US2228388A - Cathode ray amplifier - Google Patents

Description

1941- P. T. FARNSWORTH 2,228,388

CATHODE RAY AMPLIFIER Original Filed July 6, 1935 II i2 FIG. I

FLOOD GUN 22 GRID EAT SCREEN ELE OTRON GUN FIG?) 25 AMPLICATION I I I I I I I 2.8 5. VOLTS ON INSULATED GRID FIG.5

' INVENTOR 4 TTORNE Patented Jan. 14, 1941 UNITED STATES 2,228,388 I CATHODE RAY AMPLIFIER Philo T. Farnsworth, San Francisco, Calif., assign-- or, by mesne assignments. to Farnsworth Television & Radio Corporation, Dover, Del., a corporation of Delaware Application July s, 1935, Serial No. 30,111 Renewed Juncl2, 1940 8 Claims.

My invention relates to cathode ray amplifiers, and more particularly to a cathode ray amplifier suitable for production of a brilliant optical image which is greatly to be desired in the reception of television signals.

The device of the present application is an improvement on my former invention described and claimed in application Serial No. 655,784, filed February 8, 1933, for a Luminescent screen 10 and method of use, now matured into U. S. Patent No. 2,104,253 issued Jan. 4, 1938; and has, in addition to the features outlined in the former case, an embodiment of the broad method of electron image amplification described and claimed by me in my application Serial No. 29,242 filed July 1,

1935, for an Electron image amplifier.

In the former application for a Luminescent screen and method of use, I describe and claim a tube having a luminescent screen comprising thin refractory material or a refractory fabric which is positioned with respect to a source of electrons of high velocity in such a manner that when the electrons hit the luminescent screen, the points of impact are raised to incandescence. If the beam is then moved in two directions over the screen, and the' beam modulated by a television signal, an incandescent image is produced which, if sufficient power is supplied to the beam, is well adapted for use as a projection light source 30 whereby television pictures may be projected upon the usual type of viewing screen.

In this particular type of tube, the main objection is that the electron gun must of necessity have extremely high power in order to supply a 35 suflicient number of high velocity electrons in the electron beam to raise the screen to incandescence, and it is not always practical to build guns having sumcient power to attain the brilliancy desired.

40 It is therefore among the objects of the present invention:. To provide a cathode ray tube having an electron gun of relatively low power and to utilize the electrons therefrom to create a charge image which will control a much larger .45 source of electrons to form an electron image 5 means and method for producing optical images (Cl. Pas-7.5)

of sufllcient brilliance to be used for projection sources; to provide a cathode ray receiving tube operating under the broad principles of electron image amplification described and claimed in my application, Serial No. 29,242, referred to above; to provide a cathode ray receiving tube wherein a new and unique method of control is utilized; to provide a cathode ray receiving tube wherein a charge image is formed; to provide a means and method of controlling a cathode ray receiving 10 tube; to provide a means and method for forming an electron image in a cathode ray receiving tube; to provide a means and method of operating a television receiving circuit; to provide a means and method of combining electrons in two velocity categories to produce an amplified electron image, utilizable to produce a brilliant optical image; and to provide a new and novel means and method for producing an optical image for use in television receiving sets.

My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus of the present application. as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

In the drawing which accompanies this specification and forms a part thereof Figure 1 is a longitudinal sectional view of a preferred embodiment of my invention.

Figure 2 is a cross-sectional view looking toward the flood gun of Figure 1, taken on the line 2-2.

Figure 3 is a graph representing a characteristic curve of the tube shown in Figure 1.

Figure 4 is a diagrammatic hookup reduced to 40 simplest terms, showing a preferred circuit for energization of the tube shown in Figure 1.

Figure 5 is a cross-sectional view of a preferred form of insulating grid.

Other broad aspects of my invention may be as more thoroughly understood by a direct reference to the drawing and particularly to the apparatus shown in Figures 1, 2 and 4, the discussion of the operation of the device being deferred.

