US2480195A - High-frequency amplifier with controlled load impedance - Google Patents

Description

Aug. 3@, 1949.

K. POSTHUMUS 2,480,195 HIGH-FREQUENCY AMPLIFIER WITH CONTROLLED LOAD IMPEDANCE Filed May 1, 1946 2 Sheets-Sheet l Aug. 30, 1949. PQSTHUMUS 2,480,195

HIGH-FREQUENCY AMPLIFIER WITH CONTROLLED LOAD IMPEDANCE Filed May 1, 1946 2 Sheets-Sheet 2 I W r-| I: :l IR. I 0 I51 i A RECT/F/ER l -O C I I I 6 V REC TIF/ER Patented Aug. 30, 1949 HIGH-FREQUENCY AMPLIFIER WITH CONTROLLED LOAD GE Klaas' Posthumus, Eindhoven, Netherlands, as-

signor, by mesne assignments, to Hartford National Bank and Trust fiompany, Hartford,

Conn, as trustee Application May 1, 1946, Serial No 666,239 In the Netherlands J auuary 10, 1942 Section 1, Public Law 690; August- 8, 1946 Patent expires January 10, 1982 9 Claims. 1

This invention relates to a device for amplifying modulated high-frequency oscillations which comprises at least one amplifier valve and which is especially suitable for the final stage of a transmitter.

For amplifying modulated lush-frequency oseillations it is known to makeuse: of an amplifier valve whose-grid bias: isso high that during half each cycle of the alternating voltage to be amplified the: valve allows current to pass. Such an amplifier is known as a B-amplifier and at full loadit has an emeiency 01' if the amplitude: of the alternating voltage occur-.- ring in the output circuit is assumed to be equal to the anode direct voltage. In practice, how! ever, is permitted a maximum amplitude of the anode alternating voltage which is equal to from (1.8 to 0.9 times the anode direct voltage so that in practice the maximum eificiency. ot a B-amplifier does not exceed from In the use of such an amplifier for amplifying modulated high-frequency oscillations this let ficiency' of 67% will alone occur in a positive modulation peak at 106% modulation of the car'- rier wave since only in this case" the amplitude of the anode alternating voltage is equal to from 0.8 to 0.9 times the anode direct voltage. In the absence of any modulation the amplitude of the anode alternating voltage is only half the said value so that the output does not exceed 33%. For broadcast stations the mean modulation percentage over an entire day is very small because the instantaneous value of the modulationpercentage seldom vexceeds 30% or in other words the output of a B-amplifier is on the average not much in excess of 33%..

In order to obtain an amplifier having a higher output devices have been suggested in which a load impedance has energy supplied to it by at least two amplifier channels and in which the output circuit of one of these channels is connected to the: load impedance over an impedance invertingneuvork, that is to say a network whose input impedance is inversely proportional to the terminal impedance of the output circuit. The impedance inverting network may be constituted by a transmission line whose length is a quarter of the wavelength of the oscillations to be amplifled. Such amplifiers permit of obtaining an efficiency of about even with low modulation percentage The invention. has for its object to provide a device for amplifying modulated. high-frequency 0s, cillations which has ahighefficiency and in which the use of one amplifier channel suffices.

According to the invention, the anode circuit oiwhigh-irequency amplifier valve and a load impedance are interconnected by compound network whose elements are formed by impedance inverting networks, there being arranged at each point-oi connection between two or more of these networks except at the said anode circuit and load impedance at least one discharge tube whichacts as a switchand whose impedance between the anode and the cathode bridges the input or the Output circuit respectively of the impedance inverting network at the point of connecticn, said discharge tube being so controlled that with increasing amplitude of the oscillatiojns tobe amplified at least part of these tubes are rendered non-conductive successively and/or a'lternatelyrso that a stepwise decrease of the input impedance of the compound network is obtained.

