US2416513A - Multivibrator - Google Patents

Description

Feb. 25, 1947. R, D, BROWN, JR 2,416,513

MULTIVIBRATOR vFiled Nov.. 25, 1942 ouTPl/T @m JVHW J4 l 74m 1 y M 5 Patented Feb. 25, i947 UNITED STATES PATENT OFFICEl MULTIVDSRATOR Reynolds D. Brown, Jr., Blue BelLPa., assignoi` to l Philco Corporation,Philadelphia, Pa., a corporation of Pennsylvania f implication November 25, 1942, 'serial Nene-6,913 e'claims. (cl. 25o- 36) In the development of vacuum tube ampliiiers for use at high frequencies attention was early directed to the screen grid tube as a superior type of radio frequency amplifier. This superiority is inpart a result of its low effective control-gridto-anode capacity which enables its use, with good stability and high amplification, at frequencies beyond those for which the triode is suited. However, the insertion of the screening element produces a substantial increase in the tubes input capacity, i. e., in the direct interelectrode capacitance as measured between the input grid, on the one side, and all of the remaining electrodes on the other. In conventional pentodes this capacitance is about double that for triodes. This high input capacitance is not ordinarily serious in tuned amplifiers Where the input capacity of the tube may be used to tune the input circuit. But, as is Well known, in untuned wide-bandqvsystems the increased input capacity seriously limits the maximum frequency at which the system is operable, and tends, at high frequencies, to short circuit the signal source Withwhich it is directly f in shunt. j

It is a principal provide means for effectively `diminishing the input capacity of amplifier tubes having screen grids, and thereby to extend the frequency range over which the tubes are useful.

It is another object of the invention to increase the maximum frequency at which the multivibrator .type of oscillator is operable.

It is still another object of the invention to provide a multivibratorcapable of functioning 'at frequencies of the order of several megacycles.

It is a further object of the invention to pro vide a signal amplitude limiter of the multivibrator type capable of operating in the interv mediate frequency ampliiier channel of frequency modulation receivers. j

The'invention itself, as Well as other of the obiect of `4,this invention toV objects thereof, will be understood by those skilled in the art from a consideration of the following detailed description and the accompanying drawing, in which Fig. 1 is a schematic illustration of a circuit utilizing the invention; and

Fig. 2 is a schematic illustration of a multivil brator circuit embodyingthe invention.

Reference is now made to Fig. 1 in which the invention is applied to a single-stage vacuum tube Y amplifier.

Ignoringfor the moment, the pentode I, the amplifier stage will be seen to consist of a `conventional amplifier circuit comprising the pentode 2 together with an output load resistor 3. The signal to be amplified, derived from the source 4, i-s applied betweenthe cathode 5 and control grid 6 of the pentode 2, while the amp1i.

fled output signal may be derived from the anode circuit of the tubeby way of the coupling con-- denser 1. In addition to the elements mentioned, the pentode includes a suppressor grid 8 which is conveniently connected to the cathode 5, and a Vscreen grid 9 which would ordinarily be connected to a source of iixed positivepotential (not shown) The input capacity of the pentode 2 consists largely of the capacity between the control grid 6 on the one side and the` cathode 5 and screen grid 9 on the other. Capacityalsoexists between the control grid B andthe anode and suppressor grid 3, but .since the latter elements are more remote from the control grid the contributions of Vthese elements are of .less importance.

At the higher frequencies a considerable charging current flows into the control grid 5 from the signal source 4, .and this charging current is largely due tothe physical proximity ofthe cathode 5, here shown connected to ground,l andto the screen grid 9, which in conventional practice l is at ground potential so far as the radio frequency input signa-1 is concerned. As the Vsignal frequency increases in magnitude, the charging current also increases,'thus placing an abnormally heavy load upon the signal source 4, and eifectively short-circuiting the input circuit ofthe pentode'2. Y f According to the present invention, the interelectrode capacity effects between the input grid B and the screen grid 9 are substantially nullified by the application to the screen grid 9 of a signal which is similar in frequency, phase, and magnitude to the signal applied to the input grid 6. This 'may' be accomplished through theagency of an auxiliary pentode I. In the embodiment ofthe invention illustrated in Fig. 1, output signal from the pentode 2 is derived from the load resistor 3 and impressed on the input grid I0 of pentode l by way of the coupling condenser The output signal of the auxiliary pentode 1I, as taken from a point on the anode load resistor I2, is applied to the screen grid 9 of pen- 5 tode 2 by way of the conductor I3. Since a phase reversal occurs.; in ,each-l of the pentodesfithe sig-J nal applieditoathe'screengrid 9 is in ...the isame "z phase as the original signal applied to the control grid 6, and by a suitable choice of tap locations on .2.103n

