JPH0141996B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a liquid crystal display device capable of reproducing gradations.

[Prior art]

Generally, the gradation level of a video signal detected from a television radio wave or the like is corrected to an optimum gradation level by performing gamma correction or the like in a cathode ray tube television receiver, and then reproduced. On the other hand, as shown in FIG. 1, the contrast curve of the liquid crystal with respect to the applied voltage shows that as the applied voltage increases, the liquid crystal starts to turn on at a certain potential and then becomes completely on after a transient period. This transient curve changes with temperature as a, b, c
Therefore, temperature stabilization is required, especially in dynamic drive systems. Furthermore, when the power supply voltage fluctuates, the drive voltage fluctuates at the same time, reducing the display performance of the liquid crystal. Regarding this dynamic method,
It is described in detail in US.PAT.3877017 etc. In addition, even in the static method, the gradation display of TVs, etc. uses transient characteristics, so for example, the first
When using a gradation signal whose bias point is Vc in the figure, if curve b shifts to curve a due to temperature, the 50% contrast point will move to the 100% contrast point S, so the optimum value corresponding to the input gradation signal It will not be displayed. Therefore, in order to obtain the optimum display in response to such temperature-induced liquid crystal fluctuations, the bias point must be moved to Vc'. Naturally, the same can be said of fluctuations in the power supply voltage. Therefore, depending on the application, a liquid crystal display device requires some kind of stabilizing means against environmental changes such as temperature changes and power supply voltage. However, providing a temperature sensor or separately providing a regulator for this purpose results in large variations, poor stability, and poor performance. In addition, improving performance requires the use of expensive parts and requires a lot of effort for adjustment, resulting in high costs.

Therefore, even if a video signal is directly applied to a liquid crystal having the above transient curve,
There was a problem that faithful gradation reproduction could not be obtained.

〔the purpose〕

The present invention overcomes the above problems,
It is an object of the present invention to provide a liquid crystal display device that can obtain faithful gradation reproduction by providing means for applying a DC bias to a video signal at a level corresponding to the transient curve of the liquid crystal.

〔Example〕

FIG. 2 shows an embodiment of the liquid crystal display device of the present invention. 31 is a stabilizing signal generation circuit, 32 is a data signal driver, 33 is a cell unit in the n-th row and i-th column of the display body, 34 is a shift register for column selection, 3
5 is a shift register for row selection, 36 is a row signal A
37 indicates a display panel. Video signal 38 is biased to Vr by operational amplifier 39 to form output signal Vu.

On the other hand, the bias point of the 1:1 inversion buffer composed of MOS transistors 40 and 41 is set to Vc potential by the operational amplifier 42 so as to be the same as the contrast detection output voltage V _OUT . The advantage of the characteristics of this buffer is that the output bias point can be adjusted by Vc, as shown in FIG.

That is, FIG. 3 shows the input/output characteristics of the buffer made up of transistors 40 and 41. At room temperature, the bias voltage Vr serves as a reference voltage for setting the operating point of the buffer transistor. The input/output characteristics of the buffer here are in the relationship shown in the curve. Vc indicates the power supply voltage of the buffer at room temperature. At the same time, Vc is the sum of temperature change information _Vput and transistor threshold voltage _Vth . If the transistor threshold is sufficiently small, Vc is approximately
Equal to V _put .

Next, as the temperature changes, the power supply voltage Vc
When Vc' changes to Vc', the input/output characteristics of the buffer will have the relationship shown in the curve. MOS transistors 40 and 41 have exactly the same channel shape to enable 1:1 inversion. Further, 43 and 44 in the i-th column driver 32 are 4
0, 41, and therefore the output signal
A waveform in which Vu is simply inverted with the same amplitude,
It is outputted as a data output Di via a selection signal Si and a sample hold circuit formed by a gate 45.
That is, the bias voltage Vc is superimposed on the video signal Vu at the data output Di on the data output line.
In the cell unit 33, a transistor 46 samples and holds data Di through an internal capacitor 48 through a row address line An, and drives a cell 47 to display the data. Although this system can generate contrast, the stabilization circuit 31 prevents the contrast from deviating significantly from the intended contrast due to environmental changes. This circuit example
Although correction is made using Vc, another method is to directly apply the power supply voltage to Vc, feed back the output of the operational amplifier 42 to the gate of 40, use this output as the positive input of 39, and correct it using the signal bias potential. here,
As shown in FIG. 1, the Vc potential is set to approximately the center level of the liquid crystal transient curve.

FIG. 4 shows another embodiment of the present invention, in particular, an embodiment in which the drive bias voltage V _OUT of the data line drive circuit can be adjusted and the amplitude of the drive voltage of the drive circuit can also be adjusted.