According to one embodiment of the invention the compound network may be obtained by con-. meeting each ot a group of points over impedance invertingnetworks to each of a second group of points, the anode circuit of the high-frequency amplifi r valve and the load impedance being connected respectively totwo of the said points.

Theemcieney of. a normal B-amplifier is proportional to the amplitude of the oscillations to be amplified, At themaximum value thereof the emcieucy is about 67%, at half the value and so forth This is due to the 'fact that at the highest amplitude oil the oscillations to be amptified, the anode voltage of the amplifier valve is controlled completely but at halt the azaliueonly to the extent of 50% and so forth. The term {complete control of the anode voltest value Ea max of the anode alternating voltage.

Ea the output energy a mnx is equal to the maximum value of the energy the tube must be capable of supplying.

In the device according tof the invention the impedance Ra is not constant but varies stepwise at varying amplitudes of the oscillations to be amplified. At small amplitudesvRa is higher, for example several times higher than would be the case with a corresponding B-amplifier. This results in the anode voltage Es being completely controlled and the efficiency being high even at a small amplitude. In order that at increasing amplitude of the oscillations to be amplified increase of the output energy may be possible the impedan-ce Ra is decreased discontinuously to a lower amount at which the control of the anode voltage, and thus the efficiency, fall back slightly but the output energy does'not vary. At a further increase of the amplitude of the oscillations to be amplified the output energy may therefore again increase untilthe complete control of the anode voltage Ea is again attained, the impedance Ra being then again reduced discontinuously and so forth. This procedure may be repeated any number of times until the last discontinuity causes the impedance Ra to assume the same value as that which this impedance would possess permanently with the corresponding B-amplifier.

It is obvious that at amplitudes of the oscillations to be amplified between zero and the maximum value the mean efficiency is considerably higher in the method described as compared with a normal B-amplifier.

In order that the invention may be clearly understood and readily carried into effect it will now be described more fully with reference to the accompanying drawings.

The embodiment of the device according to the invention which is illustrated in Fig. 1 comprises a final amplifier valve U of an amplifier for modulated high-frequency oscillations and a load impedance R, which may be for example an antenna, to which the tube U supplies energy.

Between the anode circuit of the valve U and the load impedance R is connected a compound network whose elements are formed by impedance inverting networks R0, R1, R2, Rs, R1, R2, R3

R1, R2, and R3", which with the exception of R0 comprise three groups each of which consists of three networks connected in series and thus forming a circuit which is connected in parallel with the network R0. At the points'of connection between the networks R1 and R2, R2 andRs, R1 and R2' and so forth are arranged'respectively discharge tubes A and B, A and so forth which act as switches and whose anode circuits are so connected to the networks that the impedances between anode and cathode of the tubes A, B, A and so forth respectively bridge the input and output circuits respectively of the networks R1, R2, R1 and so forth at the points of connection, or in other words at the said points of connection the networks are terminated by the anode-cathode impedances of the tubes A, B,

A and so forth. The input impedance of the compound network forms the load impedance Ra: of the valve U, the resistance R constituting the terminal impedance of the network. For the sake of simplicity the connection lines between the: tubes and networks are shown as single conductors and the sources of supply and control voltages: are omitted.

The references R0, R1, R2, R1 and so forth designate impedance inverting networks, that is to say networks having the property that the in-- put impedance is inversely proportional to the terminal impedance of the output circuit and the output impedance is also inversely proportional.

to the terminal impedance of the input circuit. The product of the input and output impedance respectively and the terminal impedance is constant and it is hereinafter assumed that for the impedance inverting network R0 and. R1. R2 and so forth respectively the product of the input impedance and the terminal impedance is equal to R02, R12. R22 and so forth respectively. The value R0, R1 and so forth is hereinafter referred to as surge impedance. In addition for imped ance inverting networks, for example for the network R0, the following applies.

in which V designates the voltage across the input terminals of the network R0 and i the current passing through the terminal impedance of the output circuit. This current is out of phase with the voltage V. is independent of the value of the terminal impedance and at a given surge resistance it consequently only depends on the input voltage V. The same applies to the other networks R1, R2. R1, and so forth.

