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Патент USA US2131393

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Patented Sept. 27, 1938
UNITED STATES PATENT OFFICE
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2,131,393
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ELECTRIC WAVE `AMPL‘IFYING" SYSTEM
Albert L. Stillwell, Westfield, N. J., assîgnor to
Bell Telephone Laboratories, Incorporated,
New York, N. Y., a corporation yof New York
Application December- 5, 1936, Serial No. 114,392
1s claims. (o1. 179-171) ,
This invention'relates to wave amplifying'sys
Figs. 3 to 5. show specific forms of networks
tems.
`
that may be used as imped-ance elements in cir
An object of the invention is to adjust feedback . cuits such as those ofFigs. 1 and 2.
E
in an amplifier, for example for gain adjust
. he amplifier shownV may be, for example, a'.
5..;ment o-r transmission equalization, without-pro
stabilized feedback amplifier of the general type
ducing undesired variation- of the amplifier input in which a portion of the output wave is fed back
or output imped-ance.
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in gain-reducing phase and in amount sufficient
One specific aspect of the invention is an am
to reduce distortion below the distortion'level "
plifier having ‘two impedances connected alter-_Y without feedback. Such feedback is disclosed,
10 .nately between its grid and load circuits andv for example, in the copending application of HL’
also alternately between its incoming and plate S. Black, 606,871, filed April 22, 1932, for Wave
circuits. (Connecting'two impedances alternate
translation system, ` now Patent No. 2,102,671,
ly between two circuits signifies connecting one Dec~ 21, 1937,. and in the article by Black on
of the impedances, one of the circuits, the other- Stabilized feedback amplifiers, published in Elec
15 Vimpedance and the other circuit successively in trical Engineering, January 1934, pages 114 to 15y
a closed loop, the impedances thus alternating
as to their connection between the circuits.)
The amplifier- I is shown as of the Vacuum tube
The impedances may be caused to vary in
type and may have a single stage or any desired
versely, to control feedback in the amplifier, for number of the-tandem connected stages, G and
20 example to vary negative feedback for adjusting P> designating the grid of the first tube and the
120.
gain or equalizing transmission while maintain
ing constancy of the amplifier impedances faced
by the load and incoming circuits.
„
With sufficient feedback these amplifier im
25 pedances can be made to approach asymptotical
ly a constant resist-ance (matching the resistances
0f the attached load and incoming circuits, re
spectively) even when their valuesv without feed-'f
back would vary considerably with frequency, for .
30 example because of variation with frequency in
transmission efficiency of input and output trans-f
formers in the grid and plate circuits, respective
ly.
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.-
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plate of the last tube. The amplifier may have
in its input circuit »an input transformer 2, and
may have in its output circuit an output trans
former 3. Waves to be amplified by the amplifier
are supplied by incoming line ork circuit L’; ` and
the amplified waves are Atransmitted to outgoing
line or circuit L.
'25 "
The amplifier circuit has ~two «_
feedback paths Zn and Z21,'shown as two-ter.
minal transmission control networks of gener
alized impedances. The amplifying path of the
amplifier may be referred to as the ,ii-circuit, and
the feedback circuits may be referred to as the
30V
» ß-circuits, or the ,B1-circuit and the pf2-circuit,
'
In another specific aspect `the invention is an
respectively, the significance of ,ul and ß being as
indicated in the Black application and article
35 amplifier with two feedback circuits, each sym
metrical or balanced with respect to ground, -and mentioned above.
The feedback through path Zn may be, forV
each by its feedback, affecting the> gain in the ‘
example, negative feedback with liß1>>1 and the
feedback through path Z21 maybe-for example,
40 .amplifier input or output impedance .withfre-V' negative feedback with pß2>>1~ The feedbacks 40
quency and feedback through the other tendingy are applied to the amplifier through the input
same sense as the other, but feedback through ‘
one tending to produce a given variation of the
to produce a complementary or neutralizing vari
ation, so that the amplifier` input and output
impedances» are unchanged upon variation of.v
45 frequency. For example, the feedback through
one feedback circuit may be series-series nega
tive feedback and the feedback through the otherl
may be shunt-shunt negative feedback, and two
inverse reactive networks may be connected in
50 the two feedback circuits, respectively, for giv
ing a desired variation of the amplifier gain'with
frequency while maintaining the amplifier input
and output impedances constant.
Other objects and aspects of the invention willv
55 be apparent from the following‘description _and
claims.
