US1868162A - Stabilized amplifier system - Google Patents

Description

July 19, 1932. H. F. ELLIOTT I STABILIZED AMPLIFIER SYSTEM Filed Oct. 10, 1928 4 Sheets-Sheet INVENTOR. BY W ATTORNEYS.

July 19, 1932. H. F. ELLIOTT 1,868,162

STABILIZED AMPLIFIER SYSTEM Filed Oct. 10, 1928 4 Sheets-Sheet 2 JED QSUL

Qwmmn H 26 H /a/ flaro/a 5 67/6919 I N VEN TOR A TTORNE y 19, 1932- H. F. ELLIOTT 1,868,162

STABILIZED AMPLIFIER SYSTEM Filed Oct. 10. 1928 4 Sheets-Sheet s 41 2 M/ f 9 0 42 T .46 ;1|-/ 40 49 4 BY w ATTORNEYS.

July 19, LLI

STABILIZED AMPLIFIER SYSTEM Filed Oct. 10, 1928 4 Sheets-Sheet 4 v flare/d E 5/0016 INVENTOR.

A T T ORNE YS.

Patented July 19, 1932 NITED 5 STA-TIES TO RADIO CORPORATION OF AMERICA, OF NEW. YORK, Y., A

DELAWARE PATENT, E f

HAROLD r. ELLIOTT, or 2.51.0 Am, CALIFORNIA, nssrenon, BY Mnsnn ASSIGNMENTS,

oonronnfrron '0]? I STABILIZED AMPLIFIER srs'rniy i Application filed Octdber ljO, 1928. Serial No. 311,491.

Thus there is an electronic emission electrode, in the form usually of an electrically heatedfilament or cathode; as well as a plate or anode on which the electrons are received. By the'aid of an external source of potential, the anode is kept positive as regards the cathode, and when this is done, a considerable space current can flow through the tube space, the circuit being completed through the external connection between the anode and the cathode.

It has been found that the quantity of electron flow between these two electrodes can be easily controlled by providing an electrode in the tube space, the potential of which can be varied with reference to the cathode. This control electrode is usually in the form of a grid. If variations in potential diiference' exist between the cathode and this grid, as for example those due to the signaling impulses to be amplified, then corresponding variations in the space current result. It is this feature that has been utilized for the amplification function. The

magnified current variations can be impressed on an impedance or transformer, and the resultant variations in potentlal differencev can if desired,be again amplified by a succeeding stage. y

I Such amplifiers operate efficiently and without serious drawbacks when only nominal amplification is attempted, but when the degree of amplification is high, as'in cascade amplification, there areefi'ects'that dis-" turb' the successful operation of the-system.-

These may be due to the inherent capacity couplings between the tubeelectrodes, which although minute produce appreciable results when several stages of. amplification are used. Due to these capacity couplings, the

output circuit reacts in the input circuit,and

succeeding stages of amplifiers react onpreceding stages. These reactions lead tothe formation of oscillations which may be of several types. If the amplifier employs tuned circuits, as is frequently done in am-' plification at radio frequencies, oscillations may be formed at the frequency-to which the"- circuits 5 are tuned. Various neutralizing systems have been devised to overcome this form" of oscillations. However, the means which are employed for neutralizing irequently aggravate another form of insta bility, and lead to the generation of'parasitic oscillations whose frequency depends upon the stray c'apacitances and leakage inductances of the system. These parasitic oscillations may be so strong as to render the amplifier' entirely-inoperative. I

' Substantially similar parasitic effects are noted in connection with push-pull ampl'r.

fiers, used either for radio or audio frequencies. Such push-pull systems have-leakage inductances and stray capacitances formingcircuits resonant at high frequencie's,iand

therefore especially pronerto sustain such oscillations.

