JPS6118755B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an image display device such as a television using a matrix type liquid crystal display panel.

[Prior art]

Various display devices have been devised or put into practical use using the electro-optic effect of liquid crystals. These are due to the combination of alignment characteristics of liquid crystal molecules, dielectric anisotropy, optical anisotropy, etc., and are commonly called DSM,
There are names such as TN, GH, etc. Common features of these liquid crystals are that they have a light-receiving display effect, that they have relatively high resistance, and that the threshold value of the display characteristics is slow or unstable with respect to each parameter. The fact that it is a light-receiving type and has high resistance is why liquid crystals are superior to other display materials and are put into practical use as display materials. driving conditions are becoming more complex and difficult. Furthermore, the short lifespan compared to DC drive is another factor that makes the drive conditions difficult.

Figure 1 shows the display characteristics of a display using a TN-type nematic liquid crystal. Figure 1 is from 1976
This is a quotation from Fig. 5 on page 51 of DISPLAY conf. In the figure, the effective value of the voltage applied to the picture element in the selected state (display state) is represented by Vs, and the effective value of the voltage applied to the picture element in the non-selected state (non-display state) is represented by _VNS . When driving a matrix address method in which multiple liquid crystal picture elements are time-divided and dynamically scanned and driven, formulas (4) and (5) on page 51 of the above-mentioned document, where the number of scanning picture elements is N and the applied display voltage is Vc, are used. Than, It is expressed as Obviously, as N increases in the above equation, V _NS approaches. In Figure 1, V _S and V _NS
and the corresponding difference in the amount of relative transmitted light decreases, and as a result, it becomes impossible to obtain a large display contrast between the selected state and the non-selected state.

As a specific example, if N is 50, then V _S /V _NS 1.15 From Figure 1, if V _NS = V _Th 3.0V _rns , then V _S 1.15×V _Th 3.45 In this case, the relative amount of transmitted light is approximately 20, which is at saturation. No. 3
It's only about 1/10th of that. That is, in this type of driving method, as the value of N increases, it becomes difficult to obtain a large contrast in the displayed image. Further, if the ratio of the applied voltage at the time of selection and the applied voltage at the expected time is b, then b = √ + 1 from equation (3) on page 50 of the literature, and there are drawbacks such as the need to increase the operating voltage as N increases. In order to solve these drawbacks, there has been a so-called active matrix liquid crystal display device using switching transistors as shown in FIG.
This can be seen in the document SID77DGESTP64-65. In the figure, 2-1 is an image signal input such as a TV video signal, 2
-2 is a synchronous separation signal, and 2-3 is a circuit for generating a control signal such as a timing lock from the synchronous separation signal. 2-4 and 2-5 are circuits for controlling horizontal lines in the vertical line area of the matrix display section and distributing and scanning display signals to each matrix picture element. 2-4 is a drain drive circuit that converts the serial image signal inputted from 2-1 into parallel and supplies it to the drain side of the transistor connected in series to each picture element. 2-5 is a gate drive circuit that uses the 2-3 output clock to sequentially control the gates of transistors connected to each picture element on and off line by line to pass the image signal to each picture element.The structure of the shift register is shown in 2-5. I will do it. Output side drains 2-6 of transistors arranged in each matrix section
is coupled to each picture element electrode of the liquid crystal display. As shown in the figure, a capacitor is arranged in parallel with the liquid crystal picture element.

As described in the document SID78 DIGEST P96-97, in the conventional matrix display using the circuit shown in FIG. 2, the liquid crystal was driven by direct current.

In Figure 2, among the electrodes that sandwich the liquid crystal in the liquid crystal matrix display section, the electrodes facing each picture element electrode are common electrodes over the entire display surface, and the potential is GND.
It is taken as a level and matches the common side electrode potential of the substrate of the MOS transistor and the capacitor arranged in parallel. For this reason, the liquid crystal material requires treatment such as doping with a redox agent in order to maintain a long DC life.

