JPH07123298A - Gamma correction circuit - Google Patents

Abstract

PURPOSE:To attain the display matching the characteristic of individual LCD by devising a gamma correction circuit for LCD so as to adjust a gamma correction value in various ways. CONSTITUTION:Plural sets of series circuits each comprising MOS transistors (TRs) M11, M12 and resistors R16, R17 are inserted in parallel between emitters of differential pair TRs Q13, Q14 in at least one (B) of three differential amplifiers A-C. The gain is changed by turning on/off the TRs M11, M12 with external signals VG11, VG12 thereby adjusting an input output characteristic (gamma correction characteristic) of the entire circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a gamma correction circuit,
In particular, the present invention relates to a video signal correction circuit which is used in a liquid crystal television receiver and outputs a video signal after inverse gamma correction.

[0002]

2. Description of the Related Art In a liquid crystal television receiver or the like for displaying an image using a liquid crystal display device instead of a CRT, C
Non-linearity of the video signal based on the gamma correction is performed by performing inverse gamma correction processing on the video signal such as a television signal gamma-corrected according to the emission characteristics of the CRT based on the difference in characteristics between the RT and the liquid crystal display It is necessary to restore the signal to the original state, amplify it, and further perform signal processing in accordance with the VT characteristic (voltage-transmittance characteristic) of the liquid crystal.

FIG. 7 shows a conventional example of this type of gamma correction circuit. This gamma correction circuit is composed of three differential amplifiers AC. The differential amplifier A includes a differential pair of transistors Q11 and Q12 and a constant current source I11, and a collector resistance R11 of the transistor Q11 is provided. The reference voltage VH is applied to the base of the transistor Q12.

The differential amplifier B is a differential pair transistor Q1.
3, Q14, emitter resistors R12 and R13, and a constant current source I12, and the reference voltage VM is applied to the base of the transistor Q14.

The differential amplifier C is a differential pair transistor Q1.
5, Q16, emitter resistors R14 and R15, and a constant current source I13, and the reference voltage VL is applied to the base of the transistor Q16.

Then, each of the transistors Q11, Q13,
The input video signal VIN is commonly applied to the bases of Q15, and the collector outputs of these transistors are commonly derived by the load resistor R11 to become the circuit output VOUT.

FIG. 8 is a diagram showing the input / output characteristics of the circuit of FIG. 7, and FIGS. 8B to 8D are input / output characteristic diagrams of the differential amplifiers A to C of FIG. Is a combination of these three input / output characteristics. Therefore, the characteristic shown in FIG. 8A represents the comprehensive gamma correction characteristic of the gamma correction circuit in FIG. Therefore, when the voltage of the input signal VIN is in the range of VL to VH, the circuit output VOUT is shown in FIG.
The output voltage shown in (3) is derived, and this is given to the liquid crystal television receiver.

[0008]

In the conventional gamma correction circuit of this type, since the gamma correction characteristic which is the input / output characteristic of the circuit is fixed, this cannot be done when the gamma value needs to be adjusted. There's a problem.

Therefore, the present invention has been made to solve the problems of the conventional ones, and an object thereof is to provide a gamma correction circuit whose gamma correction characteristics can be adjusted appropriately. is there.

[0010]

A gamma correction circuit according to the present invention is a gamma correction circuit having a plurality of differential amplifiers to which a video input signal is commonly input and which have different input / output characteristics. At least one of
It is characterized in that a plurality of series circuits of a switching element and a resistance element are inserted in parallel between the emitters of the differential pair transistors, and the gamma characteristic can be freely changed by controlling ON / OFF of the switching element.

In another gamma correction circuit according to the present invention, a plurality of series circuits of a switching element and a resistance element are inserted in parallel between at least two of the differential amplifiers, respectively, between the emitters of the differential pair transistors. It is characterized by further comprising control means for controlling activation of the two differential amplifiers.

[0012]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention.
The same parts as in FIG. 7 are indicated by the same reference numerals. The difference from the circuit of FIG. 7 is that the gain of the differential amplifier B is variable.

That is, the differential pair transistors Q13 and Q
Between the 14 emitters, a resistor R1 for determining the gain
6 and R17 are inserted, and the resistors R16 and R17 are inserted.
In order to control the connection state of the resistors R16 and R17,
In series with N-channel MOS transistors M11, M12
Are connected.

Then, each MOS transistor M11, M
VG11 and VG12 are provided as the gate control signals of 12, and the transistors M11 and M12 are ON / OFF controlled according to these control signals VG11 and VG12. Therefore, the circuit connection of the resistors R16 and R17 is turned on and off, and as a result, the gain of the differential amplifier B can be changed.