It is one of the objects of my inventionto correct these conditions, by ensuring that neither the amplifying or neutralizing sys tem' will be cap pporting undesired oscillations. v t

' I accomplishthi'siresult by the u' df a damping element, such as a resistance, which is coupled to the system inisuch a manner" that it is effective only in theparasitic os"cillation circuits,-and has noefi'ect whatever in the useful load circuits. This damping" element is used in addition to the 'neutrali'z ing system or other means which may be employed for maintaining stability atthe-frequencies which the system is desig'ne'd to'jam g plify.

be made more easilyapparentifrom a cona fewforms in the drawings accompanying and forming part of the present specification. shall now proceedtodescribe these 7 invention possesses manyother advantages, andhas other objectswhieh may" not to be taken in a limiting sense, since the scope of my invention is best defined by the appended claims. 7 V

Referring to the" drawings:

Figure 1 is a wiring diagram illustrating. a tuned radio frequency amplifier systemv employing one of" the neutralizing systems" which havebeenproposed fr'prevent1ng-os= cillations at the frequency to which the system is tuned, but in which the amplifier'is prone to parasitic oscillations, .aswill be ex.-

plained hereinafter;

Fig 2 is:.-the=equivalent.Wheatstone alters natiiig' current. bridge Of-Fig. 1;

F igs..3 and 4:5 aresi-mp lified diagrams i1.-

lnstrating thje' manner in which parasitic oscillations are formed'in the circuits ofiFig..1.:1 Eig 5' is aYdia-gram. showing the; tuned neutralizedamplify-ing circuit of; Fig. 1 with thef addition of. stabilizing system: for suppressing. parasitic oscillations;

Fig. 6 is a: diagram: of the equivalent Wheatstone; alternating. current. bridge: of

act-site: suppress parasiticoscillations;

Fig. 8 is a Wiring' diagram: of. an alterna.-

. tive formrof my stabilizingsystem;

Fig. 9 is a diagramnshowing an. application of my. stabilizing; system t'o the prevention. of parasitic oscillations a Y push-pull.

. Eig..10..-is1a=. diagram of .a stabilizing :systemv in combination with a; neutralizing: circuitdesignate the two sections .of the secondary? of the tuned radio frequency transformer inra, pushfpull. amplifier and.

' Fig. l1is: a simplifieddiagram for explaining the operation of the systemshown in:

Fig, .10.

Themanner .in.=which.= parasitic. oscillations arev fonmedinr amplifier: c rcuits. and. the action; of. my inventlon in suppressing them:

may now be considered in detail.

7 Fig. 1;.illustrates;a= tuned radio frequency amplifier of two "stages incorporating 1 one: of the neutralizing systems. which. have." been. proposed for preventing oscillations at the, frequency to whichthe system. is tuned as an; amplifier. Coils 20. ,;-and25 c'onstitute thel 'primaryandisecondaryof a; radio frequency transformer whichistuned to resonance at impressed upon-thegrid-13 of the electronic 1 emission. amplifier-1 11; .1The=.coils;15. andz,

which. form the. secondary are usually made substantiallyequalf and are connected: at their center. point E, tothe: cathode 12 of: the am' plifier. 11,. through; the medium; of: ground connection .16. and 17.

Z is a. diagram; more: clearly showing: themanner; in which .my stabilizing, system.

posite to the point atv which the. grid. is.

i (A C battery'may. onmayf not be included. in. the: ground O l? cuit.) The variationsinpotential which are impressed between grid 13 and cathode 12 cause Variations in the current in the anode ing the capacitive coupling 2'0whichexists betweemt'he' grid 13, and anode 14: of the amplifier l1,energ.y.w.o uld be fed back from the output circuit to the resonant input circuit and the system would: oscillate at .a frequency substantiallythesameas thatto;

