US2139366A - Electron discharge device - Google Patents

Description

Dec. 6, 1938. E. w. HEROLD ELECTRON DISCHARGE DEYI O E Filed July :51, 1934 2 Sheets-Sheet 1 INVENTOR EDWARD WHHEROLD I TTO Patented Dec. 6, 1938 PATENT OFFICE ELECTRON DISCHARGE DEVICE Edward W. Herold, Bloomfield, N. J., assignor to Radio Corporation of America, a. corporation of Delaware Application July 31, 1934, Serial No. 737,692

10 Claims.

My invention relates to electron discharge devices, more particularly to electron discharge devices having a negative transconductance characteristic.

A negative transconductance tube of the type to which my invention relates is one having a plurality of electrodes and in which an increase in voltage on one electrode under the conditions that all other voltages remain unchanged decreases the current in the circuit of another electrode. Tubes of this type have various applications.

In certain types of negative transconductance tubes known as retarding field tubes, the oath: ode is surrounded by a positive grid to which a current normally flows during operation and outside which is a low potential element or group of elements. A rise in the potential of the low potential element or group of elements causes a 20 decrease in current to the grid surrounding the cathode. The tubes of this type which have heretofore been used are of the tetrode or two-grid type, with a cathode surrounded by an inner positive grid which in turn is surrounded by a negatively biasedsecond or control grid and an outer positive electrode or anode. The tube is usually operated at the saturation point of the cathode current, so that the electron emission is limited by the temperature of the cathode and during normal operation a voltage is applied to the second or control grid of such a value that the electron flow from the cathode divides equally between the inner grid and the anode. v A rise in the control grid potential increases the number of electrons reaching the anode and decreases the number returned to the inner grid. Conversely, as the potential applied to the control grid decreases more electrons return to the inner grid and less reach the anode. It is apparent that by varying the voltage on the control grid the ratio between number of electrons reaching the inner grid and the anode can be varied. 7

However, the operation of this two-grid type of tube is unreliable because of the necessity of operating at cathode temperatures lower than the normal operating temperature in order to obtain limited electron emission and high inner grid resistance, which is necessary to provide a tube having a high negative transconductance, that is a tube in which the ratio of current change in one electrode to the voltage change in another electrode is high. Another defect of this type of tube is that because the positive inner grid fails to collect all of the electrons returning from the space between the outer grid and the inner grid when the outer grid becomes more negative during operation some of the electrons collect in the space between the cathode and the inner grid and afiect the cathode current so that the negative transconductance from the outer grid to the inner grid is less than the positive transconductance from the outer grid to the outer anode, whereas it is often desirable to have these transconductances equal. The inner grid to outer grid capacitance in this type tube is quite high, which limits the use of the tube in current controlled negative resistance circuits as well as in amplifier applications. The use of this type of tube is also limited because it does not as a rule have symmetrical characteristics, the inner grid current being higher than the outer anode current at the point where the transconductances of the outer grid to the inner grid and to the anode are equal in magnitude.

It is therefore an object of my invention to provide an improved electron discharge device of the negative transconductance type, which has a high negative transconductance, symmetrical transconductance characteristics, is reliable in operation and can be used without limitation in amplitying and current controlled negative resistance circuits.

Another object of my invention is to provide an electron discharge device of the type described having increased amplification characteristics and an increased operating range over the usual negative transconductance tube.

More specifically it is an object of my invention to provide an electron discharge tube which is in effect a pair of tetrodes, one having a negative transconductanceand the other a positive transconductance.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawings in which Figure 1 is a perspective view with parts broken away to show details of construction of an electron discharge device embodying my invention; Figure 2 is a longitudinal cross section of the mount assembly shown in Figure 1; Figure 3 is .a transverse cross section taken along the line 33 of Figure 2; Figures 4 and 5 are transverse cross sec.- tions of modifications of the mount assembly shown in Figure 1; Figure 6 is a circuit employing an electron discharge device made in accordance with my invention; Figure 6a is a slightly modified circuit employing an electron discharge device made according to my invention, and Figures '7, 8, 9 and are curves showing characteristics of an electron discharge device made in accordance with my invention.

The electron discharge device shown in Figure 1 is provided with an envelope ll having the usual stem and press l2, on which is the mount comprising electrodes supported between a pair of insulating spacers l3 and I4, preferably mica. A conventional electron emitting thermionic cathode I5 is positioned within and concentric with a tubular anode 16, both of which are mounted between the insulating spacers. The other electrodes include three successive grids or foraminous electrodes concentric with and inside the tubular anode IS, an outer screen grid ll, which is optional and which may or may not be used, control grid I8 usually biased. negatively, and inner screen grid I9 which may be operated at the same potential as anode l6.

