JPH0678176A - Clipping circuit - Google Patents

Abstract

PURPOSE:To obtain a fine image suppressing the generation of black blur by feeding back an output signal from an output terminal to the input terminal of a clipping circuit using an emitter terminal as an output terminal. CONSTITUTION:Out of a pair of transistors(TRs) 1, 2, the base terminal of the TR 1 is connected to the collector terminal of a TR 8 whose emitter is connected to an input Vi through a resistor 14 and optional reference voltage is impressed from a voltage source to the base terminal of the TR 2. The emitter terminals of the TRs 1, 2 are connected to the base terminal of the TR 8 through a resistor 13 to feed back an output V0 to the input V1 of the TR 1 through the TR 8. In this constitution, output voltage close to the reference voltage is linearly changed from input voltage and a fine image suppressing the generation of black blur can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a clipping circuit for performing waveform conversion (clipping) of a video signal from a video camera or the like.

[0002]

2. Description of the Related Art Conventionally, as a clipping circuit for clipping a video signal from a video camera or the like, there is a clipping circuit using a transistor element as shown in FIG.
As shown, such a conventional clip circuit includes a first transistor having a base terminal as an input terminal for inputting a video signal and a second transistor having a base terminal connected to a reference voltage source, The collector terminals of the first transistor and the second transistor are both connected to a positive constant voltage source, the emitter terminals of these transistors are both connected to a negative constant voltage source, and the emitter terminal serves as an output terminal. It is a thing.

[0003]

However, since the conventional clip circuit as described above is composed of only a pair of transistors as described above, the output voltage near the reference voltage depends on the V _BE characteristics of the transistors. Does not follow the input voltage linearly as shown in FIG. 9, but has a waveform in which a nonlinear component is mixed in the output voltage with respect to the linear input voltage as shown in FIG. 10. Therefore, when clipping the image signal, In addition, there is a problem that the image corresponding to the low level region of the signal becomes blackish, and a solution has been desired.

The present invention has been made in order to solve the problem in the conventional clip circuit, and a good image without so-called black underexposure in which the output voltage near the reference voltage changes linearly with respect to the input voltage. It is an object of the present invention to provide a clip circuit capable of obtaining

[0005]

[Means for Solving the Problems] To achieve the above object,
A first invention of the present application includes a first transistor having a base terminal as an input terminal for inputting a video signal, and a second transistor having a base terminal connected to a reference voltage source, and the first invention. A clip circuit in which the collector terminals of the transistor and the second transistor are both connected to a positive constant voltage source, the emitter terminals of these transistors are both connected to a negative constant voltage source, and the emitter terminal is an output terminal, A feedback circuit for feeding back an output signal from the output end is provided at the input end.

A second invention of the present application is the same as the first invention, wherein the feedback circuit includes a subtractor, an amplifier provided at a stage subsequent to the subtractor, and an attenuator provided on a feedback path. The positive terminal of the subtractor is used as an input terminal of the video signal, the output terminal of the subtractor is connected to the base terminal of the first transistor through the amplifier, and the output signal from the emitter terminal is connected to the subtractor. It is characterized by being fed back to the negative terminal of.

[0007]

In the present invention, the output signals from the emitter terminals of the first and second transistors are fed back to the input terminal connected to the base terminal of the first transistor by the feedback circuit.

Further, in the second invention of the present application, as described above, the feedback circuit is composed of a subtractor, an amplifier provided at a stage subsequent to the subtractor, and an attenuator provided on the feedback path. The operation of the invention will be described with reference to the drawings. FIG. 5 is a block diagram showing the configuration of the second invention of the present application, where σ is the non-linear component in the conventional clip circuit. In FIG. 5, 3 is a subtractor, 4 is an amplifier, and 5 is an attenuator. Is.

As shown, in the present invention, the output signal v _O
Is partially fed back to the input side at the rate of the attenuation rate β. That is, the input signal v _i input from the positive terminal of the subtractor 3
Is amplified by the amplifier 4 after the feedback signal is subtracted in the subtractor 3 and input to the base terminal of the first transistor.