whichthe resonant-circuit.istuned It is possible that an amplifier. of-a single stage; might; be proportioned sot-hat.v oscillations would not be formed,.butzwhentmore than, one stage is employed, or when .thegamplification: is of more. than. a nominal amount, such: oscillations are certain tobea forrmed,.unless= means are takentoprevent'. their formation. In tlieheutralizingsystem illustrated .in Fig; 1 this is accomplishedzby the use .ofcapacitor- 24,..which feeds-energy from the output. circuit. back. to the end? of zthe input 'circuittopy connected] H 1 Since the functioningof suchaneutraliz-.- ingsyste'm' is? welli'understood,.it is only neces-.; sary to refer: briefly-to the equivalent Wheat stone alternatie' current bridge; illustrated-- in: Fig.2. Here numeral 20 represents the gridanode capacitance and 24.-is the neutnalizing, capacitor. asinFig. 1.. Numerals15-and and; 28;: designatesthe tuning. capacitor,, all. as I in. Fig; 1.. v Numeral? 20 representszithe primary of the; transformer andi18 repre-g sents-the primary of-the-transformer10f the; suc ee ingzst ge. W n' A potentialacross. C and D,--Itherefore,.does

not causerany current :to I circulate" inthe: res ona'nt circuit. 1525'.28;.. Si=1ice-no1 current? circulates in this circuit, therexis: nol'transferof'energy from. coil 18', .in ..the"-outpi1t: circuitback to coil 20%, ofthevprecedi-ngsstage;1 :The system: is, therefore, neutralized? against the transfer of energy fromonestage BKCkTtO the preceding. stage; There is,-ihoweyer,.a;.atranse fer" of. energy from theoutputfside'of each amplifying:tuberbacktto the input side ofit'he same. tube The amount. of regeneration?- which may thus be produced depends upon the parallel opposing leakage inductance of 15 and 25. If a mutual inductance l .2 of these coils were 100%, no potential would be applied to the grid as a result of the curcurrents flow in the resonant circuit and the amount of regeneration is usually not sufficient to cause instability. In fact, it may be of advantage in reducing the losses in the system.

The bridge system shown in Fig. 2 is neutralized against the transfer of energy from a preceding stage to a succeeding stage as well as in the reverse direction. Currents which" circulate in the resonant circuit 2528, cause no fiow of current in the inductor 18, since C and D are at equal potential when the bridge is balanced. Hence, no transfer of energy takes place from circuit 152528 to circuit 7980-81 in either direction so" long as the bridge is balanced. This'is the function of all the various neutralizing'systems which have been proposed for preventing oscillations'at the resonant frequency in tuned radio frequency amplifiers. V

The above discussion deals only with the manner in which the neutralizing circuits serve to prevent oscillations at the frequency to which the system is tuned as an amplifier. It will now be shown that the addition of neutralizing circuits aggravates another condition of instability and causes the system to I generate parasitic oscillations at a frequency which is dependent upon the parallel opposing leakage inductance of 15 and and the stray capacltances to ground 26 and 27,-of

the various inductances, capacitors, wiring, etc., which comprlse' the amplifier and its associated circuits. The system has two degrees of freedom, one forthe currents to be amplified whlch flow through coils 15 and 25 in series, as shown by arrows 3838; and 4 the other for currents flowmg through the same coils in parallel as shown by the dotted arrows 3939. It is the latter free; period which leads to the formation of parasitic oscillations as will now be explained.-

Fig. 3 shows the circuits of Fig. 1 withthe part-s rearranged to bring out the manner in which they combine to formthe oscillatory circuits in which the parasitic oscillationsare formed. For practical purposes, the system of Fig. 3' may be reduced to the circuits shown in Fig. 4. Numeral 89-represents the leakage inductance of coils 15 and 25 taken in parallel opposing. Numeral 90 is the net capacitance of the network comprising capacitors 26, 27 and 28. Numerals 18 and 88 are the leakage inductance and distributed capacitance of the primary of the succeeding stage. Numeral 91'is the grid-anode capacitance of the amplifier 11, plus capacitors 24 and 28, taken in series. This system constitutes an unneutralized amplifier stage tuned for frequencies which are usually well above those which the system is intended to am-' plify. It is, in fact, a Hartley oscillator, but with increased feed back. The manner in which oscillations are formed andsustained in such an unneutralized amplifier circuit' is thoroughly explainedin an article entitled, The audion oscillator, by R. A. Heising, which was published in the Journal of the American Institute of Electrical Engineers, for April and May, 1920. The

