US1764564A - Audion amplifier - Google Patents

Description

June I 17, 1930.

L. M. HULL AUDION AMPLIFIER Filed June 29, 1525 Patented June 17,1930

roams, s'EY AUDION AMPLIFIER 7 Application filed. Juneae, 1925. 'SeriaI'JNo. 40,435.

I This invention relates to audion amplifiers and particularly to amplifier networks arranged to control or to eliminate retroactive currents through the'grid p'late capacity.

It has been proposed to construct an amplifier network as an alternating current l/Vheatstone bridge in which the input and.

the output circuits of theamplifier stage com- I prise conjugatearms which are electrically iisola'ted so far as alternating currents are concerned, i 1 7 An object of'this invention is to provide an audion amplifying stage in which retroactiolnfrom output circuit uponinput circuit P be made to vary in predetermined fashion with the frequency, may be made substan tially independent of frequlency or may be suppressed over a'wide rang of frequencies.

Specifically an object of the invention is to provide an audion circuit in whichinput and output circuits are made conjugate arms of an alternating current Wheatstone bridge which has three substantially pure capacity balancing arms and a fourth" balancing arm comprising a composite impedance the ef fective Capacity of which is a function Q-l nated entirely and the" balance may be 'ob-.

frequency,

Specific embodiments of my invention are 7 illustrated in theaccompanying drawings in ---30' which v I Fig. 1 is a circuit diagranr of an audion ampl fylng stage constructed 1n accordance with my invention; A a I Fig. 2 is a bridge diagram of theimpedance .elementsof the alternating current network Fig. 3 is a circuit diagram of an audion detector which due to the so-called regenerative effect also acts as a radio frequency amplifier; and V 7 Fig. 4 is a diagram of a cascade amplifier. As will be seenfrom Fig. 1, the output cir:

cuit of the stage is arranged in the usualman' ner, and is there shown as comprls ng a coil L which may be the primary of 'a coupling transformer for the succeeding staga' The input circuit is shown as an impedance L of the composite arm,

cuit is not connected across the input terminals G, F of the tube T, but is connected across the terminals G, 2, which terminals I designate as the input terminals of the amplifying stage. Thepath from the lower input terminal 2 of the stage to thelower tube 7 terminal F is provided bya condenser C shunted by'an impedance L This path 21? forms one arm of the alternating current Wheatstone bridge and will be designated as the composite arm to distinguish from-the three capacity arms Ca; C and Gg which comprise, respectively, the inherent grid-plateca pacityofthetube, an external capacityj'connected between the plate P andfthe lower stage terminal 2, and the capacity between the grid and filament terminals. As indicated in 1 Q IC LEWIS M. Humon Boonron; new JERSEY, nssienon TO Rania EREQ ENGYLABORAQ rnoonronnrnn, or BOONTON, new JERSEY, aconraonn'rron on N W JER- Fig. 1', the capacity C rbetwe'enthe grid and I filament terminals may be composed of the inherent tube'capacity G 'plus anyexternal capacity G which is shunted across the tubecapacitv. Theexternal' capacity maybe variable. if desired, for purposesof adjust merit, but this control isnot essential. In fact, the external capacity C may be elimitainedbya proper choice of the constants of the other arms of the bridge. The plate battery may be shunt'edby a'largecapacity CB to provide a by-pass for the alternating cur-' rent... i

Thegnetwork ofFigl' theiform of a. I

Vl heatstone bridge as will be evident" from an inspection of Fig.2. It is -'apparen.t that theconditionqforbalanceis given for allfre q quencies greater than'the resonant frequency where C is the-eifective capacity of the con l posite arm 11 G v U The arms ,C and G; are pure capacity arms hencetheir ratio in equation (1) is invariant frequency; C can be .made a pure capacity by properly biasing the grid of the audion, and the efiective capacity of the composite arm may be substantially independent of the frequency or it may be made to vary with the frequency, as desired. It is shown both by experiment and by elementary circuit theory, that for all frequencies substantially greater than the value the effective capacity C of the arm F is given approximately by the equation:

where e 21r frequency. Thus if the inductance of the choke coil L is so chosen that 1ts inductive reactance (0L is large compared with the capacitive reactance of the capacity over a given range of frequencie's, the effect ve capacity C' Wlll be closelyequal to C and Wlll be practically 1ndependent of frequency over this range. Ex-

plicity, suppose that in a given range of frequencies L 1s made so large that L is greater than a i quencies." For instance if I wish to sup press retroactive currents by a complete bridge balance over the band of wave lengths 200 to 1000 meters FQAZ 10 to 1.88 X 10 with a given audion, I-may make C of the order of magnitude 200 micromicrofarads. L equal to or greater than 1. L henries, select C and C, so that Equation (1) is satisfied, using the value 200 10- for C and the bridge circuit is completely balanced, to an accuracy of one part in one thousand, with consequent suppression of retroactive currents, over this specified wave band. I have found it convenient, when a balance substantially independent of frequency over the whole range of radio frequencies is desired, to useas L the secondary winding of an ordinary commercial audio ffrequency transformer. C may comprise only the distributed capacity of these windings or may include an external condenser as well. This use of an audio frequency secondary L is convenient when my invention is applied to a circuit of the so called reflex type, in which the grid circuit of an audion is fed simultaneously from an audio-frequency transformer and a radio-frequency transformer connected in series. i

The useof the choke coil L across thearm C not only permits the reflexing of the balanced stage but has the additional and important function of placing the direct current potential of the grid at a predetermined value for preventing extraneous noises and blocking in the audion. So far as the con struction of a stage which is balanced independent of frequency is concerned, the pres ence of the coil L is not essential, but for the satisfactory reception of signals the choke is of decidedvalue, particularly when several stages are cascaded.

The invention is not'limited however to amplifiers in which the inductance L and capacity C are so related that the effective capacity C is practically invariant with frequency; By proper design of these parts it is possible to secure a substantially constant amplificationat all wave lengths even though the coupling transformers are of the type which normally result in a decrease in the amplification rate .with increasing wave length. At the higher wave lengths it is possible to operate with adegree of unbalance which would cause sustained oscillations or objectionable reaction at the lower ranges. The unbalance may be so chosen that a predetermined regenerative effect supplements the nonregenerative amplification as the latter fallsoif at the lower frequencies.

From'Equation (2) it will be evident tha the effective capacity of the composite arm will vary rapidly with the frequency when the inductivereactance L w is greater than, but comparable'with, the capacity reactance 1 0 50.)- The following specific example may be cited: If L is made equalto 5 00 microhenries and C made 200 micromicrofarads, the effective capacity C of the arm'to F is 177 micromicrofarads at 200 meters, 110 micromicrofarads at 4:00 meters, and above 596 meters the reactanceiofthis arm is no longer capacitilve but inchicti've. Thus if the arms (1, and C were chosen to provide a balance at 200 meters, retroaction fromfthe plate circuit to the grid circuit would increase progressively withthe wave length above this point. At wavelengths above 596 meters noteven an approximate balance occurs, since one arm of the bridge isno longer capacitive, and if the output circuit is tuned or has a large induc which is the conventional Armstrong regenerative detector having a plate variometer Ii which causes retroaction through the gridplate circuit with consequent regenerative increase in 'thetuned input circuit L1G. In accordance with my invention a balancing network comprising C G and the composite arm L' C is added to'this circuit. At frequen'cles for whlch the cond1t1on of balance is satisfied no regeneration can occur for any value of inductance ,L in the plate circuit Onthe other hand, when the bridge is un-, balanced, more or less regeneration occurs, depending upon the extent'of unbalance and upon the size of L It is possible so to adjust the capacities of the bridge arms that a variation in the balance condition with wave length occurs which produces maxi mum regeneration over a substantial band of wave lengthswithout altering the value of L This effect is impossible in an ordinary regenerative circuit where, as is Well known, it is necessary to increase the plate circuit inductance progressively with increasing Wave length in order to maintain the maXimum regeneration.