FIG. 3 shows a circuit for driving the display device of FIG. 2.

Specifically, examples can be found in SID78 DIGEST P94-95. In FIG. 3, 3-1 corresponds to 2-1 and is an image signal input. 3-2 is a low-pass filter, 3-3 is an amplifier, 3-4 is an A/D converter,
3-5 is a data encoder, and 3-8 is a serial/parallel conversion shift register. The image signal input 3-1 is converted into an image signal of a band corresponding to the display performance of the corresponding front panel through a low-pass filter and an amplifier, and then converted into a digital code by an A/D converter. 3-8 outputs the converted image digital image data to each data line of the matrix in parallel. The parallel output data is input to a D converter provided for each data line and restored to an analog image signal. At this time, the gain of the D/A converter output signal is controlled by the gain control circuit 3-9, and the (voltage -
Contrast) Adjustment is made so that the contrast of the correlation characteristic and the image signal match. Further, the D/A output is input to a buffer amplifier 3-12. 3-12
The offset bias level adjustment circuit 3-10 adjusts the reference level of the image signal so that it corresponds to the vicinity of the threshold value of the liquid crystal, and outputs the image signal to the data line. 3-6 is a synchronization separation circuit, 3-7 is a timing signal generation circuit, and 3-13 is a circuit for controlling the clock line of the matrix display section, which corresponds to 2-5. 3
-16 is a display matrix section, and its configuration is the same as 2-7 in FIG. 2, so its illustration is omitted. In the circuit example shown in FIG. 3, the liquid crystal is driven by direct current as in FIG. 2. In addition, in FIG. 3, a gain control circuit 3-9 and a offset bias level control circuit 3, 10 are applied to the image signal supplied to the data line of the matrix display section to adapt the signal level to the characteristics of the liquid crystal display. making things possible. The method of control in this case is to provide a D/A converter for adjusting the gain and a buffer amplifier for adjusting the offset for each data line, and to adjust each using the same control signal line. As is clear from the figure, the number of D/A converters and buffer amplifiers equal to the number of data lines is required, making the data line driving circuit extremely complicated. Furthermore, the amplifiers constituting each D/A or buffer must have the same gain and other amplification characteristics. Matching the characteristics of each amplifier without adjustment is close to possible in device manufacturing, and therefore adjustment must be made for each amplifier in advance. In such conventional display devices, liquid crystal driving is based on direct current driving, and therefore, the liquid crystal material needs to be treated to maintain its direct current life. Also, if AC drive is attempted, bipolar image signals are supplied to each switching transistor, so the transistor structure
In cases where the display device is extremely complex, such as a CMOS structure, and requires a large number of picture elements, such as a television, it is impossible to realize a display device with virtually no holes. Ta.

〔the purpose〕

The present invention overcomes the above-mentioned problems by supplying a common electrode potential with a midpoint in the operating voltage range of a switching transistor in a so-called active matrix liquid crystal display device, and switching with a polarity based on the midpoint potential. It is an object of the present invention to provide a liquid crystal image display device having an active matrix structure that is substantially capable of current driving by providing means for supplying a video signal that is inverted.

〔Example〕

Hereinafter, the present invention will be explained by specific examples using the drawings. Of course, the present invention can be implemented using other circuit systems, but these also naturally belong to the present invention.

FIG. 4 is a block diagram showing an overall view of a television receiver configured according to the present invention. In the figure, 4-1 is a tuner section that selects the frequency of a predetermined channel from the received radio waves input by the antenna. 4-2 is a circuit from the intermediate frequency amplifier to video detection; 4-4 is a circuit for audio intermediate frequency, output, etc.; and 4-5 is a circuit for separating horizontal, vertical, etc. synchronization signals from the video signal. Reference numeral 4-3 denotes a video amplification circuit block according to the present invention, which outputs a liquid crystal display image signal to a subsequent stage matrix display section data signal latch circuit 4-8. 4-9 is a data line drive circuit. 4-6 and 4-7 receive the output of the synchronizing signal separation circuit 4-5 and supply a data latch signal to 4-8 and a timing signal for driving the clock line (horizontal line) of the matrix display section to 4-10, respectively. 4-1
Reference numeral 1 denotes a power supply B, which supplies a common electrode voltage to be described later to the common electrode 4-13 (dotted chain line). 4-1
2 represents a matrix type liquid crystal display panel.

