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' M. M. LEVY ‘ 2,412,95 AMPLIFIER OF ELECTROMAGNETIC ENERGY Filed Sept. ll,_ 1942’ 3 Sheets-Sheet FIG.’ /~ | . . AHill, -. ' - _ m . ' \ I ‘ ' - IW FIG. 2. I ‘ . ! F+nof F . , F/GI 3. 3 . H 3 3 G . o l - f I ,» l | 2f | - . sf 4]‘ FREQUENCX ' ' FIG. 4. is 165m’ @Ef A I ‘512,4 lwjm' ‘ A TTOPNEY ' ea, 24, 1945. M, M‘LEVY AMPLIFIER 2,432,9Q ELECTROMAGNETIOENERGY Filed.Sept. 11, 1942 ' 5 Sheets-Sheet 2 1/5766. Q l'°__.__.‘ is ' - l- 42 A M/VENTOI? ‘40 4M Dec, 249 mg. M, M, LEW _ 2,4123% AMPLIFIER OF ELECTROMAGNETIC ENERGY Filed Sept. 11, 1942 3 Sheets-Sheet ‘3 ICU/(5.9.. Ii lo 5%; i [ll-4f_ D 2; I II I l IIF l Z _____ I ' >RP. I 7 i4 % I‘; y, F/G. /4. - Aha/Am“) A 7' TOR/VF Y Patented Dec. 24, 1946 2,412,995 ‘ 'UNlTED STATES PATENT OFFIE 2,412,995 AMPLIFIER 0F ELECTROMAGNETIC ‘ENERGY Maurice Moise Levy, London W. C. 2, England, as signor to Standard Telephones and Cables Lim ited, London, England, a British company Application September 11, 1942, Serial No. 458,060 In Great Britain June 6, 1941 11 Claims. (C1. 179-—171) 1 2 The present invention concerns the improve ment of the signal-to-noise ratio of thermionic According to a preferred embodiment of the in_ vention an ampli?er is provided with two separate feedback paths, the ?rst of which produces a ?xed negative feedback independent of frequen ampli?ers, particularly those used for amplify ing impulses. It is applicable to obstacle detect ing systems in which short trains of high fre— quency electromagnetic waves re?ected from ob ‘ cy, and the second produces a feedback the am plitude of which is preferably constant, but the stacles have to be received and detected and the phase of which varies with frequency, this phase resulting impulses ampli?ed. being preferably opposite to that of the negative feedback for the fundamental frequency, and for . It is a common experience that little advan tage is gained by amplifying signals of very low _ all its harmonics, or alternatively for all the odd power level on account of the noise produced in the ampli?er itself, or in the preceding detecting equipment, or otherwise, when this noise is of the same order of level or higher than that of the signal. It is also well-known that the noise pro harmonics. In another embodiment the ampli?er is pro vided with a single feedback path including a Wheatstone bridge of impedances, one of which includes the input circuit of a delay network. In this case, a feedback, variable both in amplitude duced may be reduced if the receiver is provided with means to eliminate frequencies not required and phase, is produced and this, feedback should for reproducing the signal. A common arrange preferably be zero for the fundamental frequency, ment is for the receiver to be made selective by and for all its harmonics, or for the odd ones only. tuning it more or less sharply to the carrier fre 20 These objects and features will be more clearly quency of the trains of waves being received. understood by a reference to the following de The more sharply it is tuned the lower will be the tailed description and the accompanying draw noise, but the more will the detected impulses ings, in which: be distorted, until ultimately the distortion may Fig. 1 shows some periodically repeated trains become so great that the amplitude of the im 25 of waves; pulses will be reduced. Fig. 2 shows the frequency spectrum for such The object of the present invention, therefore, periodically repeated trains of waves; is to improve the signal-to-noise ratio of an am Fig. 3 shows part of the gain frequency char-9 plifying system without at the same time caus acteristic of an ampli?er in accordance with the ing appreciable distortion of the signals being invention; transmitted. those frequencies necessary for de?ning the out Figs. 4 and 5 show block schematics of ampli ?ers with feedback; Fig. 5A shows a modi?cation of a detail of Fig. ; line of the signal (of of narrow bands of frequen cies in the immediate neighbourhood of these back voltages; Another object of the invention is to provide a selective ampli?er in which currents of only Figs. 6, 7 and 8 are vector diagrams of the feed frequencies), are ampli?ed, other currents being substantially not ampli?ed. In particular the ? Fig. 9 shows the schematic circuit of an ampli er; currents to be ampli?ed may be some fundamen Fig. 10 shows the schematic circuit of an arti tal frequency and any desired number of the 40 ?cial line; harmonics thereof. Fig. 11 shows another vector diagram of feed According to the principal feature of the in back voltages; and vention, the desired type of selective ampli?ca Figs. 12, 13 and 14 show circuits for compensat tion is produced by means of a delay network, ing the effect of the attenuation of a delay net which may be associated with a thermionic am pli?er, and the variations of the change of phase with frequency in the network are employed to obtain the required properties. The network may be connected in