JPH02250408A - Clip circuit - Google Patents

Abstract

PURPOSE:To improve the linearity and high frequency characteristic near a clip level by providing a switch circuit sending only a part over a prescribed level in an input signal and a differential circuit subtracting the transmission output of the changeover circuit from the input signal, and obtaining the clipped output from the differential output of the differential circuit. CONSTITUTION:A clip circuit consists of a changeover circuit 21 sending only a part of a prescribed level or over in an input signal Vin and outputting it, a differential circuit 22 subtracting an output Vin' of the changeover circuit 21 from the input signal Vin, and a feedback circuit 23 quickening the switching operation of the changeover circuit 21. That is a signal part over a prescribed level or over is extracted from the input signal by the changeover circuit 21 operated at the level and the input signal is subtracted by the extracted part signal to obtain the signal whose prescribed level is clipped. Thus, the linearity and the high frequency characteristic near the clip level are improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a technique that is effective when applied to a clip circuit, and furthermore, to a modulation input circuit of an RF modulator. It is about effective techniques.

[Conventional technology] FIG. 4 shows an example of the configuration of a conventional clip circuit.

In the figure, 1 is a color video signal source, 2 is a white clip circuit configured together with a clamp circuit, and 3 is an R
F modulator, 4 is RF multiplied signal high frequency carrier signal)
It is an oscillator.

The white level clipping circuit 2 includes npn type bipolar transistors Ql, Q2°Q5. Q6 and pnp bipolar Q3. It is configured together with a voltage clamp circuit using Q4 and the like.

After the input signal Vin from the signal source 1 has its lower limit level set to a constant clamp voltage VBI by Ql,
@PnP which is input to the base of pnp bipolar transistor Q3 via the emitter follower circuit by Q2
The bipolar transistor Q3 constitutes an emitter-coupled bipolar differential circuit together with the same npn bipolar transistor Q4. A voltage VB2 that determines the clip level is input to the base of Q4 via Q6 and Q5. As a result, the upper limit level of the input signal Vin is adjusted to a constant clip level VB2. In this way, the predetermined clip level VB2 and clamp voltage V
The signal provided with BI is given to the RF modulator 3 as a modulation input signal. The RF modulator 3 modulates the carrier provided from the high frequency carrier signal oscillator 4 with the modulation input signal, thereby generating R
Outputs the F video signal Rout.

Regarding the above-mentioned white clip circuit, for example, see "Semiconductor Circuit Manual JP" published by Ohmsha Co., Ltd.
213-215, published November 20, 1975, etc.

[Problems to be Solved by the Invention] However, the inventors have found that the above-mentioned technique has the following problems.

In other words, this type of conventional clipping circuit has a configuration that performs clipping operation by directly utilizing the element characteristics of the transistors that make up the bipolar differential circuit.
The effect of the nonlinearity of the transistor's element characteristics directly appears on the clipping characteristics, as shown in Figure 5 (A).
As shown by the solid line in , there is a problem in that the linearity deteriorates near the clip level. When a video signal having such a rip characteristic is modulated into an RF video signal, a problem arises in that the differential gain of the video signal decreases and the resolution of the image deteriorates.

In addition, a pnp type bipolar transistor Q5. which has poor high frequency characteristics. Since it was constructed using Q6, there was a problem in that the clipping operation could not be performed reliably in the high frequency region, as shown in FIG. 5(B), for example. When a video signal having such a lip characteristic is modulated (negatively modulated) into an RF video signal, so-called overmodulation occurs partially, resulting in deterioration of sound quality due to discontinuation of the audio carrier.

An object of the present invention is to improve linearity and high frequency characteristics near the clip level in, for example, a white clip circuit for video signals. Our goal is to provide this technology.

The above and other objects and novel features of the present invention will become clear from the description of this specification and the accompanying drawings.

[Means for Solving the Problems] Representative inventions disclosed in this application will be summarized as follows.

That is, by extracting a signal portion of a predetermined level or higher from an input signal using a switching circuit that operates based on the level, and subtracting the input signal by this extracted partial signal, a level-clipped signal of a predetermined level or lower is obtained. That is what it is.

[Operation] According to the above-mentioned means, the linearity between input and output, especially the linearity near the clip level, does not depend on the linearity of the element, and the restrictions on the type of element to be used are relaxed, for example. The clipping operation can be performed using an npn type bipolar transistor which is advantageous for high frequency operation.

This achieves the objective of improving linearity and high frequency characteristics near the clip level in, for example, a white clip circuit for video signals.

