JP2828650B2 - Video output circuit of television receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is also applicable to a case where a positive bias voltage is applied to a first grid of a cathode ray tube to use a screen display with high brightness.
The present invention relates to a video output circuit of a television receiver in which an electron emission characteristic of a cathode does not deteriorate.

2. Description of the Related Art In general, a television receiver has a video output having a load resistance and a cascode-connected video output transistor and a video drive transistor suitable for high-frequency amplification for driving a cathode ray tube. A circuit is provided.
This is described, for example, in "Basic Electronic Circuit I" (issued by Maruzen on March 30, 1978). Conventionally, in order to drive a cathode ray tube of a receiver, generally, a first grid of the cathode ray tube is grounded, and a video signal voltage for controlling the magnitude of a beam current is applied to the cathode via the video output circuit. I was
In such a connection, after turning on the power of the television receiver, until the cathode ray tube reaches the cathode temperature at which normal electron emission is possible, the cathode of the cathode ray tube is maintained for a few seconds during which almost no beam current flows. When the video signal is sent to the video output circuit at the lowest moment, the cathode voltage is much lower than the power supply voltage of the video output circuit and becomes almost 0 V at the lowest moment. However, since the first grid is grounded, current is not forcibly drawn from the cathode.

[Problems to be Solved by the Invention] On the other hand, in recent years, in large-sized color television receivers,
An example in which a positive bias voltage of several to several tens of volts is applied to the first grid of a cathode ray tube to display a screen with high luminance has begun to appear. In such a receiver, there has been observed a tendency that the electron emission characteristics of the cathode of the cathode ray tube deteriorate frequently. As a result of investigating the cause of this accident, the present inventors have estimated as follows.

As described above, after turning on the power of the television receiver, until the cathode ray tube reaches the cathode temperature at which normal electron emission is possible, the image is projected on the cathode of the cathode ray tube for a few seconds during which the beam current hardly flows. When a signal is sent, the cathode voltage
At the lowest moment, the voltage is much lower than the power supply voltage of the video output circuit and becomes almost 0V. In such a state, if a positive bias voltage is applied to the first grid of the cathode ray tube as described above, the temperature of the cathode of the cathode ray tube is still low, and the normal electron emission capability is low. Although there is almost no electron emission, electron emission is performed by a field emission phenomenon for strong electrolysis by a very close first grid.
The emitted electrons collide with the first grid at a high speed because no space charge is formed, and release the adsorbed gas from the surface of the first grid metal plate. The released gas causes the cathode to emit electrons. It was presumed that this would lead to a characteristic deterioration accident.

The present invention solves such a problem, and an object of the present invention is to provide a cathode-ray tube having a high brightness display with a positive bias voltage applied to the first grid of the cathode-ray tube. It is an object of the present invention to provide a video output circuit of a television receiver which largely suppresses the occurrence of an emission characteristic deterioration accident.

Means for Solving the Problems In order to achieve the above object, a video output circuit of a television receiver according to the present invention includes a load resistor, a video output transistor, and a video drive transistor connected in series between power supply terminals, The connection point between the load resistance and the video output transistor is connected to the cathode ray tube.When the power of the television receiver is turned on to the base of the video output transistor, the cathode temperature of the cathode ray tube causes normal electron emission. Means to which a time constant circuit for stopping supply of a base bias voltage for operating the video output transistor until the temperature at which the video output transistor is enabled is reached is provided.

[Operation] According to the above means, after the power supply to the television receiver is turned on, until the cathode temperature of the cathode ray tube becomes possible to perform normal electron emission, that is, the television receiver After the power is turned on, the time constant circuit connected to the base of the video output transistor
Since the supply of the base bias voltage for operating the video output transistor is stopped, the power supply voltage of the video output circuit is directly applied to the cathode of the cathode ray tube. Since the power supply voltage applied to the cathode of the cathode ray tube is higher than the positive bias voltage applied to the first grid, electrons are forcibly discharged from the cathode by field emission during a period in which normal electron emission is not performed from the cathode. The cathode is not drawn out, and the occurrence of an accident of deterioration of the electron emission characteristics of the cathode of the cathode ray tube can be greatly suppressed.

Embodiment FIG. 1 is a circuit diagram showing a configuration of an embodiment of a video output circuit of a television receiver according to the present invention.

In FIG. 1, 1 is a video output transistor, 2 is a video drive transistor, 3 is a load resistor, 4 is a cathode of a cathode ray tube, 5 is a first grid of the cathode ray tube, 6 is a relay, 7
Denotes a transistor, 8 denotes a resistor, and 9 denotes a capacitor. A circuit portion including the relay 6, the transistor 7, the resistor 8, and the capacitor 9 forms a time constant circuit.