In accordance with my invention I provide inside the inner screen grid IS, a pair of anodes 20 preferably in the form of slats mounted on opposite sides of and edgewise to cathode l5 and electrically connected by conductor 2|. A pair of electrostatic shields 22 in the form of extended rectangular plates extending longitudinally of the mount and coextensive and parallel to the cathode, are positioned in close proximity to the cathode and are electrically connected together by conductor 23. They are also connected either to the cathode or to a source of negative potential with respect to the cathode, as shown respectively in Figures 6 and 6a. The purpose of these shields is to prevent electrons from the cathode l5 from reaching the inner anodes 20 without first passing between the shields 22 and thru the screening grid l9, which serves also as a space-charge grid. Control grid I8 is ordinarily operated negatively in order to prevent a grid current.

The operation of the tube is dependent upon the control ofthe electron flow from the cathode to the inner and outer anodes. With a given voltage applied to the control grid l8 the electron flow from the cathode will divide between the inner anode 20 and outer anode l6. As the control grid I8 is made more negative more of the electrons are turned back to the anodes 20, thereby increasing the current in these anodes whereas if the control grid I8 is made suificiently positive substantially all of the electrons will pass thru this grid to the outer anode IS, the current in the inner anodes 20 decreasing to substantially zero. Practically all of the electron flow reaching either the inner or the outer anode must first pass thru the grid [9. The control grid I8 is biased to normally cause substantially equal currents to fiow in the outer and inner anodes. The grid [9 serves both as a space charge grid and as an inner screen grid.

In the tube described and made in accordance with my invention the control grid is screened from both the inner and outer anodes with consequent low capacitance to these anodes. The plate impedance of each anode is high, which is desirable for amplification purposes. The inner anodes have a high resistance because of the shields 22 which prevent the inner anode potential from influencing to any large extent the cathode current and because of the screen grid l9 which reduces the effect of the inner anode potential on the effective potential of the control grid I8. It is not necessary therefore to operate the cathode at less than normal operating temperature as in the usual retarding field negative transconductance tetrode to obtain a high inner anode resistance. Negative transconductance of the inner anodes is unaffected by returning electrons influencing the cathode current to any great extent because electrons returning to the cathode are mostly all collected by the internal anodes to each side of the cathode. The control grid to anode (either outer or inner) capacitance is made very small by the shielding grids l1 and I9 and since the inner anode current is only that returned by the retarding field of the control grid, the inner anode current is practically equal to the outer anode current at the point where the transconductances are equal, thus the tube has a more symmetrical characteristic with respect to the transconductance and the inner and outer anode currents than the usual negative transconductance tube.

With respect to secondary emission from the internal anodes I have found it possible in a tube made in accordance with my invention to operate the space-charge screen grid l9 at such a low positive potential that in normal operation the potential on the inner anodes never swings below the potential on the screen. Because of this, secondary emission from the inner anodes is of small consequence since the secondary electrons return to the inner anodes, which are at a higher potential than the screen grid. Because of the small voltage impressed on the space-charge screen grid l9, secondary emission from this grid is also very small. Under these conditions a suppressor grid is often unnecessary. However, in the modification shown in Figure 4 the inner anodes 30, which are in the form of rods, have positioned around them suppressor grids 3| which effectively prevent any undesirable effects from whatever small secondary emission there may be from the anodes 30. The cathode shields 32 are curved inwardly around the cathode.

In Figure 5 the cathode shields 32 are also curved and the inner screen shield 33 is formed to lie closely adjacent to the inner anodes 29 and the shields 32, thus providing very effective shielding of the inner anodes from a direct electron current from the cathode. The control grid 34 and screen grid 35 have an elliptical shape.

The application of an electron discharge device made in accordance with my invention to a pushpull circuit is shown in Figure 6. One side 35 of an input circuit is connected to the control grid l8 and the other side 31 connected to the cathode l5 thru the biasing battery 38. The inner screen grid l9 and outer screen grid I! are biased positively with respect to the cathode by means of the battery 39, the inner and outer anodes having the same positive potential applied thereto by means of the source of voltage supply 39 and 40. In this arrangement, as the current to the anode l6 decreases the current to the anodes 20 increases, the reverse being also true, so that the well known push-pull action is obtained in the transformer M by using only a single tube instead of two tubes which are used and must be carefully watched in the conventional push-pull circuit to obtain satisfactory results.