An envelope I is provided at one end with an W electron gun assembly comprising a perforated anode '2, a control electrode 6 and an indirectly heated cathode assembly 5. These elements are supported in the usual manner by leads sealed through a stem 6. In this respect it should be pointed out here that the particular type of electron gun utilized is immaterial in the practice of my present invention, any such apparatus capable of producing a beam of electrons having a relatively small cross section at high velocities being perfectly satisfactory.

Immediately after the electrons leave the aperture of the anode 2, a deflection system is provided positioned to influence the beam. In this particular embodiment I show horizontal deflection plates I energized by oscillator 9, and vertical deflection plates l0 energized by oscillator II, the two sets of plates being positioned so as to deflect the beam in two directions when energized, magnetic deflection systems of course being fully equivalent. Around this portion of the beam I also prefer to position a focusing coil i2 energized by focusing source I 4 under the control of a rheostat l5, and prefer to adjust the current in the coil so that the beam emitted from the electron gun will be focused at the far end of the tube in the plane of an insulated grid I6 positioned perpendicularly to the axis of the tube and to the beam when at rest.

The grid I6 is provided with a metal base I1 and is covered with an insulating layer I9, so that its exposed surface is of insulating material.

While I do not wish in any way to be bound by any particular method of forming the insulating surface, I find that a very satisfactory combination is to smoke a nickel screen of fine mesh with fumes from burning magnesium so that magnesium oxide is deposited upon the material of the grid. I also prefer to so proportion the wires of the grid to the open spaces that when the magnesium oxide is deposited on the wires, the open portions of the grid will be substantially equal to the area of the portions of the plane occupied by the grid material. The grid 16 is preferably provided with a lead wire 20 passing through the wall of the envelope l, and is closely adjacent a heat screen 2| facing the transparent and 22 of the tube. In order that the grid and the heat screen be maintained parallel and in flxed relation, I prefer to fasten them together by peripheral glass beads 24. I then provide the heat screen with a lead 25 passing through the wall of the tube envelope. The heat screen is preferably made under the teachings of my U. S. Patent No. 2,104,- 253, or with the modification described and claimed in my later application entitled Incandescent light source, since matured into Patent No. 2,066,070, issued December 29, 1936. In either of these examples the screen is made from refractory material of extremely low mass, either in the form of an extremely thin refractory sheet, or in the form of a refractory fabric, and I prefer to utilize a knitted fabric of refractory wire of approximately .00025" diameter with a mesh of at least the number of elements desired in the reproduced image. Any type of screen adapted to become luminous on electron impact is satisfactory however.

Intermediate the electron gun assembly and the grid-screen assembly I prefer to position a low velocity electron flood gun assembly. The

ported by heating leads 29 and cathode lead 30, the upper end of each indirectly heated cathode being also supported by a spacing bar 3| attached to the envelope by means of connection 32. Each cathode assembly is provided with alow velocity anode 34 which, in this instance-I prefer to make in the form of aspiral grid surrounding the indirectly heated cathode. The low velocity anodes are provided with lead wires 35.

In operation, I prefer to energize the electrodes as shown in Figure 4, but I wish it to be distinctly understood that the numerical values given herein are simply exemplary of one particular specific tube; they are given only to show voltage relationships and are not in any 'way to be construed as limitations on either the Ime'ansor method involved.

The anode 2 of the electron gun assembly is energized by a high velocity anode source 36; the cathode 5 is heated in the customary manner and is grounded; the grid 4 is provided with a negative bias from bias source 31; theinsulated grid I6 is provided with from 1 to 5 volts positive bias, preferably variable, from a source 39; and the heat screen 2| is provided with a positive potential from a source 40, preferably of from 20 to 200 volts.

In measuring a tube connected in circuit as above described statically, and recording the characteristics which are produced by varying the potential on the insulated grid I6, a characteristic curve is obtained, as shown in Figure 3.

When the electron gun is energized, a beam of electrons having relatively small cross section issues from the anode 2 and is accelerated through the tube until it impacts the insulated grid IS. The focusing coil I2 is so energized that the area covered by the spot on the grid I6 is of the dimensions necessary for the production of a television image. Inasmuch as the high velocity gun is adapted to produce a relatively small number of high velocity electrons they will, when they impact the insulated grid l6 on the surface thereof, release secondary electrons therefrom. In the meantime, upon energization of the low velocity electron gun, the entire surface facing the gun, of the grid [6, will be flooded with low velocity electrons, and these electrons will uniformly charge the insulating surface negatively and the charges will be fixed thereon, as is pointed out in my application for an electron image amplifier, referred to above.

