US1530649A - Electric circuits - Google Patents

Description

March 24, 1925. 1,530,649

W. L.' CASPER ELECTRIC v CIRCUITS Filed A ril 20, 1921 2 Sheets-Sheet 1 I I IV/ I'am L. Casper March 24, 1925.

2 Sheets-Sheet 2 Filegi April 20, 1921 m d m r 2 8 M h r.

flW/fmrl Gas 961; a7 W 2 Patented Mar. 24, 1925.

UNITED STATES PATENT OFFICE.

WILLIAM L. CASPER, 0F BROOKLYN, NEW YORK, ASSIGNOR T0 WESTERN ELECTRIC COMPANY, INCORPORATED, or main YORK, N. Y., A CORPORATION OF NEW YORK.

ELECTRIC CIRCUITS.

Application filed April- 20, 1921. "Serial No. 482,883.

To all whom it may concern:

Be it known that I, WILLIAM L. CASPER, a citizen of the United States of America, residing at Brooklyn, in the county of Kings, State of New York, have invented certain new and useful Improvements in Electric Circuits, of which the following is a full, clear, concise, and exact'description.

"This invention, relates to transmitting and amplifying electromotive forces and has for I an object to secure a desired shape of the curve which represents the relation between the amplification or gain of a vacuum tube repeater circuit and the frequency. This object also comprehends reducing the distortion produced by a repeater circuit and due either to the inherent capacity between the electrodes of an audion repeater or to the capacity between the windings of repeating coils or to both.

In the case where the audion type repeater is coupled to the line by means of a repeating coil or transformer, the object is accomplished by employing a transformer winding having an inductance of such a value that the inductance in connection with the capacity above mentioned resonates at a desired frequency. It has been found that for voice current transmission the preferable value for this resonance frequency is near the upper limit of the important voice fre quencies, and may have a value of approximately 5000 cycles per second In this case it has been found that the curve which represents the gain of the repeater for different frequencies of transmitted current is substantially fiat throughout the important part of the voice frequency range, which means that the gain is constant over that range, and the desired reduction in distortion is secured.

Before proceeding with a detailed description of the drawings, reference may be made to sections 60, 69 and 70 of the treatise on The thermionic amplifier, by Van Der Bijl (McGraw-Hill Book 00., N. Y., 1920), wherein it is explained that the potential variations impressed on the grid when power is supplied to the input circuit of a vacuum tube can be influenced by the electrostatic capacities between the electrodes-of the tube, and that in ordinary tubes when the operat- 21cf 10 the effective input reactance as measured between the filament and the grid is the reaetance of a capacity greater than the electrostatic capacity between filament and grid by an amount directly proportional to the" electrostatic capacity between grid and plate, and increasing with the amplification constant of the tube and'decreasing with the resistance in the output load. This effective input reactance is inversely proportional to the frequency. Therefore when the tube is fed from a circuit containing impedance which does not decrease with frequency the amplification obtained, as measured by the ratio of the output electromotive force of the tube to the electromotive force applied to the feeding circuit, will ordinarily decrease as the frequency is increased, for the reason that the current through the feeding circuit will increase and therefore the voltage drop in the feeding circuit will increase.

As explained in section 71 of the Van Der Bijl treatise, when the frequency is very high, so that the impedance due to the electrostatic capacity between the filament and the plate is no longer small compared to the internal plate circuit resistance with which it is in parallel, the effective input capacity of the tube although independent of the con stants of the external output circuit is dependent upon the electrostatic ca acities between grid and filament, grid an plate, and filament and plate, and reduces the input impedance to such a low value that the input current, and consequently the voltage drop in the feeding circuit, is increased to such a degree that the electromotive force across the grid and filament, and therefore the amplification obtained by using the circuit comprising the tube and the feeding circuit therefor is lowered.

Thus, as set forth in U. S. patent application of F. L. Casper, Serial No. 366,581, filed March 17, 1920, entitled Electric circuits,assigned to the assignee of this appli-' cation, the general effect of the-effective input capacity of the tube is to decrease the effective input impedance of the tube, and therefore to decrease the amplification of the repeater circuit, when the frequency becomes high, and conse uently tocause distortion ing range of frequencies is such that by unequal amp ification of different frequencies.

small. However, under certain conditions,

for instance if the frequencies impressed on a tube are as high as 1000 cycles per secnd, the effect of the capacity may be quite appreciable and where the frequency varies may cause an appreciable amount of distortion in the current waves to be repeated, due to the decrease of the degree of amplifica tion with increase of frequency.

