US2196844A - Wave amplifying system - Google Patents

Description

April 9, 1940. J, M, w s-r WAVE AMPLIFYING SYSTEM Filed April 26, 1939 /Nl ENTOR J. M. WEST ATTORNEY l PatentedApr; 9, 1940 a a 1 I I "35 will be regarded herein as the amount of .feed- "l -b li t d rs l su phide brother temperature UNITED STATES OFFI E -Q f i1 Bell Telephone Laboratories, Incorporated,

New York, N. Y.', a corporation of New York I Application April 26, 1939, Serial ;No.p 270;054

. I 7'C laims. (01. 179 -171) I I v This invention relates towave amplifying sys a variable impedance element common'to the [L j terns, as for example, vacuum tube amplifiers. circuit and the p circuit for varying the amplificae Objects 'of-the invention-rare to control transtion of the amplifier while maintaining .98 conmission properties ofsuch systems, for example Stant. I I v 5 gain anddis'tortion introduced by the systems, to Forexample, in a regulating -amplifieriwith an 52 facilitate application of feedback in the systems, I equalizer i the B C c t compensate Variaand to,reducetsinging'tendencyor maintain proptions o-fcable'losses due to temperaturevariaer margin againstsinging in the systems. tions, the desired regulationfo'f the amplification In one specifieaspect the invention is applied of the amplifierisaccomplished withoutmaterial to an amplifier with a forwardly transmitting variation 01" 3 by varying a impedance common 10- wave amplifying portion and a' feedbackpath to the a circuit and the ecircuitsoas to change from the output tothe input of the amplifying it and {3 simultaneously and substantially equally portion for producing negative or gain-reducing bufiiHVBISe Y- Forin ca he m ififl ybfi feedback around thefamplifying portion, for exa three-stage amplifier with this simultaneous ,u I I ample to stabilizethe gain or reduce themodulacircuit a ndfi C 1 fited by 3 Va iation introduced by; the amplifier, in accordance .Ible mpedanc CO m to t m circuitand-the, with the'principles' described byII-I. S. Black in his {3 circuitgvarying' the meal negative feedback on' I article entitled Stabilized feed-back amplifiers, the third sta e and su st ly q y u i B'eli System Technical Journal, January 1935i, and, versely varying the negative feedback around all,"

in his Patent 2,102,671, December 21, 1937. q I of't he stages I I w With sufiicient feedback, the amplification of n c ordan -W m ature o th inv n n such an amplifier varies substantially asthenegae' a single variable elementin a feedback amplifier tive reciprocal of p, the propagation through the V the am i c fv amplifier While MB feedback path or 13 circuit, and-is substantially f em nsfixed. Thi$haslmairked advantagesvover 5 independent of t, the amplificationof theamplius of a te va elements in t rc fying portion era circuit; for-example, the decibel 'a e e irc ta a cess tyiSavQidgain of the amplifier variessubs'tantially as the' ed for calibration of an .element in thelp. circuit. decibel loss in the feedback path. It is usual to a d an element in e e i u t at v t6 each controlthe-amplificationofthe amplifier for any to m them k; n t moreo-ver,

'39 given. frequency by changing [i -for suchxfrequeny flange! Ofthe mp ifi be0mingfln$l7ab16due t However, this change causes Variation inth'e failure (if oneiotf them Q dfeedback factor or 18 gain (i. e., in 5 the complex I e, mpe anc iniaac rdan e W th quantity by which a voltage is multiplied in a e invention'may 60 y t l 'a i single journey around the feedback loop, which for e p c ya flm -1" fi a t back), and the. variation in the'feedback'may' be. a d pende t resistance, a be u Th a le -I objectionable. For example, it may unduly erem p i l h j d li yt yl nyr duce the singing margin or the amountof ridodu-v u tab e aut at ato as for instanc a" lation reduction or. amplifier gain stabilit on: pi o twir r pi t5 a 1j u1a 0r-- in tained. by the feedbackiaction'; or it may Qbj ec-I Other objects and features oftheinvention will I I tionably changethe'amplifieninput 'or output b app rentl r m; th fo low ng'd s riptionand. impedance orvtheIefiect (discussed hereinaften 'claims- :I I i which has beenl called them? finitude-eifect, an w 7 I II effect relating to departure of the amplification *F acilcuit diagram of a m lifiet f am Q of the amplifier from the value I known type, for-facilitating explanation oi thef4 4 I a 1 I I I invention andof an example of its application; j V? I I ,f I "Fig. 2 is ablock schematic circuit diagram of I B an; amplifier embodying a form'of'the invention; .(due to thefinitude of as). and