An equilibrium point, however, will be reached depending in value upon the energization of the base wire and upon the leakage of electrons through the insulator to the base wire. The leakage time, which will be referred to later, should be made as long as possible or at least in the order of the period of one complete scanning cycle. When, however, the high velocity electrons hit the insulating surface and the electrons are knocked off, due to secondary emission therefrom, more electrons will leave that point than are received; therefore, the grid at the point covered by the spot will become positive in an amount governed by the control of the beam by a television signal applied to the grid 4. As the development of a uniform negative charge on the grid results in the formation of a space charge in the flood gun beam between the flood gun and the grid, the positive potential developed at the point of impact of the spot will reduce the space charge and allow electrons from the flood gun to pass through the meshes of the grid and impact the heat screen 2| immediately behind it. There are plenty of electrons available from the flood gun, and many I times the number of electrons pass through the screen at the point of impact as are contained in the high velocity beam. Thus a large amplifica- 5 tion takes place and the screen is raised to incanture'fleld upon the grid, at each successive area liberating electrons from the flood gun through the grid onto the heat screen at that particular 5 point. If, then, the high velocity eelctron beam be modulated by the application of a television signal to the grid 4, a visual image will be produced upon the heat screen 2| corresponding to the picture which it is desired to reproduce.

20 After elementary areas of the grid have been traversed by the high velocity beam and the area changed in charge in accordance with the modulation of the high velocity beam and the beam passes on, these areas gradually return to the equi- 25 librium potential in accordance with the leakage factor of the insulator.

Therefore, in addition to .the amplification obtained by the release of large quantities of low velocity electrons from the space charge in front 80 of the grid, I also obtain an amplification'eifect as far as the optical result is concerned because of the fact that the charge image on the grid l6 remains there after the scanning beam has passed, thus allowing electrons from the low velocity gun 36 to pass through to impact the screen after the scanning beam has gone by.

In order for the device to be most efiective the leakage time should be adjusted so that any elementary area will return to the equilibrium poten- 40 tial just before the scanning beam passes over it again, and when this happens the elementary area is recharged to a. new potential corresponding to the next picture cycle.

In practice it is not of course possible to adjust 45 this time exactly, but it is possible to obtain a very definite storage of the charge'during thescanning' cycle. In practice I find I can easily attain amplifications of more than 1000; that is, for every electron in the high velocity scanning beam I can.

50 release 1000 or more electrons through the grid to the heat screen, thus greatly reducing the diffi- 55 Reference to the characteristic curve shown in Figure 3, however, will immediately suggest another mode of operation. While I do not wish to be bound by the theory herein, I believe that the explanation of the characteristic curve is-relatively simple. It will be seen that amplification is obtained which is both positive and negative, line ll representing zero amplification, maximum positive amplification in the tube described taking place at 2.8 volts positive on the grid base wire 65 and maximum negative amplification taking place at approximately five volts positive thereon. Over the region where amplification is positive I believe that electrons are leaving the insulating surface due to secondary emission, the electrons emitted 70 probably joining the space charge created by the equilibrium charge.

As the positive potential on the insulated grid is increased, the electrons leaving the portion of the insulator facing the high velocity gun are 75 not allowed to join the space charge but return through the grid to the back side thereof and charge the insulator there negatively. As the control on the low velocity electrons will be greatest for the back side of the insulator the curve becomes reversed, the negative charge acquired 5 overcoming the positive charge on the front of the insulator, thus producing the curve shown in Figure 3. It is therefore possible to utilize an unmodulated high velocity beam, an unmodulated low frequency beam, and control the tube entirely by television signals applied directly to the base wire of the insulated grid. There are a number of portions of the curve of Figure 3 on which the tube will operate, as can readily be seen by those skilled in the art.

