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' M. M. LEVY
‘
2,412,95
AMPLIFIER OF ELECTROMAGNETIC ENERGY
Filed Sept. ll,_ 1942’
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‘ A TTOPNEY '
ea, 24, 1945.
M, M‘LEVY
AMPLIFIER
2,432,9Q
ELECTROMAGNETIOENERGY
Filed.Sept. 11, 1942
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M, M, LEW _
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AMPLIFIER OF ELECTROMAGNETIC ENERGY
Filed Sept. 11, 1942
3 Sheets-Sheet ‘3
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Patented Dec. 24, 1946
2,412,995
‘ 'UNlTED STATES PATENT OFFIE
2,412,995
AMPLIFIER 0F ELECTROMAGNETIC
‘ENERGY
Maurice Moise Levy, London W. C. 2, England, as
signor to Standard Telephones and Cables Lim
ited, London, England, a British company
Application September 11, 1942, Serial No. 458,060
In Great Britain June 6, 1941
11 Claims.
(C1. 179-—171)
1
2
The present invention concerns the improve
ment of the signal-to-noise ratio of thermionic
According to a preferred embodiment of the in_
vention an ampli?er is provided with two separate
feedback paths, the ?rst of which produces a
?xed negative feedback independent of frequen
ampli?ers, particularly those used for amplify
ing impulses. It is applicable to obstacle detect
ing systems in which short trains of high fre—
quency electromagnetic waves re?ected from ob
‘ cy, and the second produces a feedback the am
plitude of which is preferably constant, but the
stacles have to be received and detected and the
phase of which varies with frequency, this phase
resulting impulses ampli?ed.
being preferably opposite to that of the negative
feedback for the fundamental frequency, and for
.
It is a common experience that little advan
tage is gained by amplifying signals of very low
_ all its harmonics, or alternatively for all the odd
power level on account of the noise produced in
the ampli?er itself, or in the preceding detecting
equipment, or otherwise, when this noise is of the
same order of level or higher than that of the
signal. It is also well-known that the noise pro
harmonics.
In another embodiment the ampli?er is pro
vided with a single feedback path including a
Wheatstone bridge of impedances, one of which
includes the input circuit of a delay network. In
this case, a feedback, variable both in amplitude
duced may be reduced if the receiver is provided
with means to eliminate frequencies not required
and phase, is produced and this, feedback should
for reproducing the signal. A common arrange
preferably be zero for the fundamental frequency,
ment is for the receiver to be made selective by
and for all its harmonics, or for the odd ones only.
tuning it more or less sharply to the carrier fre 20
These objects and features will be more clearly
quency of the trains of waves being received.
understood by a reference to the following de
The more sharply it is tuned the lower will be the
tailed description and the accompanying draw
noise, but the more will the detected impulses
ings, in which:
be distorted, until ultimately the distortion may
Fig. 1 shows some periodically repeated trains
become so great that the amplitude of the im 25 of waves;
pulses will be reduced.
Fig. 2 shows the frequency spectrum for such
The object of the present invention, therefore,
periodically repeated trains of waves;
is to improve the signal-to-noise ratio of an am
Fig. 3 shows part of the gain frequency char-9
plifying system without at the same time caus
acteristic of an ampli?er in accordance with the
ing appreciable distortion of the signals being
invention;
transmitted.
those frequencies necessary for de?ning the out
Figs. 4 and 5 show block schematics of ampli
?ers with feedback;
Fig. 5A shows a modi?cation of a detail of
Fig. ;
line of the signal (of of narrow bands of frequen
cies in the immediate neighbourhood of these
back voltages;
Another object of the invention is to provide
a selective ampli?er in which currents of only
Figs. 6, 7 and 8 are vector diagrams of the feed
frequencies), are ampli?ed, other currents being
substantially not ampli?ed. In particular the
? Fig. 9 shows the schematic circuit of an ampli
er;
currents to be ampli?ed may be some fundamen
Fig. 10 shows the schematic circuit of an arti
tal frequency and any desired number of the 40 ?cial line;
harmonics thereof.
Fig. 11 shows another vector diagram of feed
According to the principal feature of the in
back voltages; and
vention, the desired type of selective ampli?ca
Figs. 12, 13 and 14 show circuits for compensat
tion is produced by means of a delay network,
ing the effect of the attenuation of a delay net
which may be associated with a thermionic am
pli?er, and the variations of the change of phase
with frequency in the network are employed to
obtain the required properties. The network may
be connected in tandem with the ampli?er, or
45
work.
