US2155844A

US2155844A - Magnetron amplifier

Info

Publication number: US2155844A
Application number: US91204A
Authority: US
Inventors: Serrell Robert
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1936-07-17
Filing date: 1936-07-17
Publication date: 1939-04-25
Anticipated expiration: 1956-04-25

Description

Patented Apr. 25, 1939 UNITED STATES MAGNETRON AMPLIFIER Robert Serrell, Haddonfield, N.

Radio Corporation of Delaware Application July 17,

8 Claims.

My invention relates to magnetron amplifiers. More particularly my invention is a magnetron amplifier in which the magnetic field is automatically compensated to maintain optimum amplification.

A magnetron is comprised of a cylindrical anode, and a small cylindrical cathode concentrically mounted within an evacuated envelope. A magnetic field is applied so that its lines of force surround the cathode and are substantially parallel thereto. If the cathode is heated and voltage applied between cathode and anode, electrons emitted from the cathode and under the influence of the magnetic field will follow curved paths to or towards the anode.

The curvature of the path depends upon the physical arrangement of the cathode and anode, the applied voltage, cathode emission and density of the applied magnetic field. If the'magnetron 20 elements are suitably proportioned, the number of electrons reaching the anode can be varied from a maximum to zero by simply increasing the strength of the magnetic field.

One of the objects of my invention is to provide 25 means for utilizing the relation between the anode current and the applied magnetic field to effect uniform amplification over a wide frequency band in a magnetron.

Another object is to provide means for maintaining a linear relationship between the applied field and anode current in a magnetron amplifier.

A further object is to provide, in a magnetron amplifier, an input circuit which varies the magnetic field and a resistive anode circuit which includes means, controlled by the varying anode voltage, for maintaining a predetermined relationship, over a frequency range of a million cycles and upward, between input and output currents in said amplifier.

My invention may be best understood by reference to the accompanying drawing in which Fig. 1 r is a schematic circuit diagram of a magnetron amplifier embodying one application of my invention, and

Fig. 2 is a set of characteristic curves showing the relation between the anode current and applied magnetic field of a magnetron.

In Fig. 1 an input circuit is represented as a solenoid I. The solenoid may be coaxially posi- 0 tioned with respect to the electrodes of a magnetron 3. The magnetron 3 is comprised of an evacuated envelope 5, a cathode 1, a cylindrical anode 9 coaxially arranged with respect to the cathode, and a main magnetic field supplied by a suitably energized solenoid it which is shown J., assignor to America, a corporationof 1936, Serial No. 91,204

partly in section. The cathode is energized by a battery I I which is grounded. The anode 9 is connected through an output circuit resistor 33 to the positive terminal of an anode battery is. The negative terminal of this battery is grounded. An output circuit I1 is connected between the anode 9 and ground I9. The resistive output circuit may be made substantially independent of frequency over a range of a million cycles and upward. That is the output circuit is substan- 10 tially independent of reactive components, or has an impedance which is substantially independent of the frequency of the applied currents.

A compensating magnetic field for the magnetron is supplied by a solenoid 25. This solenoid may surround the magnetron 3 and be coaxially and symmetrically arranged with respect to the input solenoid l and main solenoid iii. The sole noid 2| is connected between the anode 23 of thermionic tube 25 and the positive terminal of an anode battery 21. The negative terminal of the battery 2'! is connected to the anode Q or the magnetron 3.

The suppressor grid 29 of tube is connected to the cathode 32 which is energized by a battery 25 33. The screen grid 35 is connected to the positive terminal or a battery 31. The negative terminal of the battery 31 is joined to the positive terminalof the magnetron anode battery 55. The control grid 39 of thermionic tube 25 is biased by 3 an appropriate connection located intermediate the terminals of the anode battery i5, or the control grid may be suitably biased by'a separate battery.

The relation between anode current in milli- 5 amperes and the magnetic field in gauss of a conventional magnetron is represented by the graph A of Fig. 2. This characteristic is based upon an anode circuit of negligible resistance and a constant anode potential. If a resistance is 4 included in the anode circuit of the magnetron, the variation of anode current with variations of the applied field will alter the characteristic curve. Because the anode potential is no longer constant but varies, a magnetic field of varying strength is required to maintain the characteristic A. That is, as the anode potential varies, other factors remaining constant, the characteristic curve changes, as indicated by the graphs B and C of Fig.2.

Such Variation in the characteristic curve would be undesirable in an amplifier because a less abrupt slope or shorter slope would decrease the amplification factor and a curving or non-uniform characteristic would introduce distortion. 55

If the characteristic A is to be maintained and including cathode and anode electrodes, a source distortion avoided, the main magnetic field must be compensated.

