US2256293A

US2256293A - High frequency electron discharge device

Info

Publication number: US2256293A
Application number: US306781A
Authority: US
Inventors: Salzberg Bcrnard
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1939-11-30
Filing date: 1939-11-30
Publication date: 1941-09-16
Anticipated expiration: 1958-09-16

Description

Sept. 16, 1941. B. SALZBERG HIGH FREQUENCY ELECTRON DI-SCH-ARGE DEVICE Filed NOV. 30, 1939 2 Sheets-Sheet 1 I INVENTOR. BERNA RD SALZBERG ww dr ATTORNEY.

p 1 1941- 2 B. SALZBERG 2,256,293

HIGH FREQUENCY ELECTRON DISCHARGE DEVICE Filed NOV. 50, 1939 2 Sheets-Sheet 2 Fly. 4 2 1 1 5 INVENTOR. BERNARD SALZBERG ATTORNEY.

Patented Sept. 16, 1941 cars s'r HIGH FREQUENCY ELECTRON DISCHARGE DEVICE eras Bernard Salzberg, East Orange, N. J., assignor to Radio Corporation ofZAmcrica, a corporation of Delaware Application November 30, 1939, Serial No. 306,781

7 Claims.

When conventional electron dischargedev-ices,

such as radio tubes, are used at high frequencies the input loading increases as the operating frequency increases. This loss in the input circuit of the tube is the result of the increase of the input conductance and is due to a major extent to the cathode lead inductance, because this cathode lead inductance is common to the plate and grid return circuits.

It is now generally known that the inductance of the cathode lead of an amplifier or converter tube, which is common to both plate and control .g-rid circuits, represents a degenerative coupling between these circuits. This degeneration exhibits itself, when ordinary tubes are used, as a resistance which appears in shunt with the input terminals of the tube and which decreases as the frequency increases. The common cathode lead inductance may be due to the length-of the internal cathode lead, or to the length of the connecting wire, or to both. -The effective input conductance is, to a first approximation, directly proportional to this inductance. It also depends to a first approximation on the's uare of the operating frequency and on the cathode transconductance of the tube. The cathode transconductance is .equal to the sum of the tube transconductance and the product of the tube transconductance by the ratio of the screen grid to plate current. form of input loading probably accounts for a considerable portion, if not actually the major portion of the input loading in many commercial tubes.

It is, therefore, the principal object of my invention to provide an electron discharge device and associated circuit in which loading effects, due to the cathode lead inductance, are eliminated or substantially reduced.

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 tube and circuit diagram illustrating the problem solved by my invention, Figure 2 is a diagrammatic representation of-a tube and circuit made according to my invention, Figure 3 .shows a tube diagrammatically represented and its associated circuit made in accordance with my invention, Figure 4 is a longitudinal section of a modification of a tube and its associated circuit made according to my invention, Figure 5 is a longitudinal section .of another form .of electron discharge device made according to my invention and Figure 6 is a transverse section along line 6-6 of Figure 5.

In Figure 1 is shown the schematic arrangement of a typical amplifier stage. In this figure the tube is provided witha cathode Ill, prefer-ably indirectly heated (heater not shown) control grid .ll, screen grid 12, suppressor grid l3 and anode or plate it. The input circuit I5 is connected between the .grid .H and the cathode ill, the cathode being coupled to the secondary or the input circuit by coupling condenser 01. The output circuit is is connected between plate 14 and the usual voltage supply source H, the .bypassing condensers c3 and C2 being used between plate, screen and cathode, the voltage source 1 1 supplying the usual voltages for biasing the control grid and screen grid. The common cathode lead inductance is represented by an inductance Land may be either due to the length of the internal cathode tube lead, or to the length of connecting wire, or to both; It is because of the inductance of this lead and connecting wire that the degenerative action exhibits itself.

