JPH04337973A - Gamma correction circuit - Google Patents

Abstract

PURPOSE:To provide the gamma correction circuit which can set an amplification degree without depending on an input voltage. CONSTITUTION:A first feedback circuit including a transistor 11 and a resistor R1 and a second feedback circuit including a transistor 12 and a resistor R2 are connected to a differential amplifier circuit 10. When the level of an output voltage VA from the differential amplifier circuit 10 is lower than a polygonal point setting voltage V1, the transistor 12 is turned off and the differential amplifier circuit 10 is turned to a negative feedback amplifier circuit. When the level of the output voltage VA from the differential amplifier circuit is higher than the diagonal point setting voltage V1, the transistor 12 is turned on and the differential amplifier circuit 10 is turned to a voltage follower. Thus, the gamma correction circuit can be obtained to set the amplification degree without depending on an input voltage VIN.

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 used in a video camera.

0002

2. Description of the Related Art FIG. 10 shows an example of a conventional gamma correction circuit.

The gamma correction circuit shown in FIG. 10 includes a differential amplifier circuit 50 and corner setting circuits 64 and 68.

The differential amplifier circuit 50 includes transistors 51 and 52, current sources 53a and 53b, which flow the same operating current Ie through the transistors 51 and 52, respectively.
It consists of a resistor Re connected between the emitter of transistor 51 and the emitter of transistor 52 and a load resistor Rc connected to the collector of transistor 52. Collector of transistor 51 and load resistance R
A power supply voltage Vcc is applied to c.

The bending point setting circuit 64 is composed of a transistor 61 whose base and collector are connected, a transistor 62, a resistor R8, and a current source 63. The emitter of the transistor 61 and the emitter of the transistor 62 are connected, and a current source 63 is connected to the point where these emitters are connected. The resistor R8 is connected between a connection point A between the load resistor Rc included in the differential amplifier circuit 50 and the collector of the transistor 52, and the collector of the transistor 61. The power supply voltage Vcc is applied to the collector of the transistor 62, and the first turning point setting voltage Vr1 is applied to the base.

[0006] The break point setting circuit 68 is a break point setting circuit 6
It has the same configuration as 4, and is composed of a transistor 65 whose base and collector are connected, a transistor 66, a resistor R9, and a current source 67. The emitter of the transistor 65 and the emitter of the transistor 66 are connected, and a current source 67 is connected to the point where these emitters are connected. Resistor R9 is connected between connection point A and the collector of transistor 65. The collector of the transistor 66 is connected to the power supply voltage Vcc, and the base is connected to the second voltage.
A bending point setting voltage Vr2 is given, respectively. The second turning point setting voltage Vr2 is set higher than the first turning point setting voltage Vr1 (Vr1<Vr2).

The input voltage VIN of the gamma correction circuit is applied to the base of one transistor 51 included in the differential amplifier circuit 50. A bias voltage that determines the operating point of the differential amplifier circuit 50 is applied to the base of the other transistor 52 included in the differential amplifier circuit 50 .

The output voltage of the differential amplifier circuit 50 is
c appears at the connection point A between the collector of transistor 52 and the collector of transistor 52. The output terminal 30 is connected to the connection point A.
The voltage VA is output as the output voltage VOUT of the gamma correction circuit.

When the output voltage VOUT of the gamma correction circuit is lower than the first turning point setting voltage Vr1 (VOUT <
When Vr1<Vr2), the transistor 62 included in the break point setting circuit 64 is turned on, and the transistor 61 is turned off. Further, since the transistor 66 included in the break point setting circuit 68 is turned on and the transistor 65 is turned off, the break point setting circuits 64 and 65 included in the differential amplifier circuit 50 are turned on.
8 is not connected. Therefore, the amplification degree G0 of the gamma correction circuit is the resistance Rc and the resistance Re
Letting the resistance values of Rc and Re be respectively expressed by the following equation.

0010

[Math. 1] G0 = Rc / Re ... Formula 1

0011
]The output voltage VOUT of the gamma correction circuit is higher than the first turning point setting voltage Vr1, and the second turning point setting voltage Vr2
(Vr1<VOUT<Vr2),
Transistor 62 included in bending point setting circuit 64 is turned off, and transistor 61 is turned on. Transistor 66 of break point setting circuit 68 is on, transistor 65
remains off. At this time, resistor R8
and the load resistance Rc are connected in parallel. Since the parallel-connected composite resistance of these resistors R8 and load resistor Rc becomes a new load resistance of the differential amplifier circuit 50, the amplification degree G1 of the gamma correction circuit at this time is equal to the above amplification degree G.
becomes smaller than 0.

