JP4125303B2 - Voltage-current converter and light-emitting device - Google Patents

Description

  The present invention relates to a voltage-current converter used in a light-emitting device such as an exposure device or a display device of an electrophotographic apparatus, and in particular, arranged in a signal transfer path for supplying a video signal as a voltage signal as a current signal to each pixel. It is related with the preferable voltage-current converter.

  A display device is taken as an example of a light emitting device.

  2. Description of the Related Art In recent years, an organic electroluminescence (EL) display device, which is one of the flat panel displays that have been attracting attention, emits light by a current value supplied from a data signal source by an organic EL element as a light emitting element disposed in each pixel. There is a type whose luminance is determined. In this type, it is necessary to arrange a voltage-current converter in a data signal source to convert a video signal, which is a voltage signal, into a current signal. An example of the voltage-current converter is shown in FIG. 9 of Patent Document 1 as its circuit configuration.

  FIG. 3 shows an example of the circuit configuration of the voltage-current converter. In the figure, T0 and T1 are n-type transistors, Vin is an input voltage, Iout is a current output from the circuit, V1 is a first potential, and S1 is a control signal.

  In the device of FIG. 3, the transistor T1 outputs a current Iout having a value corresponding to the gate voltage by a gate potential set by the voltage Vin input via the transistor T0 provided as necessary.

  However, when the device of FIG. 3 is disposed on each of a plurality of data signal lines corresponding to a plurality of pixel columns, the output current Iout is changed with respect to the input voltage Vin at the same level due to variations in the threshold value Vth of the transistor T1. Variation will occur.

  In order to solve such a problem, the present inventors first invented a voltage-current converter having a circuit configuration as shown in FIG. In the figure, T0 to T3 and T6 are n-type transistors, T7 is a p-type transistor, S1 to S3 and S6 are independent control signals, C1 and C2 are capacitors, and V1 and V3 provide first and third potentials, respectively. A potential source (potential source), Vin is an input terminal to which an input voltage is input, and Iout is an output terminal to which an output current is output.

  The operation of the apparatus of FIG. 4 will be described with reference to the timing chart of FIG. In the figure, VG (T1) represents the gate potential of the first transistor T1, and VS (T1) represents the source potential of the first transistor T1.

  First, at time t1, the control signals S2 and S3 become the high level H, and the second transistor T2 and the third transistor T3 are turned on. As a result, the gate potential VG (T1) of the first transistor T1 is precharged from the first potential source V1.

  Next, at time t2, the control signal S3 of the gate of the third transistor T3 becomes the low level L, so that the third transistor T3 is turned off, and the first transistor T1 is self-biased to the turn-on voltage, and is gentle. Draw a curve and discharge. As a result, VG (T1) is discharged over a sufficient period of time toward the third potential V3 until reaching the threshold value Vth of the transistor T1. Here, Va = Vth.

  After the self-bias period ends, at time t3, the control signal S2 becomes L and the second transistor T2 is turned off. At time t4, the control signal S1 becomes H and the input control transistor T0 is turned on. As a result, the gate of the first transistor T1 becomes the potential Vb ′. Here, Vb 'is the sum of the threshold value Vth of the first transistor T1 and the C1, C2 capacitance divided voltage Vc of the input voltage Vin. At time t5, the control signal S1 becomes L and the input control transistor T0 is turned off.

  At an appropriate time after time t5, the control signal S6 becomes H and the sixth transistor T6 is turned on as necessary, and is set at times t4 to t5 via the sixth transistor T6 and the first transistor T1. An output current Iout corresponding to the gate potential VG (T1) of the first transistor T1 is obtained.

  In the voltage-current converter of FIG. 4, there is no variation other than the threshold value Vth of the first transistor T1, and if sufficient self-bias is applied, a voltage-current conversion characteristic that is uniform among a plurality of voltage-current converters can be obtained. Should be able to.

Japanese Patent Laid-Open No. 2002-40074

  However, in the voltage-current converter of FIG. 4, the charge Q injected into the gate of the first transistor T1 is C2 × (Vpre−Vth). Here, Vpre indicates a voltage precharged to the gate of the first transistor T1 through the transistors T2 and T3 which are turned on from the first potential V1. The electrification Q must be discharged during the self-bias period from time t2 to t3. Actually, since the discharge period is finite, the gate potential VG (T1) after self-bias is Va '. Va ′ is the sum of the threshold value Vth of the first transistor T1 and the self-bias residual voltage Vr. Therefore, when applied to a display device as an example, even when black display is performed in an EL element, a current corresponding to the self-bias residual voltage Vr flows to the pixel having the EL element. That is, it becomes impossible to perform black display by completely blocking the current supply to the pixel, and the display contrast is lowered.

