JP2929293B2 - Clip circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a technology effective when applied to a clipping circuit, and further to a modulation input circuit of an RF modulator, and is used, for example, for a white clipping circuit of a video signal. And effective technology.

[Prior Art] FIG. 4 shows a configuration example 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
The RF modulator 4 is an RF signal (high-frequency carrier signal) oscillator.

The white level clip circuit 2 is configured together with a voltage clamp circuit using npn-type bipolar transistor transistors Q1, Q2, Q5, Q6 and pnp bipolar transistors Q3, Q4.

After the lower limit level of the input signal Vin from the signal source 1 is suppressed to a fixed clamp voltage VB1 by Q1, the input signal Vin is input to the base of the pnp bipolar transistor Q3 via the emitter follower circuit by Q2. The pnp bipolar transistor Q3 forms an emitter-coupled bipolar differential circuit together with the same npn-type bipolar transistor Q4. The base of Q4 has a voltage VB2 that determines the clip level.
Is input via Q6 and Q5. Thus, the upper limit level of the input signal Vin is adjusted to the fixed clip level VB2. Thus, the predetermined clip level VB2
And a signal having the clamp voltage VB1 is supplied to the RF modulator 3 as a modulation input signal.
The RF modulator 3 outputs an RF video signal Rout by modulating the carrier supplied from the high-frequency carrier signal oscillator 4 with the modulation input signal.

For the white clip circuit described above, see, for example, "Semiconductor Circuit Manual"
13-215, published on November 20, 1975.

[Problems to be Solved by the Invention] However, the present inventors have clarified that the above-described technology has the following problems.

In other words, the conventional clip circuit of this type has a configuration in which the clipping operation is performed by directly using the element characteristics of the transistors constituting the bipolar differential circuit, so that the effect of the non-linearity of the element characteristics of the transistors is clipped. Appears directly in the characteristics. As a result, as shown by the solid line in FIG. 5A, there is a problem that the linearity is deteriorated near the clip level. When a video signal having such a clip characteristic is modulated into an RF video signal, there arises a problem that the differential gain of the video signal is reduced and the resolution of an image is reduced.

In addition, since the pnp type bipolar transistors Q5 and Q6 having inferior high frequency characteristics are used, the clipping operation cannot be reliably performed in a high frequency region as shown in FIG. 5B, for example. There was a problem. When a video signal having such a clip characteristic is modulated (negatively modulated) into an RF video signal, so-called overmodulation is partially generated, and sound quality is degraded due to interruption of an audio carrier.

An object of the present invention is to provide a technique for improving linearity and high-frequency characteristics near a clip level in, for example, a white clip circuit of a video signal.

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

[Means for Solving the Problems] The outline of a typical invention among the inventions disclosed in the present application is as follows.

That is, by a switching circuit that operates at a level, a signal portion of a predetermined level or more is extracted from an input signal, and the input signal is subtracted by the extracted partial signal to obtain a level-clipped signal of a predetermined level or less. That is.

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

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

[Example] Hereinafter, a description will be given with reference to a preferred example of the present invention.

In the drawings, the same reference numerals indicate the same or corresponding parts.

FIG. 1 shows an embodiment of a clipping circuit according to the present invention, wherein 1 is a video signal source, 2 is a clipping circuit configured with a clipping circuit, 3 is an RF modulator, 4 is a high-frequency carrier oscillator, Vcc Is a power supply potential.

The main part of the clip circuit 2 is a switching circuit 21 for transmitting and outputting only a portion of the input signal Vin that is higher than a predetermined level.
And a differential circuit 22 for subtracting the output Vni 'of the switching circuit 21 from the input signal Vin, a feedback circuit 23 for speeding up the switching operation of the switching circuit 21, and the like.

The switching circuit 21 is configured by a differential circuit of npn-type bipolar transistors Q7 and Q8, and an input signal Vin whose level is shifted by a resistor R1 to one differential input (Q7 side) of the differential circuit.
And a predetermined DC level VB2 is applied to the other differential input (Q8 side). Then, as shown in (A) and (B) of FIG. 2, the input signal Vin is transmitted and output only during a section where the level of the input signal Vin exceeds the DC level VB2. Vin 'indicates the transmission output.