the impedances 3 and I2 the magnitude of the signal supplied to the screen grid 9 may be made equal to that present on the control grid. When .,Y the alternating voltages appearingcnthegrigdsaz `6 and 9 are equal in magnitude. and of like phase, 151Ml far beyond its normal'range, Y

it is evident that no charging. current can flow from they control grid 6 to the-screelr-gridandai thus one cause of the objectionable chargingcune; rent is eliminated. However, it is also within the scopefv the invention izo-'compensate wholly or..20 in-tpartior the capacity"ieiliects between the con`` complis'he'd byY supplyingl 'tol-the screen* grid' 9 a signal of greater magnitude than that-'presenten the control grid 6. f Under theseconditi'ons charg- 25. ing current will ow fromfthe"screengri'df9,asa source; lto the control grid 56,. l'and by suitably adjusting the magnitude "of" the? voltage supplied=by'*"" the" pentode'v I"v it is readilyfp'ossible -to'compensate for"k the charging current-i takenffbythe control '30 gridato-cathode?circuit'cf the fpento'deZ, v' Sirr'ii'' larlyfit is possibletocompensateforth charging current 'drawn by other Joffth- `:electrodesif or 'by the distributed-capacity associated r withl the'F con# ductors'connectedito thegri'd. 35 Asidev from :its desiredcapacitycompensatingfI" effect; th 'applicationoof the. ini-'phase r'signal f'tof the screen'- gridl 9 producesinoisubstantial change-- in-the operationfof thezfamplifie'nfrand no vdis-ri` advantageous "veiectsff 'are '-Sintroduced'.v This" is largely be'causel the screenfgridetoeanode'fmutual L conductan'ceofthe'itube is smallclomparedzt'o thei control: grid-toganodemutualr-conductance; and 1consequentlyffthecontrolaeffectof vthescreen?. is i negligible. .At 1lowfsign'al requenciesiwhereftube. :45 input capacity 1' would@ have nolf effect 1on1v circuit iperformance; theapplication;.ofil the: signal voltjagefto the :screenr'S hasfsnoisubstantial .-ieect ioni. ff th'epgain 'of .'thitubeie However,'t;at;th'e1h glrsignal.: I frequencies wheretrgain:wouldinormallyffall ofi L: rapidly :witlr frequencyzthe'zpresent;circuit tendszto reduceanthe: effectivesr zinputu-,capaciw and* toil: maintain 'iavorablef operating-5:onditions;r within. 'the'amplie circui Referring nowftozitheedetailsfsciithevcircuit::ofi: g

Figi T1, .it iwi-1libeznotedffthatathe input? signal :to .1 5" the. auXiliarymentode it is derived .from an Y inter-1f mediatepointr onffthesresistor, if This practice-f-zv `is .preferred because thro-ugh this#,expedient the f e distributed capacities'fto ground of -the .fcoupling- 'f condensenl I yand the?leadfwiremassociatedftherewitlf arenot connecteddirectly; in shunt with-.the anodev circuit. i'offtithearripliiiersipentodel 2, 'andnz this Sway th'e` output-'icapacity"1ofathis pentodei is f not adversely effected by the presence ofthe265 `aux-iii'aryrpentode'lz Infpracticfit Willorma'lly I bedesira-ble toL employ intermediate "taps'onf bo'th f1 l th'resistors andu |32 (asshown) pinv order to ree strict the .over-alhg'air of the; auxiliaryiypentode' circuitjtoj; that:;whijch .is just .jnecessary-"totcom' p 70 pensatelforcthe.capacitygeffects hereinbefore "de' scribed aJns-.the #circuit shown, plate, v.and screen. f potentialifor thepentodeol., .andiscreenpotentiali.. for Lfthe... pentode .2, .is ...derived. .from a .common q source B+. This is a convenient Connectfm, bli 75 'tot obviously separate sources may be employed if desired.

