US2311981A - Electron control device - Google Patents

Description

Feb. 23, 1943. p, swo l-l 2,311,981

ELECTRON CONTROL DEVICE Filed July 25, 1940 2 sheetssheet l INPUT 8 OUTPUT INVENTOR T. FARSWORTH Feb. 23, 1943.

P. T. FARNSWORTH 2,311,981

ELECTRON CONTROL DEVICE Filed July 25, 1940 2 Sheets-Sheet 2 F G. 3 A CURRENT TIME A FIG.4

CURRENT INVENTOR T. FARNSWORTH ATTORNEY Patented Feb. 23, 1943 ELEGIRON CONTROL DEVICE Philo T. Farnsworth,

Fort Wayne, Ind., assignor to Farnsworth Television and Radio Corporation, a corporation of Delaware Application July 25, 1940, Serial No. 347,413

3 Claims.

Y i'I'his invention relates to electron control deces.

The principle of effecting electron multiplication by causing successive secondary emission by electron impact has proven an efiective means for intensifying an electron flow. In tubes of the image dissector and photomultiplier type, this principle has been used most advantageously. In such arrangements, a series of secondary-emissive surfaces is maintained at increasing positive potentials and an electron emitted from one surface is accelerated to strike against a next succeeding surface with a velocity sufiicient to eject a plurality of secondary electrons. This action is repeated in succeeding stages, producing increasing intensification of the electron stream. For example, this increase may be at a rate of the order of 3 to 7, depending upon the voltage between stages and, of

course, upon the electron-emissive material em-' ployed for the electrodes.

Heretofore, this principle of amplification has been useful in the simple form above described only for the purpose of amplifying an electron stream such as, for example, the modulated electron stream produced at the scanning aperture of an image dlssector tube or produced from the cathode of a photoelectric cell.

It is desirous to utilize the electron multiplication principle not only for simple arrangements providing current amplification but. to provide voltage amplification in various electron control arrangements, such as voltage amplifiers, oscillators, modulators, detectors and the like.

Heretofore, certain electron multiplier arrangements have been utilized wherein the first secondary electron emitting electrode constituted the anode of a conventional thermionic tube. In

such arrangements, the electron fiow to the first secondary-emissive electrode was varied by impressing a signal voltage upon the control grid disposed between the cathode and this electrode. While this method is both simple and useful for some applications, it is subject to certain. serious limitations hereinafter more specifically described The total output current of an electron multhermionic amplifier of the conventional type comprising a plurality of stages. If the current in the first stage of such an amplifier coinprises an A. C. component and a D. C. component, whereby a portion of the latter is not modulated by the A. C. signals, both components will be proportionately amplified in subsequent stages. In a resistance capacity coupled thermionic amplifier of the conventional type, it is possible capacity coupled amplifier can obtain higher values than in the direct coupled amplifier. Since substantially the same conditions prevail for an electron multiplier as for a direct coupled thermionic amplifier, the disadvantage of such devices as hitherto constructed and operated for voltage amplification is evident.

In order to reduce the unmodulated portion of the D. C. component in an electron multiplier, it

has been proposed to operate the conventional thermionic input electrode structure of the multiplier in the so-called retarding field region,

that means, at an extremely low plate current.

However, if the total output current is limited to a given value, this measure also has no particular advantage for voltage amplification since it is impractical to utilize a current multiplication ratio of over 20 for a total permissible output current of 5 milliamperes. At 5 milliamperes output current and a multiplication ratio of 20,

the input current to the multiplying structure is 250 microamperes. A thermionic input electrode structure having a mutual conductance of 500 micromhos can be readily designed for an anode current of 250 microamperes, and no advantage would be derived from the use of a weaker plate current and a poorer mutual conductance, which would then only necessitate an increase to the above value by means of electron multiplication.

It is an object of the'present invention to provide an improved electron control device utilizing the principle of secondary emission electron multiplication and suitable for use as a voltage amplifier, oscillator, modulator, detector or other electron control device.

In accordance with the 1 present invention, there is provided a. vacuum tube device comprising a plurality of electrodes disposed in se-- ries. Means are further provided for effecting the passage of an electron current successively between the electrodes, which means includes means for applying. to successive ones of said electrodes successively higher operating potentials providing predetermined potentialgradients therebetween. These means may include, for the purpose of developing an initial electron current, any suitable source of electrons such as,

for example, a thermionic cathode'or a photo-. electric cathode.

electron control device, embodying the present invention, utilized as an amplifier;

Fig. 2 is an exploded view of certain elements of the device of Fig. 1;

'Fi'g. 3"is a graph illustrating the operating characteristics of a' conventional thermionic grid control type of electron multiplier and of a multiplier in accordance with the present. invention; and

Fig. 4 is a graph illustrating the operating characteristics of a resistance capacity coupled amplifier comprising several stages.