It has also bee-n found in the case where the vacuum tube repeater is connectedto the incoming line by means of a transformer that further distortion and a lowering of v the gain of the repeater occur due to electrostatic capacity effects between the windings of the insulated conductors of the transformer. It appears that these undesirable effects of the transformer capacity are caused by the reactance of this capacity acting as an effective shunt across the grid and the filament, this capacity being in effect in parallel with the effective input capacity of the tube. The effective input capacity of tubes in ordinary use varies from 20 M. M. F. .to 250 M. M. F. depending on the type of tube and the output impedance into which it is working. Input transformers in ordinary use have capacities varying between 15 and 350 M. M. F.

hen a wide range of frequencies is impressed upon the input terminals of the tube is comparable to or lower than the impedance component in multiple with that capacity reactance component.

In accordance with this invention when it is only desired to transmit one frequency or to transmit a narrow range of frequencies the undesired effect of the grid-cathode capacity and the transformer capacity is neutralized by including effectively in series with these capacities an inductance tending to resonate with these capacities, the in ductance being given such a value that the amplification frequency curve of the repeater has a maximum point at the frequency'to be transmitted. W'hen it is desired to amplify a considerable range of frequencies and to have the amplification frequency curve of the repeater flat throughout this range the undesirable effects of the tube capacity and of the transformer capacity for these frequencies are neutralized by inserting effectively in series with these capacities an in-.

ductance tending to resonate with. the capacities and having such a value that the tendency of the capacities to cause the undesirable decrease in the amplification is neutralized for the frequencies for which it is of the mostimportance that drooping of the amplification frequency curve be eliminated. When voice frequencies are to be transmitted and the amplification frequency curve is to be flat throughout the range of the important voice frequencies transmitted, this range including. among other frequencies, the band of frequencies between about 1000 and 3000 cycles per-second, the value given the inductance is preferably such that the frequency at which the inductance would resonate with a capacity of the valueof the effective inputcapacity of the tube combined with the transformer capacity is considerably higher than the average frequency of such band. For it has been found that by having the inductance of such value that it would resonate with a capacity of the value referred to above almost at the upper limit of or even considerably above this frequency band, the amplification frequency curve of the repeater circuit can be made substantially flat throughout this band, while the curve can be made to rise markedly with frequency in the upper part of such band by giving the inductance such a value that it would resonate,with a capacity of the value referred to above at frequencies well below the upper limit of this frequency band. In certain cases where-it-is desired that the amplification frequency curve rise with frequency in a given frequency range, to compensate for increase of attenuation with frequency in the line in'which the repeater is connected, for instance, the inductance is given a value higher than that ne'cessary to obtain a flat amplification frequency curve for the given frequency range. Thus, if it is desiremfsto have thelc'urve rising in the upper part of the frequency band mentioned above, the inductance is given such a value that itwould resonate with a capacity equal to that of the effective input capacity of the tube combined with the electrostatic capacity in the transformer at a frequency below the upper limit of the frequency band 'mentioned. The amount by which the inductance is increased to obtain the rise in the GO .has a capacity component which may be repeater circuit as viewed from the source which feeds it will ordinarily have a minimum impedance point in the neighborhood of the frequency at which the resonance condition referred to above occurs. It should be understood that the principle of amplification control set forth especially with referenceto a voice frequency range may also be applied to amplification control at other frequencies, as for instance at the frequencies commonly used for carrier current.

This invention will be better understood by reference to the following detailed description taken in connection with the accompanying drawing in which Fig. 1 represents an embodiment of this invention; Fig. 2 is a modification thereof; Fig. 3

illustrates how an inefficient transformer coupling may be secured to' serve the purpose of this invention; Fig. 4 illustrates one way, and Fig. 9 a preferable way, in which the additional inductance for neutralizing undesirable effects of capacities 7 of the tube are connected to an incoming.

- can be obtained by means of a winding on the core of an input transformer for an amplifying vacuum tube; Fig. 5. shows, in

schematic, a repeater circuit comprising a voltage step up input transformer. Fig. 6'

shows an equivalent simple circuit in which the transformer has been replaced by its equivalent unity ratio T network; Fig. 7 shows curves indicating variations of im .pedance with frequency in 'thecircuit of Fig. 6; and Fig. 8 shows curves indicating the variations of amplification with frequency in the circuit of Fig. l for various values of inductance in series with the input transformer.