'50 Objecticnablevariation of the ieedbackcan 'be I Figs. Band 4-are block schematic diagrams of 56: i avoided by making "equal but inverse or comple modifications of theamplifier of Fig. 2; mentary changesin LL! and. p,- so that s remains Fig; 1 shows a typical three-stageamplifier em- '7 constant. a "j plo ying-series type negative feedback, with its In onespecifieaspcct the invention is astagain adjustable by a v"arial:1le impedance Z"v in 'bilized feedback amplifier'with means comprising the p -circuit I'he amplifier comprises vacuumj 5f networks Zr and Z2. source or sending circuit of electromotive force E v tubes V-I, V-2 and V 3 cascaded by coupling It amplifies waves from a and impedance Zin and transmits the amplified waves to a load circuit or impedance Zout. The

third stage has a cathode network or impedance ter. Their designations are taken as their vector values. In Fig. 1

in+ ziut insertion gain- Z v M-i- MW and the insertion gain of the amplifier given by: y

Substituting from (1) and (5 in (,2) gives:

I Z Z i V fl3 3 m3 03 fim-lm] Solving (6) for eg: gives:

Substituting from (7)"in (1)"yields:

Substituting from (10) in (3) gives: I

where S1111, Smz and Sm3 are the transconductances V Z Z of tubes Vl; v 2, and v-3.1 smaa(z,, Fig. 2 is like Fig. 1 except: (1) the cathode net t' =,5',, Z S,, Z g or impedance of the third tube has been made L +,g (zm+ ifl into'or replaced by'one composed of impedances ZV+ZKZ+Z2 Em and ZKZ in series which have the same con 7 (11) gurations, respectively, as Z 1 and Z 2 or which x have the impedance-frequency characteristics of ysolvmgul) for I Zm and ZK2 the same shapes as those of Z 1 and e SMZ SMZZ E' Z 2, respectively,- over the frequency range for 3 Z Z Z which e is to be maintained constant; and (2) Z S "F +Z +ZKZ+Z,; instead of leaving Zv in circuit and shunting Zxz 1 2 t- Z Z with a variable impedance for producingVaria- ZTTZE tions in [L equal but inverse to those produced in e g l v by variation of Zv, Zv is omitted and the two (12) points where the variable elements across Zxz and Substituting from (12) in (8) and rearranging Z 2 would respectively connect to the" junction of terms'gives:

Sm1 1 m2 2 3 Z Z Q V+ K2+ fi2 7,1 1+5 ZS Z SW13 v m (13) 7 1 M 1+ ZVZKZ 2+ sz) M In V+ K2+ fl2 2x1 withZKzfand the junction-of Z 1 and Z 2 are connected through a single variable element or impedance Zv, or in other words, Zv is omitted and a variableim'pedance Zv is shunted across Zxz andZ z. 4 When ZK2 is K times as large as Z and Zv'is equal to (1+K) Zv the new configuration shown in-Fig. 2 will give exactly the same insertion gainas that indicated-above for Fig. 1. If ZKI is K times'as large as Zpl, Z'Kz is K times as large as Z 2, and SmZm is large compared to unity, p will be substantially independent of the setting of Zv. Proof of this, and derivation of an expression for the insertion gain in Fig. 2 are given below.

f Referring to Fig; 2 we can set down the following expressionszt Since the denominator in (13)v is equal to l-lc and 3 is much greater thanunity 'we may sim- Forthe reference condition without the amplit I fier between the source and load we have:

By inspection from (13):

the circuitof Fig. 1.

This is, therefore, a set of conditions th'atmakes ,ufi (as regards both its phase and its magnitude) independent of Zv.

If Equation (20) is satisfied and in addition the condition 1 g v=(1+K)Zv (22) then I I V in'lT aur 1 xeferenoa fl1+ fl2 ZIV v-F 52 which can be shownto be the same as that for Expression (18) is satisfied if the local negative feedback around thelast stage is much greater than unity. Thus,,with considerable lo-- cal' negative feedback and considerable over-all negativerfeedback, an will be substantially independent of adjustment, of Zv, provided Zv is shunted across equal percentages of the cathode I net impedance ZK1+ZK2 and the ,3 circuitiimpedance Z 1+Z,q2, (it being understood that Zm and Z 2 have their impedance-frequency character I istics the, .same shapes as those of: Z a-and Z 2,

pendent of the setting of Zv.

respectively. The greater the local feedback, the

more nearly will the over-all feedback be inde- The ,8 circuit network composed of impedances 2 1,2 2 and Zv may be made such as to give the amplifier a desired, insertion gain-frequency characteristic, A procedure often facilitating this is first to determine the impedances Zn 'Z z and Zv required in the case of Fig. .1 for the desired insertion gain-frequency characteristic,

from the expression given above foninsertion gainof the amplifier in Fig. 1, then to determine Zv from the relation Zv= (1+K) Zv,- given above as that for making the insertion gains of the amplifiers of Figs. 1 and 2 the same. The p circuit network of impedances Z Z 2 and Zv may thus be made to give the amplifier of Fig. 2 an insertion gain which, over the used frequency is flat; or it may be chosen to make the amplifier insertion gain vary in some chosenlinear or nonlinear manner with frequency.

