JPH04236578A - Clamp circuit for video signal - Google Patents

Abstract

PURPOSE:To realize the clamp circuit at a low cost in which fluctuation of a pedestal level in a video signal is less. CONSTITUTION:A transistor(TR) Q3 amplifying a video signal is inserted between a clamp circuit C2 and a power supply Vcc supplying a charging current to the capacitor C2. Since the above TR Q3 is on the way of operation from an off-direction to an on-direction at a clamp point of the video signal, the impedance is equivalently low. Thus, a voltage drop due to a current charging the above capacitor C2 is less at the clamp point thereby suppressing fluctuation in the pedestal level.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal clamp circuit, and more particularly to a peak clamp circuit using a diode.

0002

2. Description of the Related Art An example of a conventional video signal clamp circuit of this type is shown in FIG. That is, in FIG. 2, Q1 represents an NPN type transistor to which a video signal is applied to its base electrode and constitutes a video amplifier that amplifies the input video signal. The collector electrode of the transistor Q1 is
Power supply +Vc via coil L1 and collector resistor R1
c, and between the emitter electrode and the reference potential point there is an emitter resistor R2 and this emitter resistor R2.
A series circuit of a resistor R3 connected in parallel to the voltage feedback capacitor C1 is connected to the voltage feedback capacitor C1.

A diode D1 is connected in the opposite direction between the collector electrode of the transistor Q1 and the power supply +Vcc, and a diode D2 is connected in the opposite direction between the collector electrode of the transistor Q1 and the reference potential point.

Further, one end of a capacitor C2 for level clamping the blanking signal is connected to the collector electrode of the transistor Q1, and the other end is connected to a coil L2.
and is connected to the cathode of a cathode ray tube (CRT) via a resistor R4. Note that a spark gap element S1 is connected between the cathode of the CRT and a reference potential point.

On the other hand, a resistor R5 is connected between the other end of the clamp capacitor C2 and the power supply +Vcc, and furthermore, a resistor R5 is connected to the other end of the clamp capacitor C2.
The anode terminal of a switching diode D3 that is turned on when the blanking signal arrives is connected. The cathode terminal of the switching diode D3 is connected to the emitter terminal of a PNP transistor Q2 whose collector electrode is connected to a reference potential point.

The base electrode of the transistor Q2 has one end connected to a power supply +Vcc and the other end connected to a sliding terminal of a potentiometer RV1 connected to a reference potential point. Further, an emitter resistor R6 is connected between the emitter electrode of the transistor Q2 and the power supply +Vcc, and a capacitor C6 is connected between the emitter electrode and the reference potential point.
3 is connected.

In the above configuration, when a positive video signal Vi is applied to the base electrode of the transistor Q1 constituting the video amplifier, this video signal is inverted and amplified by the transistor Q1 and output to the collector terminal. The video signal output to the collector terminal of the transistor Q1 is applied to the cathode electrode of the CRT via a clamping capacitor C2, and further via a coil L2 and a resistor R4.

Here, the transistor Q2 constitutes a constant voltage circuit, and the diode D3 is turned on at a blanking portion (sync chip) of the video signal Vo output to the transistor Q1 constituting the video amplifier. At this time, a rapid current flows from the power supply +Vcc to the reference potential point via the resistor R1, coil L1, capacitor C2, diode D3, transistor Q2, and capacitor C3. Therefore, capacitor C2 is charged, and the terminal voltage of capacitor C2, that is, the collector terminal of transistor Q1, is clamped to the charging voltage at that time.

[0009]

[Problems to be Solved by the Invention] According to the conventional video signal clamp circuit described above, a sudden current flows from the power supply +Vcc through the resistor R1 at the clamp point when charging the clamp capacitor C2. , a voltage drop occurs across the resistor R1 during this period. Moreover, since the charging current of the clamping capacitor C2 increases or decreases depending on the swing width of the video signal, the potential of the clamping point changes to A1 as shown in FIG.
to A2, and when the contrast of the image is large (when the swing width of the video signal is large), a phenomenon occurs in which the pedestal level P sinks, resulting in the inconvenience that so-called black level sinking occurs in the reproduced image. Occur.

[0010] In order to avoid such inconveniences, it is conceivable to insert a push-pull emitter follower circuit into the charging path of the clamp capacitor for improvement, but this would increase the number of components in the circuit and Cost increases are inevitable.

The present invention has been made to eliminate the above-mentioned disadvantages, and can prevent the voltage drop phenomenon at the clamp point when charging the clamp capacitor without increasing costs. The objective is to provide a video signal clamp circuit.

0012

[Means for Solving the Problems] A video signal clamp circuit according to the present invention, which has been made to achieve the above object, includes an active element that amplifies a video signal applied to a control input terminal, and a video signal that is amplified by the active element. A clamp capacitor that receives a video signal at one end, an output circuit to which the video signal to be output is supplied to the other end of the capacitor, and a switching diode inserted between the other end of the capacitor and a reference potential point. This clamp circuit is characterized in that an active element for amplifying a video signal is interposed between a capacitor and a power supply that supplies charging current to the capacitor.

