JP3290264B2 - 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 a video signal processing circuit when a video signal is displayed on a liquid crystal display device.

[0002]

2. Description of the Related Art A gamma correction circuit in a liquid crystal display device is disclosed in Japanese Patent Application Laid-Open No. 1-175467.
Hereinafter, this will be described with reference to FIG. In FIG. 4, when the voltage at the input terminal 3 is sufficiently smaller than the reference voltages V11 and V12, a current substantially equal to the constant current I12 flows through the transistors Q18, Q17 and Q16. However, since this current passes through the transistor Q15 of the first voltage comparison circuit A,
No current flows to the transistor Q13 side, and the level shift circuit does not operate. Therefore, the output terminal 4 has the same voltage as that of the emitter of the emitter follower Q11. As the voltage at the input terminal 3 rises and approaches the reference voltage V11, the current flowing through the current mirror circuits Q13 and Q12 increases and the potential at the output terminal 4 rises.

When V11 <voltage of input terminal 3 <V12,
The voltage at output terminal 4 is equal to the emitter voltage of Q11 + I12.R11
Approaching and lifted sharply. When the voltage at the input terminal 3 approaches the reference voltage V12, the current supplied to the transistor Q17 decreases, so that the output terminal 4 and the transistor Q11
The voltage difference between the emitter and the emitter decreases.

When the voltage at the input terminal 3 becomes sufficiently higher than the reference voltage V12, the second voltage comparison circuit B is also inverted, so that no current is supplied to the transistor Q17 and the first voltage comparison circuit A and the level shift circuit do not operate. Therefore, there is no potential difference between both ends of the resistor R11, and the output terminal has the same voltage as the transistor Q11. FIG. 5 shows the gamma correction characteristics of FIG. 4 obtained by such an operation.

In such a gamma correction circuit, when the input signal level is near the reference voltage V12, the operating current of the first voltage comparison circuit A decreases. As a characteristic of a general circuit, there is a problem that when the operating current decreases, the frequency characteristic of the circuit deteriorates, so that the frequency characteristic changes depending on the level of the input signal. Further, since a differential circuit of a PNP transistor and a current mirror circuit of an NPN / PNP transistor are used for an input signal path, there is a problem that frequency characteristics are deteriorated.

[0006]

In the conventional gamma correction circuit described above, when the input signal level is near the higher reference voltage, the operating current of the first voltage comparison circuit A decreases and the frequency characteristic is reduced. There is a problem that the frequency characteristic changes depending on the level of the input signal because of deterioration.
This invention has good gamma correction characteristics,
It is an object to provide a gamma correction circuit having excellent frequency characteristics.

[0007]

In order to solve the above-mentioned problems, an input signal, a reference voltage, and a reference voltage are applied to the bases of respective transistors in a differential matrix circuit including first to third transistors and an emitter resistor. By inputting an input signal that is clipped by voltage, each of these input signals and an output signal of the differential matrix circuit output in a certain section determined by the differential matrix circuit are added to the input signal, The gamma correction characteristic is obtained.

[0008]

According to the above means, when the input signal is sufficiently lower than the reference voltage, there is no output from the differential matrix circuit, and there is no addition of the input signal to the output signal of the differential matrix circuit. When the input signal voltage approaches the reference voltage, the output of the differential matrix circuit is added to the input signal from a voltage determined by the differential matrix circuit and the three input signals.
At this time, as the input signal voltage increases, the added amount of the output of the differential matrix circuit also increases. When the input signal voltage becomes higher than the reference voltage, the output of the differential matrix circuit decreases, and the output of the differential matrix circuit disappears from a certain voltage determined by the differential matrix circuit and three input signals. Thus, a desired gamma correction characteristic can be obtained.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram for explaining an embodiment of the present invention. In FIG. 1, a transistor Q1 whose base is connected to an input terminal 1 and a constant current source I
1 forms an emitter follower 2, and the resistor R1 forms an adder 3. Transistors Q5 to Q7, resistor R2
R5, constant current sources I5 and I6 are differential matrix circuits 4
The collectors of the transistors Q6 and Q7 are connected to the output terminal 5 and between the resistor R1 and the current source I2.

The emitter of the transistor Q1 is connected to the base of the transistor Q6. The transistor Q4 whose base is connected to the reference voltage 6 and the constant current source I4 constitute an emitter follower 7. The emitter of transistor Q4 is connected to the base of transistor Q7. The transistor Q2 whose base is connected to the input terminal 1, the transistor Q3 whose base is connected to the reference voltage 6, and the constant current source I3 constitute a voltage comparison circuit 8. The emitters of the transistors Q2 and Q3 are connected to the base of the transistor Q5. The first level at the position where the addition of gamma correction is started and the second level at the end position are set by the differential matrix circuit 4 and the three input signals.