In the device shown in Fig. l the discontinuous reduction of the anode load impedance Re is obtained as follows: It is assumed that the tubes A, B, A and so forth are so controlled that they are conductive and possess a low impedance between anode and cathode and thus practically bring about a short-circuit of the impedance inverting networks at the points of connection of these networks where the tubes concerned are arranged. The terminal impedances of the networks R1, R2 and so forth formed by the tubes A, B, A and so forth are consequently very low and the input impedances of the impedance invertingnetworks at the tube U and the resistance R are therefore very high, or in other words the networks R1, R2. R3, R1, and so forth do not allow the passage of current. For the input impedance Z1 of the network Ru applies:

RIOS:

Rres is therefore negative if one of the magnitudes R1, R2 and R3 or all three are positive. The parallel combination of the network Ro with the circuit Ri, R2 and Rs is equivalenttma new:

w nk-having a surgeresistance that is tosaygthat:thecsurgewresistances R and Rtes filfi. in 'efiect connected inpara llele Thesurge.

I'ESiSIEE.-11f8f:Rt resulting. fro this; parallelscombia nation-isrsmallerwthaniRo if-Rresis positivm Under; ;these.'. conditions the; anode. Joad impedance Re .-is .;equa;1: to. the inputimpedance. Zr ofnthe; composition of networks Ro-,. R1-.R.2 and Ra,,=.

being smaller than the variation of the anode alternating voltage Es.

of the tube 'U in Fi'g. 1 as a function-"of the am pli-tude' P" of the-oscillations tobe ampllfied by the" -t-ube Since the amplification is substantials ly linear P also represents the amplitude of the current I that passes through the. output. inn. pedance R1 4 designates the maximumivalue of th oscillationsto be. amplified.and.of...the.. out: put current I and P P2 and P3 :designate the levels at. which the anodel loadaimpedance Ra oi the tube U'is successively. reduced discontinu ously the dimensions of the. impedance inverting networks being assumed to be. such .that-the re, sultingsurge resistance RresOf eachotthencirr cuitsRi, R2, R3, R1", R2, Rswand R4", R21, Razz, is equal to Rt. The maximum' Clll'IEHtT'I" passing through the load resistanceR," that" is'flto' say the current at'coinplete control' of: the anode voltage oj th e tube U is therefore increased;;each"time that one of the circuits R1; R2, Rzor R ",R",'R.' o 'RflflR", Rs" respectively isadded flby the same amount Elma: designates the amplitudemf. the anqdeiale tion -"of-theanodeimpedance-Ra 0f the tube dl.

Asexplained before, in the amplitude region between 0 and Piin which the networkQRo is alonc switched into the circuit thi impedance-1s In" "the region-between the, l m 3 1.9 OINPZ and Pi'iol' Ps"a nd .P4 respec fl i R'ir are: connected iinrparallel-z and-the: anode respectively. Fig.2 shows how at each discontinuity; the. anode alternating voltage Eefa'lls back from com letecontrol'to a smaller value.

In order to avoid a discontinuity inthe output; it is'n'ecessaury that directly before and after each discontinuous change of the impedance Re the outputenergyIa2Rzi:I R (Ia designating the alternatin'g current'component of the anode cur-- rent. Qf the-tube U) should be substantiallythe same.- Since Ra} falls discontinuously'this' is to be takengto 'mean that" Ia must increase discontinuously. Ifpath'e tube U iS'. a triode an increase may occur-can; however, be neutralized by inverse feedback. If the 'tubes A, B, A and "so forth are rendered non-conductive in a manner which is to some extent continuous, the distortion to be neutralized by inverse feedback is considerably sma1l'r,.than. would. be the case with decisively discontinuous variations.