,
'
In the valve circuits of Figs. 9, 13 and 14 certain
well understood coupling and operating arrange
ments' are indicated for completeness, but these
arrangements are unessential as regards the in
may form part of a feedback path therein. In 50 vention, and may be modi?ed to suit particular
most cases the delay network may have appre
cases. Thus, grid and plate batteries are indi
ciable attenuation without destroying the selec
tivity, and accordingly can be constructed in a
simple and convenient form; for example, it may
consist of an arti?cial line.
cated by the usual symbols without implying that
these supplies must necessarily be provided in
this manner.
Rp represents a plate circuit re
sistance of suitable value and g a grid resistance
2,412,995
3
4
of high value, the shunting effect of which is
fundamental and all the harmonics. It will be
seen to consist of a- number of very sharp peaks
negligible.
Similarly K represents a large cou
pling condenser of negligible impedance. Fur
ther, although for simplicity the valves have been
occurring at frequencies f, 2f, 3]‘, etc. These peaks
shown as triodes, this is also not necessary, and
maximum amount of noise may be eliminated.
should be as narrow as possible in order that the
valves with any number of electrodes could be‘
’ This-kinder selective or “comb” ‘ampli?cation
Fig. 1 shows the form of a number of repeated
maybe produced by making use of the property
of suitably designed delay networks by which
used, with appropriatearrangements; f
the phase change obtained by propagation there
quently used in obstacle detection. Such trains 10 through may be made to increase progressively
trains of high frequency waves of the type fre- ' '
of waves are reflected from the obstacle and have
to be received and detected with a suitable re
with frequency, and so to pass through values
which are multiples of 1r. This property may
ceiver. It will be assumed that the frequency
of the waves is F and the frequency of repetition
be employed
of the trains of waves is f.
- Y '
'
either by direct transmission
through the network, or by making use of re
15 ?ections at the distant end, the impedance of
In Fig. 2 is shown the frequency spectrum of
such a system of repeated trains of waves and
it consists of a central frequency F accompanied
on either side by a large number of components
whose frequencies may be designated by the for 20
which is mismatched, usually by open or short
circuit. Without special means, however, good
selectivity cannot be obtained because of the
attenuation which accompanies the phase change
in any practical form of delay network due
principally to the resistance of the inductance
increases so the amplitude of the corresponding
coils, which cannot. generally be reduced su?'i
ciently even by the use of extremely bulky coils.
component tends on the whole to decrease and
In accordance with certain features of the inven
components may be eliminated without appre
ciably affecting the form of the received signal 25 tion, however, it becomes possible to use delay
networks with high attenuation without any loss
provided that n is not too small.
of selectivity, and they may therefore be given
The usual method of receiving such a train of
convenient forms; for example arti?cial‘ lines
impulses is to amplify them with a selective high
may be used. .
frequency receiver tuned to the frequency F, then
In order to explain the reason for the low
to detect them by means of a low frequency de 30
mula Fi-nf. where n can be ‘any integer. iAs’ n
tector.
A certain amount of noise will be pro
duced in the receiver and the detector, and also
from atmospherics which will be picked up to
gether with the impulses. If the impulses are at
selectivity ordinarily produced by attenuation
in the delay network, an example will be given.
One way in which the desired properties may
theoretically be produced in any ampli?er is to
a very low power level the noise may be of the 35 connect in the plate circuit of one of the valves,
in parallel with the load, the input circuit of a
same or even higher level, and in order to mini
delay network having its output terminals short
mize the effect of this noise it has been the prac
circuited, or left unconnected, whereby re?ections
tice to tune the receiver very sharply to the fre
will be obtained at the output terminals- As is
quency F. This has the effect of cutting off the
components shown in Fig. 2 on either side of the 40 well known, the input impedance of the delay
network can then be designed to pass through
frequency F. a This may be done so long as the
an in?nite value for some frequency f and for
band of frequencies passed by the receiver is not
all its harmonics, and through a zero value for
made too narrow; for example, there will be some
certain intermediate frequencies, provided the
value no of it below which any further elimina
network is made up of pure reactances, or in
tion of components will produce an undesirable
other words, has no attenuation. As already ex
distortion of the detected impulses. This is indi
plained this can never be achieved in practice
cated in Fig. 2 by the dotted lines at F+nuj and
and accordingly the impedance never becomes
F-nof. Accordingly, if the noise is still excessive
even approximately zero or in?nite, and the selec
no further advantage can be gained by reducing
tivity is accordingly bad. This will be better
the band width because then the impulses re
appreciated from the following numerical ex
ceived will become so distorted-that their ampli
tude is reduced.
ample.