The output voltage v _O in the clipping circuit according to the present invention is expressed by the following equation, where v _i is the input voltage, A is the amplification factor of the amplifier 4, and β is the attenuation factor of the attenuator 5. It

V _O = {σ / (1 + A · β)} + {A · v _i / (1 + A · β)} (Equation 1)

Therefore, as is clear from the equation (1), in the clip circuit of the present invention, the nonlinear component σ which has been a problem in the conventional clip circuit is 1 / (1
+ A · β).

[0013]

Embodiments of the present invention will be described with reference to the drawings. Figure 1
Is a block diagram showing a configuration of an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a specific configuration of the embodiment.

As shown in FIG. 2, the clip circuit of this embodiment includes a pair of transistors 1 and 2 whose collector terminals are both connected to a positive constant voltage source, and the emitter terminals of these transistors are both connected to a resistor 7. Is connected to a negative constant voltage source via the, and the emitter terminal serves as an output terminal. An arbitrary reference voltage is applied by the voltage source 6 to the base end of one of the pair of transistors 2.

The base end of the other transistor 1 of the pair of transistors is connected to the collector terminal of a transistor 8 forming a feedback circuit via a resistor 14, and the collector terminal is connected to a positive constant voltage via a resistor 10. Connected to the source. The emitter terminal of the transistor 8 is an input terminal for inputting a video signal via the resistor 9 and is connected to a negative constant voltage source via the resistor 11.

Further, the base end of the transistor 8 is grounded via the resistor 12 and is connected to the emitter ends of the pair of transistors 1 and 2 via the resistor 13.

The operation of the clip circuit according to this embodiment having the above-mentioned structure will be described with reference to FIGS. 3 and 4. Figure 2
In the clip circuit of the present embodiment shown in FIG. 3, the circuit composed of the transistor 8 and the resistors 9 to 13 is shown in FIG.
The grounded base circuit shown in FIG. 4 and the grounded emitter circuit shown in FIG. 4 can be considered separately.

In FIG. 3 ((b) is an equivalent circuit of (a)), the collector voltage v _1a of the transistor 8 and the input voltage v _i are expressed by the following equations (hereinafter, resistor 7 and resistor 9).
The resistance values of Nos. 14 to 14 are R ₇ and R _{9 to} R ₁₄ , respectively. ).

V _1a = -R _{10 ic} (Equation 2)

[0020] _{v i = - (h 1b +} R 9) i e ... ( Equation 3)

Since i _c ≈i _e , the following equation is obtained from the equations 2 and 3.

V _1a / v _i = R ₁₀ / (h _1b + R ₉ ) ... (Formula 4)

Since R ₉ >> h _ib , the equation 4 becomes the following equation.

V _1a / v _i ≈R ₁₀ / R ₉ (Equation 5)

Therefore, the following equation is obtained.

V _1a ≈ (R ₁₀ / R ₉ ) v _i (Equation 6)

On the other hand, in the grounded-emitter circuit shown in FIG. 4, the following equation holds.

V _O ′ = {R ₁₂ / (R ₁₂ + R ₁₃ )} v _O (Equation 7)

[0029] _{v 1b '= {R 9 R} 11 / (R 9 + R 11)} i e ... ( Equation 8)

Since R ₉ << R ₁₁ and i _c ≈i _e , the equation 8 is as follows.

V _1b ′ ≈R _{9 ic} (Equation 9)

Further, since AC is v _O ′ = v _1b ′, the following equation holds.

V _1b ′ ≈R _{9 ic} = v _O ′ (Equation 10)

By the way, since v _1b = −R _{10 ic} ,
From this and Expression 10, the following expression is established.

V _1b / v _O ′ = −R ₁₀ / R ₉ (Equation 11)

Therefore, the collector voltage v _1b of the grounded-emitter circuit of FIG. 4 is given by the following equation by the equations 11 and 7.