Next, regarding the operation of the above circuit, FIG. 2A showing the relationship between the input terminal voltage Vin of the input terminal 1 and the output terminal voltage Vout of the output terminal 5, and the gamma flowing through the input terminal voltage and the resistor R1. Correction current IG = I2-IC (Q6) -IC
FIG. 2B showing the relationship of (Q7), and the input terminal voltage V
in and transistors Q5 to Q7 of the differential matrix circuit 4
FIG. 2C showing the relationship between the base voltages.

When the potential Vin of the input terminal 1 is sufficiently lower than the reference voltage 6, the transistor Q3 is turned off and the transistor Q2 is turned on in the voltage comparison circuit 8. For this reason the base of the transistor Q5, Q 6 is the same potential. Comparing the base potentials of the transistors Q5 to Q7 at this time, Vb (Q5) = Vb (Q6) <Vb (Q7). Therefore, the differential matrix circuit 4 includes the transistor Q
Only 7 goes on. At this time, if the current flowing through the collector of the transistor Q7 is set to be the same as that of the constant current source I2, no current flows through the resistor R1 and no gamma correction is added to the input signal. Therefore, the output terminal 5 has the same potential as the emitter of the transistor Q1 of the emitter follower.

As the potential of the input terminal 1 approaches the reference voltage 6, the transistors Q5 and Q6 also start to turn on, and the current flowing through the transistor Q7 decreases, so that the gamma correction current IG increases. Gamma correction current IG is resistance R
When the signal flows to 1, the input signal is added with the amount of gamma correction and output to the output terminal 5.

The transistor Q of the differential matrix circuit 4
When the base voltages of 5 to Q7 are Vb (Q5) = Vb (Q6
) = Vb (Q7), the amount of addition of the gamma correction amount becomes the maximum. The potential Vin of the input terminal 1 is equal to the reference voltage 6
When the voltage becomes larger, the voltage of the transistor Q
2 is turned off, and the transistor Q3 is turned on. The base potential of the transistor Q5 becomes the same as the base potential of the transistor Q7.

Vb (Q6) ≧ Vb (Q5) = Vb (Q7
), That is, the potential of the input terminal 1 is equal to the reference voltage 6
When it becomes larger, the transistors Q5 and Q7 approach the off state, and the current flowing through the transistor Q6 increases.
Thus, since the gamma correction current IG is gradually decreased, the addition amount of the gamma correction component at the resistor R1 is small fence.

When the potential of the input terminal 1 is sufficiently higher than the reference voltage 6, the differential matrix circuit 4
Is turned on, I2 -IC (Q6) -IC
(Q7) = 0, and no gamma correction is added to the input signal. Therefore, the output terminal 5 has the same potential as the emitter of the transistor Q1 of the emitter follower, and has the gamma correction characteristic shown in FIG.

In this embodiment, NP having good frequency characteristics is used.
Since this can be realized by the differential matrix circuit 4 having N transistors, sufficient frequency characteristics can be obtained. Where P
If desired frequency characteristics can be obtained with the configuration of the NP transistor, there is no problem even if the configuration is made of the PNP transistor. Further, since the operating current does not change depending on the input signal level as in the related art, there is no change in the frequency characteristic depending on the input signal level. Furthermore, since the central operating point of the gamma correction characteristic can be determined by a single reference voltage, it is easy to change the circuit characteristic.

In the above embodiment, the differential matrix circuit 4
Although the circuit using the outputs of the two transistors has been shown, the output of the other transistor has a characteristic opposite to the characteristic in terms of the circuit configuration. FIG. 3 shows another embodiment of the present invention utilizing this relationship.

In this embodiment, the collector of the transistor Q5 is connected to the input of a current mirror CM composed of the transistors Q8 and Q9, the output of the current mirror CM is connected to the output terminal 5 and the resistor R1, and the transistors Q6 and Q7 are connected. The portion where the collector is connected to the power supply is a portion different from the configuration of FIG.