The amplification degree G1 of the gamma correction circuit is determined by the resistance R8.
Letting the resistance value of R8 be R8, it is expressed by the following equation.

[0013]

[Math. 2] G1 = Rc ・R8 / (Rc + R8)
・Re...Formula 2

Furthermore, when the output voltage VOUT is higher than the second turning point setting voltage Vr2 (Vr1<Vr2<
VOUT) is the transistor 6 of the turning point setting circuit 68.
6 is turned off, and transistor 65 is turned on. At this time, the resistors R8 and R9 and the load resistor Rc are connected in parallel. Since the parallel combined resistance of these resistors R8 and R9 and the load resistance Rc becomes a new load resistance of the differential amplifier circuit 50, the amplification degree G2 of the gamma correction circuit at this time becomes even smaller than the above amplification degree G1. .

The amplification degree G2 of the gamma correction circuit is determined by the resistance R9.
Letting the resistance value of R9 be R9, it is expressed by the following equation.

[0016]

[Math. 3] G2 = (Rc ・R8 ・R9)
/[(R8 ・R9 +Rc ・R9 ・R8
+Rc)・Re]...Formula 3

[0017]

[Problem to be Solved by the Invention] The output voltage VA of the differential amplifier circuit 50 is determined by the current source 5 included in the differential amplifier circuit 50.
If the output current of 3b is Ie, it is expressed by the following equation.

[0018]

[Math. 4] VA=Vcc-Rc・Ie...Formula 4

[
Assuming that the maximum value (maximum input amplitude) of the voltage difference between the input voltage VIN and the bias voltage of the differential amplifier circuit 50 is VI, this voltage difference VI is expressed by the following equation.

[0020]

[Math. 5] VI = Re ・ Ie ... Equation 5

0021
] By substituting equations 1 and 5 into equation 4, the following equation is obtained.

[0022]

[Math. 6] VA = Vcc-G0 ・VI ... Equation 6

[
Since there are restrictions on the magnitude of the power supply voltage Vcc, the magnitude of the second term G0·VI on the right side of Equation 6 cannot be made too large. For this reason, if the initial amplification G0 of the gamma correction circuit expressed by Equation 1 is increased, the maximum input amplitude VI of the differential amplifier circuit 50 must be reduced. Therefore, the initial amplification degree G0 cannot be made too high.

Furthermore, when the maximum input amplitude VI is reduced, errors at the bending point become noticeable, making it extremely difficult to set the amplification degree of the gamma correction circuit. Furthermore, the S/N of the gamma correction circuit also decreases. Further, the bending point setting circuit 64 is affected by the current fluctuation of the current source 63, and the bending point setting circuit 68 is affected by the current fluctuation of the current source 63.
The accuracy of the first and second knee setpoints is affected by current fluctuations, respectively.

As described above, in the conventional gamma correction circuit shown in FIG. 10, the initial amplification G0 depends on the maximum input amplitude VI.

An object of the present invention is to make the initial amplification degree of the gamma correction circuit independent of the maximum input amplitude.

[0027]

[Means for solving the problem] First invention (Claim 1)
The gamma correction circuit consists of a differential amplifier circuit to which an input voltage is applied to one input terminal, and a first feedback circuit connected to the output terminal of the differential amplifier circuit and the other input terminal of the differential amplifier circuit. and a second feedback circuit, and the second feedback circuit is disconnected from the first feedback circuit when the output voltage of the differential amplifier circuit is smaller than the turning point setting voltage, and the second feedback circuit is disconnected from the first feedback circuit. A negative feedback amplifier circuit is constituted by a switch circuit that connects the second feedback circuit in parallel with the first feedback circuit when the output voltage is higher than the set point voltage, and the output of the negative feedback amplifier circuit is It is characterized by being the output of the gamma correction circuit.

The gamma correction circuit according to the second invention (claim 3) includes a differential amplifier circuit to which an input voltage is applied to one input terminal, and an output terminal of the differential amplifier circuit and the other of the differential amplifier circuit. A first feedback circuit and a plurality of second feedback circuits connected to the input terminal of the differential amplifier circuit are provided with a plurality of different corner point setting voltages, and the output voltage of the differential amplifier circuit is below the lowest corner point setting voltage. When all the second feedback circuits are disconnected, the second feedback circuit is connected in parallel to the first feedback circuit every time the output voltage of the differential amplifier circuit exceeds a plurality of set point voltages. A negative feedback amplifier circuit is constituted by a switch circuit that increases 1 by one, and the output of the negative feedback amplifier circuit is the output of the gamma correction circuit.