fundamental theory of oscillators is completely developed in this article'and the reduction of radio-frequency amplifiers to the oughly explained with circuit diagrams, reactance vs. frequency characteristics, and vector diagrams. It is shown in th s artlcle,

in the discussion of Figs. 21 to 24 inclusive,

how the usual, unneutralized, radio frequency amplifier stage is reduced to the Hartley oscillator. The grid-anode capacitance forms the capacitance of the oscillatory circuit, while the inductive reactances of the grid and anode oscillatory circuits, at a frequency slightly below resonance for the lower of the two, form the inductance of the oscillatory circuit. It is shown that the complete circuit will oscillate at a frequency slightlybelow the natural period of the cir.

cuit having the lower resonant frequency and that large changes in the grid-anode capacitance makevery small changes in the frequency of the oscillations. This circuit will oscillate vigorously over a wide range of circuit constants and the oscillations are difli- .cult to suppress.

'85 corresponding Hartley oscillator is thorthe multiple stage amplifier may incorporate neutralizing means is of little avail in preventing parasitic oscillations, since one stage is coupled to anotherthrough stray capacitances, as indicated by 92 and 93 in Fig. 4. In fact, as explained above, the neutralizing circuit usually aggravatesthe situation rather thanimproves it.

As hereinbefore outlined, one object of my invention is to provide effective means of suppressing'the parasitic oscillations which have hitherto rendered the above described type of neutralizedradio frequency amplifier impracticable. This I accomplish by the method illustrated in Fig. 5, wherein capacitors 29;-and 30, and resistor 31, havev been added,

to .the neutralized circuits of Fig. 1. We now have an alternating current Wheatstone:

bridge consisting of eight arms: 15 and 25,

29 and. 30,- 26' and'27 and finally and 24.

This differs very materially from the usual bridge of four arms. the eight arm bridges, each of the three pairs of. capacity arms must be adjusted to have the same react-ance ratio as the pair of inductance; arms. In other Words,

whenthe bridge is balanced. In practice,

stray capacitances 26 and 27 are usually of' small magnitude and sonearly balanced that they need not be taken into account, and the bridgereduces to six arms instead of eight. hen the bridge is balanced, points E and V F, as well as points C and D, are at equal tions whose formation was described by. the

aidof Figs. and 4. This is brought out in Fig. 7 which is the same as Fig. 4 but with the additionof the parasitic suppression circuit 293031. It will be seen that the resistor 31, which, as above shown, is conne'ct'ed'across points of equal potential for theuseful load. currents, is coupled directly 'across points of high potentialfor the parasitic oscillations. This" is due to the fact that the load currents flow in series through coils 15 and 25, while the parasitic'currents flow'in parallel through the same coils. By suitably adjusting the'value of resistor 31',

it'maybe made to absorb so much energy from the parasitic oscillation circuit that.

these oscillations are completely suppressed. It has been found in practice that for the usual radio frequency amplifier this value is not at all critical. For example,'in an amplifier designed .for broadcast reception, in the'bandfrom 550 w 1500'kilocycles it maybe of any value between 300 and 2000 7 ohms. Capacitors 29 and'30, also may vary over a considerable range. For'broadcast reception, values ranging from 30'to 50 m1 cro-micro-farads'are suitable. If capacltors 29 and 30 are made too small, the coupling between the resistor 31, and the circuit in" Which the parasitic oscillations are formed may not be sufficient to prevent such oscillationsfrom forming. If too large, the main In order to balance resonant circuit may not tunexto thehighe'st frequencies which are to beamplifi'ed. Otherwisethe limitations are very broad.

