US2097896A - Amplifying arrangement - Google Patents

Description

Nov. 2, 1937. B. SALZBERG AMPLIFYING ARRANGEMENT INVENTOR zSf/FA/ARD 6341251586 ATTORNEY Filed NOV. 15, 1933 Patented Nov. 2, 1937 UNITED STAES PATENT OFFICE AMPLIFYING ARRANGEMENT of Delaware Application November 13, 1933, Serial No. 697,783

5 Claims.

An object of the present invention is to provide a vacuum tube amplifying arrangement; which is stable and not subject to oscillation when amplifying radio waves, especially those of short wavelengths such as those from 1 to 20 meters.

A further object is to prevent oscillation in a vacuum tube amplifier and circuit in which the tube has a control grid and a screen grid and in which there is a feedback between the two grid circuits at high frequencies of the order of 60 megacycles.

A still further object of the invention is to provide a path between the screen grid of a vacuum tube and ground of substantially lower impedance than that of the lead to the screen grid, in order that when high frequency currents are applied to the control grid of the tube, the screen grid will remain at substantially the same radio frequency potential and not feed back energy into the control grid circuit or cause the tube to oscillate.

Other objects and advantages of the invention will become apparent to those skilled in the art as the description thereof proceeds. For a better understanding of the invention reference is made to the following description taken in connection with the accompanying drawing in which,

Figs. 1 and 2 are diagrammatic circuit diagrams of arrangements embodying the invention; and,

Figs. 3 and 4 are diagrammatic circuit diagrams of other amplifier arrangements embodying the invention.

Referring to Fig. 1 a two stage amplifier ar rangement is shown comprising a tetrode or screen grid tube l which may be of the RCA UY224= type having its control grid 2 connected to the high potential end of a coil 3 which is inductively coupled to a coil t in the antenna circuit 5 the coil 3 being tunable to the desired incoming signal frequency by a variable condenser 6. The tube also includes a screen grid 1 located between the grid 2 and plate 8, a suitable high positive potential being impressed on the grid 1 by a lead 9 connected thereto and to a B battery grid and cathode of a succeeding amplifier stage similar to these described and it will be understood that any desired number of stages may be connected in the manner described.

I have found that when a tube of this type having a mutual conductance of the order of 1000 to 1500 micromhos is used to amplify currents having wave lengths of 1 to 20 meters, the receiver will go into oscillation under certain conditions even when only a single stage amplifier is used. 10 Measurement revealed that the reactance of the screen grid lead within the tube at a Wave length of 5 meters had a value of over 1000 ohms and was of an inductive nature and also that it had a regenerative effect on the control grid circuit. 15 This inductance of the screen grid lead is represented in Fig. 1 by the coils l4 and I4 and the capacity between the screen grid with its lead and the other elements within the tube is represented by the condensers I5 and I5. 20

Considering an instant when the signal voltage has raised the voltage of grid 2 to the positive peak of its cycle, the electron flow to the screen increases, the increase of screen current causing a voltage drop in the screen circuit I4, 55 with an 25 immediate drop of the screen voltage. Due to the relatively large impedance in the screen circuit, this voltage drop of the screen is suflicient to cause the current to the plate to decrease, resulting in a decrease in the voltage across coil I l and an increase in the voltage of the plate. Thus, when there is an impedance of substantial value in the screen grid lead, we may have a condition such that the control grid and plate voltages are in phase and out of phase with the screen grid 35 voltage.

This is the condition necessary for oscillation to occur between (1) the input and screen grid circuits and (2) the screen grid and plate or output circuits. Oscillation between the input and output circuits can no longer occur, since the control grid 2 and plate voltage are now in phase. Usually type (2) oscillations will not be observed because the transconductance of the screen grid with respect to the plate is of too low a value in commercial tetrodes. I have found that oscillations and more especially those of type (1) may be prevented in several ways. One such way is illustrated in Fig. 1 in which adjustable condensers l6 and I6 have their opposite sides connected between the plates and the control grids, each being adjusted so that the voltage fed back from the output circuit is just equal to that fed back by the capacity between the screen grid, together with its lead within the tube, and the control grid. Since these voltages are in opposite phase, they will neutralize each other and produce no net change in the control grid voltage, thus no oscillation of the amplifier will take place. Of course, in cases where a certain amount of regeneration is desired, the condensers I6 and I6 may be adjusted so that there is a small net feedback of energy.