Referring to Fig; 1, 5 is an amplifying vacuum tube having a cathode 6, a grid 7 and an anode 8. Heating current for cathode 6 is supplied by a battery 9. Space cur-. rent for the tube is supplied from a source of voltagelO. The input electrodes 6 and line 11, 11 by means of a transformer 12. The amount of potential impressed on the transformer 12 by line'11, 11 is determined by the potentiometer 17. The output electrodes 8 and 6 of the tube are connected to an outgoing line 13, 13 by means of a transformer 14. With such an arrangement tube 5 is adapted to amplify currents impressed on line 11, 11 by source 15, the amplified currents being impressed on line 13, 13 by transformer 14.

It has been found that the effective impedance between grid 7 and cathode 6 not only has a resistance component but also appreciable even. where the frequency of the incoming current is as low. as 1000 cycles plification with increase of frequency and is therefore undesirable, especially for ultraaudio frequencies where the effect is most marked.- As is also indicated above, it has further been found that there is a decrease of amplification with increase of frequency due to the capacity effect present between the winding turns of input transformer 12. In accordance with this invention the undesirable efi'ect of the grid capacity and the transformer capacity for a given frequency of source 15 is substantially eliminated, and in some cases these capacities utilized to increase the amplification, by inserting an inductance effectively in series with the prito the necessary degree, with the remain ng effective reactance of the primary winding as modified by the secondary circuit including the effective input reactance of the tube 5.

The neutralization of the undesirable effect of the capacities upon the degree of amplification, or the increase of the amplification obtained by'resonating to the necessary degree the inductive leakage reactance of the input transformer and the remaining effective reactance of the transformer and its load may be explained by reference to Figs. 5, 6, 7 and 8.

Fig. 5 shows, in schematic, a repeater circuit including animpedance matching input'transformer having primary, secondary, and mutual impedances P, S, and M, respectively. The equivalent simple circu t wherein the transformer has been replaced by its equivalent unity ratio T network may be represented as in Fig. 6. As has been indicated above, an input transformer feeding a vacuum tube may be regarded as, in effect, two separate elements, to wit, a transformer having zero electrostatic capacity and a capacity separate from the transformer and shunted across the input terminals of the tube. In accordance with this view, the transformer in Fig. 5 is regarded as a transformer having zero electrostatic capacity, the capacity which the trans former would actuallyhavebeing represented as a capacity Z separate from the transformer, shunted across the effective input impedance Z, of the tube 5. The combined impedance of Z,, and Z is designated Z.

In Fig. 6, P, M, and S represent the primary, mutual, and secondary impedances, respectively, of the transformer of Fig. 5,

tive input impedance of the tube 5 of Fig.

5, and R is the impedance ratio of the transformer.

The impedance of the circuit in Fig. 6 as measured from BC looking east is shown in Fig. 7 as 1', :22 1 being the effective resistance and n -being the reactance. The scale to which the effective resistance is plotted is only about half as large as the scale to which the reactance is plotted. The impedance between D and B, 1. e., the impedance M P a is shown in Fig. 7 as reactance :0 There is a. resistance component in this impedance which is too small to show with the scale used. The impedance of the circuit in Fig. 6 as measured from DE looking east is the sum of 1-, m and :0 and is shown in Fig. 7 as 1*, m The impedance 1', :0 shows resonance in the sense of O reactancc at a fre queney that is determined principally by the reactance of the high side impedance S of the transformer and the reactance of impedance Z. That is, at a certain frequency the reactanceof that is the reactance of resonates with li P R JR the reactance of the impedance The circuit V is being regarded as O, the voltageldrop and across the impedance R is the total capaci- -tive drop in the resonant loop. Therefore, since the impedance l R a is small and the impedance I as J is large,-the resonant loop is, in eifect,an

anti-resonant-loop in series with the source 15, the hue 11 and the impedance Consequently, the voltage drop across the.