Preferably, an emitter follower transistor is connected between the output terminal of the differential amplifier circuit and the first feedback circuit.

The output voltage of the negative feedback amplifier circuit, a reference voltage, and a clamp pulse are applied, and a clamp fixes the output voltage of the negative feedback amplifier circuit to the reference voltage while the clamp pulse is applied. It is good to have a circuit.

Furthermore, in a range where the input voltage or output voltage of the negative feedback amplifier circuit exceeds a predetermined set voltage, the input voltage or output voltage is multiplied by a coefficient of 1 or less.
Preferably, the device further includes an e-circuit.

The Knee circuit includes, for example, a voltage divider circuit that divides the output voltage of the negative feedback amplifier circuit, and a voltage divider circuit that outputs the output voltage of the negative feedback amplifier circuit as it is when the output voltage is lower than the set voltage. and a switch circuit that outputs the output voltage of the voltage dividing circuit when the set voltage is exceeded.

Further, the negative feedback amplifier circuit is a voltage negative feedback amplifier circuit or a current negative feedback amplifier circuit.

[0034]

According to the first invention, the second feedback circuit is disconnected from the first feedback circuit when the output voltage of the negative feedback amplifier circuit is smaller than the turning point setting voltage. When the second feedback circuit is disconnected from the first feedback circuit, the negative feedback circuit operates, and the amplification degree of the gamma correction circuit is determined by the resistance value of the first feedback circuit and the resistance value of the second feedback circuit. Determined by Therefore, the input voltage of the gamma correction circuit is amplified by the amplification degree determined by the value of the first feedback resistor and the value of the second feedback resistor.

When the magnitude of the output voltage of the feedback amplifier circuit is greater than the corner setting voltage, the second feedback circuit is placed in parallel with the first feedback circuit. Since the first feedback circuit and the second feedback circuit are connected in parallel to the negative feedback amplifier circuit, the negative feedback amplifier circuit becomes a voltage follower. Therefore, the amplification degree of the gamma correction circuit changes to 1.

According to the second invention, all the second feedback circuits are disconnected from the first feedback circuit when the output voltage of the differential amplifier circuit is smaller than the lowest set point voltage. Each time the output voltage of the differential amplifier circuit exceeds a plurality of set point voltages, the number of the second feedback circuits is added to the first feedback circuit by one. As the number of the second feedback circuits increases, the amplification degree of the gamma correction circuit changes.

[0037]

According to the first invention, the initial amplification degree of the gamma correction circuit is determined by the resistance value of the first feedback circuit and the resistance value of the second feedback circuit. Therefore, the initial amplification degree of the gamma correction circuit does not depend on the magnitude of the maximum input amplitude. Unlike conventional gamma correction circuits, the input voltage and the initial amplification degree are unrelated, so the initial amplification degree can be set relatively arbitrarily, making it possible to increase the amplification degree. It is also possible to widen the input dynamic range.

Therefore, since the input voltage can be made relatively large, the S/N ratio of the gamma correction circuit is also improved.

Furthermore, unlike the conventional gamma correction circuit, the amplification degree of the gamma correction circuit is determined by the resistance value of the first feedback circuit and the resistance value of the second feedback circuit, and is not affected by the power supply voltage. can be lowered.

Furthermore, since the accuracy of the set point value is determined by the accuracy of the switching circuit, the accuracy is improved.

When the gamma correction circuit of the first invention is used in a video camera, etc., the initial amplification degree can be increased, so the amplification degree of low luminance can be increased, and the reproducibility of reproduced images can also be improved. do.

According to the second invention, a plurality of second feedback circuits are connected to the negative feedback amplifier circuit, and the output voltage of the differential amplifier circuit connects the second feedback circuits to a plurality of corner setting voltages. Since a switching circuit is included that increases the number of second feedback circuits connected in parallel to the first feedback circuit by one each time the circuit exceeds the number of points, a gamma correction circuit capable of setting a plurality of bending points is obtained.

Therefore, the gamma correction circuit has a gamma curve that is almost ideal.

Furthermore, by connecting a Knee circuit to the feedback amplifier circuit, the amplification degree of the gamma correction circuit can be made less than 1, and a gamma correction circuit having a γ curve closer to the ideal can be obtained.