And, between the power supply terminal Vcc of the video output circuit and the ground,
A load resistor 3, a video output transistor 1, a video drive transistor 2, and a resistor (not shown) are connected in series, and a connection point between the load resistor 3 and the video output transistor 1 is connected to the cathode of the cathode ray tube through a resistor (also not shown). 4 is connected. Between the ground and the first grid 5 of the cathode ray tube, a bias power supply for generating a resistance (not again reference numerals) a positive bias voltage V _G is connected in series. The time constant circuit is used from when the power of the television receiver is turned on until the cathode temperature of the cathode ray tube reaches a temperature at which normal electron emission can be performed, that is, when the power to the television receiver is turned on. after, for up to several seconds has elapsed, as it acts to stop the supply of the normal base bias voltage for operating the video output transistor 1, between ground and the power supply terminal V _1, winding and the transistor of the relay 6 series circuit 7, a series circuit of a resistor 8 and a capacitor 9 are connected, resistor 8 and the connection point between the capacitor 9 is connected to the base of the transistor 7, the power supply terminal V ₁ contacts of the relay 6 and the video output transistor 1 It is configured to be connected between bases.

In the above arrangement, when the power to the television receiver is turned on, the voltage respectively applied to the power supply terminal Vcc and the power supply terminal V ₁ of the video output circuit. At this time, in the time constant circuit, the capacitor 9 is hardly charged,
Since the voltage between both ends, that is, the base voltage of the transistor 7 is applied and is small, the transistor 7 is off, and the turning off of the transistor 7 does not drive the winding of the relay 6 and the contact of the relay 6 remains open. Therefore, a normal base bias voltage is not applied to the base of the video output transistor 1. Therefore, the video output transistor 1 is turned off, the voltage applied to the power supply terminal Vcc is applied directly to the cathode 4 of the cathode ray tube through the load resistor 3, and the video signal supplied from the video drive transistor 2 is turned off. It is blocked by the video output transistor 1 and does not act on the cathode 4 of the cathode ray tube.

A few seconds have passed since the power was turned on to the television receiver, and the voltage of the power supply terminal V ₁ passed through the resistor 8 to the capacitor 9.
When the voltage across the capacitor 9, that is, the base voltage of the transistor 7 becomes higher than the base-emitter junction voltage V _BE (0.7 V) of the transistor 7, the transistor 7 is turned on. The winding of the relay 6 is driven, and the contact of the relay 6 is closed. And the base of the video output transistor 1
The voltage of the power supply terminal V ₁ is applied via the contact of the relay 6, and a normal base bias voltage is applied to the base of the video output transistor 1. As a result, the video signal supplied from the video drive transistor 2 is amplified by the video output transistor 1 and then applied to the cathode 4 of the cathode ray tube.

In this case, the time from when the power of the television receiver is turned on to when the normal base bias voltage is applied to the base of the video output transistor 1 is mainly determined by the time of the CR consisting of the resistor 8 and the capacitor 9 of the time constant circuit. Determined by a constant.

The voltage of the power supply terminal V ₁ was, in addition to supplying a DC voltage, may be supplied to blanking signal.

FIG. 2 is a diagram showing a cathode voltage of a cathode ray tube and a first grid voltage during a period when the power of the television receiver is turned on and a transition is made to a normal operation state when the present invention is implemented.

[Effects of the Invention] As described above, according to the present invention, a case where a positive bias voltage is applied to the first grid of a cathode ray tube of a television receiver to make the screen display a high-luminance state and used. Also, there is an effect that the occurrence of the electron emission characteristic deterioration accident of the cathode of the cathode ray tube can be largely suppressed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of one embodiment of the present invention, and FIG. 2 is a diagram showing a cathode voltage of a cathode ray tube during a transition from a power-on of a television receiver to a normal operation state when the present invention is carried out. FIG. 4 is a diagram showing a first grid voltage. 1 ... Video output transistor, 2 ... Video drive transistor, 6 ... Relay.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04N 5/14-5/217

Claims (1)

(57) [Claims] 1. A video output of a television receiver in which a load resistor, a video output transistor, and a video drive transistor are connected in series between power supply terminals, and a connection point between the load resistor and the video output transistor is connected to a cathode ray tube. In the circuit, the video output transistor is connected to the base of the video output transistor until the temperature of the cathode of the cathode ray tube reaches a temperature at which normal electron emission is enabled after the power of the television receiver is turned on. A video output circuit for a television receiver, wherein a time constant circuit for stopping supply of a base bias voltage to be operated is connected.