tandem with the ampli?er, or 45 work. , ' In the valve circuits of Figs. 9, 13 and 14 certain well understood coupling and operating arrange ments' are indicated for completeness, but these arrangements are unessential as regards the in may form part of a feedback path therein. In 50 vention, and may be modi?ed to suit particular most cases the delay network may have appre cases. Thus, grid and plate batteries are indi ciable attenuation without destroying the selec tivity, and accordingly can be constructed in a simple and convenient form; for example, it may consist of an arti?cial line. cated by the usual symbols without implying that these supplies must necessarily be provided in this manner. Rp represents a plate circuit re sistance of suitable value and g a grid resistance 2,412,995 3 4 of high value, the shunting effect of which is fundamental and all the harmonics. It will be seen to consist of a- number of very sharp peaks negligible. Similarly K represents a large cou pling condenser of negligible impedance. Fur ther, although for simplicity the valves have been occurring at frequencies f, 2f, 3]‘, etc. These peaks shown as triodes, this is also not necessary, and maximum amount of noise may be eliminated. should be as narrow as possible in order that the valves with any number of electrodes could be‘ ’ This-kinder selective or “comb” ‘ampli?cation Fig. 1 shows the form of a number of repeated maybe produced by making use of the property of suitably designed delay networks by which used, with appropriatearrangements; f the phase change obtained by propagation there quently used in obstacle detection. Such trains 10 through may be made to increase progressively trains of high frequency waves of the type fre- ' ' of waves are reflected from the obstacle and have to be received and detected with a suitable re with frequency, and so to pass through values which are multiples of 1r. This property may ceiver. It will be assumed that the frequency of the waves is F and the frequency of repetition be employed of the trains of waves is f. - Y ' ' either by direct transmission through the network, or by making use of re 15 ?ections at the distant end, the impedance of In Fig. 2 is shown the frequency spectrum of such a system of repeated trains of waves and it consists of a central frequency F accompanied on either side by a large number of components whose frequencies may be designated by the for 20 which is mismatched, usually by open or short circuit. Without special means, however, good selectivity cannot be obtained because of the attenuation which accompanies the phase change in any practical form of delay network due principally to the resistance of the inductance increases so the amplitude of the corresponding coils, which cannot. generally be reduced su?'i ciently even by the use of extremely bulky coils. component tends on the whole to decrease and In accordance with certain features of the inven components may be eliminated without appre ciably affecting the form of the received signal 25 tion, however, it becomes possible to use delay networks with high attenuation without any loss provided that n is not too small. of selectivity, and they may therefore be given The usual method of receiving such a train of convenient forms; for example arti?cial‘ lines impulses is to amplify them with a selective high may be used. . frequency receiver tuned to the frequency F, then In order to explain the reason for the low to detect them by means of a low frequency de 30 mula Fi-nf. where n can be ‘any integer. iAs’ n tector. A certain amount of noise will be pro duced in the receiver and the detector, and also from atmospherics which will be picked up to gether with the impulses. If the impulses are at selectivity ordinarily produced by attenuation in the delay network, an example will be given. One way in which the desired properties may theoretically be produced in any ampli?er is to a very low power level the noise may be of the 35 connect in the plate circuit of one of the valves, in parallel with the load, the input circuit of a same or even higher level, and in order to mini delay network having its output terminals short mize the effect of this noise it has been the prac circuited, or left unconnected, whereby re?ections tice to tune the receiver very sharply to the fre will be obtained at the output terminals- As is quency F. This has the effect of cutting off the components shown in Fig. 2 on either side of the 40 well known, the input impedance of the delay network can then be designed to pass through frequency F. a This may be done so long as the an in?nite value for some frequency f and for band of frequencies passed by the receiver is not all its harmonics, and through a zero value for made too narrow; for example, there will be some certain intermediate frequencies, provided the value no of it below which any further elimina network is made up of pure reactances, or in tion of components will produce an undesirable other