[0013]

[Operation] In the video signal clamp circuit having the above configuration, an active element for amplifying the video signal is interposed between the clamping capacitor at the blanking signal level and the power supply that supplies charging current to the capacitor. Therefore, at the blanking point, the active element is in the process of operating in the on direction rather than the off direction, so its impedance becomes lower in terms of equalization. This makes it possible to reduce the voltage drop mentioned above at the clamp point.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained below based on embodiments shown in the drawings. FIG. 1 is a wiring diagram showing an example of a video signal clamp circuit according to the present invention. In this FIG. 1, symbols D1 to D3, C2, C3, R4 to R6, L2
, S1, and Q2 have the same configuration as in FIG. 2 described above, and therefore, the explanation thereof will be omitted.

Q3 is an active element (PNP type transistor) to which a video signal is applied to its control input terminal, that is, its base electrode.The video signal applied to the base electrode is amplified by the transistor Q3, and is applied to its collector electrode. It is applied to the connected level clamp capacitor C2.

Between the emitter terminal of the PNP transistor Q3 and the power supply +B, there is an emitter resistor R2 and a resistor R3 connected in parallel to the emitter resistor R2.
A series circuit of a capacitor C1 for voltage feedback and a voltage feedback capacitor C1 are connected. Further, the collector electrode of the transistor Q3 is connected to a negative power supply -Vc through a coil L1 and a collector resistor R1.
connected to c.

In the above configuration, when a positive video signal Vi is applied to the base electrode of the PNP transistor Q3 constituting the video amplifier, this video signal is inverted and amplified by the transistor Q3 and output to the collector terminal. . The video signal output to the collector terminal of the transistor Q3 is applied to the cathode electrode (output circuit) of the CRT via the clamping capacitor C2, and further via the coil L2 and resistor R4.

The above operation is substantially the same as that shown in FIG. 2 described above. In the blanking part (sink chip) of the video signal Vo output to the transistor Q3 constituting the video amplifier, a diode D is connected.
3 turns on. At this time, a current flows from the power supply +B to the reference potential point through the resistor R2, the transistor Q3, the capacitor C2, the diode D3, and the parallel circuit of the transistor Q2 and the capacitor C3.

Therefore, the capacitor C2 is charged, and the terminal voltage of the capacitor C2, that is, the collector terminal of the transistor Q3, is clamped to the charging voltage at that time.

Here, the transistor Q3 constituting the video amplifier is inserted into the charging path to the clamping capacitor C2, and is in the process of transitioning from the OFF direction to the ON direction at the clamp point of the video signal. Therefore, the impedance of this path is equalized lower,
Therefore, the phenomenon in which the clamp level is lowered due to the charging current to the capacitor C2 generated at the clamp point is reduced.

That is, as shown in FIG. 3, the phenomenon in which the potential of the clamp point changes from A1 to A2 is suppressed, and even when the contrast of the image is large (when the swing width of the video signal is large). Even if the pedestal level P sinks, the phenomenon that the pedestal level P sinks is less likely to occur.

[0022]

As described above, according to the video signal clamp circuit of the present invention, an active element for amplifying a video signal is connected between a clamping capacitor and a power supply that supplies charging current to the capacitor. Since, at the clamp point, the active element is in the process of operating in the on direction rather than the off direction, its impedance becomes lower in terms of equalization. This makes it possible to reduce the voltage drop that occurs at the clamp point.
Therefore, the inconvenience that so-called black level depression occurs in the reproduced image is suppressed.

Furthermore, according to the video signal clamp circuit of the present invention, the number of components does not increase compared to conventional circuits, and, for example, a push-pull emitter follower circuit can be inserted into the charging path of the clamp capacitor. It can be provided at a lower cost than other countermeasures.

[Brief explanation of the drawing]

FIG. 1 is a wiring diagram showing an embodiment of the video signal clamp circuit of the present invention.

[Figure 2] Connection diagram showing an example of a conventional clamp circuit

[Figure 3] Signal waveform diagram for explaining the clamp point and pedestal level of the video signal

[Explanation of symbols]

Q1, Q2, Q3 Transistor R1-R6 Resistor D1-D3 Diode C1-C3 Capacitor L1, L2 Coil RV1 Potentiometer +B
power supply

Claims (1)

[Claims] 1. An active element that amplifies a video signal applied to a control input terminal; a clamping capacitor that receives the video signal amplified by the active element at one end;
A clamp circuit comprising: an output circuit to which a video signal to be outputted to the other end of the capacitor is supplied; and a switching diode inserted between the other end of the capacitor and a reference potential point; 1. A video signal clamp circuit, characterized in that an active element for amplifying the current is interposed between the capacitor and a power source that supplies charging current to the capacitor.