"It' isflnotjl essential that the resulting surge resistance of the circuits R1, R2, R3, R15, R2", R3 and R1", R2", R3" should be equal to R0; that is to say the amplitudelevels .from P1 to P4 are not necessarily chosen at equal distances. By choosing; ;the elevel 1 P1". to below. .aahighers efiiciencyris obtained,but the..arnounts by which the-impedancesuccessiyelydecreases discontinuously at the levels P,i;j P2 and .Bs also become greater, andthis will result-inrarglififit r distortion; The levels P1 and P3 are preferably so determined by choice of the networks R1, R2-;:Riiand R1, R2, R3 that the amounts by which-the anode alternating current I must-increase. each time that. one. of, the circuitsRi, R2,,Rs,.and so forth isadded are equal, the s largest.- discontinuity foccuring, being in. this case a minimum.

lt isiapparent. from the-.ioregoing-thatthe tubes A, B, A and so forth solely actasswitches, that isgto.,say, that. either. they are :as conductive as possihle;- or -:they. are; entirely non-conductive. The=.tubes. 1A-. and B, Afand B'-and A" andBf respfictivelyqare: rendered non-conductive on-surpassingfihcamplitude level P1, P2 and P3 respectiytety 0f? the oscillations tobe amplified, levels at which, asshO-Wn'in FigZ, complete control of the anode, voltage lfl roi the tube U occurs, and in one fomn of? construction of the device according to the z inventionz':this.-. non-conductive condition is obtainedzazdue I the fact that a low-frequency voltagfinthatemay benobtained after rectification and which corresponds with the :modulation .envelopenf: etheuoscillations 'to be amplified :is supplied withinegagtivepolarity to a control electrode of the tubes concerned. The tubes. are so .arranged, for example by means of .difierent grid biasespbtained throughresistors H, i2, and .13, that theyarious-sets oftubes, for. example .the tubes.,A.- .a nd. B, A and .13" etc. can berenclered non-conductive in succession at the desired amplitude levels of the oscillations to be amplinee;- Fl this purpose; the. control :electrodes of the tubes-hand =B;A"-, B and A B" respectively maybe-interconnected;

A further modification of r thedevice according four-circuits"respectively"with-surge =resistancend the invention is 'showndn Figs-3. In this -the compound network is obtained due to'the fact that between the anode circuit of the tube U and the load impedance R is connected a number of parallel branches (two of which are shown in Fig. 3) each comprising two impedance invertin networks connected in series, designated in Fig. 3 R1 and R2, R1 and R2 respectively, at the point of connection between which is arranged a discharge tube A, A respectively that acts as a switch and whose anode-cathode impedance shunts the input and output circuit respectively of the networks concerned at the point of connection. The operation essentially corresponds with that of the device shown in Fig. 1 except that only one branch can be operative at a time so that the branches must be possessed of mutually different surge resistances. The fact that it is impossible for two branches to be connected in parallel may be recognised as follows: For a branch comprising two impedance inverting networks connected in series (for example R1 and R2) applies:

in which Vu designates the voltage across the output terminals of the second network and Vi the voltage between the input terminals of the first network. For the branch which comprises the networks R3 and R4 applies:

If the two branches, that is to say the input and output circuits of the two branches, are connected in parallel the voltages Vu and Vi are equal for the two branches respectively and this is at variance with the two above equations if In this case the value Vu=Vi=0 alone satisfies the two equations at the same time, or in other words the two branches are short-circuited on both sides. In the device shown in Fig. 3 connection of two or more branches in parallel is therefore impossible.

A further modification of the device according to the invention is shown in Fig. 4, the anode circuit of the amplifier valve and the load resistance R being connected respectively to two opposite corners of a hexagon whose sides and diagonals are formed by impedance inverting networks from R1 to R9. At all corners of the hexagon except at the two mentioned before, is arranged a discharge tube designated A, B, C and D respectively which acts as a switch and shunts the output or input circuit respectively of the networks at the point of connection concerned.