words, has no attenuation. As already ex distortion of the detected impulses. This is indi plained this can never be achieved in practice cated in Fig. 2 by the dotted lines at F+nuj and and accordingly the impedance never becomes F-nof. Accordingly, if the noise is still excessive even approximately zero or in?nite, and the selec no further advantage can be gained by reducing tivity is accordingly bad. This will be better the band width because then the impulses re appreciated from the following numerical ex ceived will become so distorted-that their ampli tude is reduced. ample. > ' Suppose that a delay network is required to produce a delay of 200 microseconds, for use r up to a frequency of, say, 1 megacycle. It could, for example, be made up of about 1000 sections of a simple low pass ?lter. If this be constructed of elements of reasonable dimensions, its atten uation could easily be 30 to 50 decibels. The designed to amplify all the frequencies Fin)‘ 60 selectivity would in this case be almost inap preciable; the diiference between the maximum where n is zero or any integer up'to and including The method characteristic of the present inven tion is to include in the receiver a selective ampli fier so designed that it substantially only ampli ?es appreciably currents of frequencies actually contained in the impulses. This ampli?er may be located in the high-frequency part of there ceiver before detection, in which case it will be no. Alternatively, the ampli?er may be connected and minimum impedance would in the worst in a position in the circuit subsequent to the de tector, when it will be designed to amplify all the frequencies of where n has all the same values except'zero. Thus the frequencies not concerned case be only 0.33% and in the best case only 3%. with de?ning the impulses will ‘be largely elimi nated and with them all the corresponding noise; manner indicated in Fig. 12. f A 'delay network ‘D (which may, for example be in the form of an arti?cial line) is provided so that the only noise which remains is that asso ciated with frequencies immediately adjacent to those which are selectively‘ampli?ed. A large improvement in signal-to-noise ratio will thus be obtained. , ' 1 ‘ - ' ‘ InFig. 3 is shown thegain frequency character , - According to one feature of the invention, the effect of the attenuation of- the netw'or‘kin re ducing the selectivity may be eliminated“ in the 'with an input transformer ‘I’! having a tapping point t on the secondary winding. The output of the network D is terminated by an impedance Z equal to the image impedance thereat, and an output transformer T2, the primary winding of istic of such an amplifier, which ampli?es the 75 which is connected in parallel with Z. The ' 2,412,995 secondary winding of T2 has the lower end con nected to the tapping point t, the upper end being connected to one output terminal 3, the other 4, od of compensating the attenuation of the net work. Many other arrangements conforming to the principles explained in connection with Fig. 12 are obviously possible. According to another feature of the invention still better selectivity may be obtained with a de being connected to ground. T! and T2 are sup posed for simplicity to be ideal unity ratio trans formers, though this limitation is not essential. Let V1 be the potential applied to the input ter lay network having attenuation by associating minals l and 2 at some given frequency, and it with a feedback path in the ampli?er. let V2 be the corresponding potential developed An embodiment employing this arrangement is across the impedance Z. Then Ill shown schematically in Fig. 4. An ampli?er A has a pair of input terminals I and'a pair of out put terminals 2. It is also provided with two sep where 5 and a are respectively the attenuation arate paths 3 and 4, the ?rst of which produces and the phase change for transmission through negative feedback N which is constant as the fre the network, assuming, for simplicity, that it is 15 quency varies and the second produces a feed symmetrical. Since the secondary winding of the back which is constant in amplitude but varies transformer T2 is connected back to the tapping continuously in phase with frequency, and may t on transformer Ti, the difference of poten contain a delay network D. tial V between the terminals 3 and 4 will be Suppose that Eis the input voltage applied to A1.V1-I_-V2, where A1 is the fraction of the the grid by the signal. Let G be the ratio which applied potential V1 tapped 01f at t. The choice de?nes the gain of the ampli?er in the absence of sign will depend upon the poling of the con of feedback, then the output voltage will be GE nections of the secondary winding of T2. Thus in the absence of feedback. Let the constant negative feedback be 25 For the best results ,8 should be independent of frequency, and on should be proportional to fre quency. If A1 be made equal to e-", then, taking the positive sign, V will assume a zero value whenever a is an odd