[Example] The present invention will be described below with reference to preferred embodiments.

In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

FIG. 1 shows an embodiment of a clip circuit according to the present invention, in which 1 is a video signal source, 2 is a clip circuit configured together with the clip circuit, 3 is an RF modulator, 4 is a high frequency carrier oscillator, and Vcc is the power supply potential.

The main part of the clip circuit 2 is a switching circuit 21 that transmits and outputs only the portion of the input signal Vin that is at a predetermined level or higher.
and the output V of the switching circuit 21 from the input signal Vin.
It is composed of a differential circuit 22 for subtracting in', a feedback circuit 23 for speeding up the switching operation of the switching circuit 21, and the like.

The switching circuit 21 includes an npn type bipolar transistor Q7.
It is constituted by a differential circuit of Q8, and an input signal Vin level-shifted by a resistor R1 is applied to one differential input (Q7 side) of this differential circuit.

A predetermined DC level VB is applied to the other differential input (Q8 side).
2 is given. As shown in (A) and (B) of FIG. 2, the level of the input signal Vin is set to the above DC level V.
The input signal Vin is transmitted and output only in the section exceeding B2. Vin' indicates its transmission output.

The differential circuit 22 includes npn bipolar transistors Q31 . Constructed using Q32,
From the input signal Vin to the output Vin of the switching circuit 21
'' is subtracted. Through this subtraction operation, the input signal Vi
As for n, the waveform of the portion whose level exceeds the above-mentioned DC level VB2 is canceled out. As a result, the input signal Vin becomes
As shown in FIG. 2(C), the waveform is shaped into substantially the same waveform as that level-clipped at the DC level VB2. The signal whose level has been clipped in this way becomes the modulation input signal of the RF modulator 3.

The feedback circuit 23 is an npn bipolar transistor Q9.
．． It is constituted by a differential circuit using QIO, and lowers the other differential input (Q8 side) of the switching circuit 21 to the low level (reference potential) side when the input signal Vin exceeds the DC level VB2. . Thereby, a type of positive feedback is applied to the switching operation of the switching circuit 21, and the switching operation of the switching circuit 21 when the input signal Vin reaches the DC level VB2 can be further accelerated.

In the above manner, the input signal Vin is set to the predetermined level VB.
The signal clipped in step 2 can be given to the RF modulator 3 as a modulation input signal.

Here, in the above-mentioned clipping circuit, the clipping operation is performed by a subtraction operation of the differential circuit, thereby eliminating dependence on element characteristics. In addition, since the structure performs clipping operation by subtraction operation, restrictions on the polarity and type of elements used are relaxed, and it is now possible to construct the device using only npn-type bipolar transistors, which are advantageous for high-speed operation. There is.

This makes it possible to improve the linearity and high frequency characteristics near the clip level in, for example, a white clip circuit for video signals, as shown in FIGS. 3(A) and 3(B).

Although the invention made by the present inventor has been specifically explained above based on Examples, it goes without saying that the present invention is not limited to the above Examples and can be modified in various ways without departing from the gist thereof. Nor.

For example, the elements used may be MOS or MOS transistors.

In the above explanation, the invention made by the present inventor was mainly applied to an RF modulator for video signals, which is the background field of application, but the invention is not limited thereto. It can also be applied to clip circuits.

[Effects of the Invention] The effects obtained by typical inventions disclosed in this application are briefly explained below.

That is, for example, in a white clip circuit for a video signal, the linearity and high frequency characteristics near the clip level can be improved.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a clip circuit according to an embodiment of the present invention. The second @ is a diagram explaining and showing the operation of the main part of the above circuit. It is a diagram. 1...Input signal source. 2...Clip circuit, 3...RF modulator, 4...Carrier oscillator. Diagram

Claims (1)

[Claims] 1. A switching circuit that transmits only a portion of an input signal that is at a predetermined level or higher, and a differential circuit that subtracts the transmission output of the switching circuit from the input signal; A clip circuit characterized in that a level clipped output is obtained from the differential output of the circuit. 2. The above switching circuit is constituted by a differential circuit of npn bipolar transistors, and an input signal is applied to one differential input of this differential circuit, and a predetermined DC level is applied to the other differential input. A clip circuit according to claim 1, characterized in that: 3. The switching circuit is constituted by a differential circuit of npn bipolar transistors, an input signal is applied to one differential input of this differential circuit, and a predetermined DC level is applied to the other differential input, A clip circuit comprising a feedback circuit that pulls down the other differential input of the switching circuit to a low level when the input signal exceeds a predetermined level.