This device permits of obtaining siX different values of the anode load impedance R8 of the tube U by the following combination of conductive and non-conductive discharge tubes.

IA, B, C, D, non-conductive IIA, B, C, D, conductive I[IA, B conductive, C, D non-conductive IV-A, B non-conductive, C, D conductive V-A, C conductive, B, D non-conductive VI-A, C non-conductive, B, D conductive By reference to Fig. 4 it can be ascertained that with the combination II the network R9 is alone switched into circuit, whereas with the other combinations one or more additional circuits .iormed by series combination of three of the sesame.

8. other networks are connected in parallel with the network R9. The tubes from A to D may be rendered non-conductive and again conductive by means of a device comprising a cathode ray tube 14 in which the beam of electrons is deflected under the influence of a low-frequency voltage that may be obtained after rectification of the oscillations to be amplified and corresponds with the modulation envelope of the oscillations to be amplified, the beam of electrons with increasing amplitude of this voltage successively striking a number of electrodes connected respectively to a control electrode of the tubes from A to D in such manner that the tubes from A to D are rendered non-conductive in succession and/or alternately, for example in the order shown by the above table.

If only four of the six possible combinations from I to VI are used, to wit the combinations from III to V1 the control of the tubes A, B, C, D can be obtained by simple means. This may be recognised by reference to the following table in which the conditions of the tubes A, B, C, D, (b=non-conductive, c=conductive) are indicated at an amplitude of the oscillations to be amplified which increases from zero to maximum. In this case the networks from R1 to R9 should be so proportioned that in the order III, V, IV, VI the combinations made use of lead to a successively lower anode load impedance of the tube U in the manner shown in Fig. 2. The amplitude levels at which a discontinuity in the anode load impedance at the tube U successively occurs are desig nated P1, P2, P3.

This table shows that at increasing amplitude of the oscillations to be amplified the tube D or A respectively is rendered non-conductive or conductive respectively at the level P2, whereas the tube C or B respectively is rendered non-conductive or conductive respectively at the level P1 and is rendered conductive or non-conductive respectively at the level P3.

By supplying a low-frequency voltage, which may be obtained after rectification of the oscillations to be amplified and which corresponds with the modulation envelope of the oscillations to be amplified, jointly with suitable bias voltages to a control electrode of the tubes A and D it may be readily ensured that at more negative values than a given instantaneous value of the low-frequency voltage which value corresponds with the level P2 the tube A is rendered non-conductive and is conductive at positive instantaneous values and the tube D inversely. The desired control of the tubes B and C can be obtained by rectifying the said low-frequency voltage without smoothening and so supplying the direct voltage pulses obtained to the control electrodes of the tubes B and C that at positive or negative instantaneous amplitudes respectively of the low-frequency voltage below a given value corresponding with the level P3 or Pi respectively the tube B is conductive and at instantaneous amplitudes above this limit is non-conductive and the tube C conversely.

Fig. 5 shows a further modification of the desistance Rrs of the circuits ofthree series-convice according to the invention whichiis ila-rgely similar to'that ShOWH-iHFigi 4. *Inthetablebe low are statedthe'sixserviceable combinationsof nch-conductive and conductive discharge=tubes from A to D.

'III-C non-conductive; A, B and D conductive IV-B, C and D non-conductive, A conductive 'V--A, C and D'non-c'ondu-ctive', B conductive VI-A, B and D non-conductive, C conductive In view of the devicesshownflin Figs. 3, 4 and 5 it may be remarked .thatthe circuit arrangement of the compound networks used in'these devices and built up from the various impedance inverting networks andof further compound'networks not described which als'omay be employed may be obtained by connecting each point of a group of points over impedance inverting networks to each point ofa second group of points,

two of the said pointshaving respectivelyconnected to them the anodecircuit-of the :highfrequency amplifier valve'U and the load im'pedance R. The other points have connected to them the dischargetubes A, B and'C-and soforth.

