JPH0477063A - Clip circuit - Google Patents

Abstract

PURPOSE:To make a clip characteristic sharp and to eliminate the need for a variable gain amplifier by devising such a circuit that a transistor(TR) is turned on when a level of an input signal to an input terminal is larger than a voltage of a variable reference voltage source and the TR is turned off when smaller. CONSTITUTION:When a level of an input signal Vin reaches a clip voltage Vref or over, a TR Q3 is turned off and a TR Q4 is turned on. Thus, a current flows to a resistor R3, a voltage drop is caused and an L level is fed to a base of the TR Q2 via a TR Q5 to turn on the TR Q2, then the increase in the potential at a node N is limited. Thus, the potential of the node N is not increased over the clip voltage Vref. The node N, that is, the emitter of the TR Q2 reaches a low impedance at that point of time and the input signal Vin over the clip voltage Vref is divided by resistors R1, R2 and the result is outputted via a buffer amplifier 10. Thus, a gain G' after clipping is decreased more than the gain G before clipping.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a clip circuit such as a white clip circuit that suppresses the level of a video signal to a constant voltage.

[Conventional technology]

The peak voltage of the video signal is held down to a certain voltage,
A white clip circuit is used to prevent the white level of the screen from becoming too high (to prevent the white from becoming saturated on the screen and causing color blurring). FIG. 4 is a circuit diagram showing a conventional white clip circuit. In the figure,
1 is the input terminal where the video signal is input, 2 is the output terminal 3
This is a buffer amplifier connected between the input terminal 1 and the input terminal 1.

Ql is a PNP transistor whose emitter is connected to the output of the buffer amplifier 2, whose collector is connected to GNDY5, and whose base is connected to the variable clip voltage source 5 via the clip voltage adjustment terminal 4. FIG. 5 shows the input signal v1n and output signal V of the white clip circuit shown in FIG. , 1 is a graph showing the relationship between .

Next, the operation of the circuit shown in FIG. 4 will be explained using FIG. 5. The level of the input signal vIn is ■re+V (
V is the voltage of the variable clip voltage source 5, r anal
rer vBE is the base-emitter voltage of transistor Q1)
In the following case, the transistor Q1 is off, and therefore, as the input signal Ir+ increases, the output signal V is linearly output as shown in FIG. On the other hand, the level of the input signal V, is V+v+n
When the voltage exceeds re'BE, the transistor Q1 is turned on, and the rise in the emitter potential of the transistor Q1 is suppressed.

Therefore, the input signal rer whose level is higher than V + V
BE ■ is clicked on v + v as shown in Figure 5.
rt4 FEE is blocked.

The clipping voltage level can be adjusted by adjusting the variable clipping voltage source 5.

FIG. 6 is a circuit diagram showing another conventional white clip circuit. In the figure, 6 is a differential amplifier, and the input signals v and n from the input terminal 1 are directly input to the 10 inputs.

Reference numeral 7 denotes a comparator, the input signal v1n from the input terminal 1 is directly input to the input, and the - input is connected to the variable clip voltage source 5 via the clip voltage adjustment terminal 4.

The output of the comparator 7 is given to a variable gain amplifier 8. The variable gain amplifier 8 is connected to a gain adjustment volume 10 via a gain adjustment terminal 9. By adjusting the gain adjustment volume 10, the gain of the variable gain amplifier 8 is changed. The output of the variable gain amplifier 8 is applied to one input of the differential amplifier 6, and the differential amplifier 6 amplifies the difference between the ten inputs and the one input and outputs it from the output terminal 3.

FIG. 7 shows the relationship between the input signal V and the output signal V of the white clip circuit shown in FIG.
This is a graph.

Next, the operation of the circuit shown in FIG. 6 will be explained using FIG. 7. When the level of the input signal vIn is less than or equal to the clip voltage V of the variable clip voltage source 5, the comparator 7 outputs "0", and this "0" is sent to the input of the differential amplifier 6 via the variable gain amplifier 8. The differential amplifier 6 receives an input signal v which is sent to ten inputs.
Io is multiplied by a predetermined amplification factor and output as an output signal ■. This amplification factor ut corresponds to the slope rer of the straight line below the voltage V in the graph of FIG.

On the other hand, the level of the input signal V1o is a clip voltage V. 1
In the above case, the comparator 7 outputs only the component of the input signal V1o that exceeds the clip voltage V2. Only the component exceeding this clip voltage V is multiplied by A (the gain of the variable gain amplifier 8) by the variable gain amplifier 8, and is applied to one input of the differential amplifier 6, so that the input signal vIn applied to the ten inputs is The amplification factor of the differential amplifier 6 decreases.

Therefore, as shown in FIG. 7, the output voltage V ouL The slope becomes gentler than the slope below the voltage V.