anti-resonant loop tends to be a large proportion of the total E. M. F. of the source 15, the value of the proportion being limited principally by the value of the impedance of the potentiometer 17 since'the effective resistance components of the impedance R an effective limiting factor however. Of course, the voltage across the receiving c1rare high. Therefore, thevoltage across cuit of the equivalent unity ratio simple circuit has to the voltage across the receivmg circuit Z 1n the actual c1rcu1t the ratio so the voltage across the receiving circuit Z in the actual repeater c1rcu1t 1s times the voltage across the receiving circuit of the equivalent unity ratio simple circuit. Thus, for this condition of antiresonance, the voltage across the input electrodes of the tube tends to be condition of anti-resonance is somewhat similar to that which has been utilized, as

described. in the U. S. patent to Nichols, 1,325,879, December 23, 1919, assigned to theassignee of this application, for the purpose of preventing, for a. given frequency of the source, diminution of the effective voltage impressed upon the input terminals due to the shunting effect of the effective input capacity of the tube. In the absence of a potentiometer, such as that shown at 17 for instance, the amplification ordinarily tends to fall off very rapidly above and below the critical frequency, where the source 15 sup- ,plies a. plurality of frequencies. With the potentiometer in circuitthe peak which the from observed values of frequency and amplification in a circuit such as is shown 1n Fig. 1, has a comparatively long radius of curvature at the maximum point, --which occurs at about 1000 cycles per second. If impedance frequency curves such as 1', m or 1", of Fig. -7 were drawn for the circuit for which curve -1 of Fig. 8 was obtained they would show a maximum impedance in the neighborhood of 1000 cycles per second. As is indicated by Fig. 7 the maximum impedance is determined very largely by the effective resistance." lVith types of transformers and tubes in use the anti-resonance frequency varies between about 400 and 1300 cycles per second. i

There is a resonance point, in the sense of minimum impedance, in r, 00 in the neigh borhood of frequency H in Fig. 7. This resonance is resonance between the impedance M SM 71? Plus a as and the impedance The inductance of the impedance is so large that it is of no importance as regards its effect upon the tuning of the cir- R R R R Since, as pointed out above, the electrostatic capacity of the transformer is being regarded as 0, this resonance may be considered as resonance between the'leakage inductance of thetransformer (that is, the inductance of the impedances M s M on the one hand and the capacity of the impedance on the other hand, if we regard tube toa'v'alue higher than the total electro motive force across DE, the amplification obtained with the repeater circuit may be above.

greatly increased at this resonance frequency. \Vhi'le the potentiometer-17 will exert a tendenc to lower the electromotive force across D due to drop in 11, the electromotive force across the input terminals of thetube is not limited to the value of the electromot-ive force across the potentiometer at this condition of resonance as it was for the condition of anti-resonance described The inductance effectively in series with the combined capacity of the tube and the actual transformer may be given the value necessary to produce resonance between these reactan'ces at the desired frequency either by properly adjusting the leakage inductance of the transformer or by including an inductance 18, additional to the leakage inductance of the transformer, effectively in series with the transformer primary.

In case source 15 supplies a plurality of frequencies and it is desired that the amplification frequency curve be flat throughout a given frequency range resonance should occur at a frequency such that the tendency of the capacities to cause undesirable decrease in the amplification is neutralized for the frequencies for which it is of most importance that drooping of the amplification frequency curve be eliminated. As indicated above, when voice frequencies are to be transmitted and it is desired that the am lification frequency curve be flat throug iout the range of the important voice frequencies transmitted, this range including, among other frequencies, the band of frequencies between about 1000 and 3000 cycles per second, the value given the inductance effectively in series with the transformer primary is preferably such that the frequency at whichthe inductance would resonate with a.

capacity of the value of the effective input capacity of the tube and the capacity of the transformer is considerably higher than .the average frequency of the band mentioned.

. For instance the frequency at which the ina capacity of.

ductance would resonate with the value referred to above is preferably in the neighborhood of 5000 cycles per second. Curve 3 ofFig. 8 is such a curve, and was obtained by giving the inductance 18 a value of 4 milhenrys, the transformer not having been specially designed to have high leakage inductance. The turns ratio of the transformer was 18000 to 436, In order to make the-amplification frequency curve rise with frequency, to compensate throughout a given frequency range, for increase of attenuation with frequency in the line in which the repeater is connected for instance, the inductancemay be made greater than it would be made 'were it desired to have the amplification frequency curve flat throughout the given frequency range. For instance, by giving the inductance such a Value that it would resonate with a capacity of the value of the effective input ca acity of the tube combined with the trans ormer capacity at a frequency below the upper limit of the frequency band referred to above, the amplification frequency curve can be made to use throuhout the upper part of that band. Curves 4, 5, and 6 of Fig. 8 are such curves. All of the curves of Fig. 8 were obtained with the same transformer. The inductances 18 employed in taking curves 1 to 6 wereO, 2. 4, 8, 10 and 1-2 milhenrys, respec-.