>
'
Suppose that a delay network is required to
produce a delay of 200 microseconds, for use
r up to a frequency of, say, 1 megacycle. It could,
for example, be made up of about 1000 sections
of a simple low pass ?lter. If this be constructed
of elements of reasonable dimensions, its atten
uation could easily be 30 to 50 decibels. The
designed to amplify all the frequencies Fin)‘ 60 selectivity would in this case be almost inap
preciable; the diiference between the maximum
where n is zero or any integer up'to and including
The method characteristic of the present inven
tion is to include in the receiver a selective ampli
fier so designed that it substantially only ampli
?es appreciably currents of frequencies actually
contained in the impulses. This ampli?er may
be located in the high-frequency part of there
ceiver before detection, in which case it will be
no. Alternatively, the ampli?er may be connected
and minimum impedance would in the worst
in a position in the circuit subsequent to the de
tector, when it will be designed to amplify all the
frequencies of where n has all the same values
except'zero. Thus the frequencies not concerned
case be only 0.33% and in the best case only 3%.
with de?ning the impulses will ‘be largely elimi
nated and with them all the corresponding noise;
manner indicated in Fig. 12.
f
A 'delay network ‘D (which may, for example
be in the form of an arti?cial line) is provided
so that the only noise which remains is that asso
ciated with frequencies immediately adjacent to
those which are selectively‘ampli?ed.
A large
improvement in signal-to-noise ratio will thus be
obtained.
,
'
1
‘
-
'
‘ InFig. 3 is shown thegain frequency character
,
- According to one feature of the invention, the
effect of the attenuation of- the netw'or‘kin re
ducing the selectivity may be eliminated“ in the
'with an input transformer ‘I’! having a tapping
point t on the secondary winding. The output
of the network D is terminated by an impedance
Z equal to the image impedance thereat, and an
output transformer T2, the primary winding of
istic of such an amplifier, which ampli?es the 75 which is connected in parallel with Z. The
'
2,412,995
secondary winding of T2 has the lower end con
nected to the tapping point t, the upper end being
connected to one output terminal 3, the other 4,
od of compensating the attenuation of the net
work. Many other arrangements conforming to
the principles explained in connection with Fig.
12 are obviously possible.
According to another feature of the invention
still better selectivity may be obtained with a de
being connected to ground. T! and T2 are sup
posed for simplicity to be ideal unity ratio trans
formers, though this limitation is not essential.
Let V1 be the potential applied to the input ter
lay network having attenuation by associating
minals l and 2 at some given frequency, and
it with a feedback path in the ampli?er.
let V2 be the corresponding potential developed
An embodiment employing this arrangement is
across the impedance Z. Then
Ill shown schematically in Fig. 4. An ampli?er A
has a pair of input terminals I and'a pair of out
put terminals 2. It is also provided with two sep
where 5 and a are respectively the attenuation
arate paths 3 and 4, the ?rst of which produces
and the phase change for transmission through
negative feedback N which is constant as the fre
the network, assuming, for simplicity, that it is 15 quency varies and the second produces a feed
symmetrical. Since the secondary winding of the
back which is constant in amplitude but varies
transformer T2 is connected back to the tapping
continuously in phase with frequency, and may
t on transformer Ti, the difference of poten
contain a delay network D.
tial V between the terminals 3 and 4 will be
Suppose that Eis the input voltage applied to
A1.V1-I_-V2, where A1 is the fraction of the
the grid by the signal. Let G be the ratio which
applied potential V1 tapped 01f at t. The choice
de?nes the gain of the ampli?er in the absence
of sign will depend upon the poling of the con
of
feedback, then the output voltage will be GE
nections of the secondary winding of T2. Thus
in the absence of feedback. Let the constant
negative feedback be
25
For the best results ,8 should be independent of
frequency, and on should be proportional to fre
quency. If A1 be made equal to e-", then, taking
the positive sign, V will assume a zero value
whenever a is an odd multiple of 1r and a maxi
mum value 2e-".V1 whenever a is an even mul
tiple of 1r; Or taking the negative sign the zero
and maximum values will be interchanged.