An alternative method for preventing oscillations is shown in Fig. 2 in which a single amplifier stage of the same type as that described in connection with Fig. 1 is shown and in which coil I! is inductively coupled to primary coil H and has one end connected by lead 35; to the control grid. Coil II is wound in such a direction that the voltage fed back by it to the control grid is equal and of opposite phase to that fed back by the screen grid. Instead of the tuning condenser 6, the plate circuit is in this case shown to be adjustably tuned to the signal by a condenser i8 shunted across primary coil H.

An arrangement especially adapted to prevent oscillation at ultra high frequencies is shown in connection with a push-pull amplifying stage in Fig. 3. The coil i9 connected to the grids is tuned to the signal frequency by condenser 20 and the coil H is similarly tuned by condenser i8. A real transmission line comprising the parallel conductors 2i and 22 connected together at one end 23 has its opposite ends connected to the screen grids l by the screen grid leads 25. A metallic contact 24 slidable along wires 2 i and 22 is connected to ground as shown and serves to adjust the electrical length of the line to any desired value. By making the length of the line, including the screen grid leads, equal to half a wave length of the incoming signal wave, its impedance will be zero and the screen grids will be kept at ground potential.

The arrangement shown in Fig. 4 is similar to that shown in Fig. 3 except that in this case the screen grids are maintained at ground potential by means of an artificial line comprising the long inductance coil 25 which is grounded at its center as shown at 2?. The screen grid leads are connected to coil 28 by slidable contacts 29, 29. In this case also, these contacts are adjusted until the length of the line between each grid 1' and ground, including the screen grid lead, is equal to a half wave length of the signal to be amplified.

While I have shown only a single stage amplifier in Figs. 2, 3 and 4 it is to be understood that this stage may be duplicated as shown in Fig. 1 and the number of stages may be further increased until the desired gain is secured.

Having described my invention, what I desire to secure by Letters Patent is:

1. A vacuum tube amplifier comprising the combination of a tube having at least a cathode, a control grid, a plate and a screen grid located between said control grid and plate, means for impressing voltage variations of high radio frequency on said control grid, a ground and a connection between said screen grid and ground comprising a transmission line having an electrical wave length of where 1 is the wave length of the applied signal frequency.

2. In a device for amplifying signal currents of high radio frequency, the combination of a vacuum tube having a cathode, signal control grid, a screen grid and a plate, a circuit connecting said control grid and cathode, means for coupling said last named circuit to a source of signal currents of radio frequency, a transmission line having one end connected to said screen grid and means for adjusting the effective length of said line to a value equal to substantially one half the wave length of the signal current to be amplified.

3. In a device for amplifying signal currents of high radio frequency, the combination of a vacuum tube having a cathode, control grid, screen grid and plate, a lead to said screen grid having a substantial inductive reactance at the applied signal frequencies and a connection conductive to direct current between said screen grid and cathode exterior to said tube and having an effective length equal to substantially one half the wave length of an incoming signal current.

4. In a device for amplifying currents having wave lengths of the order of 5 meters, the combination of a vacuum tube having a cathode, control grid, anode and a screen grid between said control grid and anode, a lead to said screen grid having an inductive reactance exceeding 500 ohms at the frequency of the currents to be amplilied and a circuit path between said screen grid and ground having an effective length of the order of 2.5 meters.

5. In an amplifying device, the combination of a pair of vacuum tubes connected in push-pull each of said tubes having a cathode, a control grid, a screen grid and a plate, a tunable circuit connected to said control grids, means for impressing currents of high radio frequency on said circuit, a transmission line having one end connected to said screen grids and its other end to the tube cathodes, said line having an effective length equal to substantially one half the wave length to which said tunable circuit is resonant.

BERNARD SALZBERG.

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