US1808150A - High frequency amplifier - Google Patents

Description

June 2, 1931. P. J. TOWNSEND 1,808,150

v HIGH FREQUENCY AMPLIFIER Filed Nov. 1, 1927 l l l l l l l flerc/m/ J 7557/75 end gvmi V ATTOEA EY Patented June 2, 1931 PATENT oFFics PERCIVAL J. TOWNSEND, OF LOS ANG-ELES, CALIFORNIA HIGH FREQUENCY AMPLIFIER Application filed November 1, 1927. Serial No. 230,311.

This invention relates to high frequency currents, and especially to the amplification of radio frequency signals carried by such currents.

The amplifier usually used for such purposes is an electronic emission amplifier, consisting of a sealed evacuated vessel in which there are several electrodes. One of the electrodes serves to emit electrons, and is usually in the form of a filament arranged to be heated by the passage of a current therethrough. Another electrode is an anode or plate to which the electrons travel, forming a space current, and which is maintained at a potential positive with respect to the filament by the aid of an external circuit connecting these electrodes. There is also a control electrode usually in the form of a grid, which is interposed in the field between the filament and plate. It has been foundthat even minute changes in the potential difference between the filament and grid cause comparatively large changes in the space current.

Therefore to secure amplification, it is necessary to subject the grid and filament to potential differences correspondingto the impulses to be amplified; and there result comparatively large variations in current fiow in the circuit connecting the filament and plate. These variations in turn can be reamplified in a succeeding stage. 7

Futhermore, such a vacuum tube can also serve as a detector for the modulations carried by the radio frequency current, by the addition of a small series condenser in the grid-filament circuit and adjacent the grid. Both these modes of operation of vacuum tubes are well-understood at this time, and need no further elucidation.

It is also common to secure a very large degree of amplification from onetube by passing back a portion of the amplified impulses to the input side, as by coupling the input and output circuits. It is one of the objects of my invention to provide a novel and efficient form of feed-back to secure these results.

In such regenerative systems, difficulty has I been experienced in the past to control the w feed-back so as to keep the system stable and as battery to prevent it from setting up self-oscillations.

It is another object of my invention to pro- .vide a novel and simple form of control for gradually froma reversed feed-back to regenerative condition as the broadcast range is traversed.

My invention possesses many other advantages, and has other objects which may be made more easily apparent from a consideration of several embodiments of my invention.

For this purpose, I have shown a few forms in the drawings accompanying and forming part of the present specification. I shall now proceed to describe these forms in detail,

which illustrates the general principles of my invention; but it is to be understood that this detailed description is not to be takenin a limiting sense, since the scope of my invention is best defined by the appended claims.

Referring to the drawings; 7

Figure 1 is a wiring diagram of a radio system embodying my invention; and

fFig. 2 is a diagram of a modification there- 0 In Fig. 1, I show a thermionic amplifier and detector 11, having a filament 12, anode 13, and control electrode or grid 14, all operating as hereinbeforeset forth. The filament is shown as arranged to be heated by. current from a source of electric energy, such The filament 12 and grid 14 are subjected to potential differences corresponding to signal impulses that exist in circuit 16-17;

this is accomplished by providing a coil 18 in circuit 16-17 that is inductively coupled to another coil 19. One terminal of this latter co-il connects directly to the filament 12; and the other terminal connects to grid 14: through the usual grid condenser 20. In

necting electrode 13 to filament 12.

- the input side.

order that the system be selective to a definite frequency, a variable condenser 21 is provided, bridging coil 19, and forming with it a tunable circuit. It is now well-understood that the potential difierences existing across the terminals of the coil 19 or the condenser 21 are a maximum for a definite frequency dependent upon the setting of condenser 21. Thus by tuning this circuit, signals of a definite frequency can be accentuated or selected from the rest. Circuit 16-17 can connect to the usual pickup circuit or to the output of an amplifier interposed between such a pick-up structure and the detector stage.

These potential differences existing across this turnable circuit cause corresponding large variations in the space current, and in the current flowing inan external circuit con- This circuit includes plate 13, coil 2-2, translating device 23 such as a phone or loud speaker or coupling coil, battery 24, and filament 12.

The system can be supplemented as is well understood, by one or more stages of audio frequency amplifiers.

Coil 22 forms one of the elementsfo-r a feed-back circuit and is shown as closely coupled to the coils 18 and 19. It is also closely coupled to a coil 25 that is connected to a variable condenser 26 which in turnconnects to the filament 12. Upon proper direction of winding of coil 25, a large potential difference is induced across both coils 22-and 25 in series which is effective to transfer-energy to This large potential difference is secured 'due to the transformer effect of coil 22 on 25. Coil22 carries theoutput current and has apotential difference due .to

the flow of this current; and coil 25 has a much larger potential difference because of the larger number of turns therein. The variable condenser 26 serves to control the degree of regeneration in a manner now to be explained. a

It is to be noted that coil 25 is adjacent coil 19, and coil 22 is spaced therefrom. Thus when condenser 26 is set at-a low value, the circuit of coil 25 has a high impedance; it is therefore of little effect to serve as a. coupling coil between the input and the-output circuits, the condition approaching an open circuit between the upper terminal of coil 25 and filament 12. Since coil 22 isspaced from coil 19 the effect is that there is little transfer of energy between the input and output circuits. However, as condenser 26 is varied to increase the capacity, coil 25 becomes more paths in the output circuit. Thus coil 22 is directly in series with the translation device 23; and coil 25 parallels a portion of the output circuit.