FIG. 5 shows an active matrix liquid crystal image display device of this embodiment. 5-1 is a gate drive circuit;
Reference numeral 5-2 denotes a drain drive circuit, which is connected to a transistor for selectively supplying an image signal to the picture element electrode for each picture element 5-3 of the matrix display section. All the electrodes of the picture elements to which the outputs of each transistor are connected are located on one of a pair of flat plates that sandwich the liquid crystal, and each electrode is electrically separated and independent on the flat plate on which the electrode is placed. are doing. Among the plates sandwiching the liquid crystal, a single common electrode is provided on the flat plate facing the above-mentioned flat plate over the entire display section. Here, the substrate potential of each transistor and the one-side electrode potential of the capacitor provided for each picture element commonly match the GND potential, but the liquid crystal display common electrode potential 5-4 is not the GND potential, and will be described later. A common electrode potential is supplied. An example of a transistor and a capacitor configured for each picture element is shown in FIGS. 6 and 7. FIG. 6 is a cross-sectional view of a single plate on the side where electrodes are separated for each picture element and arranged in a matrix.
In the figure, 6-1 is a silicon substrate. 6-2 is 6-
1 is a diffusion layer of the opposite conductivity type, and 6-3 is a diffusion layer of 6-
It is a diffusion layer of the same conductivity type as 1 and works as a stopper and a capacitor electrode. Further, 6-4 is a gate oxide film, and its film thickness is approximately 400 to 2000 Å. 6-5 is polysilicon, 6-5a is
The gate electrode of the MOS transistor, 6-5b, is the electrode of the capacitor. 6-6 is a field oxide film, 6-7 is an insulating film, and 6-8 is an aluminum electrode. In FIG. 6, the transistor that switches each picture element is composed of a silicon gate MOS transistor, and the electrode of the capacitor arranged in parallel with each picture element of the liquid crystal is connected to the entire silicon substrate and the polysilicon 6- It becomes 5b. In this case, the silicon substrate is held at GND, and as shown in FIG. 5, the one side electrode of the capacitor and the substrate potential of the transistor match and become the GND level.

FIG. 7 shows a plan view of the drive circuits arranged in a matrix, and the sectional view taken along line A-A' in the figure corresponds to FIG. In the figure, 7-2 to 7-8 correspond to 6-2 to 6-8, respectively. Further, in FIG. 7, the drain electrode 6-8b in FIG. 6 is omitted so as not to complicate the diagram. In Figure 7, one picture element is
This is the area indicated by the two-dot chain line. Therefore, the so-called pixel electrodes that apply voltage to the liquid crystal are insulated from the transistors or the signal lines 7-5a, 7-8a, etc. running vertically and horizontally, and are almost shown as two-dot chain lines on the upper side of the pattern in FIG. It is supposed to be arranged as follows.

FIG. 8 shows the relationship between signal waveforms and potentials in the case of conventional DC drive. 8-1 is an image signal supplied to terminal 2-1, and 8-2 is an image signal supplied to block 2.
-4 is a synchronization signal when sampling the image signal for each data line of each matrix. The horizontal axis t represents time, and the vertical axis V represents voltage. 8-3 represents the black level of the image signal, and 8-4 represents the white level, which correspond to the threshold voltage and saturation voltage of the liquid crystal, respectively. Voltage 0 corresponds to GND in FIG. 2 and is the substrate and common electrode potential.