tively, The ordinates of the curve, designated as amplification, are thevoltage ratio E divided by E, (see Fig. 1). The potentiometer resistance was 350 ohms and the line impedance 11 was 350 ohms. had their normal output impedance and space current.

In practice, the proper value of series inductance'to be used to raise the amplification frequency curve the desired amount at high frequencies in order to get, for instance, an amplification frequency curve flat throughout a given range of high frequencies or rising with frequency throughout a given range of high frequencies, is preferably determined by first calculating the approximate value of the inductance in accordance with the principles set forth above, and then determining by trial the proper value of inductance to give the desired amplification frequency curve. For the trial a retardation coil such as 18 is preferably used, since the inductance of the retardation coil may be made readily adjustable as for instance by varying the number of active turns in the coil. hen the coil has been adjusted until the amplification frequency curve, as plotted from observed values of amplification and frequency, is of the desired form, the inductance of the coil is ascertained either by measurement or by calculation. This amount of inductance may readily be incorporated in the transformer as leakage inductance by followin the ordinary principles of transformer esign.

It has been found that when a potentiometer is employed for adjusting thevaluc of the potential applied to the vacuum tube from the incoming line in a system embodying the series inductance for neutralizing capacity in the input circuit of the tube, it is more satisfactory to have the potentiometer connected to the primary winding ofthe input transformer, as shown in Fig. 1, than to have the potentiometer connected to the secondary winding as has been customary in systems of the prior art.

It is-not necessary that the correcting inductance additional to the-leakage reactance of the transformerbe a separate element of the circuit. Fig. 4 shows how turns 20 may be wound around the core 21 of a transfor- I The tubes mer so as to have little or no mutual inductance with the primary and secondary windings 22 and 23 of the transformer. These turns 20 should be electrically connected in series with the primary winding of the transformer so that the device of Fig. 4 is the full equivalent of elements 12 and 18 of Fig. 1.

Fig. 9 shows another, and preferable, way of combining a mutually non-interfering retardation coil and transformer in the same physical-structure. The structure comprises a magnetic core having a T shaped section 41 and a U shaped section 42 fitted together at faces 43, 44 and 45 to form a closed-ma netic structure having the legs 46, 47 and 48. On the leg 46 is wound a primary winding 49 of any desired number of turns and a secondary winding 50 of any desired number of turns. Two coilsections 511 and 52 are wound on the legs 47 and 48 respectively and are connected together to form the retardation coil. Coils 51 and 52 are composed of the same number of turns and are other- .coil 51 or coil 52 does not pass through leg 46 since leg 46 connects points of equal magnetic potential. Therefore no inductive effect is produced in either coil 49 or coil 50 by the retardation coil 51, 52. It is obvious that the transformer windings might be placed on the outer legs of the core and the retardation coil winding be located on the inner legs with the same result as is obtained in the arrangement shown in the drawing.

The combined transformer and retardation coil shown in Fig. 9 is claimed in Casper application Serial No. 347,391, filed December 26, 1919, entitled Electric coils, assigned to the'assignee of this application,

Fig. 2 is a modification of Fig. 1 except that for transformer 12 of Fig. 1, a trans former 25 has been substituted which has an inefficient coupling. A transformer with an inefficient coupling is equivalent to a. transformer 01' perfect coupling with an inductance element in series with the primary winding or with an inductance in series with the primary winding and an inductance in series with the secondary winding. If the coupling of transformer. 25 is made inefficient to the proper degree the equivalent inductances 26 and 27 in circuit with its primary and secondary'windings will constitute an inductance effectively in series with theprimary winding and of such value as to control the shape of the amplification frequency curve in the manner set forth above.