multiple of 1r and a maxi mum value 2e-".V1 whenever a is an even mul tiple of 1r; Or taking the negative sign the zero and maximum values will be interchanged. If therefore the arrangement of Fig. 12 be in , GEFM ‘and let the variable feedback be 30 It can be shown that the eifective gain of the ampli?er with both the feedbacks operating will be de?ned by the ratio . ' corporated in an ampli?er so that, for example, the terminals 3 and 4 are connected in the grid circuit of a valve, currents applied to the in put terminals l and 2 will be selectively ampli ?ed and it can be arranged so that, with the 40 If H is chosen equal to F, then the denominator positive sign, for instance, the values of on which of this expression will be a minimum equal to 1 are even multiples of 11' occur for the fundamental whenever the angle 95 is a multiple of 271', and ac frequency and all its harmonics. ~ cordingly the gain of the ampli?er will then be a If ,8 should not be independent of frequency, maximum de?ned by the ratio G. If GF and GH a suitable correcting network may be connected 45 are chosen to be large compared with 1, then a in series with D provided that it is designed so very small change in the angle ¢ due to a small that the phase change is substantially independ frequency change will make a large increase in ent of frequency, or an ampli?er with appro the denominator of the above expression; in priate gain characteristic could also be used. other words, the gain of the ampli?er will be‘ re 'Another arrangement is shown in Fig. 13 in 50 duced by a large amount, and the selectivity will which the input transformer is removed and re placed by a tapped potentiometer, and the output transformer 'is replaced by the grid-cathode be good. This can be seen more clearly with ref erence to Figs. 6, '7 and 8. Considering ?rst Fig. 6, 0A is a vector repre circuit of a thermionic valve. The operation will senting the feedback GF and this vector is drawn be practically as described with reference to 55 in the negative direction since GF is negative Fig. 12, but taking the negative sign in the eX with respect to the input voltage E. The vector pression for V. AB represents the variable feedback GH and Fig. 14 shows a modi?cation of Fig. 13 employ makes an angle ¢ with the positive direction A0. ing a pair of similar valves. The plates are con The resultant of these two vectors is the vector nected together and are fed through a common resistance Rp from the plate battery, and the control grid of one valve is connected to the tap t on the potentiometer, the control grid of the other being connected to one of the output ter minals of the delay network D. In this case, the positive sign will be taken in the expression for V and the addition of the two terms will occur in the common resistance Rp. In Figs. 13 and 14, the valves may be considered as part of the selective ampli?er itself and may be followed by such other amplifying stages as may be found convenient. It will be understood that the arrangements shown in Figs. 12, 13 and 14 are only three ex amples of the application of this particular meth OB which is equal to J and at an angle 0 with A0. As has already been mentioned, the vector GH has been assumed to be constant, for sim plicity, and accordingly as the frequency varies the point B will describe a circle of which A is the The maximum and minimum values of the vector J are given by OX and CY. The maxi mum gain of the ampli?er will occur at the point Y and the minimum gain at the point X, but un less OY is very small compared with OX the variation in the gain of the ampli?er will be small. Accordingly, the arrangement of Fig. 7 is preferable in which the vectors OA and AB have 65 centre. been made equal, in other words, making F equal to H. In this case the vector OY becomes zero ~75 and the selectivity is accordingly high if OX is 2,412,995 8 7 large compared with 1. As the frequency is varied continuously in the same direction the point B rotates continuously around the circle and the corresponding gain produced will have the form of Fig. 3. Fig. 8 shows another arrangement in which the vector GF is less than the vector GI-I. In this case thevector OB is never zero and can have a positive component at certain frequencies. Thus it will be seen that in Fig. 6 the feedback is never zero, but always. has a negative component; in Fig. 7 it is zero at each of the harmonic fre quencies and has a negative component at all other frequencies; and in Fig. 8 the feedback sometimes has a positive and sometimes a nega tive component. . Fig. 9 shows an example of a three stage ampli her in which the principles just described are ap This is accordingly another means whereby an ar ti?cial line with high attenuation may be used as a delay network without any reduction in the se lectivity of the ampli?er. In Fig. 5 is shown a different arrangement for producing a variable feedback in the ampli?er. In this case the ampli?er A has a pair of input terminals I to which