When the potential Vin of the input terminal 1 is sufficiently lower than the reference voltage 6, the transistor Q3 is turned off and the transistor Q2 is turned on in the voltage comparison circuit 8. Therefore, the bases of the transistors Q5 and Q7 have the same potential, and when the base potentials of the transistors Q5 to Q7 are compared, Vb
(Q5) = Vb (Q6) <Vb (Q7). In the differential matrix circuit 4 ', only the transistor Q7 is turned on. Since the current mirror CM is in the off state, no current flows through the resistor R1 and no gamma correction is added to the input signal. Therefore, the output terminal 5 has the same potential as the emitter of the transistor Q1 of the emitter follower.

As the potential of the input terminal 1 approaches the reference voltage 6, the transistors Q5 and Q6 also start to turn on, and the current flowing through the transistor Q5 increases, so that the output current of the current mirror CM increases. IG also increases. Gamma correction current IG is resistance R
When the signal flows to 1, the input signal is added with the amount of gamma correction and output to the output terminal 5.

The base voltages of the transistors Q5 to Q7 of the differential matrix circuit 4 'are Vb (Q5) = Vb (Q
6) = Vb (Q7), the amount of addition for the gamma correction is the maximum. The potential Vin of the input terminal 1 is equal to the reference voltage 6
When the voltage becomes larger, the voltage of the transistor Q
2 is turned off, and the transistor Q3 is turned on. The base potential of the transistor Q5 becomes the same as the base potential of the transistor Q7. Vb (Q6) ≧ Vb (Q5) = V
When the relationship of b (Q7), that is, the potential of the input terminal 1 becomes higher than the reference voltage 6, the transistors Q5 and Q7 approach the off state. Since the collector current of the transistor Q5 decreases, and thereby the gamma correction current IG decreases, the added amount of the gamma correction in the resistor R1 decreases.

When the potential of the input terminal 1 is sufficiently higher than the reference voltage 6, the current mirror CM is turned off, no current flows through the resistor R1, and no gamma correction is added to the input signal.
Therefore, the output terminal 5 has the same potential as the emitter of the transistor Q1 of the emitter follower, and has the gamma correction characteristic shown in FIG.

Also in this embodiment, the differential matrix circuit 4
'Can be realized by an NPN transistor having a good frequency characteristic, so that a sufficient frequency characteristic can be obtained, and the operating current does not change depending on the input signal level as in the related art. There is no change. Further, since the central operating point of the gamma correction characteristic can be determined by a single reference voltage, the same effect is obtained such that the circuit characteristic can be easily changed.

[0025]

As described above, according to the gamma correction circuit of the present invention, since the operating current does not change with the input signal level, not only can the frequency characteristic change depending on the input signal level be prevented, but also the circuit can be prevented. The frequency characteristics as a whole can also be improved.

[Brief description of the drawings]

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

FIGS. 2A, 2B, and 2C are characteristic diagrams for explaining the operation of FIG.

FIG. 3 is a circuit diagram for explaining another embodiment of the present invention.

FIG. 4 is a circuit diagram for explaining a conventional gamma correction circuit.

FIG. 5 is a characteristic diagram for explaining the operation of FIG. 4;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Input terminal, 2,7 ... Emitter follower, 3 ... Adder, 4,4 '... Differential matrix circuit, 5 ... Output terminal,
6: Reference voltage, 8: Voltage comparison circuit, Q1 to Q8: Transistor, CM: Current mirror.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-64-60062 (JP, A) JP-A-1-175467 (JP, A) JP-A-4-351071 (JP, A) JP-A-57-167 188182 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H04N 5/202

Claims (2)

(57) [Claims] An input signal is supplied to a first input, a predetermined reference voltage.
A second input, a <br/> signal clipped by the reference voltage the input signal to the third input, if these first to third input
A predetermined voltage level from the reference voltage around the reference voltage.
Means for setting the first and second operating level having the width of the Le, from the first to third input, the voltage les of the input signal
When the bell becomes higher than the first operation level, output starts, the voltage level of the input signal becomes maximum at the reference voltage, and the voltage level of the input signal is higher than the reference voltage.
When the voltage level of the input signal becomes higher than or equal to the second operation level, a correction signal whose output becomes 0 is generated , and the correction signal is converted into a voltage. gamma correction circuit for a voltage converting means <br/> for, characterized by comprising an adder for adding the voltage level of the voltage level and the first input of the voltage converted by the voltage converting means. Wherein said voltage conversion means includes a first transistor for inputting the first input to the base, a second transistor to be input to the second base inputs <br/> of said third inlet
Third transistor and, a gamma correction circuit according to claim 1, wherein a differential matrix circuit consisting of the connected emitter resistors be configured between the emitter of the transistor for inputting a force to the base.