~
Fig. 1 is a schematic diagram of an amplifier
60
and output coils or. transformers. This pro
cedure includes the vcoils within the ,iL-circuit >of
the feedback loops, thus reducing their distor
tion by feedback.
As a result their modulation
and distortionl requirements will ber less severe,
and »ingeneral they will be cheaper to build andwill be able to provide much higher gains when
desired.
v
It is seen that the path Zai is in serial relation 50->
to the line L' with respect to the amplifier in
put and is in serial relation to the line L'with
respect to the amplifier output. Thus, the feed
back through path Zzl is a series feedback at the
input side of the amplifier and aseries feedback`
at the output side of the amplifier. Therefore, ~
the feedback through path Z21 will be ~referred to
as av series-series feedback, and p-ath Zzi will be
circuit embodying a form of the invention;
Fig. 2 shows a modified form of amplifier cir
referred to as a series-series feedback patho
cuit;
circuit.
and
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21
2,131,393
It is seen that the path Z11 is in shunt rela
tion to the line L' with respect to the amplifier
input and is in shunt relation to the line L with
respect to the amplifier output. Thus, the feed
back through path- Z1: is a shunt feedback at the
input side of the amplifier and a shunt feedback
at the output side of the amplifier. Therefore,
the feedback through path Z11 will be referred
to as a shunt-shunt feedback, and path Z11 will
10 be referred to as a shunt-shunt feedback path
or circuit.
The amplifier circuit forms a bridged Tnet
work connecting lines L’ and L. The transform-_
ers 2 and 3 are two of the arms ofthe Tyand
15 they are bridged by an arrn Z11 of the bridged T-
With considerable negative feedback the am
plifier input and output impedances can be made
to remain respectively substantially equal to the
impedances of the incoming and outgoing lines,
notwithstanding inverse variation of Z11 and Zai
andl notwithstanding variation of the amplifi
cation from the amplifier input circuit to the
amplifier output circuit (due, for example, to
variation of the impedances or amplification con
stantsof the amplifier tubes as a result of chang 10
ing tubes or types of tubes, or to variation of
the transmission eiiiciency of transformers 2 and
3 withA frequency).
Z11 and Zn may be inversely variable resist
Whenever, in, theA passive condition of: the.. am
plifier circuit, each, 0f the four arms of the
ances when they are to be used for gain ad 15
justments in; which the gain changes are the
same for all frequencies in the frequency range
of interest and Z’L and Z1. are resistances.
` bridged T network hasitsimpedance equal to
20 thatÁ of each of the circuitsL and LI, (or in other
plifier gain. with frequency, for. example, for 2U.
network, Thethird arm, of: the Tis the arm; Z21.
- When'Zn and Z21 aretoserve for varying am
words,> whenever. the primary-to-secondary. im
pedance of transformer 2., the secondary-to-pri
maryimpedance of. transformer 3, the impedances
transmissionY equalization, they may be suitableA
reactive inverse impedances, for instance, as
shown-in Figs. 3«to 5 described. hereinafter. With `
Z’L and> ZL of the lines L'; and: L, and the im.
considerable negative feedback in the amplifier,
theV amplifier gain in- decibels will be
25
25 pedances Z11 and Zv21 areV all equal: and, indeed,
whenever Z11.Z21 isrequal. tothe product of. the
impedance of L’> andthe impedance of the am-..
plifier output circuit and also equal. tothe prod-.
uct of the impedance of the load circuit L and'.
30 the impedance of. the amplifier input; circuit,
then the system is in a condition which may be
called the condition vof passive ba1ance._ In. this
condition, conjugaciesexist between. the incom.
ing circuit L' or source of waves to be amplified
35: and the output transformer 31er amplifier outputl
circuit, and between the loadycircuit or receiv...
ing circuit L and theamplifier input circuit or
input transformer 2, and.y between impedance
Z21 and impedancev Z11. Thus, these conju
gacies can be maintained even ifv Z21v and Z11`
are varied, provided` they are varied inversely
(i. e., so that their product remainsy constant).
Further, with> considerable" amounts of. negative
feed-back inthe amplifier and relatively largeV
45.; value` for Z11, these- conjugacies` practically are
n maintained even when the values of the- am
The equalizer'A can effectively replace both' in
putV and. output. bridges of anÁ amplifier using
suchï bridges to maintain constant amplifier. in 305
put. and output impedances with varying feed
back, yet it is incorporated in a single bridge net
work and the line-loss (i. e., the loss in transmis
sion between the amplifier and the lines) is; cor
respondingly less.