If therefore the arrangement of Fig. 12 be in
,
GEFM
‘and let the variable feedback be
30
It can be shown that the eifective gain of the
ampli?er with both the feedbacks operating will
be de?ned by the ratio
.
'
corporated in an ampli?er so that, for example,
the terminals 3 and 4 are connected in the grid
circuit of a valve, currents applied to the in
put terminals l and 2 will be selectively ampli
?ed and it can be arranged so that, with the 40 If H is chosen equal to F, then the denominator
positive sign, for instance, the values of on which
of this expression will be a minimum equal to 1
are even multiples of 11' occur for the fundamental
whenever the angle 95 is a multiple of 271', and ac
frequency and all its harmonics.
~
cordingly the gain of the ampli?er will then be a
If ,8 should not be independent of frequency,
maximum de?ned by the ratio G. If GF and GH
a suitable correcting network may be connected 45 are chosen to be large compared with 1, then a
in series with D provided that it is designed so
very small change in the angle ¢ due to a small
that the phase change is substantially independ
frequency change will make a large increase in
ent of frequency, or an ampli?er with appro
the denominator of the above expression; in
priate gain characteristic could also be used.
other words, the gain of the ampli?er will be‘ re
'Another arrangement is shown in Fig. 13 in 50 duced by a large amount, and the selectivity will
which the input transformer is removed and re
placed by a tapped potentiometer, and the output
transformer 'is replaced by the grid-cathode
be good. This can be seen more clearly with ref
erence to Figs. 6, '7 and 8.
Considering ?rst Fig. 6, 0A is a vector repre
circuit of a thermionic valve. The operation will
senting the feedback GF and this vector is drawn
be practically as described with reference to 55 in the negative direction since GF is negative
Fig. 12, but taking the negative sign in the eX
with respect to the input voltage E. The vector
pression for V.
AB represents the variable feedback GH and
Fig. 14 shows a modi?cation of Fig. 13 employ
makes an angle ¢ with the positive direction A0.
ing a pair of similar valves. The plates are con
The resultant of these two vectors is the vector
nected together and are fed through a common
resistance Rp from the plate battery, and the
control grid of one valve is connected to the tap t
on the potentiometer, the control grid of the
other being connected to one of the output ter
minals of the delay network D. In this case, the
positive sign will be taken in the expression for
V and the addition of the two terms will occur
in the common resistance Rp.
In Figs. 13 and 14, the valves may be considered
as part of the selective ampli?er itself and may
be followed by such other amplifying stages as
may be found convenient.
It will be understood that the arrangements
shown in Figs. 12, 13 and 14 are only three ex
amples of the application of this particular meth
OB which is equal to J and at an angle 0 with
A0. As has already been mentioned, the vector
GH has been assumed to be constant, for sim
plicity, and accordingly as the frequency varies
the point B will describe a circle of which A is the
The maximum and minimum values of
the vector J are given by OX and CY. The maxi
mum gain of the ampli?er will occur at the point
Y and the minimum gain at the point X, but un
less OY is very small compared with OX the
variation in the gain of the ampli?er will be
small. Accordingly, the arrangement of Fig. 7 is
preferable in which the vectors OA and AB have
65 centre.
been made equal, in other words, making F equal
to H. In this case the vector OY becomes zero
~75 and the selectivity is accordingly high if OX is
2,412,995
8
7
large compared with 1.
As the frequency is
varied continuously in the same direction the
point B rotates continuously around the circle
and the corresponding gain produced will have
the form of Fig. 3.
Fig. 8 shows another arrangement in which the
vector GF is less than the vector GI-I. In this
case thevector OB is never zero and can have a
positive component at certain frequencies. Thus
it will be seen that in Fig. 6 the feedback is never
zero, but always. has a negative component; in
Fig. 7 it is zero at each of the harmonic fre
quencies and has a negative component at all
other frequencies; and in Fig. 8 the feedback
sometimes has a positive and sometimes a nega
tive component.
.
Fig. 9 shows an example of a three stage ampli
her in which the principles just described are ap
This is accordingly another means whereby an ar
ti?cial line with high attenuation may be used as
a delay network without any reduction in the se
lectivity of the ampli?er.
In Fig. 5 is shown a different arrangement for
producing a variable feedback in the ampli?er.