In accordance with .my invention one way of solving the .diificulties presented is by means of the .circuit arrangement and tube shown in Figure 2. Here the tube is provided with two separate leads, the inductan'ces of which are represented by L1 and L2. 'Iwo A. C. return paths are indicated at I and II. In the arrangement shown the two leads are provided, one for the plate and screen-grid return, and the other for the controlgrid return. If the mutual inductance between these tw'oleads is negligible, the common cathode impedance is substantially eliminated and the input loading due to this cause will be practically non-existent. However, if tubes provided with two cathode leads are used in practical amplifier .or converter stages, the input loading would not necessarily be greatly improved. The reason is that if the impedance of the path TI is small as compared with the impedance of path I, as it may be in an actual circuit, most :of the A. C. plateand screen grid return current will flow through path II and hence the lead for the grid return. Iiit does so, then the object of the two leads is not fully realized for the first oathode lead again constitutes a common inductance. With my invention, however, this difliculty is removed -by inserting choke coils or resistors as inicated at I9, 20, 21 and 22. In this way the conductance of about 100 amhos.

'rectly to the cathode within the tube.

-metal plate 28.

A. C. return current is prevented from flowing through the path II to the cathode lead L1 and is forced to flow through the separate return lead L2. The impedances 2i and 22 can be used alone or the impedances l9 and 20 used alone. These additional circuit elements present a high impedance to the return currents of the operating radio frequencies and preferably though not necessarily a low impedance to the DC. return currents.

The operation of the tube can be still further improved by connecting the by-passing condenser l8 directly to the cathode as shown in Figure 3 at l8.

In one successful example of my invention in which the screen grid lead was by-passed to the control grid-cathode return lead the impedance 2| was a 60,000 ohm resistor and impedanc 22 a 5,000 ohm resistor, the by-pass condensers were all of a value .002 mfd. With this arrangement I measured an input conductance of about 130 umhos at 60 me. as compared to 500 cmhos in a conventional tube for the same value of plate current. The improvement was therefore, of a ratio of 4 to 1.

Carrying the above teaching further, the cathode may be provided with as many leads as there are electrodes in addition to the cathode. These leads can then be each separately connected as shown in Figure 2 bymeans of by-pass condensers to one of the other electrodes using suitable chokes or resistors to keep the radio frequency currents out of common paths due to common leads.

However still further improvement can be obtained. The A. C. screen grid current in the arrangement above still flows through the first or. control grid cathode lead, and since the ratio of the screen grid to cathode current was about 1 to, 5 in the tube tested, the degenerative effect of the screen grid current gave rise to an input This would leave roughly 30 ,umhos for the input conductance of the tube if the degenerative effect 'of the green grid current could be eliminated. The over-all improvement might then be of the order of 16 to 1 under these conditions.

A tube in which this improvement can be obtained is shown in Figure 3 in which the tube is provided with two cathode leads and an internal by-pass capacitor connected inside the envelope directly between the screen grid and cathode. The object of this by-pass capacitor is to prevent the effect of the screen grid impedance by connecting this electrode for A. C. di-

This eliminates the degenerative efiect of the screen grid current and insures better. shielding at high frequencies. The capacitors can be of two types; namely, commercial capacitors mounted on the tube structure, or built-up capacitors which make use of mica spacers at each end of the mount. This last arrangement is shown in Figure 4.

Here the cathode l0, control grid ll, screen grid [2, suppressor l3 and anode I4 are mounted coaxially and in the order named between a pair of mica spacer members 25 and 25'. Alternate metal plates 28, 3|, 34 and

micas

30 and 33 at the bottom of the mount provide the by-passing condensers required for the purposes'set forth above.

It will be noted that the screen grid I2 is connected by means of connector 29 to the The cathode is connected to the metal plate 3| by connector 32 and the suppressor grid to the metal plate 35, so that in effect a by-pass condenser is connected be tween the cathode and screen and between the cathode and suppressor which last grid may be maintained at a potential less than that of the screen and higher than that of the cathode through the impedance l3. This arrangement of icy-passing condensers can be duplicated at the top of the mount by means of the alternate metal and mica plates 28', 3|, 34', 30, and 33, the connections being made by connectors 29', 32' and 35'. The cathode isagain provided with two

leads

26 and 21. The impedances i9, 20', 2! and 22, as well as 13, being shown as resistors instead of inductances as shown in Figure 2. It is understood that impedances 2 I and 22' can be used without impedances l9 and 20, and that impedances l9 and 20 can be used without 2| and 22 if desired.