  An object of the present invention is to provide a voltage-current converter capable of making the output current as close to zero as possible when the voltage-current conversion characteristics are uniform and the input voltage Vin is zero.

A first aspect of the present invention is a voltage-current conversion transistor having a gate for receiving a voltage signal input from an input terminal, a drain for outputting a current signal from an output terminal, and a source,
A gate potential setting circuit for setting the Gate of the voltage-current converting transistor to a predetermined first potential,
A source over the ground potential setting circuit for setting the source over scan of the voltage-current converting transistor to a predetermined second potential or third potential,
A storage capacitor having one end connected to the gate of the voltage-current conversion transistor and the other end set to the third potential;
Comprising
After the gate of the voltage / current converting transistor is set to the first potential, the source potential setting circuit is configured to discharge the charge of the storage capacitor through the voltage / current converting transistor kept on by the gate potential. Sets the source of the voltage-current conversion transistor to a second potential that accelerates the discharge compared to the third potential,
The voltage-current conversion device , wherein the source potential setting circuit sets the source of the voltage-current conversion transistor to the third potential after the period during which the charge of the storage capacitor is discharged ends. It is.

A second aspect of the present invention is a light emitting device, comprising: a light emitting unit having a plurality of light emitting elements; and a signal source that supplies a current signal to the light emitting unit,
The signal source includes the voltage-current converter of the present invention.

  According to the present invention, even when the self-bias period of the voltage-current conversion transistor that determines the output current is short, the output current can be cut off almost completely with respect to the zero setting of the input voltage. Accordingly, even when an element that reacts to a minute current is driven by current, it is possible to reliably stop the operation of the element. Even when a plurality of voltage-current converters according to the present invention are used in parallel in a display device or the like, uniform voltage-current conversion characteristics can be obtained without being affected by variations in threshold values of transistors.

  A circuit configuration of a preferred embodiment of the voltage-current converter of the present invention is shown in FIG. In the figure, T0 to T6 are n-type transistors, T7 is a p-type transistor, C1 and C2 are first and second capacitors, S1 to S6 are independent control signals, and V1 to V3 are first to third potentials. And is also a potential source that provides those potentials. Vin is an input terminal to which an input voltage is applied, and Iout is an output terminal to which an output current is supplied.

The voltage-current converter in FIG. 1 includes a voltage-current conversion transistor T1 having a gate that receives a voltage signal input from an input terminal Vin, a drain for outputting a current signal from an output terminal Iout, and a source.
Gate potential setting circuits T2, T3, T7 for setting the gate of the voltage-current converting transistor T1 to a predetermined first potential V1,
Source potential setting circuits V2, T4 , T5, V3 including a setting transistor T4 for setting the source of the voltage-current conversion transistor T1 to a predetermined second potential V2,
Comprising
The second potential V2 is different from a potential V3 of a terminal coupled to the gate of the voltage / current converting transistor T1 via a storage capacitor C2.

  In the apparatus of FIG. 1, the source potential setting circuit may further include a transistor T5 that connects the terminal V3 and the source of the voltage-current conversion transistor T1.

  The gate potential setting circuit may include a transistor T2 that connects the gate and the drain of the voltage-current conversion transistor T1.

  Further, the source potential setting circuit causes the gate potential to transition to a potential at which the absolute value of the gate potential of the voltage-current conversion transistor T1 set to the first potential V1 is further reduced. Is preferred.

In the device of this embodiment, the gate is connected to the other terminal of the first capacitor C1 to which the voltage Vin is input and one terminal of the second capacitor C2, and the drain is connected to the output terminal from which the current Iout is output. A first transistor T1,
A second transistor T2 having a source connected to the other terminal of the first capacitor C1 and the gate of the first transistor T1, a drain connected to the output terminal Iout, and a gate controlled by an independent control signal S2,
A third transistor T3 having a source or drain connected to the drain of the first transistor T1, a drain or source connected to the first potential V1, and a gate controlled by an independent control signal S3;
A fourth transistor T4 having a drain connected to the source of the first transistor T1, a source connected to the second potential V2, and a gate controlled by an independent control signal S4;
A fifth transistor T5 having a drain connected to the source of the first transistor T1, a source connected to the other terminal of the second capacitor C2 and a third potential V3, and a gate controlled by an independent control signal S5;
Is provided.

  Here, the first, second, and third potentials may be set so that a potential difference between the third potential and the second potential is equal to or greater than a voltage remaining in the gate potential after self-bias of the first transistor. .

  The third potential may be a common potential of the input voltage.

  The first to fifth transistors may be thin film transistors formed using a non-single crystal semiconductor.

