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Janâ l0, 1950
H. G. BELJÈRS ETAL
2,494,426
AMPLIFIER WITH NEGATIVE FEEDBACK
Filed July 2, 1946
AÜENZ
Patented `Ían. 10,A 1950
2,494,426 ’
UNITED STATES PATENT oFEl-CE ~
AMPLIFIER WITH NEGATIVE FEEDBACK
Hugo Gerrit Beljers, Willem Six, and Jan te Win
kel, Eindhoven, Netherlands, assignors-t'o Hart
ford National Bank and Trust Company, Hart
ford, Conn., as trustee _ ~ ‘
Application July 2, 1546; Serial No. 680,890
In the Netherlands March 4, 1943 -.
section 1, Public Lau1 690, August s, '1946
Patent expires March 4, 1963
2 Claims. (Cl. 179-171)
This invention relates to an amplifier with l'
-negative feedback the amplification of which is ' yil, from the output terminals of the amplifier
between which> the- output voltage e3 is set up.
variable with frequency._ The desired variation
The
ampliñcation, factor of the amplifying. tube
vof amplification with frequency may be obtained
jl and the network 2 without _negative feedback,
~in known mannerv by including in the negative Cil that
is to say the relation which exists between
feedback circuit a network thetransmission fac
Vthe voltage e2 and the voltage e between the
tor ,8 of which in respect of its magnitude exhib
' grid and the cathode of the amplifying tube l
its a variability with` frequency reciprocal to that
is assumed as ,B1 and the amplification factor of
of the desired amplification. In fact, for the
the amplifier 3
amplification'p.' of an amplifier with negative
feedback there applies the formula `
. .8_3 as
63
u
>
Consequently
»1:_1‘’
1+ßß'.
4
'
in which u represents the -amplincatlon without
negative feedback whichfor ,c߻1 changes to
'
a
c*
#B51
1+ußßf‘ "
Now, according to the invention, the networks
2 and 8 are constituted in such manner that
,
.. „Lag“ 1
from which it appears that the- amplification p.
lis( reciprocal to the transmission factor ß of the 20
ßß1=1c=constant andreal, so that the above
mentioned relation changes to
ÍC
network in the negative feedback circuit. Con
.
6":.1-llL-uke‘
sequently, by constituting this network in a suit
4from which it follows lthat the phase displace
able manner it is possible to obtain any desired
vment of the negative feedback voltage ek relative
variation of amplification with frequency. How
ly to the input voltage e1 is invariable with the
25
ever, the network will also produce a phase dis
properties of the network 8. It will now always
placement variable with frequency in the nega
be possible for ßßi to be made exactly constant.
tive feedback circuit and this detrimentally af
, A very small phase displacement is already ob
fects the stability of the amplifier, since in deñ
, tained, if ßßi is maintained constant as far as
ite frequency ranges the negative feedback
possible.
changes to positive feedback with the danger in 30 In Figure 2 an impedance Z1 is substituted for
volved for self-oscillating of the amplifier.
-' the network 2 shown in Figure 1 and an imped
The present invention purports to provide steps
ance Z3 for the network 8. In series with Z3 is
whereby the detrimental iniiuence of the network u connected, in addition, an impedance Z2. Z2 and
in the negative feedback circuit upon the phase
Za are constituted by impedances Z2', and Z3', re
displacement of the negative feedback voltage 35 spectively,
which are shunted by a resistance R.
is avoided,
For the negative feedback voltage eu applies.
According to the invention, this object is
as before,
achieved by including a network in the output
circuit of an amplifying tube of the amplifier.
which network is constituted in such manner 40
that the amplification factor ßi, of the amplifying
tube and the network together substantially sat
isñes the relation ßßi=constant and real.
In order that the invention may be more clear
wherein S represents the mutual conductance
ly understood and readily carried into effect, it
of the amplifying tube and wherein it is assumed
will now be described more fully by reference to
that the internal resistance is high relatively to
Z1 and
the accompanying drawing.
Figure 1 shows a diagram of the principle of
an amplifier according to the invention. This
Zvi-Za
amplifier comprises an amplifying tube l coupled 50 this expression may read:
to output terminals 4, 5 through a network 2 and
an amplifier 3. The voltage e1 to be amplified is
AZlZ,
,=_Za_
supplied to input terminals 6, 1. The input cir
cuit has also supplied to it a negative feedback
voltage ek which is derived, through a network
“S22-Pza el
Z123
Hf‘szz-tz.
2,494,426
3
As before, between ek and e1 there exists a
phase relation independent on the network in
the negative feedback circuit„if the product [3,31
is constant. Since
cathode and having a value which is low relative
to the internal resistance of said tube, a second
amplifying stage whose input is connected across
said output impedance of saidi first-stage, and a
negative feedback network having a transmission
, factor variable with frequency, said network in
cluding a ñrst negative feedback impedance Zz
- connected across'the output of said second stage
this will be the case if
ZiZ3=constantí
Z2+Z3 :constant-.24
Z1Za=constant implies that’the impedances' Zi
and Z3 must be inverse.
_
_
_,
The second condition
Zz-l-Záf--constant
may-be
fulñlled by constituting Z2 and Za m‘themanner
and a second negative feedback impedance Z3
interposedin series between the output of said
secondrstage‘andsaid first feedback impedance,
„ .andëmeans to apply said electrical oscillations in
series with the feedback wave developed across
' said‘first‘fèedback impedance as an input to the
grid‘ 'of said‘ tube, said impedances having re
spective, values,- substantially satisfying the rela
tions.»ZiZs"-'-constant, and Z2+Za=constant~
2. An arrangement as set forth in claim 1
wherein impedance Z2 is„constituted by the paral
lel connection of an impedance having a value
Z‘z’ `and~a"'ñrst resistance-‘havinga'value Ri and
wherein ifnpedance‘Zav is constitutedïby aniim-`
»It willi not.alwaysbe` possible to. fulñllY exactly
.the-condition ZzZa, since-„Z3 comprises the parallel
pedance havinga‘value-Za" and a second'resi'stß
ance also ‘having a‘valueR, the respectivevalues
resistancefR; which' makes itLnecessary for Z1 il of f impedances Zz’v and ïZs'” substantially 'satisfying
„toc` comprise a serieseresistance;
Now, - Z1 com
the relation Z2”>Z3"‘=R2É
prises;alsostlie-capacity whichexists. between the
- .anode anditheccatho'de-eof: thealamplifying tube ‘~ I
JANÍTEWINKEL'.
and with this no resistance can be connected in
series. However, the phase displacement re
sulting from this inaccuracy is small. The con
dition Z2+Z3=constant may alternatively,Y be
satisfied by replacingîZzíby a resista-nce R' which
is high relatively lto.theifripedance Z3. In this
case the resistanceR. parallel toY Z3’ may be
REFERENCES/1 Crrnllf,i
The following referencestare of record in the
ñle of this patent:`
Number
Whatwe claim is:
2,196,844
" -'1'. An am'plifyin'gosysftem fc'nàelectricalr oscil
impedance. Z1 connected‘between said `anode v»and
Üate
FOREIGN PATENTS
lations comprising a first amplifying stage in
cluding an electronvdischarge tube provided with l
a cathode, a grid andia'n anode and an output
Name
West ___-„Y________ _.. Apr. 9, 1940
Number
Y 484,332,
CountryY
Date»
`
GreatBritaincnc-- Nov. 19, 1937
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