This is illustrated by Figs. 6a and 61) in whichtwo groups of points $1,112, aa and bi, barbs respectively areconnected by straight lines each representing' an impedance inverting network. Re-

ferring to Fig. 6a, theanode circuitof thetube Uis connectedto the point- (11 and the load impedance Rto the point bi. "If the other points (12, as, be and b3 are assumed to beconnected to discharge tubes this figure isiden tical with-Fig. 4.

Similarly, Fig. 6b-is' idntica'1 with-Fig. 5.

Instead of arranging one tube -=A, B, A and so forth respectively 'atth'e points of connection between the networksf it is possibleadvantageous- "1y to arrange two push-pullconnected tubes, no

anode voltage beingrequired for the tubes. A suitable arrangementforthis=is shown in Fig. '7. L1 and L2 designateinterconnectedcoils which b'y means of condensers C1and'C2 respectivelyare tuned to the frequency of the oscillation tobe amplified. The anodes ofthe'two'pull-push'connected discharge tubes -A1 and Aware directly connected to the ends of coil L2. Thecontrol electrodes of thetu-bes A1 "and Aaareinterconnected and consequently receive "the same controlvoltage. The input terminals'k are connected to the input or output terminals respectivelyof'theimpedance inverting network at the point" of connection between the networks. When the-tubes A1 and A2 are non-conductive it is'easy to ascertain that the impedance between the terminals 70 has a very low value, whereas the'impedance-is high when the-tubes A1 and A2 are conductive.

As mentioned before;'- the resulting surge -re- 'nected networks, such'forexampleas the circuits R1, R2 and. Rs used in the devices-shownin Figs. 1 and 4, must be positive; this may be 'ensured'by choosing one of the networks-R1, R2 or Rsorall three to be negativethat isto say building them up in such manner that'the outputcurrent' i-is 90 out of phase with the voltage Vand hence is opposite to that ofthe 'networkRo. A favourable form ofconstruction of these circuitsis'obtained by using for the networkRz afilter network which brings about a' phasefdisplaceme'nt opposite to that ofthenetwork-Ro, R1 being-equal "to R2=R3. The circuit arrangementof this is R2 31 theitidtlCtane 1i and "the-"capacity C RIO which areccrinectedisirriparallel with ithe'tube": A "and B esp actively formxian oscillatory' circuit which: is "tuned'to the oscillation to. be amplified andwhich'has a constanthigh impedance. This 5 "circuit-may therefore bexomittedsoi that the simple circuit -arrangement shown in Fig. Bb-is obtained. Fig. 9*shows a circuit diagram ofthe modification of 'the' devi'ce according to Ethe-invention shown inFig; 1, the networks fromRi to R3 l0 bing all chosen tobe'identical and built up in the mannershown-in Fig. 8b. Inste'ad of-usi'ng impedance inverting networks built u'p from concentrated inductancesand fca-paei tiesmse -may- 'also' be made-(of impedance 1. ln'verting networks havinga distributed inductance and capacity} for example transmission -lines-havi-ng-alength equal to an odd number cf huarter wavelengths 'of thecSciIIations to be amplified. As an alternative. each impedance ing 'vrting network may be constituted by itself by an-odd numbero-f series-connected impedance inverting networks.

I claim:

1. Apparatus for amplifying amplitude-modue lated. high frequency-oscillations comprising an "electron discharge tube provided with a cathode, a-g'rid' and an anode and circuits therefor the oscillations to be am-plified beingapplied-to'said "grid, a-load inember, -impedance network means arrangedto couplethe anode circuit of saidtube tosaid member,-- m"eans to adjust the impedance of said network means in a-step-wise manner, the input impedance of said network means form- "ing the anode impedance ofsaid tube and the '35 'load'member constituting the terminalimpedance'of said networkmeans, and control means responsive to the varyingxamplitude of said oscillations and coupled to said impedance adjusting'means to adjust the impedance of said netgcwork in the inverse direction whereby an increase in the amplitude-froma minimum to a-maximum value results in a step-wise reduction'in the anode impedance-of said tube.

