Electron Gun for a Color Cathode Ray Tube

The present invention relates to an electron gun for a color cathode ray tube, and more particularly to an in-line electron gun for a color cathode ray tube, having an improved focus electrode forming a quadruple lens.

An electron gun for a cathode ray tube is installed in a neck portion of a funnel connected to a panel and emits an electron beam to collide with a fluorescent layer. The general structure of the electron gun is as shown in FIG. 1.

A cathode 11, a control electrode 12 and a screen electrode 13 which constitute a triode, and a focus electrode 14 and a final accelerating electrode 15 which constitute a main lens, are sequentially formed in the electron gun. As shown in FIG.2A and FIG. 2B, focus electrode 14 is constituted by a first focus electrode 14a in which three vertically elongated electron beam passing holes 14H are formed on an outlet plane 14c, and a second focus electrode 14b in which three horizontally elongated electron beam passing holes 14H' are formed on an inlet plane 14d. A predetermined voltage is supplied to each electrode. A predetermined voltage Vf is supplied to first focus electrode 14a, and a dynamic focus voltage Vd (referenced to focus voltage Vf supplied to first focus electrode 14a) which is synchronous to a deflection signal, is supplied to second focus electrode 14b. Final accelerating electrode 15 receives an anode voltage Ve which is higher than focus voltage Vf.

As a predetermined voltage is supplied to each electrode, in a conventional color cathode ray tube constructed as above, a pre-focus lens is formed between screen electrode 13 and first focus electrode 14a of focus electrode 14, a quadrupole lens is formed between first focus electrode 14a and second focus electrode 14b in accordance with the supply of dynamic focus voltage Vd to second focus electrode 14b, and a main lens is formed between second focus electrode 14b and final accelerating electrode 15. Accordingly, when the electron beam emitted from cathode 11 scans the center of the screen, the quadrupole lens is not formed because there is no potential difference between first and second focus electrodes 14a and 14b, so that the electron beam emitted from cathode 11 passes through the main lens and then is landed on the center of the screen, Meanwhile, when the electron beam emitted from cathode 11 scans the periphery of the screen, a parabola-type dynamic focus voltage Vd synchronous to the deflection signal is supplied to the second focus electrode, so that a quadrupole lens is formed between first and second focus electrodes 14a and 14b, the electron beam emitted from cathode 11 is vertically elongated by the quadrupole effect when passing through the quadrupole lens. The vertically elongated electron beam passes through the main lens to be finally focused and accelerated, and then is deflected by the deflection yoke. Here, the electron beam is compensated so as to revert back to the circular shape by a non-uniform magnetic field, to scan the periphery of the fluorescent film. However, in such a conventional electron gun, a distorted electron beam is compensated by a non-uniform magnetic field of the deflection yoke, but since three in-line quadrupole lens are mutually interfered, a convergence characteristic is not good. Therefore, the resolution of a cathode ray tube having a conventional electron gun as above, becomes deteriorated.

The object of the present invention is to provide an electron gun for a color cathode ray tube which improves a convergence characteristic of an electron gun and minimizes the interference among three in-line electrostatic lenses, to thereby improve the resolution of the cathode ray tube.

To achieve the above object of the present invention, there is provided an electron gun for a color cathode ray tube wherein a cathode emitting red, green and blue electron beams, a control electrode, a screen electrode, a focus electrode and a final accelerating electrode, each of which have electron beam passing holes, are sequentially arranged at predetermined distances by a supporting means, said electron gun being characterized in that said focus electrode is divided into a first focus electrode and a second focus electrode, wherein said first focus electrode comprises an electron beam inlet plane in which a portion for shaping a central electron beam passing hole and portions for shaping outer electron beam passing holes are formed evenly, and an electron beam outlet plane in which a portion for shaping a central electron beam passing hole is formed convexly in respect to portions for shaping outer electron beam passing holes, and said second focus electrode comprises an electron beam inlet plane in which a portion for shaping a central electron beam passing hole is formed concavely in respect to portions for shaping outer electron beam passing holes, and an electron beam outlet plane in which a portion for shaping a central electron beam passing hole and portions for shaping outer electron beam passing holes are formed evenly.

Further features and advantages will become more apparent from the following and more particular description of the preferred embodiment of the invention as illustrated in the accompanying drawings in which the same reference characters generally refer to like parts throughout the views, and in which:

FIG. 1 is a sectional view of a conventional electron gun for a cathode ray tube;

FIG. 2A shows an electron beam outlet plane of the first focus electrode of FIG. 1;

FIG. 2B shows an electron beam inlet plane of the second focus electrode of FIG. 1;

FIG. 3 is a sectional view of an electron gun for a color cathode ray tube according to the present invention; and

FIG. 4 shows a quadrupole lens formed between the electron beam outlet plane of a first focus electrode and electron beam inlet plane of a second focus electrode as shown in FIG. 3.

As shown in FIG. 3, an electron gun 20 for a color cathode ray tube is constituted by a cathode 21, a control electrode 22 and a screen electrode 23 which together constitute a triode, and a focus electrode 24 and final accelerating electrode 25 which together constitute a main lens.