The differential circuit 22 is configured using npn-type bipolar transistors Q31 and Q32 in the RF modulator 3, and performs an operation of subtracting the output Vin 'of the switching circuit 21 from the input signal Vin.
By this subtraction operation, the waveform of the portion of the input signal Vin whose level exceeds the DC level VB2 is canceled. As a result, the input signal Vin is shaped into a waveform substantially the same as the level clipped at the DC level VB2, as shown in FIG. 2C. The signal thus level-clipped becomes the modulation input signal of the RF modulator 3.

The feedback circuit 23 includes npn-type bipolar transistors Q9 and Q10.
And the input signal Vin
Is higher than the DC level VB2.
The other differential input of 1 (Q8 side) is pulled down to the low level (reference potential) side. As a result, a kind 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.

As described above, a signal obtained by clipping the input signal Vin at the predetermined level VB2 can be provided to the RF modulator 3 as a modulation input signal.

Here, in the above-described clip circuit, the dependence on the element characteristics is eliminated by causing the clip operation to be performed by the subtraction operation of the differential circuit. In addition, since the clip operation is performed by the subtraction operation, restrictions on the polarity and type of the element to be used are relaxed, and it is possible to configure only the npn-type bipolar transistor which is advantageous for high-speed operation. I have.

Thus, for example, in a white clip circuit of a video signal, as shown in FIGS. 3A and 3B, it is possible to improve the linearity and the high frequency characteristics near the clip level.

The invention made by the inventor has been specifically described based on the embodiments. The present invention is not limited to the above-described embodiments, and it is needless to say that various changes can be made without departing from the gist of the invention. Absent.

For example, the element used may be a MOS or a MOS transistor.

In the above description, the case where the invention made by the present inventor is mainly applied to the RF modulator of a video signal, which is the application field in the background, has been described. Can also be applied to the clipping circuit.

[Effects of the Invention] The effects obtained by typical ones of the inventions disclosed in the present application will be briefly described as follows.

That is, for example, in a white clip circuit of a video signal, it is possible to obtain an effect that linearity and high-frequency characteristics near a clip level can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a clip circuit according to an embodiment of the present invention, FIG. 2 is a waveform diagram for explaining the operation of the main part of the circuit, and FIGS. FIG. 4 is a diagram showing the operation characteristics obtained, FIG. 4 is a diagram showing the configuration of a conventional clip circuit, and FIGS. 5A and 5B are diagrams showing the operation characteristics of the conventional clip circuit. 1 ... Input signal source, 2 ... Clip circuit, 3 ... RF modulator, 4 ... Carrier oscillator.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Satoshi Tanaka 5-22-1, Josuihoncho, Kodaira-shi, Tokyo Within Hitachi Microcomputer Engineering Co., Ltd. No. 111 Inside the Takasaki Plant of Hitachi, Ltd. (56) References JP-A-51-130117 (JP, A) JP-A-59-97213 (JP, A) JP-A-54-181850 (JP, A) 61-125220 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H03G 11/00 H04N 5/40

Claims (1)

(57) [Claims] 1. A switching circuit for transmitting a portion of an input signal having a level equal to or higher than a predetermined level, and a subtraction circuit for subtracting a transmission output of the switching circuit from the input signal. A first differential circuit including a first npn bipolar differential pair transistor in which the input signal is supplied to a dynamic input and the predetermined level is supplied to the other differential input. A second differential circuit including a second npn bipolar differential pair transistor having the differential input receiving the input signal and the other differential input receiving the transmission output of the switching circuit; The input signal is supplied to a differential input, the predetermined level is supplied to the other differential input, and when the input signal becomes equal to or higher than the predetermined level, the first differential circuit constituting the switching circuit Other than the above A feedback circuit comprising a third differential circuit including a third npn bipolar differential pair transistor that acts to reduce the potential applied to the other differential input to a low level side, wherein the feedback circuit comprises: When the input signal becomes equal to or higher than the predetermined level, the switching circuit operates to further speed up the switching operation of the first differential circuit including the first npn bipolar differential pair transistor constituting the switching circuit, A clip circuit which obtains a level clip output having improved linearity and frequency characteristics near a clip level by an output of the second differential circuit of the subtraction circuit.