US1882128A - Radiofrequency amplification system - Google Patents

Description

OCt. l1, 1932. E, W FEAR|NG RDIOFREQUENCY AMPLIFICATION SYSTEM Filed June l, 1927 A TTORN Patented Oct. 11, 1932 UNITED STATES ATENT FFI-QE 'EDWARD w. FEARING, or EAST ORANGE, naw JERSEY RADIOFREQUENCY AMPLIFCATION SYSTEM Application led Tune 1,

This invention relates to the neutralizing of capacity couplings between the grid and plate circuits of a vacuum tube, which couplings result in an undesirable reaction of the plate circuit on the grid circuit. y

More particularly this invention has for its v object to provide a more simple, efficient and exact neutralizing circuit arrangement which may be readily adapted for use in any type of vacuum tube and which prevents the feed bach of energy from output terminals'of the tube to the input terminals of the tube. This circuit may be readily adapted for use with any form or type of vacuum tube which has been heretofore used without a neutralizing arrangement for neutralizing the feed bach effects. This may be done with a minimum of changes and a maximum of efficiency in the resultJ obtained.

lt is well lrnown that a. vacuum tube operates on the principle of capacity effect within an enclosed envelope in which a substantial quantity or all the gas has been removed. IThe purpose of the removal of this gas has been to enhance the capacity and electronic effectsl produced by electrical discharges. Usually these discharges are increased by the use of a filament or other heated body to throw off electrons in the direction of the elements.

Where a plurality of elements are used within an enclosed and evacuated envelope a plurality of effects occur, and oftentimes some of these effects are disadvantageous to the result desired because they increase the amplification of the circuit to such a point i that the tube becomes overloaded.

Ordinarily within the envelope of a three or four element vacuum tube a control element has been used in capacity relation to a second element which is used in the load circuit. The variations of conductivity between these two elements necessarily vary the capacity between one of these elements and third or fourth element thereby affecting; the relative ratio of voltage tension between them and thereby causing variation in amplification. y n

lt is an object of this invention to retain sufficient feed-backeiects of a vacuum tube 1927. Serial N0. 195,829.

to produce a reasonable amplification and tow counteract the feed back effects of the vacuum tube such as would produce an over-abundance of amplification caused from feed-back from the output circuit to the input circuit of the vacuum tube and also to substantiallyfmaintain this amplication constant regardless of frequency.

Another object of this invention is to provide a vacuum tube with a neutralizing cir- 60 cuit coupled to both the input and the outputi circuits of the vacuum tube, and having a ratio of coupling therewith which dii'l'ers from unity whereby the amplification is kept more nearly constant in respect to frequency. rlliis difference is determined by the ratio off current of one of the circuit coils in proportion to tne current in the neutralizing coil. rl`his` invention relates to the application of a neutralizing circuit which is coupled with, but has a. different number of turns in its; coupling coil than there is in the input coil of the vacuum tube. This same proportion, having a ratio differing from unity or one hundred percent coupling, would also be present in connection with the output coil and the neutralizing coil coupled thereto.

This invention relates also to the application of a neutralizing circuit to a vacuum tube in which the voltages and current induced therein are different from the feed back voltages and feed-back currents which circulate in both the output and input circuits simultaneously, differing accordingto the ratios of couplings between the input and the output coils to those of the input and-f3 output neutralizing coils coupled therewith. This ratio may be adjusted to the point where any variation of coupling due to variation of frequency may be compensated for.

Another object of this invention is to em?? body the desirable features of a. separate neutralizing` or counter feed-back circuit with a plurality of vacuum tubes. Thus a maximum elhciency results and proper adjustment for` the preventing of undesirable feed-backs at" one frequency will automatically produce a neutralization of inherent feed-backs at all frequencies at which the apparatus is used.

A further object of this invention is to improve the operation of a vacuum tube when used with a second circuit coupled therewith and to overcome the difficulties encountered due to variations in the amount of coupling caused from variations of frequency. It has been found'that in an ordinary coupling arrangement different ratios of energy couplings result when different frequencies are impressed upon the couplings. It is an object of this invention therefore to reduce such variations in couplings to a minimum by proper arrangement and proportioning of circuits, and to compensate for their variation.