Furthermore, by providing the gamma correction circuit with a clamp circuit that fixes the output voltage of the negative feedback amplifier circuit to the reference voltage for a predetermined period, it is possible to fix the output voltage of the gamma correction circuit to the reference voltage for a predetermined period. become able to. For example, when the gamma correction circuit is used in a video camera, etc., the output voltage of the gamma correction circuit can be fixed at the pedestal level, and the accuracy of gamma correction turning point setting can be improved.

As the negative feedback amplifier circuit, a voltage negative feedback amplifier circuit, a current negative feedback amplifier circuit, etc. can be used. However, since the current negative feedback amplifier circuit is not limited to a constant gain bandwidth product (GB product), it is possible to use a negative feedback amplifier circuit. When a current negative feedback amplifier circuit is used as the amplifier circuit, the frequency characteristics of the gamma correction circuit can be expanded.

[0047]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of the present invention, and is a circuit diagram of a gamma correction circuit having one bending point.

The gamma correction circuit includes a differential amplifier circuit 10, transistors 11 and 12, transistors 13, current sources 5 and 6, and a feedback clamp circuit 20.
It is included.

The differential amplifier circuit 10 includes a first feedback circuit composed of a transistor 11 and a first feedback resistor R1, and a transistor 12 as a switching circuit.
and a second feedback circuit constituted by a second feedback resistor R2. These feedback circuits are connected to the negative input terminal of the differential amplifier circuit 10.

The feedback clamp circuit 20 includes a differential amplifier circuit 28, a switching element 26, and a capacitor 2.
5, a buffer 29 and a current source 27 are included.

The input voltage VIN of the gamma correction circuit is applied to the positive input terminal of the differential amplifier circuit 10. Differential amplifier circuit 1
An output voltage VA of 0 is applied to the bases of transistors 11 and 12. Transistor 11 is an emitter follower.

The emitter of transistor 12 and the emitter of transistor 13 are connected, and the collector of transistor 12 and the collector of transistor 13 are connected. A power supply voltage Vcc is applied to the collectors of transistors 11, 12 and 13, respectively. A turning point setting voltage V1 is applied to the base of the transistor 13.

When the output voltage VA of the differential amplifier circuit 10 is smaller than the turning point setting voltage V1 (VA < V1)
In this case, transistor 12 is turned off and transistor 13 is turned on. Therefore, resistor R2 no longer operates as a feedback circuit. Also, at this time, a voltage V1 -VBE (V
BE is the base-emitter voltage of the transistor 13), so the differential amplifier circuit 10 operates as a negative feedback circuit.

The amplification degree g11 in this case is expressed by the following equation.

[0055]

[Math. 7] g11=1+R1/R2...Equation 7

00
56. When the output voltage VA of the differential amplifier circuit 10 becomes larger than the turning point setting voltage V1 (V1 < VA), the transistor 12 is turned on and the transistor 13 is turned off. Therefore, the differential amplifier circuit 10 has two feedback circuits in parallel, a first feedback circuit and a second feedback circuit, and operates as a voltage follower.

The amplification degree g12 in this case is expressed as follows.

[0058]

[Formula 8] g12=1...Formula 8

The amplification degree changes from g11 to g at the point where the output voltage VA of the differential amplifier circuit 10 reaches the turning point setting voltage V1.
12 to obtain a gamma correction circuit.

The output voltage VOUT of the gamma correction circuit is also applied to the feedback clamp circuit 20. Clamp pulse CP is feedback clamp circuit 20
During the period given to , the output voltage VOUT of the gamma correction circuit is fixed to a predetermined voltage Vr. When the gamma correction circuit is used in a video camera, a clamp pulse CP is applied to the feedback clamp circuit 20 every horizontal scanning period.

The output voltage VOUT of the gamma correction circuit input to the feedback clamp circuit 20 is applied to the negative input terminal of the differential amplifier circuit 28. A predetermined voltage Vr to be fixed is applied to the positive input terminal of the differential amplifier circuit 28.

When a clamp pulse is applied to the switching element 26, the switching element 26 is turned on, and a charge corresponding to the voltage difference between the output voltage VOUT and the voltage to be fixed Vr is accumulated in the capacitor. As a result, an operating voltage is applied to the current source 27 via the buffer 29, the current source 27 is turned on, and current flows through the first feedback circuit and the second feedback circuit.