One way in which the coupling of a trans former may be made inefficient, is shown in Fig. 3 Where a sheet 29 of magnetic, material has been placed between the primary I and secondary windings 30 and 31 of a transformer. This sheet 29 will serve as a bypath for some of the lines of force between the primary and secondary windings thereby making the transformer ineflicient. By

regulating the size of this element 29 it will be possible to produce the degree of inefiicient coupling in the transformer which is necessary to give the e uivalent' inductance to control the shape of the amplification fre quency curve in themanner set forth above.

It is obvious that the coupling in a trans former may be made ineflicient in other ways than by the method just described. The preferred way is to use a core of the general form of that shown in Figs. 3 and 4, with the central leg increased in length and decreased in cross section, relatively to the corresponding proportioning of a normal transformento an extent sufficient to produce the desired degree of leakage. However, this feature is no part of the present invention, but is the invention of F. E. Field, and is claimed in his application Serial N 0. 512,147, filed November 1, 1921, entitled Electric circuits issued as Patent No. 1,507,994, September 0, 1924, assigned to the assignee of this applicationf It will be noted that in each of Figures 3, 4 and 9 the ratio of the cross-sectional area of the central core leg to the cross-sectional area of either of the end legs is smallerthan the ratio which obtains in ordinary transformer design, thus tending to give the value of the leakage inductance of the transformer a. comparatively high value.

This application is-a continuation in part of the Casper application No. 366,581-re ferred to above.

7 an efficiency having a value higher than that with which it tends to transmit the upper frequencies of said band, and said input c1r-' cult comprising a tuned clrcult for increasing the transmission efficiency at said upper frequencies while the transmission efficiency for said intermediate frequencies remains substantially unchanged from said value at i which said amplifier and transformer tends totransmit those frequencies.

2. A circuit comprising an electron discharge amplifier, an input transformer therefor-and a source for supplying electrical variations comprising a band of frequencies to said transformer, said transformer and amplifier having inductance and capacity effectively in shunt to said source and" being in parallel resonance at a frequency in an intermediate portion of said band, the Voltage across said parallel ar rangement being applied to said amplifier, said circuit comprising inductance effectively in series with said source and with at least a portion of said capacity and forming with such portion of the capacity a series circuit resonant at such a-higher frequency than said last mentionedfrequency as to 9 counteract the tendency of said shunt capacity to lower the transmission efficiency of said circuit at the upper end of said band of frequencies.

3. In combination, a line, a vacuum tube 35 amplifier coupled to said line for repeating a band of frequencies in said line, said amplifier having an anode and cathode and a control electrode, said amplifier having an appreciable capacity efiect between its con trol electrode and cathode, said capacity effect giving the transmission efiiciency of the amplifierfor intermediate frequencies a value higher than the transmission efficiency for the upper frequencies of said-band, and means for producing an inductive effect acting effectively in series with said line for resonating with said capacity at such a frequency as to substantially equalize the transmission of said intermediate frequencies and said 109 upper frequencies while the transmission efficiency for said intermediate frequencies remains substantially unchanged from said value given by said capacity effect. 4. In combination, an incoming line, an outgoing line, an amplifier for repeating Without distortion to said outgoing line a band of speech frequencies in substantially the same wave form as present ,in said incoming line,'said amplifier having input 110 electrodes between which an appreciable capacity effect exists, said capacitycifect a cting to cause the value of the transmission efliciency'of said amplifier for intermediate frequencies to be higher than the transmis- 5 sion frequency for upper frequencies of said band, and means resonating with said capacity for equalizing the transmission at said intermediate frequencies and said upper frequencies while the transmission efiimoney for said intermediate frequencies remains substantially unchanged from said value caused by said capacity effect.

5. In combination, a line adapted to be supplied with a band of speech frequencies, a translating device, a transformer primary winding connected to said line, a transformer secondary winding connected to said device and inductively connected to said primary winding, an appreciable capacity effect existing between the turns of said windings to cause the value of the efliciency'with which currents of intermediate frequencies of said band are impressed upon said device to be greater than the efficiency with which currents of frequencies near the upper end of said bandareimpressed on said device, and means creating an inductive effect acting effectively in series with said line for equalizing the transmission efficiency for said upper frequencies while the efficiency with which currents of said intermediate frequencies are impressed on said device remains substantially unchanged at said value caused by said capacity effect.

