JP2633291B2 - Blanking circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a blanking circuit that cuts off a cathode ray tube by a blanking signal and does not emit light, and more particularly, cuts off a cathode ray tube well even during a scanning period. A possible blanking circuit.

[Conventional technology]

In a raster scan type display device using a cathode ray tube (CRT), the CRT is used during the retrace period of electron beam deflection.
The video input signal is fixed at the black level so that no retrace will occur. However, even in the use condition where the black level is increased to some extent by increasing the brightness volume, the CRT is used during the deflection retrace period to prevent the retrace from appearing.
A blanking circuit for turning off the cart is provided. As a method of cutting off the CRT, a method of applying a negative cutoff voltage to the first grid of the CRT is known as a method of applying a positive cutoff voltage to the cathode of the CRT (Basic Television Technology, Masaaki Murakami) Author, Electronic Technology Publishing Co., Ltd., p. 275-277). FIG. 3 shows an example of a blanking circuit that performs a blanking operation by cutting off the CRT. In FIG. 3, transistor 2 forms a common emitter amplifier circuit, and transistor 1 forms a common base amplifier circuit. Then, a so-called cascode amplifier type video output amplifier circuit in which the collector of the transistor 2 and the emitter of the transistor are connected is formed. The video input signal is input to the base of transistor 2 and the load resistance connected to the collector of transistor 1
The voltage drop appearing at 11 drives the cathode of CRT20. The transistors 3 and 4 have a resistance of 12,
13, a blanking circuit is formed by the resistor 14. As the blanking signal, a signal which becomes “H” level during the blanking operation is input to the base of the transistor 4 via the resistor 14. The transistor 4 is turned on by the blanking signal, and a forward bias is applied to the base of the transistor 3 through the resistor 13. As a result, the transistor 3 is turned on, and the base of the transistor 1
The emitter is short-circuited by the transistor 3, and the transistor 1 is cut off. When the transistor 1 is cut off, the cathode potential of the CRT 20 reaches the power supply voltage Vcc1, the CRT 20 is cut off, and a blanking operation is performed.

[Problems to be solved by the invention]

In the blanking circuit according to the prior art shown in FIG. 3, no consideration is given to the blanking operation during the scanning period in which a video signal is present, and the high frequency of the transistor 3 for short-circuiting between the base and the emitter of the transistor 1 is not considered. The increase in the collector-emitter saturation voltage in the region has not been considered a problem.

However, in recent years, projection televisions that project images on the screen using a total of six CRTs, two for each color of red, green, and blue, due to the demand for large screens and high brightness, require adjustment pattern signals during convergence adjustment. Must be selected and projected for each CRT. CR selected for this
In order to display the adjustment pattern only on T, it is necessary to blank the CRT also during the period in which the video signal exists, that is, during the scanning period, and to select the CRT. When blanking the CRT during the scanning period as described above,
Due to the increase in the saturation voltage of the transistor 3 in the high frequency region shown in FIG. 3, a problem arises in that the high frequency component of the video signal is displayed on the CRT without blanking. This phenomenon is particularly problematic when the bio frequency band is widened.

An object of the present invention is to provide a blanking circuit capable of satisfactorily blanking a CRT during a scanning period as described above.

[Means for solving the problem]

The above object is achieved by making the emitter potential of the transistor 3 higher than the base potential of the transistor 1 shown in FIG.

[Action]

In this case, when the transistor 3 is in the ON state, the potential between the base and the emitter of the transistor 1 is in a reverse bias state, so that the transistor 1 can be cut off.

Further, in a video band in which the saturation voltage of the transistor 3 in the high-frequency region becomes a problem, the voltage between the collector and the emitter of the transistor 3 is raised, so that an increase in the saturation voltage of the transistor 3 in the high-frequency region can be compensated. it can.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to FIG. In FIG. 1, a video output circuit for driving the cathode of the CRT 20 is formed by cascade connection of a common emitter amplifier circuit formed by a transistor 2 and a common base amplifier circuit formed by a transistor 1. Video force signal 40 is transistor 2
Is input to the base of the
And the collector current of the transistor 1, and the load resistance 11
, The cathode of the CRT 20 is driven. Here, the base potential of the transistor 1 is the power supply voltage Vcc2
The voltage is fixed to a voltage Vz lower than Vcc2 by the Zener diode 6 via the resistor 15. The transistors 3 and 4 are blanking circuits that perform a blanking operation by cutting off the transistor 1. As the blanking signal, a signal which becomes “H” level during the blanking operation is input to the base of the transistor 4 via the resistor 14. When this blanking signal is at the “H” level, the transistor 4 is turned “ON”, and a forward bias voltage is applied to the base of the transistor 3 via the resistor 13 and the transistor 4. As a result, the transistor 3 is turned on, and the emitter potential of the transistor 1 becomes higher than the base potential. Therefore, the transistor 1 is cut off, the cathode potential of the CRT 20 reaches the power supply voltage Vcc1, and the CRT 20
20 is blanked. At this time, the video signal amplified by the transistor 2 flows from the power supply voltage Vcc2 through the transistor 3 to the collector of the transistor 2 as the collector current of the transistor 2. Where transistor 1
If the forward base-emitter voltage of the transistor 3 is V _BE1 , the collector-emitter saturation voltage of the transistor 3 increased by the high-frequency component of the collector current of the transistor 2 becomes Vcc2
If −Vz + V _BE1 or less, the transistor 1 can be completely cut off regardless of the video input signal 40. Therefore, the saturation voltage of the transistor 3 in a high frequency region can be compensated. When the blanking signal is at the “L” level, the transistor 4 is in the “OFF” state, and the base potential of the transistor 3 is equal to the emitter potential via the resistor 12, so that the transistor 3 is in the “OFF” state. Therefore, all the collector currents of the transistor 2 become the collector currents of the transistor 1, and an image is displayed on the CRT 20.