In addition, assuming that
a variable network isused between fixed input
and output bridges, these must be designed to.
minimum feedback loss to allow maximum feed
.back This corresponds to- maximum line loss
which ispresent alsofor maximum gain.4 How
ever, with. the equalizer- in accordance with this
invention the maximum gain corresponds to
maximum feedback. loss and hence minimum line
loss. For a 30 decibel maximum` gain require
ment` the corresponding. line loss would be of 45K
the order of; 0.3.decibel; for example.
The amplifier. circuit ofFig. 1 isv unbalanced .
plifier inputïandoutput impedances without feed- with respect to ground. That is, one side can be
back vary widely from their values for the-pas
sive condition~ of passiveA balancefthe feedback groundedso that there are no longitudinal trans
50.L tending to-restore the. conjugacies and‘maintain mission, problems.
the amplifier input and output impedances Z’FA
Fig. 21 shows a. modified- form ofl the circuit
which is balanced with respect to ground. That.
and ZF (orl impedances facing: lines» L” and- L)
at the values they would have if theimpedance of is,„eachf side oft the line isabove groundby the
same impedance. In,Fig.;2.the impedance Z11 '
the amplifier input` circuit (i. e., theprimary-to
is showndivided. into two equal parts, and the 5'5i
55 secondary impedance of transformer 2) and the
impedance of the amplifier output circuit (i. e., impedance Z21 isshown connected between the
the secondary-to-primary impedanceof. trans-former 3)» were at their values for passive bal-ance of the system.
60.y
î
With considerable amounts of.`
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negative feed-A
back, if Z11'Zz1=Z’1.-Z1., the amplifier input. and
output impedancesZ’F and ZF closely- approach
twohalves of. the'primary winding. of the input.
transformer 2 and. also; connected between` theA
two halvesofthe secondary winding ofthe out.
put transformer 3. As inthe case of. Fig. 1, with
Z11~Zz1 equalV to Z’L-ZL, the amplifier input im
pedance is.
’
Z F.
respectively; and then ifi Z'L and Z1. are equaly
resistances Re, Z'F and. ZF. each. substantially
equals R0.
.
Z11-Z21
:__
V
ZL
andthe amplifier outputv impedance is
Z
-Z ’ _
¿tè/L2!
Thus, the impedances Z11 and, Z21Y may be
Caused to vary inversely, toßcontrol- feedback iny
the amplifier, for example, for adjusting gain .or
equalizing transmission4 while` maintaining con
Where the gain of the feedback amplifierf is
to-be varied by different amounts at different
stancy ofA the amplifier input and’ output im»V frequencies, for example, to compensate for`
pedances facedby the incoming circuit andthe
load.
changes; of; line attenuationzwhich». varyy with` fre
quency, this may be accomplished, for example,
2,131,393
by adjusting continuously variable reactances,
such as air condensers or variometers or both, v
output circuit and in transmitting relation to
said input circuit, and with said output circuit in
transmitting relation to said input circuit and
in inverse networks Zn and Zn, while maintain
ingtherelation Z11-Z21= (R102 0r Z11-Z21=Z'L-ZL. said load circuit.
4. A system comprising wave amplifying means
For instance, the networks Zu and Z21 of Fig. l
having an input .circuit and an output circuit,
or Fig. 2 may be composed of resistances, induct
ances and capacities, with the capacities of net
two wave transmission circuits, and means con
necting said transmission circuits in bilaterally
work Zn adjustable and the inductances of net
work Z21 adjustable, as indicated in Fig. 3. The transmitting relation to each other with said
output circuit conjugate to one .and said input
10 adjustments may be made manually; or they may
be made automatically, for example, by pilot circuit conjugate to the other.
5.- A system comprising wave amplifying means
wire or pilot channel controlled operating means
in response to effects produced by temperature having an input circuit and an output circuit,
or weather changes that produce the line at -two transmission circuits, and means connect
ing one in conjugate relation to said output cir
"
15 tenuation changes.
Figs. 4 and 5 show examples of another type cuit and connecting said input circuit in con
jugate relation to the other, said means com
of variable network that can be used for the net
works Zn and Z21, for instance, in the manner prising passive means connecting said two trans
just described for the networks of Fig. 3. This mission circuits in transmitting relation.