  As described above, in the embodiment of FIG. 1, the input control transistor T0 is disposed between the input terminal and one terminal of the first capacitor C1 as necessary, and the transistor T0 is turned on by the independent control signal S1. Control off and control the input timing of the input voltage Vin. In addition, a sixth transistor T6 is arranged between the drain of the first transistor T1, the drain of the second transistor T2, and the output terminal as needed in order to control the output timing of the output current Iout, and independent control is performed. Control is performed by the signal S6.

  The voltage-current converter of FIG. 1 differs from the voltage-current converter of FIG. 4 in that fourth and fifth transistors T4 and T5 are provided, and the source of the first transistor (voltage-current conversion transistor) T1 in the self-bias period. The potential is set to the second potential V2, and the second potential V2 is set to be lower than the third potential V3 so that “the third potential V3—the second potential V2” is not less than the self-bias residual voltage Vr. deep. Thereby, the influence of the self-bias residual voltage can be eliminated in advance. Note that the third potential V3 may be a common potential of the input voltage Vin.

  In the following, the operation of the apparatus of FIG. 1 will be specifically described with reference to the timing chart of FIG.

  At time t1, the control signals S2, S3, and S4 are set to the high level H, and the second, third, and fourth transistors T2, T3, and T4 are turned on. As a result, the source potential VS (T1) of the first transistor T1 is set to the second potential V2 via the fourth transistor T4, and the gate potential VG (T1) is set to the second potential V2 via the second and third transistors T2 and T3. It is precharged from one potential V1 and becomes potential V1.

  Next, at time t2, the control signal S3 becomes a low level L, and the third transistor T3 constituting the gate potential setting circuit is turned off. Then, the first transistor T1 is self-biased to the turn-on voltage, and the discharge operation is performed while the gate potential VG (T1) of the transistor T1 draws a gentle curve. At this time, the source potential VS (T1) of the first transistor T1 is set to the second potential V2 by the source potential setting circuit. Therefore, until the gate potential VG (T1) of the first transistor T1 reaches the threshold value Vth of the transistor T1, the electric charge is discharged toward the second potential V2 in a shorter time than in FIGS.

  When S2 and S4 become L at time t3, the second and fourth transistors T2 and T4 are turned off, and the discharge of the first transistor T1 is completed. At this time, the gate potential is sufficiently low. That is, the potential Va has a small absolute value. Next, when S1 and S5 become H at time t4, the input control transistor T0 and the fifth transistor T5 are turned on, and the source potential VS (T1) of the first transistor T1 is set to the third potential V3. The gate potential VG (T1) is Vb. Here, “Vb = the threshold value Vth of the first transistor T1 + the self-bias residual voltage Vr− (the third potential V3−the second potential V2) + the C1, C2 capacitance divided voltage of the input voltage Vin”. At time t5, the control signals S1 and S5 become L, and the input control transistor T0 and the first transistor T1 are turned off. After that time, when the control signal S6 is set to H at an appropriate time and the sixth transistor T6 is turned on, the first transistor T1 set at the time t4 to t5 is set via the sixth transistor T6 and the first transistor T1. An output current Iout corresponding to the gate potential VG (T1) is obtained.

  Therefore, in the voltage-current converter, when the input voltage Vin is 0, the gate potential VG (T1) of the first transistor T1 is “the threshold Vth of the first transistor T1 + the self-bias residual voltage Vr− (the third potential V3−the third potential V3− 2 potential V2) ". Here, as described above, since the second potential V2 is set to satisfy “third potential V3−second potential V2 ≧ self-bias residual voltage Vr”, the gate potential VG (T1) is equal to the first potential V2 (T1). It becomes lower than the threshold value Vth of one transistor T1, and the first transistor T1 is sufficiently cut off.

  As described above, in the voltage-current converter of this embodiment, if the second potential and the third potential are set in a predetermined relationship according to the self-bias period, the self-bias residual voltage when the input voltage is zero is set. The output current can be made zero by eliminating the influence.

  Further, when a plurality of voltage-current converters of this embodiment are used in parallel, the correction effect due to the variation in the threshold value of the transistor is the same as that of the voltage-current converter of FIG. 4 and is maintained.

  The voltage-current converter of the present invention is preferably used in a display device, and therefore preferably applied to a thin film transistor in which the first to fifth transistors are configured using a non-single-crystal semiconductor thin film such as amorphous silicon. Is.

  The apparatus described above includes a light emitting unit 3 having a plurality of light emitting elements and a signal source 1 for supplying a current signal to the light emitting unit 3 in the light emitting device having the configuration shown in FIG. It can be applied to a light-emitting device characterized in that the source 1 includes the voltage-current converter of FIG.

Here, the light emitting section 3 is preferably an active matrix type organic EL light emitting display section having a light emitting element and a transistor. The pixel circuit of the light emitting unit 3 is as follows.
FIG. 7 shows a pixel circuit 2 having a light emitting element EL, a driving transistor M5, switching transistors M1 and M2 for current programming, and a switching transistor M3 for light emission control.