When current flows through the first feedback circuit and the second feedback circuit, the output voltage VOUT increases. Therefore, the difference between the output voltage VOUT of the gamma correction circuit input to the differential amplifier circuit 28 and the predetermined voltage Vr gradually decreases, and the output voltage VOUT of the gamma correction circuit becomes equal to the predetermined voltage Vr.

Since the output voltage VOUT can be fixed to a predetermined reference voltage Vr by the feedback clamp circuit 20, it becomes possible to eliminate an offset occurring in the output voltage VOUT.

FIG. 2 is a circuit diagram showing a specific example of the configuration of the gamma correction circuit shown in FIG. In the circuit shown in FIG. 2, the same components as those shown in FIG.

Differential amplifier circuit 10 is composed of transistors 1 and 2, current sources 3 and 4, and capacitor 5. The emitter of transistor 1 and the emitter of transistor 2 are connected, and these transistors 1 and 2
The emitter of is connected to the current source 3. The input voltage VIN of the gamma correction circuit is applied to the base of the transistor 1, and the power supply voltage VCC is applied to the collector of the transistor 1. A parallel circuit consisting of a current source 4 and a capacitor 5 is connected to the collector of the transistor 2.

In the gamma correction circuit shown in FIG. 2, the transistor 14 is used to ensure a voltage drop between the base and emitter of the transistor 13 and generate a fixed voltage Vr.
It is included. A power supply voltage Vcc is applied to the collector of the transistor 14, and the emitter of the transistor 14 is connected to the current source 7. The emitter voltage of the transistor 14 is applied to the differential amplifier circuit 28 as a fixed voltage Vr.

The gamma correction circuit also includes resistors r0, r1 and r2 connected in series. The base of transistor 13 is connected at the connection point between resistor r1 and resistor r2. The turning point setting voltage V1 applied to the base of the transistor 13 is the power supply voltage Vc.
It is created by dividing voltage c by resistors r0 and r1 and resistor r2.

If the voltage generated at the connection point between resistor r0 and resistor r1 is V0, the voltage V0 is a fixed voltage V
is applied to the base of transistor 14 which generates r.

The differential amplifier circuit 28 included in the feedback clamp circuit 20 includes transistors 28a and 2.
8b and a current mirror circuit 28c. The emitter of transistor 28a and the emitter of transistor 28b are connected. transistor 2
The collector of transistor 8a is connected to the collector of one transistor constituting current mirror circuit 28c. The collector of the transistor 28b is the current mirror circuit 2
It is connected to the collector of the other transistor constituting 8c.

A transistor is used as the switching element 26. The collector of this transistor 26, the emitter of the transistor 28a, and the transistor 28b
is connected to the emitter of A power supply voltage Vcc is applied to the emitter of the transistor 26 via a resistor 21.

The current source 27 includes a transistor 27a and a resistor 27b connected to the emitter of the transistor 27a.
It consists of

The output voltage VOUT of the gamma correction circuit input to the negative input terminal of the differential amplifier circuit 28 is the output voltage VOUT of the transistor 2.
A fixed voltage Vr applied to the base of transistor 8b and input to the positive input terminal of differential amplifier circuit 28 is applied to the base of transistor 28a.

When a clamp pulse CP is applied to the base of transistor 26, transistor 26 is turned on. The fixed voltage Vr is the output voltage VOUT of the gamma correction circuit.
If it is larger than , transistor 28b is on,
A current flows through this transistor 28b. Charge is accumulated in the capacitor 25 according to the current flowing through the transistor 28b, and an operating voltage is generated. The operating voltage generated by the capacitor 25 is applied to the base of the transistor 27a of the current source 27 via the buffer 29.
is turned on.

When the transistor 27a is turned on, a current determined by the resistance value of the resistor 27b flows through the transistor 27a.
7a, and the output voltage VOUT of the gamma correction circuit is fixed at the fixed voltage Vr.

In the gamma correction circuit shown in FIG. 2, the setting accuracy of the break point is determined by the accuracy of the base-emitter voltages of the transistors 12 and 13, so the break point can be set with high precision.

In the circuit shown in FIG. 2, the output current of the current source 3 is 100 [μA], the resistor R1 is 7.5 [kΩ],
R2 is 2.5 [kΩ], bending point setting voltage V1 -V
Graphs of the input/output characteristics of the gamma correction circuit when 0 is set to 250 [mV] are shown in FIGS. 3(A) and 3(B). FIG. 3(B) is a graph in which a part of FIG. 3(A) is enlarged. As can be seen from Figures 3(A) and (B), the amplification degree g11 is 4.
, g12 are 1, making it possible to obtain an extremely wide input dynamic range compared to conventional gamma correction circuits.