On the other hand, FIG. 9 shows the operating voltage range supplied to the switching transistor and the potential applied to the common electrode in this embodiment.

In the figure, 9-1 and 9-2 are both image signals.
The dashed-dot line shown in 9-6 indicates the potential supplied to the common electrode of the liquid crystal matrix display section, and the image signal applied to each matrix pixel electrode of the liquid crystal is shown in 9-6.
The inversion is repeated at a certain period based on the potential level of 6.

That is, the image signals 9-1 and 9-2 are symmetrical with respect to the common electrode potential 9-6, and the black level and white level are mutually inverted, resulting in a signal whose polarity is inverted based on the intermediate potential. It is becoming.

Regarding the repetition cycle of inversion, for example, in the case of image signals for TV broadcasting, one screen of image signal is taken as one frame, one frame is further divided into two fields, and the screen is interlaced scanned for each field. It's summery. In FIG. 9, for example, 9-
1 corresponds to one horizontal scanning line of one frame of image signal including the first and second fields. 9-
2 corresponds to the signal for one frame. 9-
2 is an image signal corresponding to the same display portion of the image signal for one frame following the signal for one frame;
and 9-2 are symmetrical with respect to the same image input.
9-3 is an image sampling synchronization signal;
The period shown in 4 corresponds to a period during which one picture element is displayed horizontally on the matrix display panel. 9
-5 corresponds to the horizontal flyback period of the television image signal. In the vertical axis of FIG.
The display body circuit voltage corresponds to 1. That is, 9
-10 is the maximum level of the operating voltage range supplied to the switching transistor.

Further, the potential supplied to the common electrode shown at 9-6 is approximately at the middle level of the operating voltage range of 9-10.

In Fig. 5, this potential level 9 is applied to terminal 5-4.
-10 is given.

Therefore, in this embodiment, the switch transistors arranged for each picture element in FIG. 5 can be constructed of, for example, a P-channel enhancement MOSFET. When setting 9-10 to the potential of the substrate 6-1,
The switch transistor may be formed of an N-type MOS.

The amplitude of the image signal 9-1 is from the broken line 9-7 to the broken line 9.
It is between -8. 9-7 is the black of the image signal, 9-8
corresponds to white. When using a liquid crystal having the characteristics shown in Fig. 1, 9-7 is in the range from V _NS to V _th ,
9-8 is taken near V _S. FIG. 1 O Vrms corresponds to 9-6. Regarding the linearity of the signal, it is sufficient to prevent the linearity of the original image signal from being distorted by the liquid crystal by interposing an amplifier that is corrected according to the correlation characteristic between the applied voltage of the liquid crystal and the display contrast. The purpose of alternately applying the image signals 9-1 and 9-2 to each liquid crystal display element electrode is to extend the life of the display by AC driving the liquid crystal. When converting the signal into an alternating current signal to drive the liquid crystal, flickering of the displayed image occurs due to alternating voltage driving of the liquid crystal. This is because the direction in which the electric dipoles of the liquid crystal molecules are oriented also changes in accordance with the reversal of the applied voltage polarity. The following method can be considered as a method for reducing flickering or making it effectively negligible. That is, it is sufficient to invert the phase at a cycle faster than the eye can respond.

(1) Invert the phase at the frame period and make the frame period approximately 30 Hz or more.

(2) Invert the phase in units of picture elements or in units of scanning lines within one frame period to increase the effective inversion period.

Furthermore, various methods can be considered depending on the above reaction.
When displaying television images in a matrix, the number of picture elements constituting the matrix may be reduced by the effective number of picture elements (or resolution) of the television video signal. At this time, for example, if a liquid crystal image is constructed from a matrix of one field (1/2 frame) of a TV video signal, the phases of the first and second field signals are inverted, and the two fields' worth of signals are transmitted to the same pixel. can be displayed at a frequency of 60Hz. Although the resolution as an image quality is reduced, flickering due to the difference in original image signals is canceled by the response performance of the liquid crystal itself, and an average image of the first field and the second field is displayed.

