US1893563A - Selective amplifier - Google Patents

Description

Jan. 10, 1933. A. 5. ms 1,893,563

SELECTIVE AMPLIFIER Filed June 8, 1929 lNV/EgJO $2 'm (JATTORNEY Patented Jan. 10, 1933.

UNITED STATES PATENT eerie ALGER S. RIGG-S, OF NEW YORK, N. Y., ASSIGNOR TO WILLIS L. PRATT, OF NEW YORK, N. Y., FRANK W. MCDONELL, 0F EYE, NEW YORK, AN D FREDERICK R. RICH, OF NEW YORK, N. Y.

SELECTIVE AMPLIFIER Application filed June 8,

This invention relates to amplifiers of electrical variations wherein one of the component parts is an electron-discharge device, but more especially the invention relates to selective amplifiers such as are used for instance in transmission and reception of radio telegraphy and telephony, or in any other application in which it is desired to amplify a specific frequency or band of frequencies.

An especial object of this invention is the provision of an amplifier system capable of amplifying a predetermined frequency to the exclusion of other frequencies.

A further object of the present invention is the reduction of the eifective electro-sta-tic capacity between grid and plate by lowering the resistance into which the plate circuit is connected, thereby eliminating to a large exicent the tendency to feed-back and oscilate.

A still further object of the present invention is the provision of a selective amplifier in which the signal static ratio is considerably increased.

Figure l is a diagrammatic drawing of an electron-discharge device V which may be of the conventional three electrode type or may be of the type described in my co-pending ap plication Serial No. 358,774, filed April 29, 1929, and/or my co-pending application Serial No. 363,201, filed May 15, 1929. The battery A serves to maintain the filament f at a temperature necessary to produce sufficient electron emission from the cathode 0. The ionization shield s surrounds the cathode as in my co-pending application Serial N 0. 358,774, filed April 29, 1929. The battery B serves to supply voltage for the plate-cathode circuit. A source of input potential, such as for instance a tuned radio-frequency transformer T, is connected at its opposite ends to the cathode c and the grid g. The output or load circuit consists of the resister Ra? shunted by the tuned circuit LC, points 1 and 2 thereof constituting the output terminals which may be connected to a detector or to the input terminals of an amplifier such as herein disclosed if more than one stage of amplification is desired.

In accordance with the well-known action 1929. Serial No. 369,383.

of the three electrode electron-discharge device, variations of the potential of the grid 9 will cause a current to flow in accordance therewith through the circuit including the plate-cathode path.

This invention anticipates the use of a resonant circuit consisting of inductance and capacity, either one of which or both may be variable (herein the capacity is shown as the variable element) as a portion of the platecircuit load. In contra-distinction to conventional practice, I introduce a current into a series resonant circuit instead of a potential into a parallel resonant circuit.

In order to more readily understand the advantages of this invention, it must be realized that the effective impedance between the opposite terminals of a series resonant circuit (to resonant frequency) is equal to the value of the resistance included between the opposite terminals. The total voltage in troduced into such a series resonant circuit drops across this resistance, shown as B and with a given current the potential across the inductance L and the condenser C may be many thousand times greater than the voltage introduced between the opposite terminals of the series resonant circuit.

In accordance with the above, I pass the alternating component of the plate circuit through a load consisting of the resister R00 shunted by the series resonant circuit L-R,;C. At the resonant frequency the effective impedance between points 1 and 3 is equal to Ron shunted only by R since i the reactances of L and C are equal and opposite. During this condition, if R is equal to B which is the optimum, one half of the A. C. plate current will flow through R00, while the other half will flow through the series resonant circuit above referred to, and although the potential across R00 may be extremely small, the potential across either the inductance or the capacity will be equal to the product of the A. C. current flowing therethrough, and the reactance of the condenser or inductance. It is therefore evident that a relatively high degree of amplification may be obtained with this system, coincidently with a high degree 'of selectivity and transient disturbance, such as for instance atmospheric disturbances, is twice that of fered to the signal frequency (if R00 is equal to R It is generally believed that the objectionable noises due to atmospheric disturbances in radio receiving systems are caused principally by impulse excitation of the resonant circuit. In other words, a transient disturbance such as static causes the tuned circuit to oscillate for a few cycles at the frequency to which it is tuned, the amplitude of the oscillation and the time duration thereof being dependent upon the decrement or effective resistance of, the tuned circuit. By use of the present system,.I am enabled to secure the same degree of signal selectivity as with conventional parallel resonant circuits, co-incidently with a very high and adjustable decrement or resistance to transient response.

If, for instance, R00 is made a variable resistance, the response to signal may be increased co-incidently with an increase in the decrement offered to transient disturbances, but at slight sacrifice of signal selectivity. Inasmuch as this invention utilizes the electron-discharge devices as current and not as voltage generators, electron-discharge devices having a low value of plate resistance and a high value of mutual conductance are desirable. Since only. the mutual conductance contributes toward the degree of amplification obtained in the present system, it is unnecessary and inadvisable to utilize electron-discharge devices having a high value amplification factor.

In order to utilize to best advantage the high decrement which this system offers to transient impulses when used in radio receiving systems, the antenna should be connected to the grid of the electron-discharge device aperiodically and not by a tuned transformer as shown on the drawin The amplifier herein shown and described consists of only one stage but more than one stage may be employed where necessary and desirable, and although I have herein shown batteries as power supply sources, it is understood that such power may be supplied in other ways, as for instance, from commercial power circuits with the necessary rectifiers and filters.

Although I have herein shown and described but one form of my present invention, I do not limit myself thereto, inasmuch as modifications and changes may be made therein without departing from the original spirit of the invention or the scope of the appended claims.

What I claim is:

1. A selective amplifier of electrical variations consisting of an electron-discharge device, a suitable source of input signal poten tial, power supply sources for cathode and anode circuits, and an anode output net-work consisting of a non-inductive resister shunted by a condenser and an inductance forming a series circuit resonant to the frequency of the input signal potential, said non-inductive resister having a value approximating that of the tuned circuit resistance at signal frequency.

2. A coupling medium for selective electron-discharge amplifiers consisting of a noninductive resistance shunted by a series resonant circuit, said non-inductive resistance having a value approximating that of the tuned circuit resistance at signal frequency output circuit of preceding electron-discharge device eing connected across the non-inductive resistance and input circuit of succeeding electron-discharge device-being connected across one of the elements forming the above mentioned series resonant circuit.

3. A selective output net-work for electrondischarge amplifiers consisting of a non-inductive resistance adapted to be connected in series with the plate-cathode path of an electron-discharge device and a series resonant circuit in parallel with the non-inductive resistance, said non-inductive resistance hav- .f

ing a value approximating that of the tuned circuit resistance at signal frequency.

7 4. In an amplifier of electrical variations,

selective to a particular frequency, one or more electron-discharge devices, suitable I power supply sources therefor, and a coupling media between said electron-discharge devices consisting of a non-inductive resister shunted by a series resonant circuit, said noninductive resister having a value approximating that of the tuned circuit resistance at signal frequency and being connected in series with preceding plate-cathode circuit, and grid-cathode of succeeding electron-discharge device being connected across a portion of said resonant circuit.

In testimony whereof, I have signed my name to this specification.

ALGER S. RIGGS.

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