  The transistors M1 and M2 are turned on, the transistor M3 is turned off, and the current signal Iout is programmed in the pixel circuit 2. Next, the transistors M1 and M2 are turned off, the transistor M3 is turned on, and current is supplied from the power supply line Vcc to the light emitting element EL to emit light. P1 is a scanning control line.

It is a figure which shows the circuit structure of one Embodiment of the voltage-current converter of this invention. It is a timing chart of the voltage-current converter of FIG. It is a figure which shows the circuit structure of an example of the conventional voltage-current converter. It is a figure which shows the circuit structure of the other example of the conventional voltage-current converter. It is a timing chart of the voltage-current converter of FIG. It is a figure which shows the structure of the light-emitting device of this invention. It is a figure which shows the pixel circuit of the light-emitting device of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Signal source 2 Pixel circuit 3 Light emission part T0-T6, M1, M2 n-type transistor T7, M3, M5 p-type transistor C1, C2, Ch capacity S1-S6 Control signal V1-V3 Potential Vcc Power supply line Vin Input terminal, Input Voltage Iout Output terminal, Output current P1 Scan control line EL Light emitting element

Claims (10)

A voltage-current conversion transistor having a gate for receiving a voltage signal input from the input terminal, a drain for outputting a current signal from the output terminal, and a source;
A gate potential setting circuit for setting the Gate of the voltage-current converting transistor to a predetermined first potential,
A source over the ground potential setting circuit for setting the source over scan of the voltage-current converting transistor to a predetermined second potential or third potential,
A storage capacitor having one end connected to the gate of the voltage-current conversion transistor and the other end set to the third potential;
Comprising
After the gate of the voltage / current converting transistor is set to the first potential, the source potential setting circuit is configured to discharge the charge of the storage capacitor through the voltage / current converting transistor kept on by the gate potential. Sets the source of the voltage-current conversion transistor to a second potential that accelerates the discharge compared to the third potential,
The voltage-current conversion device , wherein the source potential setting circuit sets the source of the voltage-current conversion transistor to the third potential after the period during which the charge of the storage capacitor is discharged ends. . The source potential setting circuit includes a potential source for providing a third potential, the voltage-current converter according the source over the scan of the voltage-current converting transistors, the transistors connected to the including claim 1. The gate potential setting circuit includes a voltage-current converter according to claim 1 or 2 including a transistor for connecting the Gate and drain of the voltage-current converting transistor. The voltage-current conversion according to any one of claims 1 to 3 , wherein the second potential is set so that a voltage across the holding capacitor at the end of the discharge period is equal to or lower than a threshold voltage of the voltage-current conversion transistor. apparatus. A gate of the voltage-current conversion transistor is connected to the other terminal of the first capacitor that receives a voltage input from the input terminal and one terminal of the second capacitor that is the holding capacitor , and the voltage current The drain of the conversion transistor supplies the current output from the output terminal,
The gate potential setting circuit is
A second transistor having a source connected to the other terminal of the first capacitor and the gate of the voltage / current converting transistor, a drain connected to the drain of the voltage / current converting transistor , and a gate controlled by an independent control signal; ,
The source or drain to the drain of the voltage-current converting transistor is connected, the first potential is provided to the drain or source, and a third transistor whose gate is controlled by independent control signals,
The source potential setting circuit is
A fourth transistor in which the drain is connected to the source of the voltage-current converting transistor , the source is connected to the second potential, and the gate is controlled by an independent control signal;
The drain to the source of the voltage-current converting transistor, the source is connected to the other terminal and the third conductive position of the second capacitor, and a fifth transistor whose gate is controlled by independent control signals,
The voltage-current converter of Claim 1 provided with. 6. The voltage current according to claim 5, further comprising a transistor having a source or drain connected to the input terminal, a drain or source connected to one terminal of the first capacitor, and a gate controlled by an independent control signal. Conversion device. 7. The transistor according to claim 5 , further comprising a transistor having a source or drain connected to the output terminal, a drain or source connected to the drain of the voltage-current conversion transistor, and a gate controlled by an independent control signal. Voltage-current converter. The voltage-current converter according to any one of claims 5 to 7, wherein the voltage-current conversion transistor and the second to fifth transistors are thin film transistors configured using a non-single crystal semiconductor. A light emitting device, comprising: a light emitting unit having a plurality of light emitting elements; and a signal source for supplying a current signal to the light emitting unit,
A light-emitting device, wherein the signal source includes the voltage-current converter according to claim 1.   The light emitting device according to claim 9, wherein the light emitting section is an active matrix organic electroluminescence light emitting element section having a light emitting element and a transistor.

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