In FIG. 4, focus electrode 24 is constituted by a first focus electrode 24a where a vertically elongated electron beam passing hole 24H is formed on an electron beam outlet plane 24f, and a second focus electrode 24b where a horizontally elongated electron beam passing hole 4H' is formed on an electron beam inlet plane 24d. In electron beam outlet plane 24f of first focus electrode 24a, a portion for shaping a central electron beam passing hole 24c is convex in respect to portions for shaping outer electron beam passing holes 24S. In electron beam inlet plane 24d of second focus electrode 24b, a portion for shaping a central electron beam passing hole 24c' is concave with respect to portions for shaping outer electron beam passing holes 24S'. That is, electron beam outlet plane 24f of first focus electrode 24a has a side-wall 24W that is convex with respect to portions for shaping outer electron beam passing holes 24S, while electron beam inlet plane 24d of second focus electrode 24a has a side-wall 24W' that is concave with respect to portions for shaping outer electron beam passing holes 24S'. It is desirable that the portion for shaping a central electron beam passing hole 24c of first focus electrode 24a is slightly advanced into portion for shaping a central electron beam passing hole 24c' of second focus electrode 24b. A predetermined voltage is supplied to each electrode in such a manner that an anode voltage Ve of 25-35 kV is supplied to final accelerating electrode 25, a focus voltage Vf being 20-35% of the voltage supplied to final accelerating electrode 25 is supplied to first focus electrode 24a of focus electrode 24, and a parabola-type dynamic focus voltage Vd higher than focus voltage Vf by as much as 0.5-1 kV is supplied to second focus electrode 24b in synchronization with the deflection signal.

The electron gun for a color cathode ray tube of the present invention constructed as above is operated as follows.

As a predetermined voltage is supplied to each electrode which constitutes electron gun 20 for a color cathode ray tube, a pre-focus lens is formed between screen electrode 23 and first focus electrode 24a, a quadrupole lens is formed between first focus electrode 24a and second focus electrode 24b in accordance with the supply of dynamic focus voltage Vd, and a main lens is formed between second focus electrode 24b and final accelerating electrode 25.

When the electron beam emitted from cathode 21 scans on the center of the screen, a parabola-type dynamic focus voltage Vd synchronous to the deflecting signal is not supplied to second focus electrode 24b, so that the quadrupole lens is not formed between first focus electrode 24a and second focus electrode 24b. Accordingly, the electron beam emitted from cathode 21 is preliminarily focused and accelerated in the pre-focus lens and then is finally focused and accelerated in the main lens, to be landed on the center of the fluorescent layer. At this time, since portion for shaping a central electron beam passing hole 24c' formed on electron beam inlet plane 24d of second focus electrode 24b is inwardly concaved, the electron beams passing through the outer electron beam passing holes are moved toward the central electron beam, That is, the outer electron beam emitted from cathode 21 is moved toward the central electron beam passing hole by side-wall 24W' of the portion for shaping a central electron beam passing hole 24c formed on the electron beam inlet plane of second focus electrode 24b where a focus voltage having a positive potential is supplied.

When the electron beam emitted from cathode 21 is deflected to the periphery of the screen, parabola-type dynamic focus voltage Vd synchronous to the deflection signal is supplied to second focus electrode 24b, so that a quadrupole lens is formed between electron beam outlet plane 24f of first focus electrode 24a having vertically elongated electron beam passing hole 24H, and electron beam inlet plane 24d of second focus electrode having horizontally elongated electron beam passing hole 24H'. The quadrupole lens formed by the central electron beam passing holes is symmetrically formed with the electron beam proceeding line, but the quadrupole lens formed by the outer electron beam passing holes is asymmetrically formed by side-wall 24W' of the portion for shaping a central electron beam passing hole 24c'. That is, since the electric field formed in the edges of the outer electron beam passing holes of first and second focus electrodes 24a and 24b, and in side-walls 24W and 24W' is not uniform, the quadrupole lens made by the equipotential line is asymmetrically formed in respect to the electron beam proceeding line. Accordingly, the electron beam emitted from cathode 21 is preliminarily focused and accelerated in the pre-focus lens, and secondly focused and accelerated in the quadrupole lens, and then finally focused and accelerated in the main lens to land on the fluorescent layer. Since the quadrupole lens formed by the outer electron beam passing holes is asymmetrically formed described as above, the electron beam having passed through this quadrupole lens is converged toward the central electron beam passing hole, and the section of each electron beam is vertically elongated. Since the magnitude of the electron beam changes in accordance to the dynamic focus voltage which is synchronous to the deflection signal, the electron beam's convergence characteristic will be in the optimum condition when the electron beam scans the periphery of the fluorescent film. Particularly, the electron beam which is vertically elongated when passing through the quadrupole lens, is compensated so as to revert back to the circular shape by a non-uniform magnetic field of the deflection yoke during deflection by the deflection yoke, thereby optimally landing on the fluorescent film.

In the electron gun for a color cathode ray tube of the present invention, the portion for shaping a central electron beam passing hole placed on the electron beam outlet plane of the first focus electrode is formed convexly, and the portion for shaping a central electron beam passing hole placed on the electron beam inlet plane of the second focus electrode, is formed concavely. Therefore, the quadrupole lens formed by the outer electron beam passing holes is formed asymmetrically, so that the convergence characteristic of the electron beam is improved, and the interference between three in-line electrostatic lenses is minimized, to thereby improve the resolution of the cathode ray tube having this electron gun.

Having described a preferred embodiment of the present invention, it will be clear to those skilled in the art that modifications and alternatives to the disclosed apparatus exist within the scope and spirit of the present invention. Accordingly, it is intended to limit the scope of the present invention only as indicated in the following claims.