By adjusting the variable clip voltage source 5, the clip voltage V can be adjusted, and by adjusting the gain adjustment volume 10, the output voltage after clipping Vout can be adjusted.
The gain can be adjusted.

[Problem to be solved by the invention]

The conventional white clipping circuit is constructed as described above, and in the white clipping circuit shown in FIG. 4, it is necessary to set the reference potential V to a level 1 vBE lower than the desired clipping level, which is inconvenient. Further, since it is a diode clip circuit, there is a problem that the clip characteristics are not uniform as shown in FIG.

In addition, in the white clip circuit shown in Figure 6, the clip level and the reference potential V are the same, and the gain after clipping is also variable, and the gain adjustment after clipping is also variable. Each can be adjusted independently, but a variable gain amplifier is required.
The number of circuit elements increases.

In addition, clip voltage adjustment terminal 4 and gain adjustment terminal 2
Two adjustment terminals are provided. Generally, once the gain after clipping is set, there is almost no need for VJ adjustment, and it can be fixed internally in actual use. Considering this, there is no need to take the trouble to provide the gain and current adjustment terminal 9.

This invention was made to solve the above-mentioned problems, and it sharpens the clip characteristics and fixes the gain for inputs above the clip level to a desired chain.
It is an object of the present invention to obtain a clip circuit which can change the clip level and has a small number of circuit elements.

[Means to solve the problem]

A clip circuit according to the present invention includes an input terminal, an output terminal, a variable reference voltage source, a differential amplifier whose one input is connected to the variable reference voltage source, and a connection between the input terminal and the output terminal. A first resistor connected, a second resistor connected between the other input of the differential amplifier and the output terminal, one electrode connected to the other input of the differential amplifier, and the other electrode connected to a predetermined potential. control electrodes are respectively connected to the outputs of the differential amplifiers, and the transistors are turned on when the level of the input signal to the input terminal is larger than the voltage of the variable reference voltage source and turned off when the level is smaller than the voltage of the variable reference voltage source. .

[Effect]

The transistor in this invention turns on when the level of the input signal to the input terminal is higher than the voltage of the variable reference voltage source, and turns off when the level is lower than the voltage of the variable reference voltage source. When the transistor is turned off, the input signal is output as is, and when it is turned on, the input signal is output from the first . It is output according to the division ratio of the second resistor. Also, by increasing the gain of the differential amplifier, the clipping characteristics become sharper. Furthermore, a variable gain amplifier is not required.

〔Example〕

FIG. 1 is a circuit diagram showing an embodiment of a clip circuit according to the present invention. Input terminal 1 is connected to one input of differential amplifier 100 via a buffer amplifier 2 and a series circuit of resistors R1 and R2. The ten inputs of the differential amplifier 100 are connected to a variable clip voltage source 5 via a clip voltage adjustment terminal 4. The output of the differential amplifier 100 is PNP)
Connected to the base of transistor Q2. The emitter of the transistor Q2 is connected to one input of the differential amplifier 100, and the collector is connected to the GND potential. Resistance R1, R
The common connection point of the two is connected to the output terminal 3 via the buffer amplifier 10.

FIG. 2 is a circuit diagram showing an example of the configuration of the differential amplifier 100. The differential amplifier 100 includes an NPNI transistor Q3.
Q4. Q5, resistor R3, and constant current source 11.12. Transistor Q3. The emitters of Q4 are connected in common,
This common connection point is grounded via a constant current source 11. The base of transistor Q3 is clip voltage adjustment terminal 4
The collectors are respectively connected to the power supply voltage ccC.

The base of the transistor Q4 is connected to the output of the buffer amplifier 2 through resistors R1 and R2, and the collector is connected to the power supply voltage V through a resistor R3. 0, respectively.

The transistor Q5 constitutes an emitter follower, and its base is connected to the collector of the transistor Q4, the collector is connected to the power supply voltage ■cc, and its emitter is grounded via the constant current source 12 and also connected to the base of the transistor Q2. has been done.

The rest of the configuration is similar to the circuit shown in FIG. Figure 3 shows the input signal Io and output signal V of the circuit shown in Figure 2.
It is a graph showing the relationship between

ut Next, the operation of the circuit shown in FIG. 2 will be explained using FIG. 3. Input signal V is applied to the base of transistor Q4, which is one input of differential amplifier 100, via buffer amplifier 2 and resistors R1 and R2. When the level of the input signal ■1□ is below the clip voltage V determined by the variable clip voltage source 5, at transistor Q3 is turned on, transistor Q4 is turned off,
Power supply voltage cc ("H" level) is applied to the base of transistor Q2 via transistor Q5 forming an emitter follower. Since the transistor Q2 turns off in response to mH, the node N is kept in a high impedance state, and the input signal V is passed through the buffer amplifier 2.10.
Buffer n is amplified by the gain of the amplifier 2.10 times G (A2xA, o (A2゜Alo is the gain of the buffer 7 amplifier 2.10)) and outputted to the output terminal 3 as an output signal V. In this example, G-A2-Al.