7 As will'be apparent from Fig. 4;, stages such as illustrated inFig. 1 may be cascaded in the usual manner when the inductance L of the first stage is coupled to an antenna circuit AG and the plate inductance Lof one stage and the input inductance Li of the succeeding stage take the form of the primary and the secondary, respectively, of a coupling transformer. 1 a

The invention is not limited to the specific circuit details illustrated in the accompanying drawings nor to the particular constants herein specified, as the I arrangement of the bridge circuit and the relative values of the elements employed may be varied-within wide ranges without departingfrom my invention.

Obviously a complete balance'or the desired approximation thereto may be obtained by varying one or more of the quantities appearing in Equation So far as I am aware it is'broadly new to provide an audion network in which the direct current potential of the grid is fixed with respect to the filament, and in which the input and output circuits are connected across conjugateterminals of an alternating current Wheatstone bridge'having one or more balancing arms whose nnpedances arevar1- ant with frequency, and of which the remaining balancing arms are substantially pure capacities; My invention therefore contem- 7 plates all arrangements of this type whether ornot the bridge is balanced and hence nonregenerative over a substantial-range of frequencies, or whether it is unbalanced and ience regenerative for a 130111011 011 the range of frequencies at which the amplifier is adapted to operate.

Iclaim: 1. In an audion amplifier network arranged' at a Wheatstone bridge which may] be balanced to controlretroactive currents, inputand output circuits of the amplifier stage forming the respective conjugate arms of the bridge, and four balancing arms, three of said arms comprising substantially pure 7 capacities and the fourth arm comprising a composite impedance having an effective ca-I 'pacity which is a functionof the frequency. 2. 1 In an amplifier, a three-electrode audion,

input and output circuits therefor adaptedjto formconjugate arms ofa VVheatstone bridge, three points of the said bridge being defined bythe grid,pla te and filament terminalsof said audion, and balancing arms connecting theplat'e andthe filament terminals respectively with a fourth bridge point, the impedanc'es comprising three of said arms'being capacities whose respective reactances at varying frequencies are substantially those of fined capacities, and theimpedance of the fourth armbeing composite and having a reactance which, within'predetermined limits within the frequency range of theamplifier, varieswith frequency at a rate substan tially different from that of a fixed capacity.

3. In an audion amplifier, an alternating current Wheatstone bridge network comprising conjugate arms which form, respectively, the input and output circuits of the amplifier stage,'threebalancing arms having the form of substantially pure capacitiesand a fourth arm comprising a composite impedance havof the frequency. p I

4. An audion amplifier-stage the impedance mesh of which is arranged in the form of an alternating current VVheatstone bridge of which the inputand output circuits of the,

amplifier stage form conjugate arms, the balancing arms'of said bridge comprising sub! stantially pure capacity between grid and plate of the audion tube, substantially pure substantially pure capacity and the other of said impedancescomprising a capacity shunt ed by aninductance, one terminalof saidining an effective capacity which is a function put circuit'being connected to the grid of said 7 audion tube and the other terminal ofsa'id input circuit being connected to a point'intermediate said two impedances in the plate circuit of said: audion tube. r

'5. In an audion a1nplifier,an

alternating f current Wheatstone bridge network comprising conjugate arms which form, respectively,"

the input and output circuits of the amplifier stage,- three balancing arms having the form of substantially pure capacities, and a fourth balancing arm comprising a capacity shunted by an inductance, whereby the effective capacity of said arm is a function of the frequency.

6. In an audion amplifier network ar ranged as a Wheatstone bridge Which may be balanced to control retroactlve currents, 1n-

put and output circuits of the amplifier stage forming the respective conjugate arms of the brldge, and four balanclng arms, three of said arms comprising substantlally pure capacitiesand the fourth arm comprising aparallel inductance-capacity circuit of Which the reac'tance of the inductive branch is comparable With, but greater than, the reactance of the capacitive branch, whereby the efiective'capacity of the composite impedance is a tunc- 'tion of the frequency. V p In testimony whereof, I afiix my LE'WIS HULL.

signature.

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