The variable impedance .Zv in Fig. Land in Fig. 2 the variable impedance Zv which is equal to (1+K) Zv, may be such that adjustment of the variable impedance Zv Changes the loss of the e circuit network composed of Z Z52 and Zv equally at all of the usedfrequenciesor. by

' desired different amounts at the difierentIuSed frequencies, (for example inthe manner .of the adjustable impedance in the variable equalizer network of Fig. 18 of H. W..Bode Patent 2,096,027,

.October 19, 1937, for ,Attenuation equalizer), so

that the adjustment of the impedance "-Z v and consequently the adjustment of Zv Willaccording ly change the insertion gain of theamplifier equally at all of the used frequencies or by desired different amounts at the difierentusedfrequencies.

trolcircuits.

In this figure, as in the case of Fig. 2, when 3 =islike 'Fig. 2 except: (1) B supply, coupling impedances I and 2, gridleak resistors 3, 4 and 5,-and stopping condensers 6, I, 8, 9 and I!) are shown; (2) thesource or 'sending'circuit j" is shown 'as'input transformer l I connected to circuit l2 and thedoad is shown as output transformer I3 connected to circuit [4, either or each of the circuits l2 and 14 being, for example, a

section of coaxial line or cable for broad fre-.

quency band multiplex carrier or television transmission; (3) cathode networks or impedances. l5

' and 16 are shown for tubes V-l and V2, re-

spectively, these networks serving, for example,

to provide grid biasing voltages or alternating current feedback voltages, or both, for these tubes; and (4) the impedance Zv is shownas atemperature dependent resistance, for exam ple silver sulphide, having itstcmperature (and consequently'its resistance) variable by means of a heating element 2,0 controlled .by pilot channel or pilot frequency operated regulator circuit 2| to vary and e substantially equallybut inversely- This maintains e constant while varying the amplification or the gain of the amplifier, for example, to compensate for, variations" of cable tem- 1 :perature and cable length. I v I The regulator circuit may be of any suitable type. For examplait may be of the type disclosed in.E.;I. Green Patent 1,918,390,July 19',

1933, R. W. Chesnut Patent 2,154,062, April '11,

1939, R. R. Blair Patent 2,100,375, November 30, 1937, R. R- Blair Patent 2,179,915, November 14,

1939, for Gain control circuits, or R. R. Blair Patent 2,178,333, October 31, 1939, for Gain con- Zv varies, the local negative feedback on the third tube varies inthe same sense as the negative feedback around. all of "the stages tends to vary due to change. of 8, and the'amount of the change inthe local feedback is such as tocausethe change inpltoequal the inverse change in [1, so that -,u13 remains] constant as is changed. v z i Q Forsimplifyingthe drawing, inFig. 3, as in the otherfigures, thecircuits for heating the cath-, odes are omitted. -In eachofthe figures they maybe of any usual or suitable type.

.Likewise, for simplifying the drawingin Figs.

-1 and ;2, and alsoin Fig. 4 about to be described,

thecircuitsincluding direct current sources, stopping,condensers,' etc., for supplying plate currents and providing grid-biasing potentials to condition theamp-lifier for operation are omitted. In, each of the figures. they may be of any usual orsuitable type, asfor example, of the general.

.type indicated in Fig. 3.

the amplifier .gain

The invention may be embodied in various forms. For example, Fig. 4 shows a modification Jef Fig. 2 in that the impedance Z1 1+Z1 2, composed of' the networks .-Zm and Z112 in the cathode-plate circuit of tUbeLV S is in the oathode-grid circuit of tube V+Zinsteadfof tube V3.

Thuathe network ZICl-FZKB providesa local negative -feedback around the last two tubes. in the-case of Fig. 2,;with considerable local negative "feedback (which obtains inFig.-4 when ative feedback: the ,cc gain around the outer "loop will beusubstantially independent of-the ad- 'justment' of impedance Zv provided ZKl divided by Zxz'equals Z divided by 2 2, and the inser- I tion gain of theiamplifier'will be thesame as'in Fig. 1 provided Zv= (1'+K)Z'v."