Referring now more particularly to the preferred embodiment of the invention which is illustrated in the accompanying drawings, in Fig. 1 there is shown schematically an. electron multiplier device embodyingan. envelope i containing a photoelectric cathode 2 and a series ofsecondary-emitting electrodes 2A, 38, 2C, and 2D, arranged in juxtaposition, as shown. More particularly, each of. these electrodes comprises an L-shaped member having disposed on the inner surface thereof a material, preferably caesium oxide, adapted to emit photoelectrons orsecondary electrons. The electrodes iii-3D are also provided with a screen portion as shown. Adjacent the electrode 3D. are positioned an electro: collecting screen l and aplane sec ondary-electron emissive electrode 4.

The electrodes 2, 3A, 2B, 3C, 2D, 4 and ID are held in position by suitable conducting rods 5A, 5B, 5C, SD, 5E, 5F, and 56, which extend through a re-entrant stem 6 formed in the envelope l as shown. Disposed opposite the opensides of the electrode assembly are controlfelectrodes I and8 (Figure 2) each of which comprises a thin metallic plate provided with aninsulating surface IA and 8A respectively, facing the elec- For the purpose of providing trode structure. an initial source of electrons, there is disposed outside the tube envelope I .a source of light in the form of an incandescent lamp ll having a lens portion I IA fused to its glass bulb. The lamp is so positioned as to direct its light upon the inner photoelectric surface of the electrode 2.

For the purpose of applying operating potentials to the electrodes 2, 3A, 3B, 3C, 3D,! and There'is disposed, outside of v the. path of said current, suitable means for l0,there is provided a voltage divider l2 having 1 its opposite ends connected to a source of unidirectional voltage indicated at 13, I4. Taps II are connectedat suitable points alongthe voltage divider by way of suitable leads to the connecting rods 5A, 5B, 5C, SD, 5E, 5F, and "G and to the several electrodes, as shown. A slidable tap I6 is provided on the voltage divider 12 a resistor ll whose function will be described shortly. An operating potential is app ied to the collector electrode III by way of a suitable repedance path for the amplified signal currents.

The photoelectric electrode 2, acting as the cathode of primary electrons, is connected to the most negative fixed tap I! on the voltage divider by means of a suitable lead, as indicated.

For the purpose ofv applying a signal to the control electrodes land 8, an lnputcircuit indicated by the terminals and -2! is provided, the terminal 2| being preferably grounded as shown. and the terminal," being connected .by way of a suitable coupling condenser 22 .to the electrodes I and l.

The electron multiplier device which has been described is suitable for use as an amplifier. particularly an amplifier of radio-frequency signals. In the operation of this device, withsufficient light impinging upon the photoelectricsurface of the electrode 2, electrons are emitted therefrom and directed toward the first electrode 3A. These electrons impinge upon the secondary emissive surface of this electrode,.thereby Y causing a plurality of electrons to beemitted therefrom which in turn are directed, by virtue of the higher operatingpotential applied thereto, to the secondary-emlssive electrode 23 where again the impinging electrons effect the emission of secondary electrons, thus effecting a multiplication of the current. This process is repeated throughout the succeeding stages, afinal multiplication taklngplace at the electrode -The screens attached to electrodes IA, 3B,.8C,

2D serve to accelerate the flow of electrons.

The signal to be amplified is applied to the terminals "and 2|v and hence, by way oft-condenser 22 and resistor l'|,=to the control electrodes 1 and 8. This signal effects a variation in the voltage gradient between the successive electrodes, and thereby. modulates the stream of f electrons with successive multiplication through the path along which. these electrons are .disposed. The modulated multiplied electron stream is collected by the electrode I 0 and produces the amplified output signal across theoutput resistor 18, to the ends of which the outputter minals23, 24 are coupled by way of blocking condensers 25 and 26, respectively. The advantages obtained by the tubev embodying the presentinvention may be best understood by reference to the curves of Figs. 3 and I In Fig. 3, curve I shows the current in the first tensification in a third multiplier stage. In each case the ordinates have been marked with the plier, according to the letters A, B and C, designating the alternating component of the current, the direct-current component and the unmodulated portion of the direct-current component respectively. These curves show that the ratio of the alternating component to the direct-current component remains constant throughout the multiplier and, while the absolute value of the mutual conductance increases, the ratio of the mutual conductance to the direct-current component remains constant. Thus, with a limited output current, a larg portion of the direct-current component is not utilized for signal amplification due to the low value of the ratio of mutual conductanceto direct-current component.

In Fig. 3, curves I, 2' and 3' show the current in three successive stages of an electron multiplier in accordance with the present invention. From these characteristics it is to be seen that the ratio of the alternating component of the current to its direct-current component increases from stage to stage since, with the particular type of electron control providedby the present invention, the ratio of mutual conductance to direct-current component increase in each successive multiplier stage, as will be shown presently.

In Fig. 4, curves I, 2 and .3 illustrate the plate current in three successive stages of a conventional resistance capacity coupled thermionic amplifier. It is readily seen, by comparison of the curves I, 2 and 3 of Fig. 3 with those of'Fig. 4, that the operating characteristics of the multipresent invention, are similar to those of a conventional thermionic voltage amplifier. However, the use of an electron multiplier for this purpose has the great advantage of being a far simpler means of accomplishing voltage amplification.