In general it is not difficult to design an auxiliary pentode circuit capable of providing, even at very high frequencies, a signal for application to the screen grid 9 which is equal to or somewhat greaterfthanthe Asignal applied toEv the input grid 6. s This is becausethe overall gain `ofithe auxiliary pentode circuit need not be large, being, in

igeneral, substantially less than unity, or of the order of unity, depending upon the gain of the amplieapentode 2. Under these circumstances, andibyproyiding theauxiliary pentode with low inip'egdanceinput-andiutputcircuits, the operatinif;; requeneyz.ci.;thpentode I may be extended Referenceishowmade to Fig..2 in .which the v invention..is .applied to a multivibrator circuit which is 'adapted for use v.asan amplitude limiter the anode-4 2 Iff' 'of '.-tuberfl 5 bytiwayiior Pa couplingff'f condenser'2.31.'-`- The'tube M'nay be provideclwith'v a suitable gri'dilea'kf.resistenrZAJanrl'anfi'ariode load' circuit cc-mprisingth seriallyconnected resistors 25 andE SimilarlythetubelS may'beprovided with a grid1l'eak1'213 and anodeloaii resistors V28 and" Z Ani inputsig-na'lofrom 'a so.urce"30''mayl if be applied". tofthe tubelf il mth-rough fan l input'grid" 3|? The correspondingy inputgridIZ'iofthe'tube'v Y l 5f may be" connected ltofground, `but if "af balanced' source 1 of signalls provided-,ifthesignal may supplied'. tov both :finput '.lgrids finbpushepull' rela;` tion, LTheoutputLSignal dfthemultivibrator maybetake'nfrom either# 'ofthe anodesfZtl-lori.'l;4 'or if :desired the 1 output fsignalwmay. -`bewaken-from the" anodes in'ipushepull rela-tio'n.- asf shown; the?. push-pull connection' giving ktwicethe output prol1 vided by :the'singlelside"connectionzi1 Thf-vacuumvtubewl'g infaddition Ato its conven= tional f function' `finif '-the'l multivibrator; lis related tothe vacuum tube I4 in much the same'wayf" asf-"the" auxiliaryyacuum'iube* iI 'of-Fig v'1 is rei'ateclrftb:itgassociated va'cuum'tubeZ. Thus Fi'gfZ utputs'igna'l frcmth'e' tube I'5is derived' from thevv junction fbetween'the anod'e'fload esisn'ftors 28A and 29, and is '-*applie'drby'f way offthe" conductor '33" to" Vthef =screerr` grid lernent 34 of tube I4. `Similarlyioutput-'sigallfromthefltube t I4 is derived fromthejunctio'nofthe'anode load resistors '25'and" 26 and is applied bytwayA of :the Y u conductor 35'toith'e'-screen'grid`-36of1thetube I5. In' additongto thev he'renbefore 'described'physe ical circuitv elementsizthe.schematicoffFig 2 also. shows, .in .dashed lines," fthe" effective capacities i: associatedgwith the-.tubeelectrodes.: Tlis the' capacityin shunt 'with`the grid leak* resist'o'r2il` represents the sum 01 .the eiectivcapa'cties from thJ'grid I'8`to`th screengridi34,"frm`the' grid plate 2| of tube I5 to the suppressor grid 31 of tube I5, and the load capacity of the tube I5. The capacity shown in shunt with the load resistor 28 represents the capacity lfrom the grid I8 of tube I4 to the screen grid 34 of tube I4. This capacity is included in the capacity in shunt with the grid leak resistor 24, but it is here isolated. The capacity shown in shunt with the load resistor 29 represents the capacity from the screen grid 34 of tube I4 to the suppressor grid 38, and to the input grid 3| of the tube I4. The capacities shown in shunt with the grid leak resistor 21 and the load resistors 25 and 26 correspond to those above mentioned.