The particular location of the damping:resistance so that it is effective in a parasitic. circuit and ineffective in auseful circuitcan be varied. For example, in Fig. 8 an arrangement is shown in which. the damping. element is inductivelycoupled tocoils 15 and. 25, rather than being capacitively coupled. as in the circuits described above. In this case, a circuit including coils 34 and v35 and damping resistance 33, is coupled to both; coils 15 and 25. The coupling tocoil 15 is. effected by a coil 34, and the coupling to coil 25 is effected by another coil 35, in'series Wit-h coil 34. The arrangement is such that equal and opposite E. M. F.?s are induced" in: coils 34 and 35 for the load currents-passing; in series through coils 15 and 25, and-consequently no damping current flows through re.- sistance 33. However, for the parasitic-currents which flow in parallel through coils- 15 and 25, the E. M. F.s induced in coils 34 and 35, are cumulative, and'strong dampw ing results for the parasitic oscillations.

In the two forms of my invention thus far. described, there are only two stages of amplification, however, the damping-scheme can be utilized with any number of stages... Fur-- thermore,'it is not limited to radio frequency systems. For example, serious parasitic. os-- cil'lations often occur. in connection with either radio or audio frequency push-pull amplifiers. In Fig. 9, Ishow an'audio frequency push-pull amplifier system, utilizing; two transformer coupled stages, in which my stabilizing scheme may be very advantageously used.

In this figure, each stage includes a pair of tubes 40 and 41. The-input circuit oftube 40, includes grid 46., coil 42, a grid bias battery 43, ground connections 44 and Y45, andv cathode 47 The output circuit includes anode 48, transformer primary 49,"B battery 50, and cathode 47 i The input circuit and the output, circuit of the other tube 41, have elements in'common with the respective circuits of tube '40, as is now well known in connection with push pull. systems. Thus the input circuit of tube 41, includesgrid 51, coil52, battery 43; ground connections 44 and 45, and cathode 53. The outputcircuitincludes anode 54, transformer primary 55, B battery. 50 and cathode 53.

The incoming circuit 56, is'shownas con-- nected to the primary of an iron cored trans-- former57, which is coupled t'o-both coils 42 r and 52. When a signaling impulse flows incircuit 56, the-grids 46and 51 are oppositelyaffected. Primary coils 49 and5'5, however,

are so arranged that their effecton the succeedin stage. including. coils 58 and. 59-, is.

cumulative. I

This. cursory review ofpush-pu'll ampllfi completed. By using a pair of series capacitors 60 and 61, I can provide a bridge 42526160 that operates as the bridge 15258029, of Fig. 5. Thus a resistance 63, can be connected between points of equipotential so far as the load currents are concerned, but across points of large potentlal difference for the parasitic currents to be supressed. Resistance 63 is for this purpose connected between coils 42 and 52, and between capacities 60 and 61.

In the audio frequency system of Fig. 9, the damping system is shoWn in each stage, and

only two stages are shown, feeding an outgoing circuit 66. However, it is apparent that the same scheme can be used for each of any number of stages, and for radio frequency push-pull systems also. For radio frequency systems, the input circuits may be tuned, and the iron cores of the coupling transformers may be omitted. One form of such a radio frequency push-pull system is shown in Fig. 10.

In that figure, the amplifier tubes 67 and 68, of each stage have input circuits that include coils 69 and 71 and output circuits including coils and 72. The system operates just as that of Fig. 9, except that a tuning capacitor 7 3 may be provided, and that the couplings between stages are affected without the aid of iron-cored coils. As in the prior systems described, leakage inductances and stray capacities cause parasitic oscillations which flow in parallel in coils 69 and 71.. By providing a pair of series capacitors 74 and 75, across coils 69 and 71, which capacitors have the proper ratios to the inductances, a selective damping bridge is formed. A resistor 76 is connected across points E and F that are equipotential for the load currents, but which have a considerable potential difference for the parasitic currents.

