US2991393A - High-transconductance cathode-ray tube - Google Patents

Description

July 4, 1961 J. c. FRANCKEN I 2,991,393

HIGH-TRANSCONDUCTANCE CATHODE-RAY TUBE Filed June 26, 1959 lNVENTOR JAN CAREL, FRANCKEN BY M f- AG NT United States Patent 2,991,393 i IHGH-TRANSCONDligEgA-NCE CATHODE-RAY This invention relates todevices comprising a cathoderay tube, the gun of which is constituted at least by a cathode, a control electrode and; an accelerating. anode,

lation 5101 6, 0.1 transconductance of the gun.

It is known that the slope of the modulation of the gun of acathode-ray tube may be increased if thecontrol voltages are applied not. only to the control electrode, but also to the accelerating anode.

This may be. ensured in a simple manner if thel'control voltages are applied to the cathode, since the cathode potential then varies both with respect to the control electrode and the accelerating anode. This effect is even intensified if the accelerating anode has a great reaction (penetration factor) upon the cathode current, since an increased voltage on the accelerating anode then results in an appreciable increase in electron current and hence increases the rise in current brought about by the control electrode.

However in practice it has been found that the increase of the penetration factor of the accelerating anode is subject to limitations, since the penetration factor may be incerased by decreasing the thickness of the control electrode and by decreasing the distance between the control electrode and the accelerating anode and cathode with respect to the aperture of the control electrode. However, the decrease in the distances between thel electrodes is limited by mechanical difficulties, while an increase of the aperture of the grid produces a larger image spot on the screen and hence results in a lower definition of the image.

It is known to decrease these difliculties by supplying an amplified signal to the accelerating anode, the control signal then being amplified outside the tube by means of an amplifier tube. This has the disadvantage that the capacitances of the lead-through and connecting Wires of the anode and of the grid of the amplifier tube detrimentally affect the band-width of the signal, so that the required anode current of the amplifier tube is high and the anode coupling resistor must be of comparatively low value.

A much more favourable solution is obtained in such a device if, according to the invention, in the gun itself the control voltages applied also control one or more auxiliary currents of electrons which strike parts of the accelerating anode, the secondary-emission factor of which is higher than unity and the secondary electrons of which are collected by a positive auxiliary electrode. The control voltages are then transmitted in amplified form to the accelerating anode so that an important increase in the modulation slope of the gun is obtained without the penetration factor requiring an unduly high value.

In order that the invention may be readily carried into eifect, one embodiment will now be described in detail by Way of example with reference to the accompanying drawing, in which:

FIG. 1 shows diagrammatically a cathode-ray tube, and

FIG. 2 shows an electron gun according to the invention.

In FIG. 1, the window is indicated by 1, the cone by and in which steps are taken for increasing the modul atented July 4,1961

2 2 and the neck of the cathode-ray tube by 3. The neck 3"contaijns an electron gun 4. and a deflection system 5. T e. gm 4 comprises a cathode with an electron-emitting area '6 a control electrode 7,. a generally cup-like accel-. crating anode 8 and a ring like auxiliary electrode 9. Since the control electrode 7, in addition to the aperture from which an electron ray 10 emergens, has one or more, in this case, two furuther apertures, auxiliary electron currents 11 also strike parts 12 of accelerating anode 8, Theparts 12 are coated .with a material having. a second-. a yemissi a or h r n n y, so hat a e dyem on c rren 13 fl s r m t e pa t 2 towards th-' 1 X :f .'Y l c e Th coat n 5 may e tained' y ub equ y v p r t a silver nd m gesiu aye n the p r 2; su ye s ar l kn n from their use in image orthicons. Since the auxiliary e ectr de 9 has. a. omp elyv h h po t po ent l, he po enti l of a ce e in anode tincr as a result of the secondary-emission current 13, which. is eate han the pr ma yu c t. ;-v Th ntr ages are supplied to cathode 6 by means of a condenser, and the cathode 6 is connected to a positive voltage of V through a resistor.

When a negative-going pulse is supplied to the oathode, the voltage-difference between cathode 6 and anode 8 will be higher so that the currents 10 and 11 increase. Due to the increased secondary emission currents 13 the anode 8 will become still more positive. The voltage variations between cathode 6 and control electrode 7 are thus transmitted in amplified form to accelerating-anode 8, so that the transconductance of the gun is greatly increased, although the penetration factor of accelerating anode 8 may have the usual value (about 20%). Since it is not necessary to provide separate sealed and leadthrough conductors to a separate tube as in the known arrangements, the capacitances of the electrodes with respect to earth may remain small, so that resistor 14 may have a comparatively high value (30 kc) and hence also the amplification of the control voltage applied to ac celerating anode 8 may be high.

Although a single embodiment has been described, it will be evident that the electrodes may alternatively be differently shaped within the scope of the invention. Thus, it is possible to take steps for preventing electronemissive material evaporated from the cathode from depositing on the secondary emitting parts of the accelerating anode.

What is claimed is:

l. A cathode-ray tube exhibiting increased transconductance, comprising a cathode source of electrons, an apertured control grid, an apertured accelerating anode having a surface portion exhibiting a secondary emission factor exceeding unity and accessible to primary electrons from the cathode, an auxiliary electrode for collecting the secondary electrons generated at the anode when bombarded by primary electrons from the cathode, an impedance directly connected to the accelerating anode, and means for applying a positive potential to the impedance and thus to the anode, and to the auxiliary electrode.

2. A cathode-ray tube exhibiting increased transconductance, comprising a cathode source of electrons, a control grid having a central aperture and a further aperture spaced therefrom, an accelerating anode having a central aperture aligned with the central aperture of the control grid and a surface portion exhibiting a secondary emission factor exceeding unity and aligned with the further aperture in the control grid and thus accessible to primary electrons from the cathode, an auxiliary electrode for collecting the secondary electrons generated at the anode when bombarded by primary electrons from ode, means for applying a positive potential to the auxiliary electrode, means for applying a fluctuating control voltage between the cathode and control grid controlling the flow of electrons through the apertures inthe control grid and accelerating anode and controlling the flow of electrons to the said anode surface portion, and means 1 for applying through an impedance to the anode a positive potential below that applied to the auxiliary electrode, whereby the anode potential fluctuates in accordance with the control voltage fluctuations so as to enhance the con? trol effect on the electron flow.

4. A cathode-ray tube as claimed in claim 3 wherein 4 the control grid has one aperture aligned with that of the accelerating anode, and another aperture aligned with the said surface portion of the accelerating anode, said auxiliary anode being annular and surrounding the control grid.

5. A cathode-ray tube as set forth in claim 3 wherein the auxiliary electrode has applied to it a very high positive potential, the control grid and accelerating anode have applied to them lower positive potentials, the cathode has applied to it an intermediate positive potential, and the control voltage is applied to the cathode.

References Cited in the file of this patent UNITED STATES PATENTS 1,993,457 Schlesinger Mar. s, 1935 2,163,210 Wienecke June 20, 1939 2,173,498 Schlesinger Sept. 19, 1939 2,341,764 De Gier Feb. 15,1944 2,919,380 Barnett Dec. 29, 1959 FOREIGN PATENTS 52,296 France Dec. 1, 1943 France Mar. 16, 1955

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