In this case the ampli?er A has a pair of input
terminals I to which a signal voltage E is applied
and a pair of output terminals 2 where an ampli
?ed signal voltage EG appears in'the absence of
feedback. In this case there is only one feedback
path which involves a, Wheatstone bridge KLMN;
the diagonal points KM are connected to the out
put terminals 2 and the diagonal points LN pro
vide the desired feedback. The arms KL, LM and
MN are composed of impedances Z3, Z4 and Z2,
respectively and the arm KN is composed of an
impedance Z! in series with the input terminals
of the delay network D, the output terminals of
a transformer Ti and the constant negative feed 20. which are left unconnected for example. This
delay network should preferably have substan
back is obtained by connecting the cathode of the
tially no attenuation. The impedances Zl to Z4
?rst Valve to a resistance BC in the cathode cir
will preferably be pure resistances and in the fol
cuit of thethird valve. The variable feedback is
lowing explanation this will be assumed.
produced by connecting a delay network or arti
Referring to Fig. 11, the vector KM represents
?cial line L to the platev circuit of the third valve, 25
the voltage applied to the diagonal points K, M of
through a transformer T2, for example. The out
the Wheatstone bridge. It is composed of two
put of the arti?cial line is terminated by a poten
vectors KL and LM in the same line which reptiometer P, the moving contact of which is con
resent respectively, the voltage drops across Z3
nected to the grid of ‘the ?rst valve through the
and Z6. The vectors KQ and QP represent, re
secondary winding of the input transformer Ti.
spectively, the voltage drops across Zl and Z2,
The arti?cial line L should preferably have an
which since ZI and Z2 are preferably both‘pure'
attenuation substantially constant over the fre
resistances, will be in the same direction. The
quency range concerned and the phase change
vector PM represents the voltage drop across-the
should also preferably be substantially propor
tional to the frequency. However, if the arti- '7 delay network D which will be at right angles
to the vector KQP because the network will have
?cial line L has a variable attenuation, it would
an impedance which is substantially a pure re
be possible to compensate this by means of a
act'ance at all frequencies, the output being open
correcting network or with an ampli?er having a
circuited or short-circuited. The point P will
suitable gain characteristic. The electrical length
of the artificial line L should'also preferably be 40 thus describe a circle with KM as the diameter,
as the frequency changes. From the point Q
chosen so that the phase change for the funda
is drawn a vector QN parallel and equal to PM
mental frequency which has to be ampli?ed is a
plied. The signal is applied to the grid through
multiple of 211-.
Thus for every harmonic of the
so that NM will be equal to the drop across Z2.
ance RC in Fig. 9 will be represented by the vec
LM being equal to the drop across vZl'l, it follows
that the resultant of LM and MN, namely, LN,
must be the difference of potential between L
and N, and accordingly must be the feedback
tor 0A, and the feedback GI-I produced by the ar
voltage.
fundamental frequency, it will also have a phase u
change which is a multiple of 211'.
Referring to ' ‘
Fig. 7, the feedback GF produced by the resist
It can be seen that as P moves round
the circle, QN will cut the vector LM in a ?xed
H can be adjusted to be equal to F by adjusting "50 point 0 and the point N will describe another
circle whose diameter is OM. Thus the vector
the potentiometer P, for example, or by other
LN will have the same properties as the vector J
means. The sum of the feedbacks produced by
in Fig. 6. By adjusting the relative values of
resistance RC and by the arti?cial line L is repre
Z! and Z2 or Z3 and Z4 the point 0 may be moved
sented by the vector OB in Fig. 7. The output
along the vector LM, and, for example, can be
may be taken from a third winding of the trans- "
made to coincide with L. In this case a result
former T2 as indicated, or in several other ways
equivalent to Fig. '7 is produced and this will
which may be more convenient in certain cases.
be the preferred arrangement.
