JPH0775118A - Picture tube drive circuit - Google Patents

Abstract

PURPOSE:To prevent the degradation of high frequency responsiveness at each color output step and a saturated state caused by a large luminance signal by limiting a current while inserting a small resistor, which does not affect the circuit, and a Zener to the cascade joint part of the picture tube drive circuit. CONSTITUTION:This circuit forms the current limit circuit of a transistor 8 composed of a resistor 13 inserted between the collector of a cascade connected red drive transistor 5 and the emitter of the red transistor 8 and a Zener 14 connected between the base of the transistor 8 and the collector of the transistor 5. The collector current of the transistor 8 is operated less than a maximum current Ic and the Zener 14 is turned off. When the collector current of the transistor 5 is increased and the collector current of the transistor 8 exceeds the maximum current Ic, the Zener 14 is turned on and the collector current of the transistor 5 is branched by the Zener 14. Then, the collector current of the transistor 8 does not exceed the maximum current Ic, and no saturated area is generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to saturation prevention of a color output transistor for driving a cathode ray tube (cathode ray tube) cathode in a picture tube drive circuit.

[0002]

2. Description of the Related Art When a television receiver is used for a long period of time, thermionic emission from a cathode ray tube cathode deteriorates,
It is known that the beam current decreases with the same drive voltage.

On the other hand, in the receiver, the beam current is detected and the DC bias voltage and amplitude of the luminance signal are changed so as not to increase the beam current above a certain value.
oBrightness Limiter) circuit and
ACCL (Auto Contrast Color)
Limiter) circuit.

Here, in the above-mentioned receiver used for a long period of time, when the beam current decreases, a large amplitude luminance signal,
Even if the color difference signal is input, the ABL and ACCL circuits do not operate, and the transistor that drives the cathode ray tube cathode is saturated. The conventional device is JP-A-6
As shown in Japanese Patent Laid-Open No. 2-199194, a clip circuit is provided in the collector of the color output transistor to prevent a saturation state.

[0005]

The above prior art is a low-pass filter including a junction capacitance of a clip diode, a resistor inserted in series with the diode, and a load resistor of a color output transistor. This deteriorates the high frequency response of the color signal.

Further, a diode having a high breakdown voltage is required.

An object of the present invention is to provide a picture tube drive circuit which prevents deterioration of high frequency response of each color output stage and saturation of a large luminance signal.

[0008]

In order to achieve the above object, a small resistor (several tens of ohms) which does not affect the circuit at the cathode junction portion of a picture tube drive circuit composed of a cathode amplifier, Insert a Zener to limit the current.

[0009]

By limiting the current of the color output transistor, it is possible to prevent the saturation state and at the same time protect the color output transistor.

[0010]

FIG. 1 shows an embodiment of the present invention.

The 17 parts surrounded by the dotted line are the essential parts of the present invention.

In FIG. 1, 1 is a luminance signal and chrominance signal processing block, 2 is a base resistor, 3 is a luminance signal transistor, 4 is an emitter resistor of the luminance signal transistor, and 5
Is a red drive transistor, 6 is a red drive transistor emitter resistor, 7 is a red drive transistor DC bias resistor, 8 is a red output transistor,
Reference numeral 9 is a DC bias resistor for the red output transistor, 10 is a collector resistor for the red output transistor, 11 is a series resistor to the cathode of the cathode ray tube 12, 13 is a current detecting resistor, and 14 is a current limiting zener. , 15 and 16 are green,
The blue circuit part. If there are not 17 parts in this figure,
The collector of the red drive transistor and the emitter of the red output transistor are directly connected.

The operation in this case will be described.

The red color difference signal output from the luminance signal / color signal processing block 1 is input to the base of the red drive transistor 5 via the resistor 2, and the luminance signal is input to the base of the red drive transistor 5 via the resistor 6 via the resistor 6. It is input to the emitter of the transistor.

The collector current of the red drive transistor 5 is determined by the emitter resistor 6 and the difference voltage between the red difference signal input to the base and appearing at the emitter and the luminance signal output to the emitter of the luminance signal transistor 3. It

Therefore, the collector output of the red output transistor 8 is determined by the collector current of the red drive transistor 5.

When a red signal with a large amplitude comes from a luminance signal with a larger amplitude and a red color difference signal, the collector current of the red drive transistor 5 increases and the collector voltage of the red output transistor 8 decreases, resulting in a saturation phenomenon. There is.

The operation of this embodiment 17 will be described below.

A resistor 13 and a red output transistor 8 base inserted between the collector of the red drive transistor 5 and the emitter of the red output transistor 8 connected in cascade.
Of the red output transistor 8 formed by a Zener 14 connected between the collector and the collector of the red drive transistor 5.

Here, the Zener voltage Vz (V) is V when the maximum current Ic (A) of the collector current of the red output transistor 8 which limits saturation and the insertion resistor 13 are R (Ω).
Select z≈Vbe + Ic × R (V).

Here, Vbe is a voltage between the base and the emitter of the red output transistor 8.

In this circuit, normally, the collector current of the red output transistor 8 operates at the maximum current Ic or less, and the Zener 14 is off.

Next, when the collector current of the red drive transistor 5 increases and the collector current of the red output transistor 8 exceeds the above-mentioned maximum current Ic, the Zener-14 is turned on. The collector current of the red output transistor 8 which is shunted by the Zener 14 does not exceed the maximum current Ic and can be prevented from entering the saturation region.

Normally, the collector resistor 10 of the red output transistor 8 is selected to be about 10 kΩ. Therefore, in order to prevent the red output transistor 8 from saturating, the maximum collector current Ic may be suppressed to about 15 mA. In order to suppress the influence on the output stage, if the resistor 13 is set to about 50Ω, a 1.4V Zener may be inserted.

Although only the red output stage has been described in the present embodiment, the same applies to the green and blue output stages.

[0026]

According to the present invention, it is possible to prevent deterioration of image quality and prevent saturation of the color output stage.

[Brief description of drawings]

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

[Explanation of symbols]

1 ... Luminance signal / color signal processing block, 2 ... Base resistor, 3 ... Luminance signal transistor, 4 ... Luminance signal transistor emitter resistor, 5 ... Red drive transistor, 6 ... Red drive transistor emitter resistor, 7 ... DC bias resistor of red drive transistor, 8 ... Red output transistor, 9 ... DC bias resistor of red output transistor, 10 ... Collector resistor of red output transistor, 11 ... Series resistor to cathode, 12 …
CRT, 13 ... Current detection resistor, 14 is a current limiting Zener, 15, 16 ... Green and blue circuit parts, respectively.

Claims (1)

[Claims] 1. A second transistor which is composed of a cascade amplifier and which is coupled to a color difference signal input source and a luminance signal input source and a DC voltage source and which drives a cathode of a picture tube. A picture tube drive circuit comprising a transistor, wherein a current detecting means composed of a resistor and a zener is provided between the first transistor and the second transistor.