I have found that suitable compensation may be obtained by a thermionic tube and circuit, such as the pentode connection illustrated in Fig. 1. As is shown, the bias on the grid 39 is fixed; under these circumstances the plate or anode current of the pentode is proportional to the square root of the anode voltage. The compensating coil 2| therefor has increments of current flowing through it which are proportional to the square root of the signal voltage across the anode resistor l3. These increments of current, which are proportional to the output signal currents, establish a compensating magnetic field which maintains the desired relationship between input and output currents.

It should be understood that the main magnetic field is initially adjusted so that the operating point of the magnetron amplifier is substantially in the center of the sloping portion of the characteristic curve A. The input currents alter the magnetic field and anode current but the compensating pentode circuit automatically varies the magnetic field, and thereby shifts the operating point with the signal to obtain the maximum linear variations of the magnetron anode current.

Thus I have described a magnetron amplifier in which the characteristic curve is utilized to its full advantage by automatically shifting the operating point with varying signal output currents. The resistive output circuitinsures an output which is substantially independent of frequency and makes amplification over a wide range of frequencies practical. Although I have shown a plurality of solenoids, it should be understood that a single solenoid winding with suitable taps, or several solenoids with or without magnetic cores may be used. Likewise any compensating circuit may be used provided it substantially follows the law that the current through the compensating solenoid is proportional to the square root of the voltage changes across the magnetron anode circuit resistor.

I claim as my invention:

1.- An amplifier comprising a magnetron having cathode and anode electrodes, means for creatin a magnetic field for said magnetron, means for varying said field by a second magnetic field established by currents to be amplified, a source of anode energizing potential, an output circuit resistance traversed by the anode currents from said magnetron, whereby variations in potential are produced across said output resistance which are proportional to variations in anode current and means including a thermionic tube responsive to said voltage variations for compensating for changes in the characteristic curve of said magnetron caused by potential variations of said anode.

2. A magnetron amplifier comprising an evacuated envelope containing cathode and anode electrodes, means for establishing a magnetic field whose lines of force surround and are substantially parallel to said cathode, means for varying said field with input currents to be amplified, means for producing voltage variations proportional to anode current variations comprising an output resistance connected to said anode, and means including a thermionic tube responsive to said voltage variations for compensating for the effects of said voltage variations on the amplification characteristic of said tube.

3. An amplifier comprising a magnetron tube of anode power and a resistive output element serially connected between said source of anode power and said anode electrode, means for establishing a magnetic field for effecting a curved path of electrons moving from said cathode to said anode electrodes, means for varying said magnetic field by a second magnetic field created by the currents to be amplified, and means including a thermionic tube responsive to the voltage across said resistive output element for correcting the effects of varying anode potentials caused by potential variations across said resistive output element.

4. An amplifier comprising a magnetron, means for creating a magnetic field for said magnetron, means for varying said field by a second magnetic field established by currents to be amplified, a resistive output element traversed by amplified output currents, and means including a thermionic tube responsive to voltage changes across said output element for establishing a compensating magnetic field by currents which vary as the square root of the voltage across said resistive output element.

5. An amplifier comprising a magnetron, means for creating a magnetic field for said magnetron, means for impressing input currents on said magnetron, a resistive output element traversed by amplified output currents, and means including a thermionic tube responsive to voltage changes across said output element for creating a magnetic field by currents in said tube which substantially vary as the square root of the voltage across said resistive output element.

6. An amplifier comprising a magnetron having a cathode and anode electrodes, means for creating a magnetic field for said magnetron, means for impressing input currents on said magnetron, a resistive output element, traversed by amplified anode currents, and means including a thermionic tube responsive to voltage changes across said output element for creating a magnetic field for compensating for changes in the characteristic curve of said magnetron caused by potential changes across said resistive output element.

7. An amplifier comprising a magnetron, means for creating a magnetic field for said magnetron, means for impressing input currents on said magnetron, a resistive output element traversed by amplified output currents, a thermionic pentode having fixedly biased suppressor, screen, and control grids, a solenoid connected to the anode electrode of said pentode and serially including the resistive output element of said magnetron whereby a compensating magnetic field is created by currents fiowing in said solenoid, and means for applying said compensating field to said magnetron.

8. An amplifier comprising a magnetron, means for creating a magnetic field for said magnetron, means for impressing input currents on said magnetron, a resistive output element traversed by amplified output currents, a thermionic pentode having fixedly biased suppressor, screen, and control grids, a solenoid connected to the anode elec trode of said pentode and serially including the resistive output element of said magnetron whereby a compensating magnetic field is created by said solenoid, and means for applying said compensating field to said magnetron to vary the total magnetron field by current increments which vary as the square root of the voltage changes in said resistive output element.

ROBERT SERRELL.