US1724498A - Amplifier - Google Patents

Description

Aug. 13, 1929. v MILLER 1,724,498

AMPLIFIER Filed Nov. 21. 1921 dnren i-or John M. Miller 3 w ef ii'o 63 I Patented Aug. is, was.

JOHN M. MILLER, 01* .WASHINGTON, DISTRICT OF COLUMBIA.

AMPLIFIER.

Application filed November 21, 1921. Serial No. 516,682.

My invention relates generally to an amplifier, particularly one using three element vacuum tubes which serve to amplify alternating currents or voltages of the'higher frequencies of those used in radio signaling, and particularly to a means for obtaining and passing on an amplified voltage from one stage to another in a radio frequency amplifier, and one of the novel features thereof resides in the provision of an amplifier circuit of a character which permits a high amplification at-short waves throughout a relatively wide band of frequencies and provision for shifting the location of this band in the frequency scale. The advantage thus secured is high amplification without the necessity of critical adjustment.

Other features will appear in the following description, reference being had to the accompanying drawing, in which:

The figure is a diagrammatic viewof one form of my amplifying system, which is merely for the purpose of illustration and is not to be considered in any limiting sense.

Reference characters a and b indicate respectively the usual receiving antenna and the secondary receiving circuit. The mannor in which the energy is collected from the radio signaling waves and the particular type of collector constitute no part of the present invention.

In the well known manner, a radio signal produces voltage variations between the grid 9 and filament f of the first amplifier tube cl. These variations produce current variations in the circuit between the filament f and the plate p. These current variations produce amplified voltage variations across the inductance coil 6, which coil, as will be hereinafter described, has a variable high in ductance and a low capacity. Such a coil offers a high impedance to these current varations if their frequency is within a wide band of frequencies.

The voltage across the coil 6 is applied between the grid '9' and the filament f of the second tube (Z; the condenser 0 serving to prevent the high positive voltage of the plate battery from acting on the grid of the second tube, but permitting the voltage variations to be transmitted.

, The high resistance indicated by reference character 7', maintains the grid at approximately the potential of the negative terminal of the filament, but does not reduce apfurther similar stages of amplification may be used, but in the particular circuit I have shown, the third tube (2 is represented in a well known manner as a detector tube, in which case the condenser 0 should be of suitable capacity, and resistance 1" of proper resistance and connected to the positive terminal of the filament.

The plate circuit of the detector tube may contain the customary telephone receivers t, as, shown, or the input circuit of an audio frequency amplifier. In lieu of the detector tube any other means of utilizing or detecting the amplified radio frequency oscillations may be employed. The main feature of this invention over my copending application Serial No. 452,- 542, filed March 15, 1921, and a division thereof resides in the employment of coils e and 6 having a suitable variable high inductance and low capacity, and I prefer to provide for this variable feature by winding these coils as a long single layer solenoid on a cylinder of small radius and from a fine insulated wire, but it is to be understood that my invention is not to be limited to any particular form of winding, as the essential point is that the winding be of variable inductance and low capacity. High inductance is possible through the very large number of turns obtainable with a fine, insulated wire on a long cylinder, and low capacity is secured through the fineness of the wire, the small radius of the turns and the single layer of turns. In this construction the insulation of the wire is then removedfrom the outside surface of the wire along a line parallel to the axis of each coil to permit the sliding contacts 8 ands to make contact at any point along the co1ls.

The coils are so wound that the number of turns in the plate circuit of the tube can be varied by adjustment of the members 8 and s. In effect this varies the inductances of the coils. The inductances of the coils in combination with the coil and tube capacities act as a parallel system of inductance and capacity, and because of the relative high inductance and low capacity this system has a high impedance for currents in the plate circuit of frequencies over a relatively broad band about the resonant frequency of the system, and hence, high amplification Will be secured over such a band of frequencies. By shifting the contacts s and s the resonant frequency, and hence, the band of high amplification, can be shifted to longer or shorter wave lengths 'While still maintaining a broad band.

I prefer to mechanically connect the sliding contacts 8 and s in such a manner that they move together along the coils, so that only one adjustment device is required. It is desirable to connect the sliding contacts to the positive terminal of the plate battery so that the contacts themselves, the unused turns of the coils e and e, and any other conductors in contact at this point will be connected into the amplifier circuit at a point where the alternating voltage to ground is low, which tends to reduce the capacity of the coils.

To obtain the best results it is desirable to use vacuum tubes having a low capacity between the tube electrodes, in particular a low capacity between the grid and plate of the tube, since the tube capacities in effect introduce a capacity which is in parallel with the capacity of the coil.

While I have illustrated two stages of amplification, it is obvious that one stage or any number of stages may be employed. Further, the amplification secured by the means described may be utilized in various ways other than that shown.

Having fully described my invention What I claim as new is:

In a radio frequency amplifier including a plurality of vacuum tubes in cascade, each of said tubes having grid-filament and platefilament circuits, the combination of a circuit of variable resonance in the grid-fila ment circuit of the first vacuum tube, a coil of high inductance and low capacity in each of the plate-filament circuits of the remaining vacuum tubes, a high resistance in the grid-filament circuit of each of the remaining tubes, condensers connected between the plate of each tube and the grid of each succeeding tube and mean Within the platefilament circuit of each tube for adjusting the effective length of each of said coils to such a value that the coil acts as a pure inductance with substantially no capacity through a relatively broad band of frequencies both above and below the frequency to which the resonantcircuit in the gridfilament circuit of the first tube is tuned.

JOHN M. MILLER.

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