a signal voltage E is applied and a pair of output terminals 2 where an ampli ?ed signal voltage EG appears in'the absence of feedback. In this case there is only one feedback path which involves a, Wheatstone bridge KLMN; the diagonal points KM are connected to the out put terminals 2 and the diagonal points LN pro vide the desired feedback. The arms KL, LM and MN are composed of impedances Z3, Z4 and Z2, respectively and the arm KN is composed of an impedance Z! in series with the input terminals of the delay network D, the output terminals of a transformer Ti and the constant negative feed 20. which are left unconnected for example. This delay network should preferably have substan back is obtained by connecting the cathode of the tially no attenuation. The impedances Zl to Z4 ?rst Valve to a resistance BC in the cathode cir will preferably be pure resistances and in the fol cuit of thethird valve. The variable feedback is lowing explanation this will be assumed. produced by connecting a delay network or arti Referring to Fig. 11, the vector KM represents ?cial line L to the platev circuit of the third valve, 25 the voltage applied to the diagonal points K, M of through a transformer T2, for example. The out the Wheatstone bridge. It is composed of two put of the arti?cial line is terminated by a poten vectors KL and LM in the same line which reptiometer P, the moving contact of which is con resent respectively, the voltage drops across Z3 nected to the grid of ‘the ?rst valve through the and Z6. The vectors KQ and QP represent, re secondary winding of the input transformer Ti. spectively, the voltage drops across Zl and Z2, The arti?cial line L should preferably have an which since ZI and Z2 are preferably both‘pure' attenuation substantially constant over the fre resistances, will be in the same direction. The quency range concerned and the phase change vector PM represents the voltage drop across-the should also preferably be substantially propor tional to the frequency. However, if the arti- '7 delay network D which will be at right angles to the vector KQP because the network will have ?cial line L has a variable attenuation, it would an impedance which is substantially a pure re be possible to compensate this by means of a act'ance at all frequencies, the output being open correcting network or with an ampli?er having a circuited or short-circuited. The point P will suitable gain characteristic. The electrical length of the artificial line L should'also preferably be 40 thus describe a circle with KM as the diameter, as the frequency changes. From the point Q chosen so that the phase change for the funda is drawn a vector QN parallel and equal to PM mental frequency which has to be ampli?ed is a plied. The signal is applied to the grid through multiple of 211-. Thus for every harmonic of the so that NM will be equal to the drop across Z2. ance RC in Fig. 9 will be represented by the vec LM being equal to the drop across vZl'l, it follows that the resultant of LM and MN, namely, LN, must be the difference of potential between L and N, and accordingly must be the feedback tor 0A, and the feedback GI-I produced by the ar voltage. fundamental frequency, it will also have a phase u change which is a multiple of 211'. Referring to ' ‘ Fig. 7, the feedback GF produced by the resist It can be seen that as P moves round the circle, QN will cut the vector LM in a ?xed H can be adjusted to be equal to F by adjusting "50 point 0 and the point N will describe another circle whose diameter is OM. Thus the vector the potentiometer P, for example, or by other LN will have the same properties as the vector J means. The sum of the feedbacks produced by in Fig. 6. By adjusting the relative values of resistance RC and by the arti?cial line L is repre Z! and Z2 or Z3 and Z4 the point 0 may be moved sented by the vector OB in Fig. 7. The output along the vector LM, and, for example, can be may be taken from a third winding of the trans- " made to coincide with L. In this case a result former T2 as indicated, or in several other ways equivalent to Fig. '7 is produced and this will which may be more convenient in certain cases. be the preferred arrangement. It will be seen by this arrangement the arti As already explained-however, the desired re ?cial line can have relatively large attenuation sult can only approximately be attained in prac and accordingly it could be constructed with ele tice because of the attenuation of the delay net ments of reasonable dimensions. For example, work D which cannot be reduced to zero. vAc suppose the ampli?er has a gain of 60 decibels in cordingly, a variation of the method may be the absence of feedback, and suppose that the at adopted which is shown in Fig. 5A, In this case tenuation of the arti?cial line is 20 decibels, the value of the feedback GH can be such as to cor 65 the diagonal of the Wheatstone bridge KN is com posed of an impedance Zi, connected in parallel respond to 40 decibels. The difference