In the form of the invention shown in Fig. 10, neutralizing circuits are also included. The neutralization for the grid- plate capacitances 101 and 102 is affected by the aid of capacitors 77 and 7 8, forming with coils 69 and 71 and 70 and 72, and the other capacitances of the system, an alternating lVheatstone bridge having four inductive and eight capacitive arms as shown in Figure 11. Since the transformer of a push-pull system are usually made so that the two'primar'ies and two. secondaries are fully balanced, it is only necessary to balance each pair of capacities in order to have the entire bridge in balance. Thus, for a fully balanced system, L =L L70:.L72, 101 102, 77 787 74 and .0 0 I In the following claims, the term load current? is used to include those currents essential to the proper operation of the system, such for example as the curents to be amplified and .the neutralizing currents, but notsuch currents as arise from parasitic oscillations at frequencies determined by the strap capacitances and leaking inductances of the system. '1 i 1 Iclaimzf I 1, 1. In an amplifier, the combination of an electron dischargedevice' havinginput and output electrodes, a tuned inputcircuit for said device including inductanceand capacity, one end of said inductance being connected to one of said input electrodesand an intermediate. point on said. inductance being-connected to another of said input 619C?" trodes, a capacity connected inseries between the other end of said inductance and one of said output electrodes, an impedance shunting said input circuit, and a nQnTreactive" circuit includinga damping resistance andconnecting said intermediate point with a point on said impedance that is equi-potential therewith with respect to load currents in said input circuit.

2.v In an amplifier, the combinationof-v an electron discharge devicehaving input and output electrodes,an input circuit connected to said input electrodes and including an input transforme ya pair of capacitiesin series shunted across said transformenand a non-reactive damping circuit connected between said capacities and to aninterniediate point on said transformer, said .two capacities and the two portions of said transformer constituting a balanced Wheatstone bridge.

8. In an amplifier, the combinationof an electron discharge device having input and output electrodes, a tunable input circuit connected to said input electrodes and including an input transformer, a pair of capacities in series shunted across said transformer, and a damping resistance connected between said capacities and to an intermediate point on said transformer, the points of connection of said resistance being equi-potential with respect to load currents in said input circuit.

4. In an amplifier, the combination of an electron discharge device having input and output electrodes, an input circuit connected to said input electrodes and including an inductance, one end and an intermediate point of said inductance being connected to said input electrodes, a neutralizing capacity connected to the other end of said inductance and toone of said output electrodes, a'pair ofcapacities in series shunting said inductance, and a dam-plngresl-stance connected to an intermediate point on said inductance and between said capacities.

-5. In an amplifier, the combination of an electron discharge device having input and output electrodes, a tunable input circuit including an input inductance and a variable capacity, one end and the midpoint of said input inductance being connected to said input electrodes, aneutralizing capacity connecting the other end of said inductance and to one of said output electrodes, a pair ofsubstantially equal capacities connected in series across saidinductance and an aperiodic damping circuit connected to the midpoint of said inductance and between said capacities.

' 6. In an amplifier,the combination of a plurality of electron discharge devices having input electrodes, a common input circuit connected to said electrodes, a plurality of V capacitors in series-directly shunted across said input circuit, and aperiodic damping means connected between said capacitors and to an intermediate point on said input circuit, the points of connection of said damping meansbeing equipotential with respect'to useful load currents in said input circuit.

7. In an amplifier, the combination of a plurality of electron'discharge devices having input and output electrodes, a commoninput circuit connected to said input electrodes, a reactive connection between the out-. putelectrode" of each of said devices and the input electrode of the other of said devices, a plurality of capacitors in series directly shunted across said input circuit, and aperi'odic damping means connected between said capacitors and toan intermediate point on said input circuit, the points of connection of said damping means being equipotential with respect to useful load currents in said input circuit. I

In testimony whereofI have hereunto set my hand.

HAROLD F. ELLIOTT.

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