6. A system comprising wave amplifying means 20
20 typeisa terminated bridged T constant impedance
having an input _circuit and an output circuit,
structure, which can be used as a two terminal
network, for example, in the bridged T network a wave source and a load circuit, and a single
of Fig. l or Fig. 2. In the networks Zn and Z21 bridge circuit including said source and said cir
cuits and connecting said _output circuit in con
of Figs. 4 and 5 the inverse impedances are desig
jugate relation to said source and said input 25..
25 nated Zn’ and Zzl’. rI-‘he impedances designated
K in these networks have the value K, K being circuit in conjugate relation to said load circuit.
7. A wave amplifying system comprising a
a complex quantity, and Z11’~Z21'=K2. The am
bridge circuit composed of six branches, an am
pliñer gain adjustments, for example, for com
pensating for line attenuation changes that vary pliñer having its input and output circuits re
30 with frequency, may be made for instance, as spectively connected in two o-f said branches hav
indicated by the arrows in Figs. 4 and 5. These
figures show the networks Zn and Zar com
posed of resistances, inductances and capacities;
and in Fig. 4 the arrows indicate a capacity in
35 network Zn and an inductance in network Z21
as adjustable; and in Fig. 5 the arrows indi
cate two of the capacities of network Zn and
two of the inductances of network Z21 as ad
justable. The impedance product K can be,
40 chosen to give a prescribed or desired average
ing a common junction point, and a source o_f
waves to be ampliñed by said ampliñer and a
load circuit for said ampliñer respectively con
nected in two others 0f said branches having a
common junction point', the branches including
said input circuit and said source having a 'com
mon junction point, and the branches including .
said output circuit and said load circuit having a
common junction point.
8. A bridge circuit composed of six branches,
an ampliner having its input and output cir
slope of the amplifier gain-frequency character
istic for the normal setting of these adjustable cuits respectively connected in two of said
elements, and then the characteristic can be .branches having a common junction point, and
a source of waves to be amplified by said am
raised and lowered by adjustment of these ele
45 ments. If necessary for obtaining the desired plifier and a load circuit for said amplifier re
spectively connected in two others of said
characteristics, resistance as well as reactance
elements of the networks Zn and Zzi can be made branches having a common junction point, the
adjustable.
f
45
remaining two branches respectively consisting
of two inverse networks comprising reactances,
one of said remainingtwo branches connecting 50»
said input and output circuits and the other con
output circuits, a wave source for association
necting said two junction points.
with said input circuit, a load circuit for asso
9. A bridge circuit composed of six branches,
ciation with said output circuit, and two irn
an ampliñer having its input and output circuits
pedances connected alternately between said in
55 put and load circuits and also alternately be- ~ respectively connected in two of said branchestween said source and said output circuit, said having a common junctionk point, a source of`
What is claimed is:
50,
.
l. Wave amplifying means having input and
load circuit being in shunt relation to one of . Waves to be amplified by said amplifier and a
said impedances and in serial relation to the load circuit for said amplifier respectively con
other impedance, with respect to said output nected in two others of said branches having a
common junction. point, the remaining two
60 circuit, and said source being in shunt relation
to said one impedance and in serial relation to branches respectively consisting of two inversely
the other, with respect to said input circuit.
2. The method of operating a transmission
controlling system including an ampliiier and
65 two impedance networks connected alternately
between its input and receiving circuits and also
alternately between its output and sending cir
cuits, which comprises causing the impedances
of said networks to vary inversely.
3. A system comprising wave amplifying means
70
having an input circuit and an output circuit,
a wave source and a load circuit, and means
connecting said source and said load circuit in
bilaterally transmitting relation to each other
75 with said source in conjugate relation to said
adjustable impedances, one of said remaining
two branches connecting said input and output
circuits and the other connecting said two junc
tion points.
65
10. A bridge circuit composed of six branches,
an amplifier having its input and output circuits
respectively connected in two of said branches
having a common junction point, and a source
of waves to be amplified by said ampliñer and 70
a load circuit for said ampliñer respectively con
nected in two others of said branches'having a
common junction point, the product> of the im-Ü
pedances of the latter two branches being equal
to the product of the impedances of the remain 75
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2,131,393
ing two of said-“six branche's,„one .of ls'aid§re-'~ armsfandhaving 'said input section in fa branch`
maining two branches connecting said `inputzand connected across that arm and an adjacentarm.
