JPH0736610B2 - White clip circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a white clip circuit for preventing overmodulation of video modulation.

[Conventional technology]

The white clip circuit prevents overmodulation in video modulation. In this white clip circuit, if the white level of the composite video (video composite) signal is clipped by the pre-stage processing unit only for the purpose of preventing overmodulation, the image quality may be degraded. It needs to be done to a limited extent.

Generally, in the white clip circuit, as shown in FIG. 2, a differential amplifier 4 is installed in a stage next to the clamp circuit 2 for setting the sync tip level of the composite video signal to a predetermined level, and the output of the differential amplifier 4 A clipping circuit 6 for clipping the upper limit level of the composite video signal is connected to the side.

The clamp circuit 2 is composed of a transistor 8, a capacitor 10 and resistors 12, 14, 16 and 18, and the input terminals 20 and 22 formed between the connection point of the capacitor 10 and the resistor 12 and the ground side line are combined with each other. A video signal is added. A drive voltage V _CC is applied from the power supply between the power supply terminal 24 formed on the positive side line and the ground side line, and the bias voltage formed at the voltage dividing point between the resistors 14 and 16 and the transistor 18 is applied to the transistor. 8 is added to the base, and the transistor 8 has an input terminal 2
Only the sync tip level section in the composite video signal added to 0 and 22 becomes conductive.

The differential amplifier 4 is a constant current source between the emitters of a pair of transistors 26 and 28 having a common emitter and the ground side line.
The differential output taken out from the collector side of the transistor 28 is totally fed back to the base of the transistor 28. That is, the differential amplifier 4 constitutes an all-feedback differential amplifier.

The clip circuit 6 includes transistors 32, 34, 36, 3
8, 40, 42, constant current sources 44, 46, 48 and resistors 50, 51. The transistors 32, 34 and the constant current source 44 form a differential amplifier, and the transistors 36, 38 and the resistor 50,
Reference numeral 51 constitutes an active load composed of a current mirror circuit, and transistors 40 and 42 and constant current sources 46 and 48 constitute a level shift circuit. The upper limit level of the white signal in the composite video signal is regulated.

Then, output terminals 52 and 54 are formed between the bases of the transistors 28 and 32 and the ground side line. According to such a white clip circuit, in the composite video signal as shown in FIG. The chip level V _S is fixed to a constant level by the clamp circuit 2, and the white level V _W is cut to a constant level and output.

[Problems to be solved by the invention]

In this white clip circuit, when a composite video signal is applied to the input terminals 20 and 22, the current I flowing in the transistor 8
_{Between C1} and the base current I _B of the transistor 26, I
Relationship of _C1 = a · I _B (where a is a constant). There is a relation of I _B = (1 / β) I _C2 between the base current I _B and the collector current I _C2 of the transistor 26, and between the current I _C1 and the current I ₁ flowing in the constant current source 30. _{is, I C2 = I 1/2} = constant becomes relationship. Therefore, for the currents I _C1 and I ₁ , I _C1 =
(A / 2β) I ₁ and the current I _C1 is the current amplification factor β
Inversely proportional to.

Then, the base-emitter voltage V _{F of} the transistor 8
Is determined by the current I _C1 flowing through the collector of the transistor 8, so that if the current amplification factor β changes, the current I
_{Since C1} changes and the voltage V _F changes, the current amplification factor β affects the clamp potential of the clamp circuit 2.

Therefore, when such a white clip circuit is mass-produced in a semiconductor integrated circuit, there is a drawback that the white clip level becomes uneven due to the nonuniform current amplification factor β of the transistor 26.

Therefore, it is an object of the present invention to provide a white clip circuit in which the non-uniformity of the current amplification factor β in manufacturing a transistor is compensated and the white clip level is made uniform.

[Means for solving problems]