FIG. 4 shows another embodiment, and is a circuit diagram of a gamma correction circuit having two bending points. In the circuit shown in FIG. 4, the same components as those shown in FIGS. 1 and 2 are designated by the same reference numerals, and the explanation thereof will be omitted.

The gamma correction circuit shown in FIG. 4 includes transistors 15 and 16. The emitter of transistor 15 and the emitter of transistor 16 are connected, and these emitters are connected to current source 8. Power supply voltage Vcc is applied to the collector of transistor 15 and the collector of transistor 16. The base of transistor 15 is connected to the base of transistor 12.

The emitter of transistor 15 is connected to resistor R3.
It is connected to the base of transistor 2 via. Transistor 15 and resistor R3 constitute a third feedback circuit of differential amplifier circuit 10.

In the gamma correction circuit shown in FIG. 4, resistors r1, r3 and r
A series resistor circuit consisting of 4 is included. A predetermined voltage V0 is applied to one end of the resistor r1, and a power supply voltage Vcc is applied to one end of the resistor r4. Also, at the connection point between resistors r3 and r4, transistor 1
The bases of transistor 13 are connected to each other, and the base of transistor 13 is connected to the connection point between resistors r1 and r3.

The first turning point setting voltage V1 and the second turning point setting voltage V2 are both determined by the magnitudes of the power supply voltage Vcc and the predetermined voltage V, and the resistance values of the resistors r1, r3 and r4. The point setting voltage V1 is determined by the voltage division between the resistors r1 and the resistors r3 and r4, and the second turning point setting voltage V2 is determined by the voltage division between the resistors r1 and r2 and the resistor r4. The value of the first corner setting voltage V1 is lower than the value of the second corner setting voltage.

The output voltage VA of the differential amplifier circuit 10 is the first
When the voltage is lower than the set voltage V1 (VA < V1
<V2), transistors 12 and 15 are off and transistors 13 and 16 are on, so the first amplification g21 of the gamma correction circuit is expressed by the following equation.

[0084]

[Equation 9] g21=1+(R2 +R3)・R1/R2
・R3...Formula 9

The output voltage VA of the differential amplifier circuit 10 is the first
higher than the second turning point setting voltage V1 and lower than the second turning point setting voltage V2 (V1 < VA < V2 )
Since transistors 12 and 16 are turned on and transistors 13 and 15 are turned off, the second gamma correction circuit
The amplification degree g22 of is lower than the first amplification degree g21,
It is expressed by the following formula.

[0086]

[Formula 10] g22=1+R1 ・R2 /(R1+R2
)・R3...Formula 10

The output voltage VA of the differential amplifier circuit 10 is
When the voltage is higher than the set voltage V2 (V1 < V2
<VA), transistors 12 and 15 are on,
Transistors 13 and 16 are turned off, and differential amplifier circuit 10 becomes a voltage follower. The third at this time
The amplification degree g23 is even lower than the second amplification degree g22 and is expressed by the following equation.

[0088]

[Formula 11] g23=1...Formula 11

In the circuit shown in FIG. 4, the output current of current source 3 is 100 [μA], the resistor R1 is 14 [kΩ], and R
2 to 4.5 [kΩ], R3 to 3.5 [kΩ],
The first turning point setting voltage V1 −V0 is set to 400 [mV
], the second turning point setting voltage V2 −V0 is set to 700 [
FIG. 5 shows the input/output characteristics when the voltage is set to [mV]. As can be seen from Fig. 5, the amplification degree g21 is 8, g22 is 2,
g23 becomes 1, resulting in a gamma correction circuit having two types of breaking points.

By increasing the number of feedback circuits, it is possible to further increase the types of amplification degrees to be changed.

FIG. 6 shows still another embodiment, and is a circuit diagram of a gamma correction circuit having two bending points. The gamma correction circuit shown in FIG. 6 can have an amplification degree of 1 or less. In FIG. 6 as well, the same components as those shown in FIGS. 1 and 2 are given the same reference numerals, and the description thereof will be omitted.

The gamma correction circuit shown in FIG. 6 includes a knee circuit 35 for setting the amplification degree of the gamma correction circuit to 1 or less. Knee circuit 35 includes resistors R5 and R
6 and a transistor 36. Also included is a transistor 17.
The base of the transistor 12 is connected to the base of the transistor 12, and the emitter is grounded via a current source 31. A power supply voltage Vcc is applied to the collector of the transistor 17.