For high frequency operation, such as for the broadcast range and for the higher frequencies above that range, the use of a bypass condenser around phones 23 is not essential. It is found in practice that the impedance of the translating device 23, such as telephone receivers or the primary of an audio frequency transformer, varies greatly to radio frequency currents; that is, owing to the distributed capacities of such a device certain radio frequencies are allowed to pass through this device. In that case these currents pass from plate 13 through coil 22 and back to --fila1nent 12 through the capacity ;of the translating device 23. This energy .passing through coil 22 induces radio frequency 7 potentials across coil :25 which feeds back to coil 19. The amount of feed-back is .de-

pendent on the value of 'capacity 26.

In the case where the impedance of the translating devlce .23 is such as to exclude these radio frequencies they are forcedtotake the :path through plate 1 3, coil 22, coil 25 and capacity 26; the amount-of feed-back to coil 19 again depending'on the value of capacity .o-fcondenser26. p I

I found that for the ordinary broadcast range and when the usual storage battery .tubes are used, coil 22 can be from three to five turnsand coil 25 from eight to fifteen or more turns and condenser 26 can be about .00025 inicrofarads. These values of .course depend onthe degree oficoupli-ng between coils 19,, .22, 25. :F or the smaller dry cell tubes, the member of turns can be increased. Due to the step-up .eifectof coils22.and25,.andthe precise-control of regeneration by condenser in the series; this is merely .an ralternative, the

theory of operation being thesame as ifibut onetube were used. Coil 18 is coupled asbefore to coil 19., formingatunablecircuit with condenser 21. Tube 2'? affected by this circuit is a radio frequency amplifier, the input of which is affected by coils .28 and .29., associated with the output circuit of succeeding tube 30, shown in this instance as a detector. The output circuit of tube 27 includes @011 33 thatserves to :affect the tunable circuit 34 in the input circuit of tube 30. The output circuit of tube .30 includes plate 35,, 003L129,

translating device 36 such as a loud speaker bridged by a small capacity 37, a portion of the common B battery 38, and filament 39.

In this form, I also take care of instability that may resultin a multi-stage amplifier at the high frequency end of the range. lVhen signals at the high frequency end of the range are tuned in, the circuits are unstable, and undesired oscillations may result. Under such circumstances, positive regeneration is a detriment, for it assists the oscillations. lVith the present scheme, the coil 29 opposes the coil 19. Therefore a reversed feed-back effect is secured. Condenser 40 in circuit with coil 28 however can be adjusted just up to the point where instability begins. When signals at lower frequencies come in, the capacity is increased so that the positive regenerative effect of coil 28 becomes larger and larger, and finally it overcomes the reversed effected of coil 29 and the net result is positive regeneration at the lower frequencies. Thus it is seen that the system can be varied through a range between negative regeneration to positive regeneration merely by gradually increasing the capacity of condenser 40. It is seen that the number of turns of coil 28 should be greater than that of coil 29 so that a step-up transformer efiect can be secured between these two coils, whereby the negative regeneration of coil 29 can be finally overcome for the lower frequency signals.

It is to be noted that coil 29 is in series both with the plate 35 and device 36 and therefore serves as a direct medium for transfer of energy between the output of tube 30 and the input of tube 27 The circuit for coil 28 parallels a portion of this branch of the output circuit, said portion including coil 29 and device 36 so that this coil circuit can also affect the input of tube 27. The variation in the effect of coil 28 is secured as before, by varying condenser 40. In this instance, coils 28 and 29 can be wound in the same direction to secure an opposed efiect, because the position of the terminals of coil 29 is reversed with respect to plate 35 and translating device 36, as compared with the position of the terminals of coil 22 with respect to plate 13 and device 23. In this way, the coils 28 and 29 are opposed, instead of being cumulative, as are coils 22 and 25 in Fig. 1.

In the form illustrated in Fig. 1, it is possible to make condenser 26 fixed at a medium value, whereby the regeneration is at an efficient point for the entire range without the necessity of adjustment; the value of capacity 26 being then low enough to prevent oscillation of the detector circuits, and the amount of regeneration of the system is con trolled in one or more of the other stages. It is also of course possible, when oscillations are desired to be generated, to fix the value of capacity 26 by a fixed condenser so as to throw the system into oscillation for the entire range.

I claim:

1. In combination, one or more electronic emission devices having a plurality of electrodes, an input circuit connected to some of the electrodes, an output circuit connected to some of the electrodes, and means for feeding back energy from the output circuit to the input circuit, comprising a pair of inductances and a capacity connected in series and between the input and output circuits, the inductances being inductively coupled to the input circuit, said output circuit including a translating device in series with one only of said inductances..

2. An inductively coupled regenerative system for one or more electron emission devices, having an input and an output circuit, characterized by'the provision of a pair of cumulatively wound coils in an output circuit, coupled to an input circuit, one of said coils being more closely coupled than the other, and means for varying the efiect of i said more closely coupled coil on the input circuit.

3. An inductively coupled regenerative system for one or more electron emission devices, having an input and an'output circuit, characterized by the provision of a pair of coils in an output circuit, both being coupled to the input circuit, a translating device in series with one of said coils, and the other of said coils being in parallel with the translating device, and variable impedance means in series with said other coil and forming a circuit therewith the impedance of which is variable.

4. The combinationas set forth in claim 3, in which said other coil is placed closer to the input circuit than the said one of the coils, and has a larger number of turns, whereby a stepup effect is obtained between the two coils by energy passing through that coil which is in series with the translating device.

5. The combination as set forth in claim 3, in which the variable impedance means is a variable condenser.

In testimony whereof I have hereunto set my hand.

PERCIVAL J. TOWNSEND.

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