JP2872020B2 - Gamma correction circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gamma correction circuit,
In particular, the present invention relates to a video signal correction circuit used in a liquid crystal television receiver and which performs reverse gamma correction on a video signal and outputs the result.

[0002]

2. Description of the Related Art In a liquid crystal television receiver or the like which displays an image using a liquid crystal display device instead of a CRT, a C
Inverse gamma correction processing is performed on a video signal such as a television signal that has been gamma-corrected according to the light-emitting characteristics of a CRT based on the difference in characteristics between the RT and the liquid crystal display, and the non-linearity of the video signal based on the gamma correction Must be restored and amplified, and further signal processing must be performed in accordance with the VT characteristic (voltage-transmittance characteristic) of the liquid crystal.

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

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

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

The transistors Q11, Q13,
The input video signal VIN is commonly applied to each base of Q15, and the collector outputs of these transistors are commonly led out 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 the input / output characteristics of the differential amplifiers A to C of FIG. Is the sum of these three input / output characteristics. Therefore, the characteristic shown in FIG. 8A indicates the overall gamma correction characteristic of the gamma correction circuit of 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.
Is derived and supplied to the liquid crystal television receiver.

[0008]

In a conventional gamma correction circuit of this kind, since the gamma correction characteristic, which is the input / output characteristic of the circuit, is fixed, it is not possible to adjust the gamma value if it is necessary. There's a problem.

Therefore, the present invention has been made to solve such a problem of the conventional device, and an object of the present invention is to provide a gamma correction circuit capable of appropriately adjusting a gamma correction characteristic. is there.

[0010]

SUMMARY OF THE INVENTION Each of a first and a second
First, second and third differential amplifiers having input terminals
Having the video signal of the first to third differential amplifiers.
Commonly supplied to the first input terminal, low, medium, and high level
The first, second and third differential amplifiers
Respectively supplied to the second input terminals of
The respective outputs of the first to third differential amplifiers are combined and the combined signal is
Gamma correction circuit configured to supply the signal to the LCD
Path, at least to the second differential amplifier.
Between the emitters of the transistors that make up the amplifier
A first means for changing the impedance.
Means by changing the combined impedance by means
The second difference of the composite signal with respect to the video signal
Can change the waveform of the part where the output of the dynamic amplifier becomes dominant
It is characterized by having such a configuration.

In another gamma correction circuit according to the present invention, at least two of the differential amplifiers are turned on / off in response to an external control signal between the emitters of a differential pair transistor. And a control means for controlling the activation of these two differential amplifiers by inserting a plurality of series circuits in parallel.

[0012]

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

FIG. 1 is a circuit diagram of one embodiment of the present invention.
7 are denoted by the same reference numerals. The difference from the circuit of FIG. 7 is that the gain of the differential amplifier B is configured to be variable.

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

Then, each MOS transistor M11, M
Twelve gate control signals VG11 and VG12 are provided, and the transistors M11 and M12 are turned on / off in accordance with the 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 becomes variable.

FIG. 2 shows a general notation of the differential amplifier B shown in FIG. 1. In FIG. 2, a series circuit of a resistor RXi (i = 1 to n) and a switching MOS transistor MXi forms a differential pair transistor Q71. , Q72 are connected in parallel in n sets. Here, 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 increased so that the on-resistance R0 of the transistor MXi is 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}. Note that r0 is the NPN transistor Q71, Q72
Is the impedance of

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

Further, all the transistors MX1 to MXn
Becomes maximum, Gmax = Rc / {(RE1 + RE2)} RX1
RXn｝. Note that ‖ indicates parallel.

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

In the embodiment of the present invention, the circuit shown in FIG. 2 is applied to the differential amplifier B constituting the gamma correction circuit shown in FIG. 1. In FIG. And the input / output characteristics shown in FIG.

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

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

FIG. 6 is a circuit diagram showing another embodiment of the present invention, and the same parts as those in FIG. 1 are denoted by the same reference numerals. In this example, two differential amplifiers B (of 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 connected to a switch. S
The configuration is such that the activity can be freely controlled by W1 and SW2.

The combination of the on / off states of the switches SW1 and SW2 allows four types of circuit configurations to be selected, and further allows the transistors M110, M120, M111 and M121 to be turned on / off, thereby obtaining various gamma correction characteristics. Can 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 the 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 correction characteristics suitable for each LCD, and it is possible to achieve high quality LCD display.

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

[Brief description of the drawings]

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

FIG. 2 is a diagram generally showing a part of the circuit of FIG. 1;

3A is a diagram illustrating an example of a change in input / output characteristics of the circuit in FIG. 2; FIG. 3B is a diagram illustrating an example of a change in input / output characteristics of the circuit in FIG. 1;

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

5 is a diagram illustrating an example of a change 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.

FIGS. 8A to 8D are diagrams illustrating examples of input / output characteristics of the circuit of FIG. 7;

[Explanation of symbols]

 AC 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)

(57) [Claims] 1. Each having first and second input terminals.
A first differential amplifier, a second differential amplifier, and a third differential amplifier.
To the first input terminal of the first to third differential amplifiers
Supply common and precede low, medium, and high level reference voltages
The second input of the first, second and third differential amplifiers;
And the first to third differentials
The outputs of the amplifier are combined and the combined signal is sent to the LCD
In a gamma correction circuit configured to supply
At least this amplifier is constituted for the second differential amplifier.
Changes the combined impedance between the emitters of the
A first means for changing the position of the first means.
The impedance of the video signal is
The output of the second differential amplifier of the combined signal
It is configured so that the waveform of the dominant part can be changed.
And a gamma correction circuit. 2. An amplifier according to claim 3, wherein said amplifier is
Composite impedance between emitters of constituent transistors
A second means for changing the data, the second means
By the of the composite signal for the video signal
The waveform where the output of the third differential amplifier is dominant is
It is configured so that it can be changed further
The gamma correction circuit according to claim 1. 3. The first means comprises a resistor and a switch, respectively.
A plurality of circuits comprising a series circuit of
Between the emitters of the transistors that make up the
And the switches in the plurality of circuits are selected.
2. The method according to claim 1, wherein the conductive state is established.
Gamma correction circuit.