Furthermore, the gamma correction circuit shown in FIG. 6 includes a transistor 18 and a transistor 19 whose base and collector are connected. transistor 1
The emitter of transistor 8 and the emitter of transistor 19 are connected. A collector of transistor 19 is connected to current source 9 . Collector of transistor 19 and current source 9
The base of the transistor 36 is connected at the connection point with the . Also, the base of the transistor 18 is a resistor r3.
It is connected to the connection point between and r4.

The transistor 18 guarantees a voltage drop in the base-emitter voltage VBE of the transistor 17, and the transistor 19 guarantees a voltage drop in the base-emitter voltage VBE of the transistor 36.

The output voltage VA of the differential amplifier circuit 10 is the first
When the voltage is lower than the set voltage V1 (VA < V1
<V2), the transistor 12 is turned off and the transistors 13 and 17 are turned on, so the first amplification degree g31 of the gamma correction circuit is expressed by the following equation.

[0096]

[Formula 12] g31=1+R1/R2...Formula 12

[
When the output voltage VA of the differential amplifier circuit 10 is higher than the first turning point setting voltage V1 and lower than the second turning point setting voltage V2 (V1 < VA < V2), the transistors 12 and 17 are activated. On, transistor 1
3 is off, the gamma correction circuit becomes a voltage follower. The second amplification degree g32 at this time is lower than the first amplification degree g31 and is expressed by the following equation.

[0098]

[Formula 13] g32=1...Formula 13

The output voltage VA of the differential amplifier circuit 10 is
When the voltage is higher than the set voltage V2 (V1 < V2
<VA), transistors 12, 17, and 36 are turned on, transistor 13 is turned off, and the knee circuit 3 is turned on.
Since the output voltage VOUT of the gamma correction circuit is divided by the resistors R5 and R6 of 5, the third amplification degree g33 of the gamma correction circuit is lower than the second amplification degree g32, and becomes 1 as expressed by the following equation. The following is true.

[0100]

[Formula 14] g33=R6/(R5 +R6)...
Formula 14

In the circuit shown in FIG. 6, the amplification degree of the gamma correction circuit can be set to 1 or less, so that the gamma correction circuit has a γ curve close to the ideal.

In the circuit shown in FIG. 6, the knee circuit 3
5 is connected to the output side of the differential amplifier circuit 10, but it may be connected to the input side of the differential amplifier circuit 10.

In the circuit shown in FIG. 6, the output current of current source 3 is 200 [μA], the output current of current sources 4, 5, 6, and 7 is 100 [μA], and the resistor R1 is 12 [kΩ].
], R2 is 4 [kΩ], R5 is 5 [kΩ], R6
5 [kΩ], the first bending point setting voltage V1 - V0 is 200 [mV], the second turning point setting voltage V2 - V0
FIG. 7 shows the input/output characteristics when the voltage is set to 350 [mV]. As can be seen from Fig. 7, the amplification degree g31 is 4,
g32 is 1, g33 is 0.5, and the gamma correction circuit has an amplification degree of 1 or less.

FIG. 8 shows still another embodiment, and is a circuit diagram of a gamma correction circuit. In FIG. 8 as well, the same components as those shown in FIGS. 1 and 2 are designated by the same reference numerals, and the description thereof will be omitted.

In the gamma correction circuit shown in FIG. 8, a current feedback amplifier circuit 40 is used in the differential amplifier circuit.

The current feedback amplifier circuit 40 is composed of a current mirror circuit 41, a transistor 42, and a current source 32.

The input voltage VIN of the gamma correction circuit is applied to the base of transistor 42. Current feedback amplifier circuit 4
0 includes a current-voltage conversion circuit 41. This circuit 41 converts the current flowing through the transistor 42 into a voltage, which is output from the current feedback amplifier circuit 40 and applied to the base of the transistor 11.

Similar to the circuit shown in FIG. 1 or 2, the degree of amplification changes depending on whether the output voltage VA of the current feedback amplifier circuit 40 is less than or equal to the set point voltage V1.

In the gamma correction circuit shown in FIG. 8, since the current feedback amplifier circuit 40 is used, unlike the voltage feedback amplifier circuit, there is no restriction that the gain bandwidth product (GB product) is constant. Therefore, the frequency characteristics can be widened.