Further and more definite objects will be observed in connection with the following specifications, claims and drawing in which,

Figure 1 represents a simple schematic arrangement of an ordinary three electrode amplifier and the simplied neutralizing circuit coupled thereto;

VFigure 2 shows an analytical circuit arrangement corresponding to the diagram in Figure 1;

Figure 3 shows the application of my neutralizing circuit to a push-pull7 amplifier;

Figure l shows a modified neutralizing circuit which should be applied to Fig. 3 in place of the one shown therein;

Figure 5 shows a still further adaptation of my arrangement with a push-pull amplifier;

Figure 6 shows a method of winding the wires which form the coupling coil;

Figure 7 shows another method of obtaining this same result where a plurality of coils yare desired to be wound concurrently and having a maximum of coupling efficiency.

As is well understood by those skilled in the art, ordinary neutralizing circuits are ineffective in certain conditions and at such frequencies will not neutralize to sufficient extent the energy which finds its way from the output circuit back into the input circuit unless additional adjustments are made besides those which occur during the act of tuning. Difiiculty is encountered when mechanically arranging the values and parts so that a complete neutralization will occur at all frequencies at which the apparatus is desired to be used.

Such a diiiiculty occurs in a circuit of the vacuum tube 1 with input circuit 2 composed of a coil or impedance 8 and output circuit 4, composed of impedance 5. The filament 6 is of the customary type forming a common return in this ease for the output circuit 4 and the input circuit 3. A source of potential is shown at 7 in the output circuit. Capacity 8 represents the grid-plate capacity.

Feed-back is prevented by coupling the circuit 9 to the vacuum tube circuit. Coils 10 and 11 are coupled with coils 3 and 5 respectively and a series condenser 12 is prop-n erly proportioned to capacity 8.

n order to overcome the dilhculty encountered when using one of the coils 3 or 1() to constitute a tuned circuit and thereby establish cause for variations for energy circulating through these circuits, the tuned circuit 14 is coupled, as nearly as possible, equally to these two circuits. The tuned circuit 14 is made up of a variable capacity 15 and the coil 16. It might well be made up of a variable coil 16 and a fixed capacity 15, or both elements might be made adjustable. In any event it is desirable to have coils 3, 10 and 16 as closely coupled as possible. A method of obtaining this result is to be described further on. The same is true of the coupling circuit 17 which is shown as coil 18 and variable capacity 19 coupled as closely as possible and equally to coils 5 and 11. From these two tuned circuits and the manner of coupling to the vacuum tube and neutralizing circuit it will be seen that any energies existing in tuned circuit 17 will affect equally the coils 5 and 11 to thereby feed back energy in an equal amount to coils 2 and 10 where each will be substantially ceunteracted by the other. The same will be true of any energy which is stored in the circuit 14 which may attempt to react through the capacity 8 on energy or voltage applied to the coil 5 to cause undesirable oscillations,

The terminals 2O of coil 16 may be connected to the source of alternating current energy desired to be amplified either in the form of an antenna or in the form of another amplifying circuit or other device. rlhe terminals 21 of the tuned circuit 17 may be connected to apparatus in which it is desired that the amplified signals will circulate.

In order to accomplish this result where all frequencies within a wide range may be impressed upon the amplifier, it is necessary that these circuits, be correctly proportioned in all their elements and details. All of the elements must be proportioned in proper ratio with each other. Heretofore it has been considered that it is only necessary to have one of the coils in the amplifier with relation to one of the coils in the counter feed-back circuit in the correct ratio. It has been found that this prior method of proportioning the circuit is inaccurate and does not hold for all conditions and all frequencies which are impressed upon these circuits.

These two circuits may be proportioned so that all of the different elements will have exactly the same value, that is, the coil 2 will be of exactly the same diameter and the saine number of turns as coil 10, coil 5 will have exactly the same diameter and the same number of turns as coil 11, and the capacity 12 will be exactly the same size as the inherent capacity 8 between the grid and plate of the vacuum tube 1. This provides a means f of neutralizing the inherent effects between substantially unity. In order to neutralize for. all frequencies, direct conductive relations between the two circuits may be eli1ninated and the two circuits may be separately coupled as shown in Figure 1. This provides means whereby, the effects of the tuned v burn out the filament.

circuits on the capacity of the plate and grid of the vacuum tube are the same as the effect of these circuits upon the condenser 12 in the counter feed-back circuit and therefore undesirable oscillations are prevented in thevacuum tube circuit. This permits the variation of all the constants of the two circuits in the correct ratios'.