between with which are the input terminals of the delay the maximum and the minimum ampli?cation network D which in this case is supposed to have will be about 46 decibels, because the maximum attenuation, and may thus be in the form of an negative reaction OX in Fig. 7 is equal to G(F+H) arti?cial line. It is desirable that the impedances and since these two vectors have been chosen to Zl, Z2, Z3 and Z4 should be chosen so that the be equal, the vector OX will be 6 decibels greater delay network may bev terminated in its char than GH. acteristic impedance in order to avoid undesir It will be clear from the explanation just given able re?ections at the input terminals. The ex, that the attenuation of the arti?cial line L is com pensated by part of the gain of the amplifier. 75 planation of the operation of the circuit in terms ti?cial line L will be represented by the vector AB. I 2,412; 995 9 10 of‘ a vector diagram is'in'this case rather com‘ plicated and can be more easily understood in said delay network, and circuit means for alge braically-‘adding the output voltage of said delay the following way. Assume ?rst of all that the delay network is in?nite in length. The im pedances in the bridge can be adjusted so that network to a proportionfof the input voltage ap plied to its input terminals, in which the said circuit means comprises a- potentiometer, having a fraction of the output voltage is transmitted as a feedback to the input of the ampli?er and so an intermediate tapping point, the said potenti ometer being connected across the input termi nals of ‘the said delay network, and said therm that the feedback is negative. This feedback then is equivalent to the feedback GF in Fig. '7. ionic valve ampli?er comprises a thermionic Now assume that the network has a ?nite length, 10 valve, the control grid of which is connected to re?ection will take place at the output terminals an output terminal of the said delay network, and the re?ected current will be transmitted back and the-cathode of ‘which is connected to the said to the input and a part will be transmitted to intermediate tapping point. the input of the ampli?er. This re?ected current 3. A selective thermionic ampli?er circuit com then plays the part of the vector GH in Fig. '7. prising a'thermionic Valve ampli?er, a delay net By adjusting the impedances in the bridge the work having attenuation and terminated at its effects corresponding to Figs. 6 and 3 can also output terminals with an impedance substantially be-produced. equal to its image impedance thereat, means for Experience shows that a circuit combining ‘the coupling said output terminals to an input circuit principles of Figs. 5 and 5A gives good results 20 of said‘ ampli?er, means for applying said peri in practice if the values of the impedances of odically repeated impulses to input terminals of the bridge are correctly chosen. said delay network, and circuit means'for alge The delay network 01' arti?cial line used for braically adding the output voltage of said delay these circuits should preferably have a constant network to a proportion of the input voltage ap attenuation and a phase change which varies in plied to its input terminals, in which the said proportion to the frequency, as already stated circuit means comprises a potentiometer having above. One form of the delay network D may be an intermediate tapping point, the said potenti similar to that of an arti?cial line such as is ometer being connected across the input termi shown in Fig. 10 consisting of a number of sec nals of the said delay network, and said therm tions having series elements of inductance and 30 ionic valve ampli?er comprises two thermionic resistance and shunt elements of capacity and valves, the plates of which are fed from the plate resistance, and having mutual inductance be supply through a common resistance, the con tween adjacent sections, as indicated. Various trol grid of one of the said valves being con other forms are, however, also possible and Fig. nected to the said tapping point and the control 10 has been given just as an example. grid of the other valve being connected to an out The arti?cial line is only one example of a put terminal of the said delay network. delay network that can be used in these circuits. 4. A selective thermionic ampli?er circuit for Other types of network will occur to those skilled amplifying periodically repeated electrical im in the art, as also will other arrangements in ac pulses comprising a thermionic ampli?er having cordance with the principles of the invention. 