Furthermore, using the method (2), the polarity direction is switched for each pixel so that the image display signal within one frame has both positive and negative polarity, and the alternating current period of each pixel is changed. When the linearity of the amplifier or the linearity of the switching characteristics of the transistor provided in each picture element cannot be sufficiently obtained in the operating voltage range (range of 9-11 to 9-10) in units of 1 frame. However, non-linearity from the viewpoint of display effects can be effectively ignored.

FIG. 10 shows one embodiment of a circuit that realizes the above description. 10-2, 10-3, 10-4 are image signal amplifiers, 10-5, 10-7, 10-8 are fifth
This corresponds to block 5-2 in the figure. 10-6 is a changeover switch circuit, and the output of 10-6 is shown in Fig. 5-5-
8 image signal inputs.

The operation will be explained below.

10-1 is an original image signal input, 10-2 is an initial stage amplifier, and 10-9 is an amplification factor adjustment terminal. Liquid crystal voltage. 10-2 should have an amplification factor characteristic (FIG. 1) that matches the slope of the contrast characteristic.
10-3 and 10-4 are differential amplifiers. 10-
The same signal, ie, the 10-2 output, is coupled to the positive input terminal 3 and the negative input terminal 10-4. 1
The 0-3 negative polarity input terminal and the 10-4 positive polarity input terminal are coupled to form a 10-10 terminal. 10
-3 and 10-4 are preset to values that provide substantially the same characteristics as amplifiers. 10-10
The terminal is used to adjust the brightness of the image displayed by the liquid crystal, and a variable DC voltage is applied to the terminal. for example,
The force signals 10-3 and 10-4 correspond to FIG. 9, 9-1 and 9-2, respectively. At this time, 10-9
adjusts the difference between 9-8 and 9-7, that is, the amplitude, in other words, the contrast of the displayed image. 10-10
adjusts the difference between 9-7 and 9-6. 10-
The gains of 3 and 10-4 may be set appropriately. 10
-6 is a switch circuit, and as mentioned above, 10-3 and 10-4 are used when supplying an AC drive signal to the liquid crystal.
This is a circuit that switches each output television receiver and selectively outputs the output.

As the switch element, various types of transistors such as bipolar or MOS transistors can be considered, but when a semi-conductor is used for the display substrate and the block 2-4 is housed inside the semiconductor substrate as shown in FIG. It is also desirable to have a quasi-joint structure with a similar structure, such as a so-called transmission gate structure. The same applies to the subsequent circuit 10-7. 10-5
is a circuit that generates signals for controlling the switch element 10-7, for example from left to right, and is constituted by a shift register. 10-8 is a circuit for storing and holding the image sampling signal sampled by the switch and distributing it to each picture element electrode. 10-8 and subsequent parts correspond to the liquid crystal matrix display section including the drive section, that is, FIG. FIG. 11 shows yet another embodiment. Figure 11 shows 10-2, 10-3 in Figure 10,
This is a modification of the amplifier 10-4. 1
1-1 and 1-2 are image signals with substantially the same amplitude and opposite polarities.

The amplifier circuit on the upper side of the figure (transistor 11-3,
11-5) and the lower amplifier circuit (transistor 11
-3, 11-14) are designed to have the same circuit configuration and amplification characteristics. 11-4, 11
-8 is a variable resistor for gain control of the amplification system, which adjusts the contrast of the liquid crystal display image. 11-4,
11-8 is moved by 11-10 shown in broken lines and can be manually adjusted from the outside. 11-7, 11-9
There is a resistor capable of controlling the output potential level, that is, changing the brightness of the liquid crystal image display, which is linked by the broken line 11-11 and can be manually adjusted from the outside. However, potential levels of 11-7 and 11-9 operate in opposite directions, and their respective outputs maintain symmetry about level 9-6 as shown in FIG. 9, 9-1 and 9-2.