As illustrated in FIG. 1, the white clip circuit of the present invention divides a power supply voltage (V _CC ) into a voltage dividing circuit (resistor 1
4, 16 and 18) connects a series circuit composed of a capacitor (10) and a resistor (12) to a first transistor (8) whose base voltage is set, and a composite video signal is connected to the connection point of the capacitor and the resistor. In response to this, the clamp circuit (2) which clamps the sync tip level of the composite video signal to a constant level and takes out the composite video signal whose clamp tip level is clamped to the constant level from the connection point of the first transistor and the capacitor. ) And a transistor pair consisting of a second and a third transistor (26, 28) having a common emitter, the first constant current source (30) being connected to the emitter side of this transistor pair, and A second constant current source (46) is connected between the base of the third transistor and the reference potential point, and an emitter is provided between the second constant current source and the power source. A fourth transistor (40) is connected in series on the current source side, the collector of the third transistor is connected to the base of the fourth transistor, and the fifth transistor (58) is connected to the second transistor. ) Are connected in series, and a current equivalent to the base current of the second transistor taken out through the fifth transistor is supplied to the first current mirror circuit (transistor 6).
A current supply circuit (56) for flowing into the base of the second transistor through 0, 62), and an output is taken from the emitter side of the fourth transistor commonly connected to the base of the third transistor. All feedback differential amplifier (4) and the sixth and seventh transistors (32,
A transistor pair having a common emitter of 34) is installed, a third constant current source (44) is connected to the emitter side of the transistor pair, and a second current mirror circuit (transistor) is connected to the transistor pair as an active load. 36, 38), an eighth transistor (42) is connected between the base of the seventh transistor and the power source, and a second transistor is connected between the base of the seventh transistor and the reference potential point. Four
Constant current source (48) is connected, the fourth transistor of the full-feedback differential amplifier is commonly connected between the base of the sixth transistor and the power supply, and the fourth transistor of The collector of the sixth transistor is connected to the base, the composite video signal output from the all-feedback amplifier is applied to the base of the sixth transistor from the emitter side of the fourth transistor, and the eighth A clip voltage is applied to the base of the transistor from the voltage dividing circuit of the clamp circuit, and the upper limit level of the composite video signal is clipped by the clip voltage.
And a clip circuit (6) adapted to take out a composite video signal output having a white level set to a constant level from the emitter side of the fourth transistor.

[Action]

Therefore, the present invention suppresses the potential change on the clamp circuit side by supplying the base current component that changes due to the unevenness of the current amplification factor β from the current supply circuit to the transistor on the clamp circuit side of the all-feedback differential amplifier. ing.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an embodiment of the white clip circuit of the present invention, and the same parts as those of the white clip circuit shown in FIG. 2 are designated by the same reference numerals.

As shown in FIG. 1, this white clip circuit has
A clamp circuit 2 for clamping the sync tip level of the composite video signal applied to the input terminal 20 to a constant level, and an all-feedback differential amplifier 4 for passing the composite video signal having a constant sync tip level set by the clamp circuit 2. ,
A clipping circuit 6 for clipping the upper limit level of the composite video signal output through the all-feedback differential amplifier 4 is installed. In the clamp circuit 2, a series circuit including a capacitor 10 and a resistor 12 is connected to the first transistor 8 whose base voltage is set through a voltage dividing circuit including resistors 14, 16 and 18 for dividing the power supply voltage V _CC , The composite video signal applied to the connection point of 10 and the resistor 12 through the input terminal 20 is received, and the sync tip level of the composite video signal is clamped to a constant level.

In addition, the all-feedback differential amplifier 4 is provided with a transistor pair composed of second and third transistors 26 and 28 having a common emitter, and a first constant current source 30 is provided on the emitter side of this transistor pair. It is connected. A fifth transistor is provided between the collector of the transistor 26 and the positive voltage side of the power supply.
58 is connected in series, and a current supply circuit 56 is installed which takes out a current equivalent to the base current of the transistor 26 through the transistor 58 and supplies this current to the base of the transistor 26 through the transistors 60 and 62 forming the first current mirror circuit. Has been done.

The second constant current source 46 is connected between the base of the transistor 28 and the reference potential point, and the output signal taken out from the collector side is connected to the base of the second constant current source 46.・ Fully returned through the emitters.

The clip circuit 6 is provided with a transistor pair in which the emitters of the sixth and seventh transistors 32 and 34 are commonly used, and the third constant current source 44 is connected to the emitter side of this transistor pair. . A second current mirror circuit composed of transistors 36 and 38 is connected to the collector side of the transistor pair as an active load. In addition, an eighth element is provided between the base of the transistor 34 and the positive voltage side of the power supply.
The collector and the emitter of the transistor 42 are connected in series, and the fourth constant current source 48 is connected between the base of the transistor 34 and the reference potential point. Further, a DC voltage taken from the middle point of the resistors 14 and 16 forming the voltage dividing circuit of the clamp circuit 2 is applied to the base of the transistor 42, and the clip level of the video signal is set.

In the current supply circuit 56, a transistor 58 identical to the transistor 26 is installed as an equivalent element on the collector side of the transistor 26, and the current obtained by the transistor 58 is supplied to the base of the transistor 26.
A current mirror circuit consisting of 0 and 62 is installed.

That is, the all-feedback differential amplifier 4 receives the composite video signal by connecting the emitter side of the transistor 8 which is the output point of the clamp circuit 2 to the base of the transistor 26, and receives the composite video signal.
By receiving the output signal taken out from the collector side on the base side of the total feedback amplifier, a total feedback amplifier for passing the composite video signal is constructed.
The composite video signal obtained through the above is added to a clipping circuit 6 for clipping its upper limit level.