Here, if a constant voltage is applied to Vc and V _OUT is the output level from the photo sensor transmitted through the liquid crystal, the output of the operational amplifier 42 will fluctuate due to fluctuations in the intermediate potential of the liquid crystal, and the output voltage of the operational amplifier 42 will change. becomes a potential such that the intermediate voltage of the inverting buffer is approximately equal to V _OUT . Here, the reason why it is assumed to be almost equal is that the voltage amplification factor of the operational amplifier 42 is Gv,
If the intermediate voltage of the inverting buffer is Vk and the output voltage of the operational amplifier 42 is _V42 , then _V42 = (Vk - V _OUT ) x Gv. However, since the amplification factor Gv of the operational amplifier 42 is very large and is in the tens of thousands, For example, if Gv=10000
When V ₄₂ becomes 5V, Vk – V _OUT = (5V) ÷ 10000
= 0.0005V, which means that approximately Vk≒V _OUT . Such a voltage V ₄₂ is the + of the operational amplifier 39.
For example, when applied to the input terminal, the output Vu becomes a driving waveform that oscillates symmetrically in accordance with the video signal 38 with V _OUT as the center. In this example, in particular, the driver 32 and the inverting buffer are connected to complementary MOS transistors 56 and 5.
An example configured with 7, 61, and 62 is shown. In order to make the amplitude of the drive voltage of the drive circuit adjustable, the amplification factor of the operational amplifier 39 may be made adjustable, and for this purpose the amplification factor setting resistor Ri or Rf may be made variable. FIG. 4 shows an example in which the resistance Ri is manually variable. By making it possible to adjust the amplitude of the drive voltage in this way, the screen contrast can be set arbitrarily, and the user can adjust the contrast as needed depending on the surrounding brightness to avoid eye strain. You can see it with easy-to-read contrast. Although Fig. 4 shows an example in which the contrast is adjusted using a manual variable volume control, the amplification factor of the operational amplifier 39 can be electrically varied by detecting the ambient brightness using a photo sensor such as a solar cell or a cadmium cell. You can also do it.

Incidentally, the level of V _OUT can be adjusted by a simple circuit as shown in FIG. 6, which allows the user to adjust the level by hand while actually looking at the screen. The operational amplifier 72 is used as a buffer amplifier, and the volume 71 is connected to its input.
Set V _OUT by connecting and adjusting it manually. The advantage of this method is that the circuit in Figure 2 uses external light and therefore does not work well if there is too little or too much external light, but the circuit in Figure 6 is not limited to such conditions. However, it is effective under conditions where the liquid crystal display device can function as a display device, that is, under conditions where images can be confirmed by the human eye. This method is more suitable because the liquid crystal display device is of a light-receiving type.

Further, FIG. 5 is shown as an example of automatically setting the bias level without using external light. In this circuit, the light emitting diodes 63, 64, and 65 emit light via the buffer amplifier 70 in response to the clock pulse Cl, and the light receiving circuit 66 outputs V _OUT , which is sent to the A-D.
It is encoded by converter 67 and stored in memory 68. The value of V _OUT encoded in this manner is again converted into an analog signal V _OUT by a DA converter and connected to V _OUT of the circuit shown in FIG. In this case, the light emitting diodes 63 to 65 do not need to emit light all the time, and clock pulses are used to reduce power consumption.
By emitting light using Cl and sampling it at intervals of 0.1 to 10 seconds, it is possible to sufficiently follow normal environmental changes.

〔effect〕

As described above, the present invention has a liquid crystal sealed within a pair of substrates, and a data signal line and a data signal line on one of the substrates.
In a liquid crystal display device including a scanning signal line and a pixel electrode, a first amplification means (39) generates a data signal (Vu) by superimposing a DC first fixed bias (Vr) on an input video signal (Vd). , second amplification means (41, 42) for generating a variable second DC bias (Vc), the data signal (Vu) from the first amplification means (39) and the second amplification means (41, 42). )
A third circuit superimposing a direct current bias (Vc) from and supplying the superimposed signal (Di) to the data signal line.
It has amplification means (43, 44), and the variable second DC bias is variably adjusted to a voltage level corresponding to approximately the center of the transient curve of the liquid crystal, so that temperature fluctuations are avoided. Even if the transient curve of the liquid crystal changes due to fluctuations in the power supply voltage, fluctuations in the liquid crystal material, fluctuations in the incident light, etc., the presence of the second DC bias level ensures that the contrast can always be corrected appropriately. Since deviations in gradation levels can be easily corrected, faithful gradation reproduction is always possible.

[Brief explanation of drawings]

FIG. 1 is a voltage-contrast curve diagram of liquid crystal. FIG. 2 is a diagram showing an embodiment of the present invention. Figure 3 is a characteristic diagram of the buffer transistor. FIG. 4 is a diagram showing another embodiment of the present invention. FIG. 5 is a circuit diagram of a first embodiment of voltage contrast voltage setting of the present invention. FIG. 6 is a circuit diagram of a second embodiment of the voltage contrast voltage setting of the present invention. Stabilization signal generation circuit...31, Data signal driver...32, Display unit nth row, i column cell...3
3. Video signal...38. Operational amplifier...3
9,42.

Claims (1)

[Claims] 1. In a liquid crystal display device in which a liquid crystal is sealed within a pair of substrates, and a data signal line, a scanning signal line, and a pixel electrode are formed on one of the substrates, a first fixed DC current is applied to an input video signal (Vd). a first amplification means (39) for generating a data signal (Vu) by superimposing a bias (Vr); a second amplification means (41, 42) for generating a variable second DC bias (Vc); The data signal (Vu) from the amplification means (39)
and third amplification means (43, 44) which superimposes the DC bias (Vc) from the second amplification means (41, 42) and supplies the superimposed signal (Di) to the data signal line.
1. A liquid crystal display device comprising: a liquid crystal display device, wherein the variable second DC bias is variably adjusted to a voltage level corresponding to approximately the center of a transient curve of the liquid crystal.