It will be seen by this arrangement the arti
As already explained-however, the desired re
?cial line can have relatively large attenuation
sult can only approximately be attained in prac
and accordingly it could be constructed with ele
tice because of the attenuation of the delay net
ments of reasonable dimensions. For example,
work D which cannot be reduced to zero. vAc
suppose the ampli?er has a gain of 60 decibels in
cordingly, a variation of the method may be
the absence of feedback, and suppose that the at
adopted which is shown in Fig. 5A, In this case
tenuation of the arti?cial line is 20 decibels, the
value of the feedback GH can be such as to cor 65 the diagonal of the Wheatstone bridge KN is com
posed of an impedance Zi, connected in parallel
respond to 40 decibels. The difference between
with which are the input terminals of the delay
the maximum and the minimum ampli?cation
network D which in this case is supposed to have
will be about 46 decibels, because the maximum
attenuation, and may thus be in the form of an
negative reaction OX in Fig. 7 is equal to G(F+H)
arti?cial line. It is desirable that the impedances
and since these two vectors have been chosen to
Zl, Z2, Z3 and Z4 should be chosen so that the
be equal, the vector OX will be 6 decibels greater
delay network may bev terminated in its char
than GH.
acteristic impedance in order to avoid undesir
It will be clear from the explanation just given
able re?ections at the input terminals. The ex,
that the attenuation of the arti?cial line L is com
pensated by part of the gain of the amplifier. 75 planation of the operation of the circuit in terms
ti?cial line L will be represented by the vector AB. I
2,412; 995
9
10
of‘ a vector diagram is'in'this case rather com‘
plicated and can be more easily understood in
said delay network, and circuit means for alge
braically-‘adding the output voltage of said delay
the following way. Assume ?rst of all that the
delay network is in?nite in length. The im
pedances in the bridge can be adjusted so that
network to a proportionfof the input voltage ap
plied to its input terminals, in which the said
circuit means comprises a- potentiometer, having
a fraction of the output voltage is transmitted as
a feedback to the input of the ampli?er and so
an intermediate tapping point, the said potenti
ometer being connected across the input termi
nals of ‘the said delay network, and said therm
that the feedback is negative. This feedback
then is equivalent to the feedback GF in Fig. '7.
ionic valve ampli?er comprises a thermionic
Now assume that the network has a ?nite length, 10 valve, the control grid of which is connected to
re?ection will take place at the output terminals
an output terminal of the said delay network,
and the re?ected current will be transmitted back
and the-cathode of ‘which is connected to the said
to the input and a part will be transmitted to
intermediate tapping point.
the input of the ampli?er. This re?ected current
3. A selective thermionic ampli?er circuit com
then plays the part of the vector GH in Fig. '7.
prising a'thermionic Valve ampli?er, a delay net
By adjusting the impedances in the bridge the
work having attenuation and terminated at its
effects corresponding to Figs. 6 and 3 can also
output terminals with an impedance substantially
be-produced.
equal to its image impedance thereat, means for
Experience shows that a circuit combining ‘the
coupling said output terminals to an input circuit
principles of Figs. 5 and 5A gives good results 20 of said‘ ampli?er, means for applying said peri
in practice if the values of the impedances of
odically repeated impulses to input terminals of
the bridge are correctly chosen.
said delay network, and circuit means'for alge
The delay network 01' arti?cial line used for
braically adding the output voltage of said delay
these circuits should preferably have a constant
network to a proportion of the input voltage ap
attenuation and a phase change which varies in
plied to its input terminals, in which the said
proportion to the frequency, as already stated
circuit means comprises a potentiometer having
above. One form of the delay network D may be
an intermediate tapping point, the said potenti
similar to that of an arti?cial line such as is
ometer being connected across the input termi
shown in Fig. 10 consisting of a number of sec
nals of the said delay network, and said therm
tions having series elements of inductance and 30 ionic valve ampli?er comprises two thermionic
resistance and shunt elements of capacity and
valves, the plates of which are fed from the plate
resistance, and having mutual inductance be
supply through a common resistance, the con
tween adjacent sections, as indicated. Various
trol grid of one of the said valves being con
other forms are, however, also possible and Fig.
nected to the said tapping point and the control
10 has been given just as an example.
grid of the other valve being connected to an out
The arti?cial line is only one example of a
put terminal of the said delay network.
delay network that can be used in these circuits.
4. A selective thermionic ampli?er circuit for
Other types of network will occur to those skilled
amplifying periodically repeated electrical im
in the art, as also will other arrangements in ac
pulses comprising a thermionic ampli?er having
cordance with the principles of the invention. 40 input and output circuits, a ?rst feed-back circuit
The arrangements shown in Figs. 4, 5, 9, 12, 13
extending between said output and input circuits
and 14 have been given by way of illustration,
including a delay network arranged to vary the
and the invention is not intended to be limited
phase of the feedback voltage dependent upon
thereto.