In the circuit shown in FIG. 8, the resistor R1 is set to 10
[kΩ], R2 is 2.5 [kΩ], and the set voltage V1 −V0 is 250 [mV]. The input/output characteristics are shown in FIG. As shown in FIG. 9, the circuit of FIG. 8 also provides a gamma correction characteristic with a break point.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of the present invention, and is a circuit diagram of a gamma correction circuit.

FIG. 2 is a circuit diagram showing a configuration example of the gamma correction circuit shown in FIG. 1;

3A and 3B are graphs showing input/output characteristics of the circuit shown in FIG. 2;

FIG. 4 shows another embodiment of the present invention, and is a circuit diagram of a gamma correction circuit.

FIG. 5 is a graph showing input/output characteristics of the circuit shown in FIG. 4;

FIG. 6 shows still another embodiment of the present invention, and is a circuit diagram of a gamma correction circuit.

FIG. 7 is a graph showing input/output characteristics of the circuit shown in FIG. 6;

FIG. 8 shows still another embodiment of the present invention, and is a circuit diagram of a gamma correction circuit.

FIG. 9 is a graph showing input/output characteristics of the circuit shown in FIG. 8;

FIG. 10 is a circuit diagram of a conventional gamma correction circuit.

[Explanation of symbols]

10 Differential amplifier circuits 11 to 13 Transistor 20 Feedback clamp circuit 35 Kne
e circuit 40 current feedback amplifier circuit

Claims (8)

[Claims] 1. A differential amplifier circuit to which an input voltage is applied to one input terminal, a first feedback circuit connected to an output terminal of the differential amplifier circuit and the other input terminal of the differential amplifier circuit, and When the output voltage of the second feedback circuit and the differential amplifier circuit is smaller than the turning point setting voltage, the second feedback circuit is disconnected from the first feedback circuit, and the output voltage of the differential amplifier circuit is disconnected from the first feedback circuit. A negative feedback amplifier circuit is constituted by a switch circuit that connects the second feedback circuit in parallel to the first feedback circuit when the voltage is higher than the set voltage, and the output of the negative feedback amplifier circuit is set to a gamma voltage. Gamma correction circuit that serves as the output of the correction circuit. 2. The gamma correction circuit according to claim 1, wherein an emitter follower transistor is connected between the output terminal of the differential amplifier circuit and the first feedback circuit. 3. A differential amplifier circuit to which an input voltage is applied to one input terminal, a first feedback circuit connected to an output terminal of the differential amplifier circuit and the other input terminal of the differential amplifier circuit, and A plurality of second feedback circuits and a plurality of different corner point setting voltages are provided, and when the output voltage of the differential amplifier circuit is less than or equal to the lowest corner point setting voltage, all the second feedback circuits are
a switch that disconnects the feedback circuit of the differential amplifier circuit and increases the number of the second feedback circuits connected in parallel to the first feedback circuit by one each time the output voltage of the differential amplifier circuit exceeds a plurality of set voltages; A negative feedback amplifier circuit is constructed from the circuit.
A gamma correction circuit in which the output of the negative feedback amplifier circuit is the output of the gamma correction circuit. 4. The gamma correction circuit according to claim 3, wherein an emitter follower transistor is connected between the output terminal of the differential amplifier circuit and the first feedback circuit. 5. The output voltage, the reference voltage, and the clamp pulse of the negative feedback amplifier circuit are provided, and the clamp pulse is applied to the negative feedback amplifier circuit.
5. The gamma correction circuit according to claim 1, further comprising a clamp circuit that fixes the output voltage of the negative feedback amplifier circuit to the reference voltage while a pulse is applied. 6. In a range where the input voltage or output voltage of the negative feedback amplifier circuit exceeds a predetermined set voltage, the input voltage or output voltage is multiplied by a coefficient of 1 or less.
The gamma correction circuit according to any one of claims 1 to 5, further comprising an e circuit. 7. The Knee circuit includes a voltage dividing circuit that divides the output voltage of the negative feedback amplifier circuit, and outputs the output voltage of the negative feedback amplifier circuit as it is when the output voltage is equal to or lower than the set voltage, 7. The gamma correction circuit according to claim 6, further comprising a switch circuit that outputs the output voltage of the voltage dividing circuit when the voltage exceeds the set voltage. 8. The gamma correction circuit according to claim 1, wherein the negative feedback amplifier circuit is a voltage negative feedback amplifier circuit or a current negative feedback amplifier circuit.