-1.

On the other hand, when the level of the input signal vIn (that is, the level of the node N) exceeds the clip voltage V, the ef transistor Q3 is turned off and the transistor Q4 is turned on. Therefore, a current flows through the resistor R3, causing a voltage drop and “L”
" is applied to the base of the transistor Q2 via the transistor Q5 that constitutes an emitter follower. Since the transistor Q2 is turned on in response to "L", the rise in the potential of the node N is suppressed. Therefore, the potential of the node N is The voltage does not rise above the clip voltage V. At this point, the node er N, that is, the emitter of the transistor Q2, is in a low impedance state, and the input signal VIn, which is above the clip voltage V, is multiplied by the division ratio of the resistors R1 and R2 (R1/
(R1, +R2) times) and output via the buffer amplifier 10. Therefore, the gain G' after clipping is Gx
R1/(R1+R2), and the gain G after clipping is
' is smaller than the gain G before clipping.

The gain of the differential amplifier 100 is determined by R3/2RE (RE is the emitter resistance of transistors Q3 and Q4), and the larger this value, the higher the gain, and the higher the gain, the higher the level of the input signal vIn becomes the clip voltage. The moment it reaches this point, the clip operation begins. In the configuration of differential amplifier 100 shown in FIG. 2, transistors Q3. The emitters of Q4 are commonly connected (
RE"O), the gain of the differential amplifier 100 is very large. Therefore, the clipping characteristics are sharper than in the case of the diode clipping circuit shown in FIG. 4. Also, the clipping voltage is variable. It can be changed by adjusting the clip voltage source 5. Also,
Since the gain after clipping is determined to a desired value by the division ratio of the resistors R1 and R2, there is no need to provide a gain adjustment terminal as in the conventional case. Furthermore, since a variable gain amplifier is not provided, there are fewer circuit elements.

In the above embodiment, the clip voltage (2) is applied to the ten inputs of the differential amplifier (OO), and the input signal V is applied to the - input.

rcr+n is given, but this may be reversed. In this case PNP)
It is necessary to make transistor Q2 an NPN transistor.

〔Effect of the invention〕

As described above, according to the present invention, a variable reference voltage source, a differential amplifier whose one input is connected to the variable reference voltage source, a first resistor connected between the input terminal and the output terminal, A second terminal connected between the other input and the output terminal of the differential amplifier.
resistor, one fr5 pole is connected to the other input of the differential amplifier, the other electrode is connected to a predetermined potential, and the control electrode is connected to the output of the differential amplifier, and the level of the input signal to the input terminal is the voltage of the variable reference voltage. Since a transistor is provided that turns on when the voltage is larger than , and turns off when the voltage is smaller than , the clip voltage can be changed by adjusting the variable reference voltage source. Furthermore, when the transistor is turned off, the input signal is output as is, and when the transistor is turned on, the input signal is output from the first input signal without providing a gain adjustment terminal as in the conventional case. It is output according to the division ratio of the second resistor.

Therefore, the first. By setting the resistance value of the second resistor to Y·m, the gain after clipping can be changed to a desired value, which has the effect of reducing the number of terminals. Furthermore, by increasing the gain of the differential amplifier, the clipping characteristics become sharper, and since a variable gain amplifier is not used, the number of elements can be reduced.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the clip circuit according to the present invention, FIG. 2 is a circuit diagram showing an example of the configuration of a differential amplifier in the circuit shown in FIG. Graphs to explain the operation of the circuit shown in the figure, Figures 4 and m6 are circuit diagrams showing conventional white clip circuits, and Figure 5 is a graph to explain the operation of the circuit shown in Figure 4. , FIG. 7 is a graph for explaining the operation of the circuit shown in FIG. In the figure, 1 is an input terminal, 3 is an output terminal, 5 is a variable clip voltage source, 100 is a differential amplifier, R1 and R2 are resistors, and Q2 is a PNP transistor. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] (1) an input terminal, an output terminal, a variable reference voltage source, a differential amplifier whose one input is connected to the variable reference voltage source, and a first differential amplifier connected between the input terminal and the output terminal. a second resistor connected between the other input of the differential amplifier and the output terminal, one electrode of which is connected to the other input of the differential amplifier, the other electrode of which is at a predetermined potential, and a control electrode that is connected between the other input of the differential amplifier and the output terminal; A clip circuit comprising transistors connected to the outputs of the differential amplifiers and turned on when the level of the input signal to the input terminal is higher than the voltage of the variable reference voltage source and turned off when the level is lower than the voltage of the variable reference voltage source.