The advantages of such a tube are readily apparent. High intensity images are produced without the use of guns requiring a large amount of electrons with restricted beam sections; large quantities of electrons are immediately available from the low velocity flood gun which does not require the production of a beam having a restricted cross section; large currents are available from the flood gun under full control of the charge image produced on grid l6 by the interaction of the high velocity gun and the bias on the insulated grid. The leakage lag occurring in the insulator provides not only an increase in the apparent brilliancy of the image, but also prolongs the fade-out of each individual picture to such a point that flicker is almost completely eliminated, there being only a gradual dying down of one image before the next one appears. The amount of amplification available is extremely high and there are no critical adjustments necessary in the tube except for the adjustment of the bias on the insulated grid, and this adjustment needs only to be sufiiciently critical as to place the tube on a. proper operating portion of the curve shown in Figure 3. :30

In the particular embodiment shown I have been able to release ten to twenty milliamperes of current in the elemental areas of the heat screen with a scanning beam supplying only a few microamperes; and while the velocity of the electrons in the scanning beam must be sufllciently high to create secondaries upon impact, the velocities need not be high as compared with the velocities which would be necessary were the heat screen to be impacted directly.

- I claim: culties attendant upon producing an electron; beam of the intrinsic intensity necessary to p1"o'--,, duce the results obtained with the present device.

l. The method of electron image amplification which comprises creating a pair of unmodulated electron streams of differing cross sectional area, intercepting both of said streams by an insulating material to create charges thereon, spatially modifying the charges in accordance with theillumination of elementary areas of a picture field, and directing the resultant flow against a luminescent screen.

2. The method of amplification which comprises bombarding an insulating material with an unmodulated stationary stream of electrons of picture area, and with an unmodulated moving stream of electrons of elemental cross sectional area, and capacitatively modifying the charges produced on said material-by the combined impact of said streams in accordance with picture signal values, directly utilizing said modified charges to change the intensity of said streams, and directly utilizing the changed streams to create a visual image.

3. The method of amplification which comprises bombarding an insulating material with an unmodulated stationary stream of electrons of T5 picture area, scanning said material over successive elemental areas thereof with a second unmodulated electron stream to form combined V to create a visual image.

5. The method of amplification which com prises simultaneously" bombarding an insulating material with an unmodulated electron stream of unit cross sectional area having an average electron velocity therein, and with an unmodulated electron stream of elemental cross sectional area having a different average electron velocity, modifying the charges produced on said material in accordance with television signals, and moving the smaller stream over successive elemental areas of said material being bombarded by the larger stream, in synchronism with the signals, directly utilizing said modified charges to change the intensity of said streams, and directly utilizing the changed streams to create a visual image.

6. The method of amplification which comprises simultaneously bombarding an insulating material with an unmodulated electron stream of unit cross sectional area having an average electron velocity therein, and with an unmodulated electron stream of elemental cross sectional area having a diflerent average electron velocity, modifying the charges produced on said material in accordance with television signals, moving the smaller stream over successive elemental areas of said material being bombarded by the larger stream, in synchronism with the signals, and directing the resultant stream against a luminescent screen to produce an optical image.

7. Means for producing a visual image comprising an envelope containing a luminescent screen, a grid having an interior conducting layer and an exterior layer of insulating material positioned adjacent and parallel to said screen, means for flooding said screen through said grid with a uniform unmodulated stream of electrons impacting the entire picture area of said screen, means for scanning successive elementary areas of said screen through said grid with an unmodulated stream of electrons having a cross section of elemental area, and means for applying a signal to the conducting layer of said grid.

8. Means for producing a visual image comprising an envelope containing a luminescent screen, a grid having an interior conducting layer and an exterior layer of insulating material positioned adjacent and parallel to said screen, means for flooding said screen through said grid with a uniform unmodulated stream of electrons impacting the entire picture area of said screen, means for scanning successive elementary areas of said screen through said grid with an unmodulated stream of electrons having a cross section of elemental area, and means for applying a picture .signal to the conducting layer of said grid in synchronism with said scansion.

PHILO T. FARNSWORTH.

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