6, In combination, a line, a vacuum tube amplifier for repeating a band of speech frequencies present in said line, a transformer primary winding connectedto said line, a transformer secondary winding connected to the input of said amplifier and inductively connected to said primary windlng, an appreciable capacity effect existing between the turns of said windings tending tocause currents of intermediate frequen-' cies of said band to be impressed upon said amplifier more efficiently than currents of frequencies near the upper end of said band, said amplifier having input electrodes between which an appreciable capacity effects exists tending to cause intermediate frequencies of said band to be transmitted more efficiently than the upper frequencies of said band, and means for creating an inductive effect acting effectively in series with said line for resonating with said capacity effect at'a frequency present in said line to render said amplifier and transformer windings ca-.

, pable of transmitting said, upper frequencies and said; intermediate frequencies simultaneously, at substantially the same efiiciency.

7. In combination, an amplifier having in put and output circuits arranged to repeat in substantiallythe same wave form a wide band of speech, frequencies with substan-: tially constant transmission for all frequencies in said band, anincoming line, said amplifierhaving input electrodes coupled to said line, said input electrodes having an appreciable capacity effect therebetween acting asash'unt across said line and tending to decrease the transmission efficiency of said amplifier for said band an amount increasing'with the frequency, and means resonating with said capacity and capable of substantially overcoming the decrease in transmission efficiency caused by said capacity effect, over a widerange of frequencies of said band simultaneously; 8. In combination, a line, an electric device having input electrodes responsive to a band of frequencies in said line, means responsive to said frequencies through the intermediary of said device, said device having an appreciablecapacity effect between its input electrodes causing the value of the efliciency with which said device transmits intermediate frequencies to said means to be higher than the efficiency with which upper frequencies of said band are transmitted, and means resonating with said capacity effect for equalizing the transmission of said upper frequencies while the transmission efficiency for said intermediate frequencies remain substantially unchanged at said value caused by said capacity effect.

9. A circuit comprising an electron discharge device, an input transformer therefor and a source for supplying a band of frequencies tosaid transformer, the value of mutual inductance of said transformer being so related to the capacity effectively in shunt to its secondary as to transmit intermediate frequencies of said bandmore efficiently than the upper frequencies thereof,'said transformer having leakage inductance, forming a series resonant circuit ef- I fectively in series with said source for increasing the voltage applied to said discharge device at the upper frequencies of said band. 7

10. A circuit comprising an electron discharge device, an input transformer therefor, and a source for supplying a band of frequencies to said transformer, said circuit including capacity effectively in shunt to the secondary of said transformer, the value of the mutual inductance of said transformer being such that the impedance looking into the primary of the transformer is inductive at the lower frequencies of said band and capacitative at the upper frequencies of said band, said transformer comprising inductance effectively in series with said source and forming with said capacitative impedance a circuit resonant at a frequency 11. A circuit comprising, an electron discharge device, an input transformer therefor, and a source for supplying a band of frequencies to said transformer, said circuit including capacity effectively in shunt to the secondary of said transformer, the value of the mutual inductance of said transformer being such that the impedance looking into the primary of the transformer is inductive at the lower frequencies of said band and capacitativ'e atthe upper frequencies of said band, said transformer comprising leakage inductance effectively in series with said source and forming with said capacitative impedance a circuit resonant at a frequency in the neighborhood of the upper limit of said band.

12. Irr combination, two line sections and means for coupling said lines 'to provide a substantially constant transmission between said means comprising a transformer hav 'ing an appreciable capacity between windings and having a leakage flux sufiicient to' correct for the distorting efiect of said capacity upon said range of frequencies.

13. In a system for amplifying electrical variations comprising a band of frequencies, an amplifier comprising reactance and having a greater transmission efiiciency for certain of said frequencies than for others of said frequencies, and means comprising inductance forcooperatin with said reactance to increase the-efiieiency, at those fre- \quencies at which the amplifier alone has relatively low efficiency, to Such a value that the width of the band of frequencies which the system is capable of transmitting simultaneously with equal efficiency is increased.

14. A circuit for transmitting simultaneously a band of frequencies comprising a substantial portion of the range .from zero frequency to the upper limit of said band, said circuit comprising an electric discharge device having electrodes with an effective electrode capacity therebetween, and a transformer connected to said electrodes and having reactance resonating With said capacity at a frequency in the intermediate portion of said band.

In witness whereof, I hereunto subscribe my name this 19th day of April, A. D. 1921.

. WILLIAM L. CASPER.

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