' 2. "Apparatus" for amplifying amplitude-modu- 3 'lated high frequency oscillations comprising an electron discharge tube provided with a cathode, a grid and an anode and circuits therefor; a "load'member, a compound network arranged to couple the anode c-ircuit of said tube to said member, the input impedance of said network forming the anode impedance of said tube and the load member constituting the terminal impedance of said network, said compound network including a single impedance inverting net- '55 Work and a plurality of three series-connected impedance inverting networks all connected in parallel with said single network, a pair 'of normally closed switching devices for each of said three series-connected impedances, the devices of each pair being bridged across the two junctions "in said three series-connected networks, and control means responsive to the varying amplitude of said-oscillations for successively actuating each pair of switching devices whereby an increase in the amplitude from a minimum to a maximum 'value effects a stepwise reduction in the anode impedance of said tube. 3. Apparatus for amplifying in a class B man- 'ner amplitude-modulated high frequency oscillatlons comprising an electron discharge tube pro- Vided with a cathode, a'grid and an anode and -circuits therefor, a loadmember, a compound network arranged tocouplethe'anode circuit of said "tube "to said member, the input impedance--01 "said "compound network forming the "anode impedance of said tube and the load member constituting the terminal impedance of said compound network, said compound network including a single impedance inverting network and a plurality of three series-connected impedance inverting networks all connected in parallel with said single network, a pair of grid-controlled electron discharge devices for each of said three series-connected impedances, the devices being bridged across the two junctions in said three series-connected networks and being arranged to be normally conductive, and control means responsive to the varying amplitude of said oscillations to apply a voltage to the grids of said discharge devices to successively render each pair of discharge devices non-conductive, whereby an increase in the ampltiude from a minimum to a maximum value effects a step-wise reduction in the anode impedance of said tube.

4. Apparatus for amplifying in a class B manner amplitude-modulated high frequency oscillations comprising an electron discharge tube provided with a cathode, a grid and an anode and circuits therefor, a load member, a compound network arranged to couple the anode circuit of .said tube to said member, the input impedance of said compound network forming the anode impedance of said tube and the load member constituting the terminal impedance of said compound network, said compound network including a single impedance inverting network and a plurality of three series-connected impedance inverting networks all connected in parallel with said single network, a pair of normally conductive grid-controlled electron discharge devices for each of said three series-connected impedances, the devices being bridged across the two junctions in said three series-connected networks, grid bias means for said discharge devices, rectifier means for deriving a low frequency control voltage from said tube which varies in accordance with the amplitude of the oscillations therein, and means to apply said control voltage to the control grids of said devices, the biases on the respective control grids of said devices having values at which each pair of said discharge devices is successively rendered non-conductive whereby an increase in the amplitude from a minimum to a maximum value effects a step-wise reduction in the anode impedance of said tube.

5. Apparatus for amplifying in a class B manner amplitude-modulated high frequency oscillations comprising an electron discharge tube provided with a cathode, a grid and an anode and circuits therefor, a load member, a compound network arranged to couple the anode circuit of said tube to said member, the input impedance of said compound network forming the anode impedance of said tube and the load member constituting the terminal impedance of said compound network, said compound network including a single impedance inverting network and a plurality of three series-connected impedance inverting networks all connected in parallel with said single network, a pair of grid-controlled electron discharge devices for each of said three series connected impedances, the devices being bridged across the two junctions in said three series-connected networks, grid bias means for said discharge devices, rectifier means for deriving a low frequency control voltage from said tube which varies in accordance with the amplitude of the oscillations therein, means to apply said control voltage to the control grids of said devices, the biases on the respective control grids of said devices having values at which each pair of said discharge devices is successively rendered non-conductive whereby an increase in the amplitude from a minimum to a maximum value effects a step-wise reduction in the anode impedance of said tube, and a negative feedback circuit coupled between the output of said compound network and the grid circuit of said tube and arranged to compensate for distortion introduced'by the step-Wise variation in said anode impedance.