40 input and output circuits, a ?rst feed-back circuit The arrangements shown in Figs. 4, 5, 9, 12, 13 extending between said output and input circuits and 14 have been given by way of illustration, including a delay network arranged to vary the and the invention is not intended to be limited phase of the feedback voltage dependent upon thereto. What is claimed is: ~ frequency and a second feedback circuit extend 4 1. A selective thermionic ampli?er circuit com prising a thermionic valve ampli?er, a delay net arranged to produce a constant negative feed back voltage independent of frequency. work having attenuation and terminated at its output terminals with an impedance substan tially equal to its image impedance thereat, means for Coupling said output terminals to an input circuit of said ampli?er, means for applying said periodically repeated impulses to input terminals of said delay network, and circuit means for alge braically adding the output voltage of said delay network to a proportion of the input voltage ap 5. A selective thermionic ampli?er circuit ac cording to claim 4 in which the said delay net work has an attenuation substantially independ ent of frequency and in which the amplitude of the feedback voltage produced thereby is equal to the said constant negative feedback voltage produced in , the said second feedback path, 55 whereby the effect of the said attenuation tend ing to reduce the selectivity of the said ampli?er plied to its input terminals, in which the said may be Substantially eliminated. circuit means comprises a‘?rst transformer hav , 6. A selective thermionic ampli?er circuit com ing a secondary winding connected to the input terminals of the said delay network, said sec ondary winding being provided with an inter ing between said output and input circuits and prising a thermionic ampli?er having input and 60 output circuits, a feedback circuit extending be tween said output and input circuits and phase changing means in said feedback circuit arranged to produce an overall ampli?er gain which is a maximum for the fundamental frequency of said impulses and for a plurality of harmonics of said mediate tapping point, and a second transformer, the primary winding of which is connected to the output terminals of the said delay network, one terminal of the secondary winding of the said second transformer being connected to the said fundamental frequency, in which said feedback 2. A selective thermionic ampli?er circuit com prising a thermionic valve ampli?er, a delay net work having attenuation and terminated at its circuit includes a Wheatstone bridge, three arms of which consist of impedances and the fourth arm consists of a fourth impedance and the input circuit of a delay network, opposite diagonals of intermediate tapping point. output terminals with. an impedance substantially equal to its image impedance thereat, means for coupling said output terminals to an input circuit of said ampli?er, means for applying said peri odically repeated impulses to input terminals of 75 said bridge being connected respectively to said input and output circuits. 7. A selective thermionic ampli?er circuit ac cording to claim 4 in which the said delay net work comprises an arti?cial line. ' 2,412,995 11 v 12 . 8. A selective thermionic ampli?er circuit‘ for amplifying periodically repeated electrical__'im pulses comprising a thermionic valve ampli?er, a delay network having attenuation and terminated at its output terminals with an impedance sub stantially equal to its image impedancethereat, means for coupling said output terminals to an arm consists of a fourth impedance in series with the input of a delay network. . ~ > 10. A selective thermionic ampli?er circuit comprising a thermionic ampli?er having input and output circuits, a feedback circuit extending between said output and input circuits and phase changing means in said feedback circuit arranged to produce an overall ampli?er gain which is a input circuit of said ampli?er, means for applying maximum for the fundamental frequency of said said periodically repeated impulses to input ter minals of said delay network, and circuit means 10 impulses and for a plurality of harmonics of said fundamental frequency, wherein said feedback for algebraically adding the output voltage of circuit includes a Wheatstone bridge, three arms said delay network to a proportion of the input of which consist of impedances and the fourth voltage applied to its input terminals. arm consists of a fourth impedance in parallel 9. A selective thermionic ampli?er circuit com-. prising a thermionic ampli?er having input and 15 with the input of 'a delay network. 11. A selective thermionic ampli?er circuit ac output circuits, a feedback circuitextending be cording to claim 4 wherein said, delay network tween said output and input circuits and phase comprises an arti?cial line, means for coupling changing means in said feedback circuit arranged the input terminals of said line to the output cir to produce an overall ampli?er gain which is a maximum for the fundamental frequency of said 20 cuit of said ampli?er and means for coupling the output terminals of said line to the input circuit impulses and for a plurality of harmonics of said of said ampli?er. . fundamental frequency, wherein said feedback V circuit includes a Wheatstone bridge, three arms of which consist of impedances, and the fourth ~ MAURICE MOISE LEVY.