It isalso obvious from this arrangement that no direct contact is made with either the i input circuit or the output circuit and that energy is taken off from both these circuits by means of inductive couplings. Therefore, in case the condenser 12 breaks down or there is a short circuit between any part of the neutralizing circuit, no difiiculty will be encountered such as the burning out of the filamentvas would be the case particularly when the filament resistance switch 22 is in the .off position as shown in Figure 1.

It is necessary that the insulation between coils 3 and 10, as well as the insulation between coil 5 and coil 11 be broken down at the same time in the present arrangement to This is very unlikely to happen and therefore thisradditional fac tor of safety results. Where unusually high voltages are used in place of the battery 7, an additional capacity could be added in the connection23 which, when added to the capacity at l2 would give atotal capacitance effect in the proper proportion to the capacitance 8 of the vacuum tube.

In Figure 2 is shown a simplified circuitof the arrangement in Figure 1. All of the equivalent parts bear the same indicating numbers except that they are primed.

The proper relation between the coils and the constants of the various circuits is determined as follows: Assuming that all coils are the same diameter and of the same resistance wire, with the coils spaced the same distance apart, and preferably as close as possible, then the number of turns in coil 3 might be represented by A, the turns in coil 5 by B, and the inherent capacity between these two coils by c; this would also include other external capacities.

vCoil 10 would be represented by a, coil 11 by b, andk capacity 12` by c2. Then the relation between capacity c2 and the capacity c1 would be equal to the following expression and not simply Lemie This is true because the energy factor or measure of ability to store energy of al circuit containing capacity and inductance in parallel for a given frequency is approximately the capacity times the inductance. Therefore to get the two circuits (the vacuumtube circuit and the counter-feedbaclr circuit) to have the same stored energies,

but other factors enter into the circuits namely the transformation ratios of the couplings. Thus, power is transmitted from B to b With the voltage ratio of and from a to A with the voltage ratio of A and this voltage ratio is the direct measure tio is the direct measure of the throttling ef- A fect of the condensers.

Therefore A+BBa c A+BBa-c a+b X 1 XAOOT +1 XAXb-o In other words the total impedance of the vacuum tube circuit to the total impedance of the neutralizing circuit times the ratio of the second coil in the vacuum tube circuit to the iirst coil in the vacuum tube circuit times the irst coil in the neutralizing circuit to the second coil in the neutralizing circuit should be equal to the neutralizing capacity divided by the vacuum tube circuit capacity.

Theoreticaily these values would simply relate to the inductance of the various elements but practically it mav be assumed that the coils, if they are wound in a manand increasing the-capacity to such a pointY that; the regeneration of the circuit is substantially the same for all frequencies, thus keeping the amplification and' feed-back constant regardless of variations in inductive couplings and'increased capacitive feed-back pull or tandem amplifier circuit. Such an amplifier circuit is shown in Figure 3 where the vacuum tubes 24 and 2,5 are connected with their Vinput coils in the form of a single coupled coil 26, the halves of which are as closely coupled as is possible and the terminals of whichare arranged so that one of the tubes 24 or 25 is energized during each alternate half cycle. The ouput coil 27 is of the same nature as the input coil and is connected at its middle tapv 28 through a source of high potential 29 connected to the return to the filament 30. The filaments are energized by the customary batteries 31 which have a controlling switch and resistance as in the case of Figure 1.

Connection ismade for the input to the amplifier by means of terminals 32 to a coil 33 which may be closely coupled to both halves of coil 26 and neutralizing coil 34 similar to that described in Figure 1. The output tothe ampliers mav be connected through the terminals 35 of an output coil 36 coupled equally and as close as possible to the coils 27 and 37 of the neutralizing circuit.

The equivalent capacities of the vacuum tubes are represented by the condensers shown in dotted lines as 38 and 39. A counter-balancing condenser 40 is designed so that it bears the proper relation between these two'condensers and the coils of the respective circuits such as is described in connection with Figure 1. Assuming that the upper half ofvcoils 26 and 27 are equal to the lower halves and to the coils 34 and 37 respectivelyV thenthe capacity 40 should equal the sum of the capacities 38 and 39.