A modification 0f the arrangement shown in Figure 4 is shown in Figure 5. The metal plate 45 is a complete disc and is electrically connected to cathode Ill by connector 46. The plates connected to the screen and suppressor grids however are half discs as best shown in Figure 6, half disc 40 or segment 40 being connected by the electrical connection 4! to the suppressor grid and the disc 42 to the screen grid by means of connector 43, these metal plates being electrically insulated from each other by mica spacer 44. This arrangement could of course be duplicated at the top of the mount.

The degenerative effect of the A. C. screen grid current can be still further reduced by using tubes designed to have a low screen grid current. It is now generally known that such tubes also possess the virtues of reduced fluctuation noise and increased efficiency. 7

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

1. An electron discharge device having an envelope containing a cathode and an anode,

and a plurality of grids positioned between the cathode and anode and including a signal grid, screen grid and suppressor grid, said cathode having a plurality of leads extending between the cathode and the exterior of the envelope, mica spacers at opposite ends of the mount for spacing the cathode, anode and grids, an additional mica spacer and a conducting plate on each side of said additional mica spacer all adjacent one end of said mount, one of said conducting plates being electrically connected to the screen grid and another of said plates being electrically connected to the cathode.

2. An electron discharge device having an envelope containing a cathode and an anode, and a plurality of grids positioned between the cathode and anode and including a signal grid, screen grid and suppressor grid, mica spacers at opposite ends of the mount for spacing the cathode, anode and grids, alternate mica spacers and conducting plates being adjacent each end of said mount, one of the conducting plates at each end of the mount being electrically connected to the screen grid and another of said plates at each end of the mount being electrically connected to the cathode.

3. An electron discharge device having an envelope containing a cathode and an anode, and a plurality of grids positioned between the cathode and anode and including a signal grid, screen grid and suppressor grid, said cathode being provided with at least two leads extending between the cathode and the exterior of the envelope, mica spacers at opposite ends of the mount for spacing the cathode, anode and grids, alternate mica spacers and conducting plates adjacent one end of said mount, one of said conducting plates being electrically connected to the screen grid and another of said plates being electrically connected to the cathode, and a third plate being connected to the suppressor grid, the plate connected to said cathode being between the plates connected to the suppressor grid and the screen grid.

4. An electron discharge device having an envelope containing a cathode and an anode, and a plurality of grids positioned between the cathode and anode and including a signal grid, screen grid and suppressor grid, mica spacers at opposite ends of the mount for spacing the cathode, anode and grids, alternate mica spacers and conducting plates adjacent each end of said mount, one of said plates at each end being electrically connected to the screen grid and another of said plates at each end being electrically connected to the cathode, and a third plate at each end being connected to the suppressor grid, the plates connected to said cathode being between the 'plates connected to the suppressor grid and the screen grid.

5. An electron discharge device having an envelope containing a mount including a cathode and an anode, and a plurality of grid electrodes positioned between the cathode and anode and comprising a signal grid, screen grid and suppressor grid, mica spacers mounted at opposite ends of said mount for spacing the anode, cathode and grid electrodes, a conducting plate extending across one end of said mount and electrically connected to said cathode, a pair of semicircular conducting plates positioned adjacent said first conducting plate and insulated therefrom by means of an insulating spacer, and an electrical connection between the suppressor grid and one of said semi-circular plates, and an electrical connection between the screen grid and the other semi-circular plate.

6. An electron discharge device for use at high frequencies and having a cathode and anode, and a control grid, and a screen grid between the control grid and anode, a plurality of leads connected to said cathode, one of said leads being adapted to be connected to an input circuit and a by-passing condenser connected between the screen grid and another of said leads.

7. An electron discharge device for use at high frequencies and having a cathode and anode, and a control grid positioned between the cathode and anode, and a plurality of other grids positioned between the control grid and the anode, a plurality of leads connected to said cathode, one of said leads being adapted to be connected to an input circuit and another of said leads being adapted to be connected to an output circuit, and a condenser directly connected between the cathode and one of said other grids between the control grid and anode.

BERNARD SALZBERG.