In the preferred form of the invention. the time constants of the six R.C. circuits, which include the resistors 24 to 29 inclusive. are preferably made equal by a suitable selection of the several resistors. Under these conditions the effective capacities of the circuit disappear at all frequencies for which the equality is substantial. Even if this preferred adjustment is ignored with respect to the grid leak resistors 24 and 21, it will be advantageous to adjust the time constants of the R.C. circuits which include the resistors 25, 26, 28 and29.

In the design of amplifier and multivibrator circuits constructed in accordance with the principles of the invention, the following relations, while not advanced as limitations to the invention, will be helpful. Taking Cg as the interelectrode capacity between the grid I8 and the screen grid 34, and taking Cs as the capacity between the screen grid 34 and all the other electrodes exclusive of grid I8, in the preferred embodiment Rea R25-t' Rza is greater than zero, but less than Moreover, in general, the lower the values of R25 and R26 the higher the effective frequency of operation, but the higher the level of the minimum signal which must be applied to reach the limit effectively. The corresponding resistors associated with the second tube I5 may be similarly evaluated.

The multivibrator circuit illustrated in Fig. 2 may advantageously be employed Wherever a, multivibrator capable of generating a signal of high fundamental frequency is required. The multivibrator circuit shown is capable of being synchronized with signals from the source 30 whose frequency is well in excess of one megacycle` a frequency which formerly represented the upper practical limit for multivibrators of conventional design.

The multivibrator of Fig. 2 is particularly well adapted for use as a signal amplitude limiter in the intermediate frequency channel of a frequency modulation radio receiver. The conventional form of multivibrator, on the other hand, is largely inoperative at the intermediate frequency generally used in frequency modulation receivers. Even were the intermediate frequency made so low that this consideration did not apply, the deviation of the frequency-modulated carrier would cause it to reach frequency regions in which the conventional multivibrator is unreliable. The present invention however eliminates the difiioulties previously considered inherasians i the control grid and cathode electrodes of saidv ent in the multivibrator, and provides a nonresonant limiter system which is readily synchronized with the frequency-modulated intermediate frequency signal. It also provides an amplifier of simple construction capable of high gain while limiting the output, in addition to providing a push-pull source of limited signals using a single side input signal.

Although the invention has been described and illustrated with particular reference to certain preferred embodiments, it should be understood that numerous alterations and modifications may be made within the scope of the invention as defined in the appended claims.

I claim:

l. A multivibrator circuit adapted for operation at high frequencies, comprising a pair of vacuum tubes, each having at least a cathode, control grid, screen grid and anode, a load resistor connected to each of said anodes, said cathodes, control grids and anodes being interconnected to provide multivibrator operation, means for coupling the screen grid of the first of said tubes to an intermediate point on the load resistor of the second of said tubes, and means for coupling the screen grid of said second tube to an intermediate point on the load resistor of. said rst tube.

2. A multivibrator circuit as claimed in claim 1, wherein at least one of said tubes is provided with an auxiliary grid, and wherein a synchronizing signal is applied to said grid for controlling the period of oscillation of said circuit.

3. A multivibrator circuit adapted for operation at high frequencies,'comprising a pair of vacuum tubes, each having at least a cathode, control grid, screen grid and anode, a first pair of load resistors serially connected in the anode circuit of the first of said tubes, a second pair of load resistors serially connected in the anode circuit of the second of said tubes, a first condenser connected between the anode of said first tube and the control grid of said second tube, a second condenser connected between the anode of said second tube and the control grid of said rst tube, a connection between the screen grid of said first tube and the junction of said second pair of load resistors, and a connection between the screen grid of said second tube and the junction of said first pair of resistors.

4. A multivibrator circuit as claimed in claim 3, characterized in that the time constants of the circuits formed by the said resistors and the capacities in shunt therewith are substantially equal.

5. A multivibrator circuit as claimed in claim 3, wherein grid leak resisto-rs are connected between tubes, and characterized in that the time constants of the circuits formed by the said grid leak and load resistors and the capacities in shunt therewith are substantially equal.

REYNOLDS D. BROWN, JR.

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

UNITED STATES PATENTS Number Name Date 2,033,274 Burton Mar, 1l), 1936 1,986,331 Farnsworth Jan. 1, 1935 1,941,393 Farnham Dec. 26, 1933 2,145,368 OBrien Jan. 31, 1939 2,299,366 Ziel Oct. 20, 1942

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