11-12 is an image signal polarity changeover switch circuit corresponding to FIG. 10 10-6. The present invention can also be realized by a configuration other than the circuit described as an example. Furthermore, the contrast and brightness can be adjusted manually as described above, or the gain or bias level can be automatically controlled by automatically detecting light by matching the display degree of the liquid crystal to the reference pattern display signal level. Of course, this is also possible.

In the description of the embodiment of the present invention, as shown in FIG. 6, a silicon substrate is used as one of the flat plates sandwiching the liquid crystal, and a transistor is constructed within the silicon substrate. Therefore, it is also possible to construct each element on a glass substrate or by other methods. In Fig. 5, the transistor provided to switch each picture element is a single MOS type transistor, but in order to improve the linearity, response speed, operating voltage, etc. of the element, a PMOS transistor is used.
Switching can also be performed by combining two types of MOSFETs, MOSFETs of type and N type, into a complementary type. Of course
It is also possible to configure it with elements other than MOSFETs.

In Fig. 5, a capacitor is arranged in parallel with each liquid crystal picture element, but in this case, as mentioned earlier, both electrodes of the capacitor are not connected completely in parallel with the liquid crystal picture element electrodes, but on the common electrode side. The potentials are set separately for each. This is because by adopting the structure shown in FIG. 6, the common electrode of the capacitor can be replaced by the substrate. At this time, the polarity and magnitude of the bias potential applied to the capacitor in accordance with the image signal applied to the liquid crystal picture element are different from the bias potential of the liquid crystal picture element electrode, but the effective electrical characteristics related to display are the second This has the same effect as the case shown in the figure.

〔effect〕

As described above, the present invention includes a means for applying a potential approximately in the middle of the operating voltage range of an image signal applied to one electrode of a MOS transistor to a common electrode, and a means for inverting the polarity of the image signal with reference to the intermediate potential. Applicable
Since the image signal inverting means for supplying to one electrode of the MOS transistor and the means for supplying the horizontal scanning signal to the gate electrode of the MOS transistor are arranged, the transistor used as the switching means can only be driven with one polarity. Therefore, when these transistors are arranged in a matrix on a liquid crystal substrate, the structure is simple, so it is possible to provide a liquid crystal image display device that can substantially AC drive tens of thousands of picture elements.

[Brief explanation of the drawing]

Fig. 1...Example of voltage-display characteristics of TN type liquid crystal, Fig. 2...Example of conventional display circuit, Fig. 3...Example of another conventional display circuit, Fig. 4...One embodiment of the present invention FIG. 5 is an example of a display circuit diagram according to the present invention, FIG. 6 is an example of a partial sectional view of a display device,
Fig. 7... A plan view of Fig. 6, Fig. 8... Conventional image signal and sample clock, Fig. 9... Signal waveform diagram in the embodiment of the present invention, Fig. 10, Fig. 11... ... Practical circuit example of the present invention, 4-12... Matrix display section, 6-1... Silicon substrate, 7-8a... Data line for driving matrix display, 7-5a... Clock line for driving matrix display, 7 -5b...Capacitor electrode, 8-
1,9-1,9-2...image signal, 10-2...
Image signal amplification bridge. 10-3, 10-4...Differential amplifier.

Claims (1)

[Claims] 1 A liquid crystal is sealed in a pair of substrates, a common electrode is placed on one substrate, and a plurality of picture element electrodes arranged in a matrix on the other substrate correspond to each of the picture element electrodes. In a liquid crystal image display device in which MOS type transistors are connected and arranged in a matrix, a means for applying a potential in the middle of the operating voltage range of the MOS type transistors to the common electrode; The image signal inverting means inverts the polarity of the image signal with reference to and supplies it to one electrode of the MOS transistor, and means supplies the horizontal scanning signal to the gate electrode of the MOS transistor. LCD image display device.