In order to point out more specificallythe advantage of the electron multiplier of the present invention over multipliers hitherto constructed, it can be shown mathematically that the ratio of the over-all mutual conductance to the directcurrent component in the output stage is n times greater than the same ratio for the input electrode structure, whereby n designates the number of stages in which the electron stream has been multiplied. Since this ratio is constant in multipliers in which electron control is eilected in one stage only, a tremendous increase in the ratio of mutual conductance to direct-current component is obtained in multipliers in accordance with the present invention.

Considering now the method of electron control utilized in the present invention, it is found that the eifect of the potentials applied to the control electrodes 1 and 8 upon the intensity of the electron stream is substantially the same as that produced by potentials applied to a .conventlonal control grid disposed in the path of the electron stream. Thus, for a steady condition, the following laws are valid:

ff= wim and 1': OX lief C wherein Eeff designates the effective voltag between two successive electrodes, E9 the control electrode potential, Ep the steady potential difference between successive electrodes, u the amplification factor, i the current flowing between successive electrodes, and C a constant.

' the electrode 3B,

sliding tap IS on insulated surfaces Considering first the term Ey, it is round that a different steady potential of the-control electrode .is required for each multiplying stage to obtain the same bias potential, so that substanment or the invention, the metallic portions of the controlelectrodes l and 8 are held at the proper bias potential for the first multiplier stage, comprising electrodes 2 an 3A, by means of the the voltage divider I 2, While the control electrode is shown to be negatively biased lm'th respect to the photoelectric cathode 2, it may conveniently beadjusted to the proper polarity and magnitude or bia by adjusting the sliding tap I 5.

While the bias for the first multiplying stage is shown as what is commonly referred to as battery bias, the following stages are self-biased as will be explained presently. Opposing the open sides of the electrodes 3A, 3B, 3C, 3D, there is disposed the insulated surfaces 7B and 8B of the control electrodes 1 and 8. A number of electrons emitted by the electrode 3A impact the 1B and 8B and produce secondary emission therefrom which is attracted to which is the most positive electrode inthe close proximity of the point of km pact. According to whether the secondaryemission ratio is greater or smaller than 1, the insulated surfaces become increasingly positive or negative and finally reach an equilibrium poelectrons. The value or secondary emission just referred to determines whether the equilibrium potential will approach that of the more negative electrode 3A or hat of the more positive electrode 3B.

The thickness of the insulation 18 and 813 determines the capacity between its exposed surface and the metallic portion of the control electrodes 1 and 8. This capacity is one of the factors entering into the time constant of the self-biasing arrangement comprising the insulation 1B and 8B and'the input-resistor l1. Since the capacity is inversely proportional to the thickness of the insulation, thinner insulation is used for tubes to be operated at lower frequencies.

In'this manner, the control electrode bias for each multiplying stage automatically adjusts itself to a predetermined value with respect to the direct-current operating potentials of the adiacent electrodes.

Referring now to the second equation above set forth, it may be noted that the current flow throughout-the multiplier does not remain constant but is intensified in each successive stage.

The following then holds true for the n stage, designating any stage in the multiplier tube con- Egn the steady grid bias of the n stage, an the amplification factor of the n stage and Epn the steady voltage applied between the electrodes of the n multiplier stage.

This shows that with increasing current the term l Epn number of stages, C

must also increase, since Eg remains constant for each stage,'as previously. described. This renders three possibilities. for operation of the device: First, that the amplification factor remains constant a'nd the voltage between successive electrons is successively. increased; second, that the potentialubetween successive electrodes remains constant and the amplification {actor forsuccessive stages is decreased; and, third, that both are varied as Just described. The proper variations are readily computed from the above equation.

However, it is not the object of this description v to showthe mathematics of such a computation.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from i the invention, and it is, therefore, aimed in the appended claims to cover all such changes and cent said electrodes and disposed outside the path of said current for" controlling said electrostatic fields to control said current, means for applying an input signal to said control .means and an output electrode for utilizing said controlled current. j y

2. A vacuum tube. device comprising a'plurality of secondary-electron emissive electrodes disposed in series, means for eiiectingthe passage otan electron current successively between said electrodes, including meansior applying to suecessive ones of said electrodessuccessively higher operating otentials. roviding electrostatic fields therebetween, control means adjacent said electrodes and disposed outside and extending at least over a portion of thepath of said current for controlling said electrostatic fields to control said current, means ,for applying an input signal modifications as fall within the true spirit and scope of the invention.

Iclaim:

l. A vacuum tube device comprising a -plurallty of secondary-electron emissive electrodes disposed in series, means for effecting the passage of anelectron current successively between said electrodes, including means for applying to successive ones of said electrodes successively higher operating potentials providing electrostatic fields therebetween, control'means adiato said control means and an output electrode 3. A vacuum tube device comprising a plural: lty of secondary-electron emissive electrodes distor utilizing said controlled current,

posed in series, me'ans ior efiecting'the'passage least over a portion oi .the path of; said current for controlling said electrostatic fields to control said current, means ior applying an input signal to said control means thereby to-develop a signal current, and an output electrode for utilizing said signal current.

Pimp 'r. FARNSWOR'I'H.

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