V _1b = (− R ₁₀ / R ₉ ) v _O ′ = (− R ₁₀ / R ₉ ) · {R ₁₂ / (R ₁₂ + R ₁₃ )} v _O (Equation 12)

By calculating the collector voltage v ₁ of the transistor 8 of this clip circuit from v _1a and v _1b by the superposition theory, the following equation is obtained.

V ₁ = v _i (R ₁₀ / R ₉ ) −v _O (R ₁₀ / R ₉ ) {R ₁₂ / (R ₁₂ + R ₁₃ )} (Equation 13)

Further, the nonlinear components generated from the transistors 1, 2 sigma, and the output of the transistors 1 and 2 and v _O, because the v _O = v ₁ + σ, by transforming it, v ₁
= V _O −σ is obtained. Then, from this equation and the equation 13, the output voltage v _O of this circuit is as follows.

V _O = σ / [1+ (R ₁₀ / R ₉ ) {R ₁₂ / (R ₁₂ + R ₁₃ )}] + v _i (R ₁₀ / R ₉ ) / [1+ (R ₁₀ / R ₉ ) {R ₁₂ / (R ₁₂ + R ₁₃ )}] ... (Equation 14)

Further, R ₁₀ / R ₉ = A, R ₁₂ / (R ₁₂ +
If R ₁₃ ) = β is set, then Expression 14 becomes the following Expression, as in Expression 1 above.

V _O = {σ / (1 + A · β)} + {A · v _i / (1 + A · β)} ... (Equation 15)

Therefore, as is clear from the equation (15), the nonlinear component σ, which has been a problem in the conventional clip circuit in the clip circuit of this embodiment, is 1 /
It will be reduced to (1 + A · β).

Further, FIGS. 6 and 7 show the input / output characteristics of the clip circuit according to this embodiment. As is clear from the figures, the output of this circuit follows the input very well linearly. According to this clipping circuit, a good image without blackout can be obtained near the reference voltage.

[0046]

As described in detail above, according to the present invention,
Since the output voltage near the reference voltage changes linearly with respect to the input voltage, a good image without so-called black crushing can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a specific configuration of an embodiment of the present invention.

3A is a circuit diagram for explaining an embodiment of the present invention, and FIG. 3B is a diagram showing an equivalent circuit of the circuit shown in FIG.

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

FIG. 5 is a block diagram showing a configuration of a second invention of the present application.

FIG. 6 is a diagram showing input / output characteristics of a clip circuit according to an embodiment of the present invention.

FIG. 7 is a diagram showing input / output characteristics of the clip circuit according to the embodiment of the present invention.

FIG. 8 is a circuit diagram showing a configuration of a conventional clip circuit.

FIG. 9 is a diagram showing input / output characteristics of a conventional clip circuit.

FIG. 10 is a diagram showing input / output characteristics of a conventional clip circuit.

[Explanation of symbols]

 1, 2, 8 Transistors 3 Subtractors 4 Amplifiers 5 Attenuators 6 Voltage Sources 7, 9-14 Resistances In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (2)

[Claims] 1. A first transistor having a base terminal as an input terminal for inputting a video signal, and a second transistor having a base terminal connected to a reference voltage source. In the clip circuit, the collector terminals of the two transistors are both connected to a positive constant voltage source, the emitter terminals of these transistors are both connected to a negative constant voltage source, and the emitter terminals are output terminals. A clipping circuit, wherein a feedback circuit for feeding back an output signal from the output end is provided in the clip circuit. 2. The feedback circuit comprises a subtractor, an amplifier provided at a stage subsequent to the subtractor, and an attenuator provided on a feedback path, and the positive terminal of the subtractor is used as an input end of a video signal, The output terminal of the subtracter is connected to the first terminal via the amplifier.
2. The clip circuit according to claim 1, wherein the clip circuit is connected to the base terminal of the transistor and the output signal from the emitter terminal is fed back to the negative terminal of the subtractor.