turn the drive transistor on, while the electro-optic element is reverse biased,
2. The method of driving a pixel circuit according to claim 1, wherein a current is passed through the drive transistor while no current flows through the electro-optic element in a reverse bias state. A pixel circuit arranged at a portion where the scanning line and the signal line intersect with each other, and includes an electro-optic element, a storage capacitor, a sampling transistor, and a drive transistor,
The storage capacitor is connected between the source and gate of the drive transistor, the sampling transistor is connected between the gate of the drive transistor and the signal line,
When the sampling transistor is selected by a scanning line, it samples an input signal from the signal line and holds it in the holding capacitor,
The drive transistor drives the electro-optic element in accordance with a signal voltage held in the holding capacitor,
Correction means for detecting a threshold voltage of the drive transistor prior to current driving of the electro-optic element and holding the detected voltage in the holding capacitor;
The pixel circuit according to claim 1, wherein the correction unit applies a predetermined voltage at which the drive transistor is turned on between a gate and a source of the drive transistor.

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