The voltage current characteristics of the inner anodes and outer anode of a tube made in accordance with my invention is shown in Figures '7 to 10 inclusive. It will be observed that, as the control grid voltage becomes increasingly positive, the inner anode current decreases while the outer anode current increases, and vice versa, and that these curves are substantial opposites of each other indicating symmetry.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:-

1. An electron discharge device having a thermionic cathode, a foraminous electrode surrounding said cathode, an anode positioned to one side of said cathode inside said foraminous electrode, an electrostatic shield between said cathode and said anode, a second anode outside said foraminous electrode and another foraminous electrode positioned between said second anode and said first foraminous electrode.

2. An electron discharge device having a thermionic cathode, a foraminous electrode surrounding said cathode, a pair of anodes, one of said anodes positioned inside of said foraminous electrode and to one side of said cathode and the other of said anodes outside of said foraminous electrode, an electrostatic shield between said one of said anodes and said cathode and a second foraminous electrode between the other of said anodes and said first foraminous electrode.

3. An electron discharge device having a thermionic cathode for emitting electrons, an anode positioned to one side of said cathode for receiving electrons from said cathode, a foraminous electrode surrounding said cathode and anode, an electrostatic shield between said cathode and said anode, a second anode outside of said foraminous electrode, and a second foraminous electrode positioned between said second anode and said first foraminous electrode, and a third Iloraminous electrode positioned between said second foraminous electrode and said second anode.

4. An electron discharge device having a thermionic cathode for emitting electrons, anodes positioned on opposite sides of said cathode for receiving electrons from. said cathode, a foraminous electrode surrounding said cathode and anodes, an electrostatic shield between each of said anodes and cathode, an anode outside said foraminous electrode for receiving electrons from said cathode and a second foraminous electrode between said last anode and said first foraminous electrode.

5. An electron discharge device having a thermionic cathode for emitting electrons, anodes positioned on opposite sides of said cathode for receiving electrons from said cathode, a foraminous electrode surrounding said cathode and anodes, an electrostatic shield between each of said anodes and the cathode, grids surrounding each of said anodes, an anode on the outside of said foraminous electrode, and a second foraminous electrode between said last anode and said first foraminous electrode.

6. An electron discharge device having a thermionic electron emitting cathode, an anode positioned to one side of said cathode for receiving electrons from said cathode, a grid electrode surrounding said cathode and anode, an electrostatic shield between the cathode and anode and means for biasing said electrostatic shield whereby during operation electrons moving from the cathode to the anode must first pass thru said grid electrode, an outer anode for receiving electrons from said cathode, a second grid electrode between the first grid electrode and said outer anode and means for applying a voltage to said second grid electrode, and a voltage source for said anodes, the electrons from said cathode dividing between the first anode and the outer anode in accordance with the voltage on said second grid electrode.

7. An electron discharge device having a thermionic cathode for emitting electrons, anodes on opposite sides of said cathode for receiving electrons from said cathode, a grid electrode surrounding said cathode and anodes, an electrostatic shield between each of said anodes and the cathode and means for biasing said electrostatic shield whereby during operation electrons from said cathode are made to first pass thru said grid electrode to reach said anodes, an outer anode for receiving electrons from said cathode, a voltage source for said anodes a second grid electrode between said first grid electrode and said outer anode, and means for applying a potential to said second grid electrode to determine the ratio of electrons moving from said cathode to said outer anode and to the anodes on opposite sides of said cathode.

8. An electron discharge device having a thermionic cathode, anodes positioned on opposite sides of said cathode, a grid electrode surrounding said cathode and anodes, means for positively biasing said grid electrode with respect to said cathode an electrostatic shield between each of said anodes and the cathode and means for biasing the electrostatic shields whereby during operation electrons from said cathode are made to first pass thru said screen grid to reach said anodes, an outer anode for receiving electrons from said cathode, a second grid electrode between said first grid electrode and said outer anode, a voltage source for said anodes and a third grid electrode between said second grid electrode and said outer anode, and means for applying a potential to said second grid electrode for determining the ratio of the electrons moving from the cathode to said outer anode and the anodes on opposite sides of said cathode.

9. An electron discharge device having a thermionic electron emitting cathode, flat inner anodes positioned edgewise to and on oposite sides of said cathode for receiving electrons from said cathode, electrostatic shields coextensive with and curved inwardly around said cathode and positioned between each of said fiat inner anodes and cathode, and a screen electrode surrounding said cathode shields and fiat inner anodes and formed to lie closely adjacent to said shields and said fiat inner anodes, an outer anode for receiving electrons from said cathode and acontrol grid between said outer anode and said screen electrode.

10. An electron discharge device having a thermionic cathode, an electrostatic shield adjacent said cathode, an anode on the other side of said electrostatic shield from said cathode, a grid electrode surrounding said cathode, electrostatic shield and anode, a second anode outside of said grid, and a second grid between said second anode and said first grid.

EDWARD W. HEROLD.

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