6. Apparatus for amplifying amplitude-modulated high frequency oscillations comprising an electron discharge tube provided with a cathode, a grid and an anode and circuits therefor, a load member, a compound network arranged to couple the anode circuit of said tube to said member, the input impedance of said compound network forming the anode impedance of said tube and the load member constituting the terminal impedance of said compound network, said compound network including a single impedance inverting network and a plurality of three seriesconnected impedance inverting networks all connected in parallel with said single network, a pair of grid-controlled electron discharge devices for each of said three series-connected impedances, the devices being bridged across the two junctions in said three series-connected networks, grid bias means for said discharge devices, rectifier means for deriving a low frequency control voltage from said tube which varies in accordance with the amplitude of the oscillations therein, means to apply said control voltage to the control grids of said devices, the biases on the respective control grids of said devices having values at which each pair of said discharge devices is successively rendered non-conductive whereby an increase in the amplitude from a minimum to a maximum value efiects a stepwise reduction in the anode impedance of said tube, a negative feedback circuit coupled between the output of said compound network and the grid circuit of said tube and arranged to compensate for distortion introduced by the stepwise variation in said anode impedance, the surge impedances of the respective networks constituting said compound network having values at which the variation in said anode impedance is effected in uniform steps.

'7. Apparatus for amplifying amplitude-modulated high frequency oscillations comprising an electron discharge tube provided with a cathode,

a grid and an anode and circuits therefor, a load member, a compound network arranged to couple the anode circuit of said tube to said member,

the input impedance of said network forming the anode impedance of said tube and the load member constituting the terminal impedance of said compound network, said compound network including a plurality of pairs of series-connected impedance inverting networks all connected in parallel, the pairs of said series-connected networks having progressively diiferent surge impedances, a normally closed switching device bridged across the junction in each pair of seriesconnected networks, and control means responsive .to the varying amplitude of the oscillations for successively actuating said devices, only one of said devices being actuated at any one time,

- whereby an increase in the amplitude from a minimum to a maximum value effects a stepwise reduction in the anode impedance of said tube.

8, Apparatus for amplifying amplitude-modu- 13 lated high frequency oscillations comprising an electron discharge tube having a cathode, a grid and an anode and circuits therefor, a load memher, a compound network arranged to couple the anode circuit of said tube to said member, the input circuit of said compound network forming the anode impedance of said tube and the load member constituting the terminal impedance of said compound network, said compound network including six serially connected impedance inverting networks arranged to define an hexagon and three impedance inverting networks connected between pairs of opposing vertices of the hexagon one pair of opposing vertices providing the input and output terminals of said compound networks, four grid-controlled electron discharge devices bridged respectively across the remaining vertices of said hexagon, and control means responsive to the varying amplitude of the oscillations to render said electron discharge devices 20 Number non-conductive in an order effecting a step-wise reduction in the anode impedance as the amplitude goes from a, minimum to a maximum value.

9. An arrangement as set forth in claim 8 wherein said control means comprises rectifier means for deriving a low frequency control voltage from said tube which varies according to the amplitude of said oscillations, a cathode-ray tube having deflecting means and a plurality of electrodes arranged to be scanned successively by the beam, means to apply said control voltage to said deflection means to successively scan said electrodes in accordance with the amplitude thereof, and means connecting said electrodes respectively to the grids of said electron discharge devices.

KLASS POSTHUMUS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date 2,237,796 Smith Apr. 8, 1941 2,255,476 Thomas et al Sept. 9, 1941 2,321,269 Artzt June 8, 1943

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