What is claimed is:
~
frequency and a second feedback circuit extend
4
1. A selective thermionic ampli?er circuit com
prising a thermionic valve ampli?er, a delay net
arranged to produce a constant negative feed
back voltage independent of frequency.
work having attenuation and terminated at its
output terminals with an impedance substan
tially equal to its image impedance thereat, means
for Coupling said output terminals to an input
circuit of said ampli?er, means for applying said
periodically repeated impulses to input terminals
of said delay network, and circuit means for alge
braically adding the output voltage of said delay
network to a proportion of the input voltage ap
5. A selective thermionic ampli?er circuit ac
cording to claim 4 in which the said delay net
work has an attenuation substantially independ
ent of frequency and in which the amplitude of
the feedback voltage produced thereby is equal
to the said constant negative feedback voltage
produced in , the said second feedback path,
55 whereby the effect of the said attenuation tend
ing to reduce the selectivity of the said ampli?er
plied to its input terminals, in which the said
may be Substantially eliminated.
circuit means comprises a‘?rst transformer hav
,
6. A selective thermionic ampli?er circuit com
ing a secondary winding connected to the input
terminals of the said delay network, said sec
ondary winding being provided with an inter
ing between said output and input circuits and
prising a thermionic ampli?er having input and
60 output circuits, a feedback circuit extending be
tween said output and input circuits and phase
changing means in said feedback circuit arranged
to produce an overall ampli?er gain which is a
maximum for the fundamental frequency of said
impulses and for a plurality of harmonics of said
mediate tapping point, and a second transformer,
the primary winding of which is connected to the
output terminals of the said delay network, one
terminal of the secondary winding of the said
second transformer being connected to the said
fundamental frequency, in which said feedback
2. A selective thermionic ampli?er circuit com
prising a thermionic valve ampli?er, a delay net
work having attenuation and terminated at its
circuit includes a Wheatstone bridge, three arms
of which consist of impedances and the fourth
arm consists of a fourth impedance and the input
circuit of a delay network, opposite diagonals of
intermediate tapping point.
output terminals with. an impedance substantially
equal to its image impedance thereat, means for
coupling said output terminals to an input circuit
of said ampli?er, means for applying said peri
odically repeated impulses to input terminals of 75
said bridge being connected respectively to said
input and output circuits.
7. A selective thermionic ampli?er circuit ac
cording to claim 4 in which the said delay net
work comprises an arti?cial line.
' 2,412,995
11
v
12
.
8. A selective thermionic ampli?er circuit‘ for
amplifying periodically repeated electrical__'im
pulses comprising a thermionic valve ampli?er, a
delay network having attenuation and terminated
at its output terminals with an impedance sub
stantially equal to its image impedancethereat,
means for coupling said output terminals to an
arm consists of a fourth impedance in series with
the input of a delay network.
.
~
>
10. A selective thermionic ampli?er circuit
comprising a thermionic ampli?er having input
and output circuits, a feedback circuit extending
between said output and input circuits and phase
changing means in said feedback circuit arranged
to produce an overall ampli?er gain which is a
input circuit of said ampli?er, means for applying
maximum for the fundamental frequency of said
said periodically repeated impulses to input ter
minals of said delay network, and circuit means 10 impulses and for a plurality of harmonics of said
fundamental frequency, wherein said feedback
for algebraically adding the output voltage of
circuit includes a Wheatstone bridge, three arms
said delay network to a proportion of the input
of which consist of impedances and the fourth
voltage applied to its input terminals.
arm consists of a fourth impedance in parallel
9. A selective thermionic ampli?er circuit com-.
prising a thermionic ampli?er having input and 15 with the input of 'a delay network.
11. A selective thermionic ampli?er circuit ac
output circuits, a feedback circuitextending be
cording to claim 4 wherein said, delay network
tween said output and input circuits and phase
comprises an arti?cial line, means for coupling
changing means in said feedback circuit arranged
the input terminals of said line to the output cir
to produce an overall ampli?er gain which is a
maximum for the fundamental frequency of said 20 cuit of said ampli?er and means for coupling the
output terminals of said line to the input circuit
impulses and for a plurality of harmonics of said
of said ampli?er.
.
fundamental frequency, wherein said feedback
V
circuit includes a Wheatstone bridge, three arms
of which consist of impedances, and the fourth
~
MAURICE MOISE LEVY.
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