The operation will be described based on the above configuration.

Since the current I _C2 flowing through the transistor 26 flows through the transistor 58, the base current I _B necessary for flowing the current I _C1 flows through the transistor 58. That is, the transistor 26
By an equivalent transistor 58, the base current I _B flowing through the transistor 26 is calculated monkey.

This base current I _B flows to the transistor 62 and is supplied to the base of the transistor 26 due to the current mirror effect of the current mirror circuit composed of the transistors 60 and 62.

In particular, since the transistor 26 and the transistor 58 can be formed equivalently on the same semiconductor chip, each transistor 2
The base currents I _{B of 6} and 58 can be made equal, and the base current of the transistor 26 can be guaranteed with high accuracy.

As a result, the influence of the base current I _{B on} the current I _C1 flowing through the transistor 8 can be avoided, the influence of the current amplification factor β of the transistor 26 can be eliminated, and the unevenness of the clamp potential can be assured. The white clip level can be maintained and the image quality can be improved.

The other operations of the clamp circuit 2, the differential amplifier 4, and the clip circuit 6 are the same as those of the white clip circuit shown in FIG.

〔The invention's effect〕

As described above, according to the present invention, after the sync tip level is clamped to a constant level in the composite video signal by the clamp circuit, the video signal is added to the clipping circuit through the all-feedback differential amplifier to set its upper limit level. In addition to clipping, the base current is compensated for the transistor on the clamp circuit side of the all-feedback differential amplifier by the current supply circuit, so it is possible to suppress the non-uniformity between white clip level products due to the non-uniform current amplification factor β of the transistor. Therefore, it is possible to improve the image quality by using it in a video reproducing device.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a white clip circuit of the present invention, FIG. 2 is a circuit diagram showing a general white clip circuit, and FIG. 3 is an explanatory diagram showing a composite video signal. V _CC ...... Power supply voltage 2 ...... Clamp circuit 4 ...... All feedback differential amplifier 6 ...... Clip circuit 8 ...... First transistor 10 ...... Capacitor 12 ...... Resistance 14, 16, 18 ...... Resistance (min 26. Second transistor 28. Third transistor 30. First constant current source 32. Sixth transistor 34. Seventh transistor 36, 38 .. Transistor (second transistor Current mirror circuit) 40 ... Fourth transistor 42 ... Eighth transistor 44 ... Third constant current source 46 ... Second constant current source 48 ... Fourth constant current source 56 ... Current supply Circuit 58 …… Fifth transistor 60, 62 …… Transistor (first current mirror circuit)

Claims (1)

[Claims] 1. A series circuit including a capacitor and a resistor is connected to a first transistor of which a base voltage is set by a voltage dividing circuit for dividing a power supply voltage, and a composite video signal is received at a connection point of the capacitor and the resistor. , The sync tip level of this composite video signal is clamped to a certain level,
A clamp circuit for taking out the composite video signal whose sync tip level is clamped to a fixed level from a connection point of the first transistor and the capacitor, and a transistor pair including a second and a third transistor having a common emitter are provided. The first constant current source is connected to the emitter side of this transistor pair, and the second constant current source is connected between the base of the third transistor and the reference potential point. A fourth transistor is connected in series between the current source and the power source with the second constant current source side and the collector of the third transistor is connected to the base of the fourth transistor, A fifth transistor is connected in series to the second transistor, and the base voltage of the second transistor taken out through the fifth transistor is connected. A current supply circuit for injecting a current equivalent to a current into the base of the second transistor through a current mirror circuit is installed, and output from the emitter side of the fourth transistor commonly connected to the base of the third transistor. Is installed, and a transistor pair in which the emitters of the sixth and seventh transistors are common is installed, and the third constant current source is connected to the emitter side of this transistor pair, A current mirror circuit as an active load, an eighth transistor connected between the base of the seventh transistor and the power supply, and a second connection between the base of the seventh transistor and a reference potential point. 4 constant current source is connected,
The fourth transistor of the all-feedback differential amplifier is commonly connected between the base of the sixth transistor and the power supply, and the collector of the sixth transistor is connected to the base of the fourth transistor. Then, the composite video signal output from the all-feedback amplifier is applied to the base of the sixth transistor from the emitter side of the fourth transistor, and the component of the clamp circuit is added to the base of the eighth transistor. A clipping voltage is applied from a voltage circuit, the upper limit level of the composite video signal is clipped by the clipping voltage, and a composite video signal output with a white level set to a constant level is taken out from the emitter side of the fourth transistor. The white clip circuit is characterized by including the clip circuit described above.