Next, a second embodiment will be described with reference to FIG. Second
In the figure, the same symbols are used for the same parts as in FIG. 1, and the description of the same operation parts is omitted. In FIG. 2, a signal which becomes “L” level during the blanking operation is supplied to the base of the transistor 4 as the blanking signal. First, when this blanking signal is at the “L” level, the transistor 4 is in the “OFF” state. The base potential of the transistor 5 is obtained by connecting the power supply voltage Vcc2 to the resistor 12,
The potential divided by the resistors 13 and 16 is given. By setting the divided potential higher than the base potential Vz of the transistor 1, the base-emitter voltage of the transistor 1 becomes lower than the forward base-emitter voltage.
0 is blanked. When the blanking signal input to the transistor 4 is at the “H” level, the transistor 4 is in the “ON” state, and the base potential of the transistor 5 is a value obtained by dividing the power supply voltage Vcc 2 by the resistors 12 and 13. . By setting the divided value lower than the base potential of the transistor 1, the transistor 5 is cut off. Therefore, an image is displayed on the CRT 20. Here, when the resistance 12 is R ₁₂ , the resistance 13 is R ₁₃ , and the resistance 16 is R ₁₆ The above operation is achieved by satisfying the following conditions. In the blanking operation, the collector current flowing through the transistor 2 is supplied from the power supply voltage Vcc2 through the transistor 5. Here, when the forward base-emitter voltage of the transistor 5 is V _BE5 , the collector-emitter saturation voltage of the transistor 5 which increases due to the high-frequency component of the collector current of the transistor 2 becomes In the following case, the transistor 1 can be completely cut off regardless of the video input signal 40, and the saturation voltage of the transistor 5 in the high frequency region can be compensated.

〔The invention's effect〕

According to the present invention, the emitter potential of the transistor forming the grounded base type amplifier circuit is raised to compensate for the increase in the saturation voltage in the high frequency region of the switching transistor which operates so as to be lower than the forward base-emitter voltage. Therefore, the CRT can be completely blanked even during the scanning period in which the video signal exists. As described above, the present invention is very effective when the video signal is widened. In a projection television, a CR that projects a pattern signal for convergence adjustment is used.
Selection of T can be selected by cutting off the video output amplifier circuit, and good blanking can be performed even in a text broadcast in which a text signal is superimposed during a blanking period.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a blanking circuit as one embodiment of the present invention, FIG. 2 is a circuit diagram showing a blanking circuit as a second embodiment according to the present invention, and FIG. FIG. 3 is a circuit diagram illustrating a ranking circuit. 1,2,3,4,5 ... Transistor 6 ... Zener diode 20 ... CRT (CRT) 30 ... Blanking signal 40 ... Video input signal

Claims (2)

(57) [Claims] In a cathode ray tube display, an NPN junction type transistor (1) for forming a grounded base amplifier circuit for amplifying and outputting a video signal and driving a cathode of the cathode ray tube is used as a first transistor. Of the second transistor, PN,
A collector of a P-junction transistor (3) is connected, an emitter of the second transistor (3) is connected to a fixed potential higher than a base potential of the first NPN junction transistor (1), and a blanking signal source is connected. To the base of the second PNP-junction transistor (3) to turn on the second PNP-junction transistor (3) during the blanking period, thereby allowing the first NPN-junction transistor (3) to turn on. A blanking circuit characterized in that the cathode ray tube is blanked by cutting off 1). 2. A cathode ray tube display comprising: a first NPN junction type transistor for forming a grounded base amplifier circuit for amplifying and outputting a video signal to drive a cathode of the cathode ray tube; The transistor (1) has an emitter connected to the second transistor NP
The emitter of the N-junction transistor (5) is connected, the collector of the second transistor (5) is connected to a potential higher than the base potential of the first NPN matched transistor (1), and the blanking signal line is connected. By connecting to the base of the second NPN junction transistor (5), the second NPN junction transistor (5)
The base potential of the first NPN transistor (1) is higher than that of the first NPN junction transistor (1).
A blanking circuit characterized in that the NPN junction type transistor (1) is cut off to blank the cathode ray tube.