FIG. 2 shows a general notation of the differential amplifier B of FIG. 1. A series circuit of a resistor RXi (i = 1 to n) and a switching MOS transistor MXi is a differential pair transistor Q71. , Q72 are connected in parallel between n emitters. I is a constant current source, RC is a collector load resistance, RE1 and RE2 are emitter resistances, and VREF is a reference voltage.

The gate size (W / L) of the transistor MXi is made large so that the on-resistance R0 of the transistor MXi becomes significantly smaller than the series resistance RXi. Therefore,
The minimum gain Gmin of the gain of this differential amplifier is when all the transistors MX1 to MXn are off, and Gmin = Rc / {r0 + (RE1 + RE2) + R0}. In addition, r0 is an NPN transistor Q71, Q72
Is the impedance of.

In the above equation, RE1 and RE2 are R0,
When it is larger than r0, Gmin = Rc / (RE1 + RE2).

Further, all the transistors MX1 to MXn
The gain becomes maximum when is turned on, and Gmax = Rc / {(RE1 + RE2) ‖RX1‖ …… ‖
RXn}. In addition, ‖ indicates that they are in parallel.

Therefore, by appropriately turning on one or more of the transistors MX1 to MXn,
The gain can be properly selected between Gmin and Gmax.
The input / output characteristics can be changed as shown in FIG. However, each value of the resistor RXi is also set appropriately.

It is an embodiment of the present invention that the circuit shown in FIG. 2 is applied to the differential amplifier B constituting the gamma correction circuit of FIG. 1, and the case of n = 2 in FIG. 3B, the input / output characteristics are shown in FIG.

FIG. 4 is a circuit diagram of another embodiment of the present invention, in which the same parts as in FIG. 1 are designated by the same reference numerals. In FIG. 4, two differential amplifiers B and C of the three differential amplifiers have a variable gain structure, and both n = 2 are shown, but n may be 3 or more. Is clear.

FIG. 5 shows an example of changes in the input / output characteristics of the circuit shown in FIG.

FIG. 6 is a circuit diagram showing another embodiment of the present invention, and the same portions as those in FIG. 1 are designated by the same reference numerals. In this example, two differential amplifiers B (having a variable gain structure) shown in FIG. 1 are connected in parallel (indicated by B1 and B2), and these two differential amplifiers B1 and B2 are switched. S
It has a structure in which the activity can be freely controlled by W1 and SW2.

By combining the switches SW1 and SW2 on and off, four types of circuit configurations can be selected, and the transistors M110, M120, M111, and M121 can be selected on and off to obtain various gamma correction characteristics. To be

[0026]

As described above, according to the present invention, a plurality of series circuits of a switching element and a resistor are provided in parallel between the emitters of a differential pair transistor, and on / off of these switching elements is externally controlled. By doing so, many gamma correction characteristics can be obtained, so that it is possible to obtain the correction characteristics suitable for each LCD, and it is possible to achieve high-quality LCD display.

Further, even if the correction characteristic as designed cannot be obtained due to the variation in the resistance value at the time of manufacturing, there is an effect that the characteristic as designed can be obtained by the on / off control of the switching transistor.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention.

FIG. 2 is a diagram generally showing a part of the circuit of FIG.

3A is a diagram showing a variation example of input / output characteristics of the circuit of FIG. 2, and FIG. 3B is a diagram showing a variation example of input / output characteristics of the circuit of FIG.

FIG. 4 is a circuit diagram of another embodiment of the present invention.

5 is a diagram showing an example of changes in input / output characteristics of the circuit of FIG.

FIG. 6 is a circuit diagram of another embodiment of the present invention.

FIG. 7 is a diagram showing a conventional gamma correction circuit.

8A to 8D are diagrams showing examples of input / output characteristics of the circuit of FIG.

[Explanation of symbols]

 A to C differential amplifier Q11 to Q16 NPN transistor M11 to M14 NMOS transistor R11 to R19 resistance I11 to I13 constant current source VG11 to VG14 external control signal

Claims (3)

[Claims] 1. A gamma correction circuit having a plurality of differential amplifiers, to which video input signals are commonly input and have different input / output characteristics, wherein at least one of the differential amplifiers is provided.
A gamma correction circuit characterized in that a plurality of series circuits of a switching element and a resistance element are inserted in parallel between the emitters of a differential pair transistor, and the gamma characteristic can be freely changed by controlling ON / OFF of the switching element. . 2. A plurality of series circuits of a switching element and a resistance element are inserted in parallel between at least two of the differential amplifiers between the emitters of the differential pair transistors, respectively.
The gamma correction circuit according to claim 1, further comprising control means for controlling activation of one differential amplifier. 3. The gamma correction circuit according to claim 1, wherein the switching element is a MOS transistor.