In Figs-'2 and 4, the variable "impedance Zv ,Sm'JZ2Sm3ZK1 1) ,and considerable over-all neg may, if desired, be a temperaturedependent impedance, such as that of Fig. 3; and in Fig. 4 the adjustment of this impedance may, if desired, be automatic, for example, as in Fig.3.

In a feedback amplifier, the p43 finitude effect referred to above is the factor by which the amplification that would obtain if the feedback ;43 were infinite, or more generally if ,ufl

were equal to 1- l,8, must be multiplied in order toobtain the actual or exact value -ufi of the amplification. That is, the amplification with feedback, AF, is

1 1 1 T-7f t 1 which approaches the exact inverse (or negative reciprocal) of the propagation through the feedback path, asap approaches infinity. In controlling the gain of the amplifier by varying Zv in accordance with the invention, avoiding change of s (during gain adjustment) avoids varying the factor and thus renders departure of the amplification from independent of the setting of Zv. This is advantageous, for example, as facilitating equalization or control of transmission over the circuit in which the amplifier is connected by control of the transmission properties of the feedback path,

for instance as facilitating design of impedances Z91, Z 2 and Zv to compensate for line or cable attenuation changes, variable with frequency, due

to changes of line length or of temperature or other weather conditions to which the line or cable in Fig. 3 is subject. By way of further example, the feature of simultaneously changing a and e substantially equally but inversely is advantageous in enabling the over-all gain of the amplifier to be changed, by change of the transmission efficiency of the feedback path, while fied in said amplifying path, and meansfor varying the transmission efficiency of said feedback path and simultaneously substantially equally but inversely varying the transmission efficiency of said amplifying path, said means comprising a two-terminal temperature responsive impedance common to said paths. 2. An amplifier comprising an amplifying path,

a feedback path around said amplifying. path for producing negative feedback in said amplifier, a two-terminal impedance in said first path, a two-terminal impedance in said second path, and means for producing simultaneous inverse changes in propagation through said paths, said means comprising a two-terminal variable-impedance connected across a portion of each of said two first-mentioned impedances.

3. A wave amplifying system comprising an amplifying path, a feedback path from the output of said amplifying path to its input for producing negative feedback in said system, a twoterminal impedance in said amplifying path, a two-terminal impedance in said feedback path, said second impedance having one of its terminals common with one of the terminals of said first impedance, and means for simultaneously producing approximately equal but inverse changes in propagation through said-paths, said means comprising a two-terminal variable impedance connected across substantially equal percentages of said first and second impedances.

4. A wave translating system comprising an amplifying path with partly common amplifying portions one of which has smaller amplification than another, a feedback path for producing negative feedback around said amplifying path, a two terminal impedance so connected in said first path as to produce local negative feedback around said one portion, a two-terminal impedance included in said second path having one of its terminals common with one of the terminals of said first impedance, and means for simultaneously producing approximately equal but inverse changes inpropagation through said paths, said means comprising a two-terminal variable impedance connected across substantially equal percentages of said first and second impedances.

5. An amplifier comprising an amplifying path, a feedback path for producing negative feedback around said amplifying path, cascaded amplifying devices included in said first path, a twoterminal impedance so connected in said first path, as to produce local negative feedback around certain of said devices, a two-terminal impedance included in said second path having one of its terminals common with one of the terminals of said first impedance, and means for simultaneously producing approximately equal but inverse changes in propagation through said paths, said means comprising a two-terminal variable impedanceconne'cted across substantially equal percentages of said first and second impedances.

6. A multistage amplifier comprising a feedback circuit coupling the output circuit of the last stage of the amplifier and the input circuit of that stage for producing local negative feedback in that stage, a feedback circuit coupling the output circuit of that stage to the input circuit of the first stage for producing over-all negative feedback around all of the stages of the amplifier to control the amplifier gain, and a two-terminal temperature responsive impedance connected between such points of said first-mentioned feedback circuit and said second-mentioned feedback circuit that Variation of said temperature responsive impedance varies the propagation of said second feedback circuit and inversely varies the propagation of said last stage.

7. A multistage amplifier comprising an impedance connected in the output circuit of the last stage and having a portion in the input circuit of the first stage for producing'pver-all the input circuit of an intermediate stage for producing local negative feedback around a group p of stages, and a two-terminal variable impedance electrically connecting suchpoints of said portions that variations of said variable impedance varies the propagation of the' over-all feedback path of the amplifier'and simultaneously.

approximately equally but inversely varies the propagation of said group of stages.

JULIAN M. WEST.

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