In order to apply the formula given in connection with Fig. 1 it must be assumed that the two halves of coils 26 and 27 are substantially equal and in the proper proportions to the constants of the neutralizing circuit. The only difference in this amplifier has been found to be, instead of neutralizing a single tube by the capacity 40 the amount of energy transferred by this neutralizing circuit or the capabilities to transfer energy must be equal to the capabilities to transfer energy of both the circuits, 38, 26, 30, 29, 27 and 39, 36, 30, 29 and 27. If these two circuits are substantially identical and equal they will transfer twice as much energy as a single circuit and it has been found that this energy is transmitted in the same direction and that therefore a single neutralizing circuit is sufficient to compensate for both.

y In case high potentials are applied to the output circuit of such an amplifier a neutralizing circuit such as is shown in Figure 4 may be used where 41 and 42 are the coils which are'closely coupled to the input and output coils respectively and the capacity 43 plus 44, other things being equal, are the same as that of the condenser 40 shown in Figure 4. This is also an advantageous arrangement when used with high frequencies for the salie of symmetry.

' I-Ieretofore it has been considered that vacuum tubes were neutralized by the fact that voltages equal and opposite in effect were applied at a single point to prevent the flow of current. Practically such a condition never occurs because of leakages and other imperfect `use of instrumentalities. That usually does occur is a neutralization by the balancing of an actual fiow of current and of applied energy in the form of variation iii-magnetic field within the coils connected to the amplifiers. There is a balancing of energy rather than a balancing of potentials or a simple balancing of current. This balancing of energy must be maintained in all corresponding parts of the circuits.

A particular application of this principle to a push-pull amplifier is shown in Figure 5 where the input may be connected to the terminals 45 of an input coil 46 coupled equally with two halves of the coil 47 across which is connected a tuning condenser 48 if desired. A common return is indicated at 49 for both vacuum tubes through a high potential battery 50 and the output coil 51 for both vacuum tubes arranged as described in connection with Figure 4. Here both vacuum tubes 52 and 53 are neutralized by the circuit coupled thereto through the condenser 54. The two feed-back circuits through the vacuum tubes are as follows: vacuum tube 52, coil 47, return 49, battery 50 and coil 51; vacuum tube 53, coil 47, return 49, battery 50 and coil 51. The counter feed-back circuit which would balance these two feed-back circuits is as follows: condenser 54, upper-half of coil 51, battery 50, common return 49 and lower-half of coil 47.

The fact that these two `halves of coil 47 and 51 are as closely coupled as possible prevents any variation of effects between the two vacuum tubes 52 and 53, to a substantial degree. In addition to this tight coupling between these two coils it may be noted that a comparatively large circulatory current is flowing from the coil 50 and into the condenser 48 so that the comparatively small feedback currents have little or no effect in producing a time variation of voltages applied to the end of this coil 47. This aids to overcome any deficiencies in lack of coupling between the two halves of coil 47.

It may also be noted in connection with coil 51 that during each alternate half-cycle a substantially large load current flows through this coil and to the battery 50. This load current causes a large potential to be applied and a large flux to flow through the convolutions of this coil. The percentage of change produced on this ux and therefore the percentage of change in this voltage at which the ener is being transmitted between the feedack and the counter-feedback circuit as related to this coil 51 is therefore relatively small and this effect enhances and increases the eect of coupling between the two halves of this coil.

The output connection may be made through the coil 55 coupled as closely as possible and equally to the two halves of the coil 51 as described in connection with some of the previous modications.

It will be noted in connection with these modifications described that the coupling .coil or coils are so related `as to provide as close as possible to one hundred percent. coupling. It has been found that by a carefully designing of these coils that a coupling of as high a percentage as 95% or possibly slightly higher in eXtreme cases has been produced. To express it another way a coupling in which the leakage has been reduced to a practical minimum has been accomplished by proper proportioning of the parts and improving the mechanical arrangey ment.