output circuits and the otherA connectingsaid
15.'y The combination of a wave source, a load
two junction points`
`
circuit, and-a bridged T network for connecting
5
11. A bridge circuit composed of six branches, saidsource lfor transmission through said net
an amplifier having its‘input and output circuits work l'to said load circuit, said network compris
respectively connected'in two of saidbranchesy ing ,an amplifier and having the amplifier input
having a common junction point, and a sourcev
circuit as one arm,- the amplifier output circuit
as an adjacent arm, and two inversely variable
10~load circuit for said amplifier‘respectively con» impedances asy the remaining arms, one con 10a?
nected in two others of said branches having a nected across the amplifier input and -output cir
of waves to be amplified by said'ampliñer-anda
common junction point; one-«of 4the 4latter two
cuits, and one connected from the junction of
the amplifier input and output circuits to the
branches having its impedance equal tothe
square root of theY product of the impedances of - junction of the source and the load circuit.
- 15 ithe two remaining branches, one'. of said-:two ~
16. In a transmission system, a transmitting 15 2li.
remaining branches connecting said'input and
medium having a varying efficiency dependent
output circuits and the other connecting said two
uponv the frequency of the wave transmitted
junction points.
therethrough, an amplifier for off-setting at least
12. A bridge circuit composed of six branches,=» a portion of the transmission loss of said medium,
20- an ampliñer having its input and output circuits said-amplifier having input and output circuits,
respectively connected-in two of said branches> incoming.- and outgoing lines for connecting said
having a comme-n junction point, and;a s-curce amplifier-into said medium, and a network com
of waves to be amplifiedV by-ïsaid amplifier and# prising said .circuits and two general impedances,
a load circuit‘for said kamplifier respectively con-1I one ofzsaid circuits and one of said impedances
25 «'nected in two others of said‘branches having «a - being vvserially connected across one of said lines, 25 il;
common junction point, Veach of the latter ‘tw-o'I the other circuit being bridged across said one
branches having its Yimpedance-equal to a given circuit and the other of said general impedances
resistance, and the remaining tw-o branches hav
in serial relation', the other of said lines being
ing the product of their impedances equal- to the> v connected across the whole of said network, the
30 square of said resistance, one -of Vsaid-remain-l. network being designed to simulate the Varying 30i
ing two branches connecting said -input‘and out- Y efficiency of at least a portion of said medium and
put circuits andthe other connecting said .two l, - match the attached impedances of said lines.
junction points.
13. A wave amplifying system comprising a
35 ¿bridge circuit composed 'of six` branches, an am-`
plifier having its input and output circuits re
spectively c-onnected in two of said branches Vhav
ing a common junctionv point,` and a source of
waves to be ampliñed by saidampliñer anda
40cload circuit for said amplifier respectivelyA con
nected in two others'of said branches-having. a
common junction point„ the remaining two
branches providing feedback that- changes 4the
17. A system
comprising
wave amplifying
means having an input transformer and an out
put transformer, a wave source and a load cir
cuit,.and means connecting said source and said
load circuit. in bilaterally transmitting relation
toeach other with said source in conjugate rela
tion to said output transformer and in trans
mitting relation to said input transformer, and 40 I:
with said output transformer in transmitting
relation to said input transformer and said load
Y circuit.
18. A negative feedback amplifier having input
gain of the system for transmission- from. said .
45 ¿source to said load circuit without changing the w and output -circuits and having input and out
impedances presented to said source-:and said
load circuit,` one of ysaid remaining two branches
connecting said‘input tandoutput circuits-and
the other connecting said two junction points»V
50m; 14. A bridge network comprisinga line having
a frequency selective transmission 4characteristic 1
and having sending and receiving sections‘and an
amplifier circuit. connecting said , sections, said
put impedances, a wave source attached to said
input impedance, a load circuit attached to said
output impedance, and two inversely variable
impedances with their product approaching that
of said input and output impedances, one of said
inverse'impedances> and the load circuit being
in serial relation with respect to the amplifier
output circuit, and the ampliñer input circuit
and said one inverse impedance being in serial
relation with respect to the source, and the 55 or;
source, in parallel with the amplifier input im
pedance and said one inverse impedance in series,
pliñer circuit compensating‘for variation of the~ ' being connected across the other inverse im
line attenuation with frequency,> and said kbridge pedance and the load in series.
60*network having said output-section in one` of its
ALBERT L. STILLWEIL.
amplifier circuit comprising an amplifier.- with"
55 fïi'nput and output circuits connected for transmit
ting waves from said output circuit to said- input
circuit that produce variation of gain of said am»
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