These coils may be advantageously constructed of a triple strand conductor such as is show nin Figure 6 composed of strands 56, 57 and 58, braided, twisted or otherwise combined into a single cord or cable. If these strands are wound around together in va regular manner and as close as possible, preferably over a cylindrical form, there will result a coil having as little leakage between each section as practicable. The twisting or braiding of wire aids in the preventing of electrostatic coupling between turns to a considerable extent, it being understood that electrostatic coupling between turns permits a leakage of current between turns and there- I fore a leakage of magnetic flux between turns.

Another common and simpler method is shown in Figure 7 where conductors 59, 60 and 61 are combined together in a single cord or cable 63 and are wound in the form of a coil as above described.

It can be seen from thisdisclosure that it is practically impossible to obtain a coupling having 100% efliciency or of a coupling factor of unity.Y Therefore, there is always a transference of energy in any type of neutralizing circuit which is applied-to a vacuum tube and unless this energy finds a path which is of equal resistance, impedance and capacity, an over or under balance of the vacuum tube circuit results.

This over or under balancing can only be obviated by designing the circuit on which energy is fed back as nearly as possible similar to that which counter-feed-back energy is conducted. I-Ieretofore this .identity of circuits has not been maintained it being thought necessary to produce only equal and opposite voltages.

As explained above these equal and opposite voltages do not entirely neutralize the vacuum tube and particularly at all frequencies. This neutralization can only be accomplished when the parts and elements are proportioned according to the formula specified above. It has also been found that it is not entirely necessary to maintain a high value of coupling between the various coils in case the proper ratios of the elements in the circuits are maintained, through which the feed-back and counter-feed-back currents and energies circulate.

It has been found in practice that suiicient coupling results between two or more coils, for the purposes described herein, if each one of the coils is simply wound on one of a plurality of concentric and tight fitting cylinders, either in the form of cardboard, bakelite or other material having thin rigid walls. Any coupling which approaches in value the range of 35 to 85% is usually suiiicient in most low powered radio receiving circuits to produce the proper counter-acting effect of the feed-back current, substantially throughout the broad-cast range.

It is not intended to be limited to the eX- act method of construction and apparatus shown so long as the characteristics are confined to the principles embodied herein. A full range of equivalents is anticipated to the extent indicated in the following claims.

That I claim is:

1. In an amplifier circuit a plurality of vacuum tubes connected in tandem, the inductances of the input circuits of said tubes being conductively connected and inductively coupled and the inductances of the output circuits of said tubes being conductively connected and inductively coupled, and a single counter feed-back neutralizing circuit inductively coupled to both said input and output circuits.

2. In a radio frequency circuit, a plurality of tandem connected vacuum tube circuits each containing, inductance and capacity, a part of said capacity inherently arranged to produce a feed-back action, a single counter feed-back circuit having its total impedance equal to the average impedance of said circuits and its capacitance equal to the sum of the feed-back capacitances of said circuits.

3. In a radio frequency circuit, a plurality of tandem connected vacuum tube circuits each containing, inductance and capacity, a

ypart of saidcapacity inherenetly arranged to produce a feedback action, a single counter feedback circuit having its total impedance equal to the average impedance of said circuits and its capacitance equal to the sum of the feed-back capacitances of said circuits, said capacitance being arranged in two parts.

4. In a radio frequency circuit, a plurality of'vacuum tube circuits of substantially equal electrical constants arranged in tandem, said circuits having a plurality of paths through which feed-back currents may flow, and a single counter feed-back path coupled to each of said circuits and arranged to conduct an amount of counter-feedback energy equal to the amount of energy conducted over all of said feed back paths.

5. In a radio frequency circuit, a plurality of vacuum tube cii'cuits of substantially equal electrical constants arranged in tandem, said circuits having a plurality of paths through which feed-back currents may flow, and a single counter feed-back path coupled to each of said circuits and arranged to conduct an amount of counter-feedback energy equal to the amount of energy conducted over all of said feed-back paths and a pair of condensers 'for insulating different portions of said circuits.

6. In a radio frequency circuit, a plurality of vacuum tube circuits of substantially equal electrical constants arranged in tandem, said circuits having a plurality of paths through which feed-back currents may flow, a single counter-feedback circuit having a total of impedance sufficient to cause a flow of current at such a voltage and power factor as to cause sufficient energy to iiovv to counteract the energy applied to the feed back circuit, said counter-feedback circuit being coupled to each of said circuits.

7. In a push-pull amplifier circuit7 a pair of electron tubes having their input circuits inductively connected and inductively coupled and having their output circuits conductively connected and inductively coupled, an independent inductance in each circuit and an independent counter-feedback circuit di- Y into two symmetrical parts by means of coni densers, one of said part-s being coupled to the inductance in said input circuit and the other of said parts being coupled to the inductance in said output circuit.

9. In a push-pullamplifier circuit, a pair of electron tubes having their input circuits conductively connected and inductively coupled and having their output circuits con-v ductively connected and inductively coupled, an independent inductance in each circuit and an independent counter-feedback circuit divided into vtwo symmetrical parts by means of condens-ers, the terminals of each being cont nected to the terminals of counter-feedback coupling coils, one of said coils being inductively related to the inductance in said input circuit and the other of said coils being inductively related to the inductance in said output` circuit.

10. In a push-pull amplifier circuit, a pair of electron tubes having their input circuits conductively connected and inductively coufeo pled and having their output circuits conductively connected and inductively coupled, an independent counter-feed-back circuit comprising a pair of counter-feed-back coupling coils and a pair of condensers, said pair of condensers dividing said circuit into two symmetrical parts, the terminals of said condensers each being connected to the terminals of said counter-feedback coupling coils, the relative proportions between the inductance and capacity of said counter-feedback circuit being so adjusted as to counteract to substantially the same degree all feed-back currents within the amplifier circuits.

ll. In a push-pull amplifier circuit, a pair of electron ytubes having their input circuits conductively connected and inductively coupled and having their output circuits conductively connected and inductively coupled, an independent counter-feed-back circuit comprising a pair of counter-feed-back coupling coils and a pair of condensers, said condensers dividing said circuit into two symnietrical parts, the terminals of each of said condensers being connected to the terminals of counter-feedback coupling coils7 the relative proportions between the inductance and capacity of said counter-feedback circuit being so adjusted as to counteract to substantially the same degree all feed-back currents within the amplifier circuits and automatically compensating for variation in coupling" losses by increased energy conductivity.

12. In a radio frequency amplifying circuit, a pair of electron tubes each having grid, plate and cathode elements, circuits interconnecting said elements, impedances, resist! ances and Vcapacities disposed in said circuits, means for neutralizing the undesirable feedback currents in said circuits consisting of an independent counter-feed-back circuit coupled to the electron tube circuits wherein saidcircuits have the following proportions:

where A is the impedance of the input cir "ilo cuit, B the impedance of the neutralizing coil coupled with the input circuit, b the impedance of the neutralizing coil coupled with the output circuit, C the inherent capacity of the feed-back circuit and o the capacity of the counter-feedback circuit.

13. The method of preventing the output circuits of a push-pull amplifying stage from a'ecting the input circuit of said stage, which comprises balancing the retroactive eifects in one tube by impressing on its grid a voltage opposite in phase to its alternating plate voltage, and balancing the retroactive effects in the other tube by transferring a voltage from its plate to a point in its grid circuit of opposite alternating voltage to its grid, through a single coupling common to the two tubes.

lll. A circuit comprising two space discharge devices connected in balanced or pushpull relation, each having a cathode, an anode and a grid or impedance control element, and a single, common connection between the anode and grid circuits of said devices for neutralizing the grid-plate capacities of both of said devices.

15. A circuit for capacity-neutralizing a push-pull vacuum tube stage comprising a capacity connected between the anode of one tube and the grid of the opposite tube and of such magnitude as to neutralize substantially the retroactive effects in both tubes.

16. A circuit for capacity-neutralizing a push-pull vacuum tube stage comprising a connection from the anode of one tube to the grid of the opposite tube, said connection including capacity of a value to substantially prevent retroactive effects in both tubes.

17. In an amplifier comprising an even number of space discharge devices connected in balanced or push-pull relation symmetrically about their common cathode connection, a capacity path joining points in the output and input circuits on respectively opposite sides ofthe common filament connection, said path serving in common to neutralize eectively inter-electrode capacity effects in both halves of the circuit.

In testimony whereof I aiiix my signature.

EDWARD W. FEARING.

Images