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Патент USA US2118122

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May 24, 1938.
Filed Nov. ‘17, 1935
6 ‘Eli 4
(9X (‘K/MC
Patented May 24, 1938
Balthasar van der P01 and Klaas Posthumus,
Eindhoven, Netherlands, assignors to Radio
Corporation of America, a corporation of Dela
Application November 17, 1933, Serial No. 698,486
In the Netherlands January 26, 1933
1 Claim.
(Cl. 179—171)
valves having one or more grids, and particu
larly to the manner in which the various circuits
Fig. 2 is a diagram of the impedances illus
trated in Fig. 1 ;
Figs. 3 and 4 show circuit diagrams for con
necting a triode in accordance with the inven
associated with the amplifying valve such, for
tion; and
example, as the control grid and anode circuits
Fig. 5 is a circuit diagram of another modi?ca
tion of the invention and Fig. 6 shows a circuit
This invention relates to ampli?cation circuit
arrangements comprising thermionic amplifying
of a triode or the control grid and screen grid
circuits of a screen grid valve are connected to
the cathode of the amplifying valve.
A well known phenomenon which appears with
triodes and with screen grid valves having a
great steepness of the characteristic, particularly
when the anode circuit of the triode or the screen
grid circuit of the screen grid valve includes an
15 impedance which is low for high frequency cur
rents, consists in that there is a certain conduc
tivity between the control grid and the cathode
which is dependent upon the negative grid volt
age and which in the case of the control grid
20 and the cathode having an oscillatory circuit in
cluded between them, brings about a supplemen
tary damping in the circuit. This conductivity
obviously has the same effect as a resistance in
parallel with the input oscillatory circuit and
would cause a damping thereof.
ly with this conductivity a grid-cathode capac
ity which is also dependent upon the negative
grid voltage and which is to be imputed essen
tially to variable space charge phenomena, oc
curs between the control grid and the cathode.
The invention has for its object partly or
wholly to neutralize or to balance the said phe
ncmena, via, the decrease in control grid-cath
ode resistance and the control grid-cathode ca
pacity that is dependent upon the negative bias.
According to the invention, this object is ob
tained by so proportioning the impedance of the
part of the anode-cathode connection in the case
diagram of a screen grid ampli?er embodying the 1
Fig. 1 shows by Way of example, an indirectly
heated triode comprising an electrically heated
?lament F, a cathode K, a grid G and an anode
A. These electrodes are connected by means of
connecting wires to contact pins I, 2, 3, 4 and 5 I
?xed in the base. The said connecting leads in
clude impedances designated by 29. and Zr. In
addition, the internal tube capacity Cat, Cgf and
Cag are indicated in this ?gure. These imped
ances (for example the inductances of the con
necting wires) and the capacities which are ale 20
ways present bring about a mutual coupling be
tween the anode and the control grid circuit.
Owing to this mutual coupling a certain apparent
admittance is set up between the terminals 3 and
4. The impedance Zr, which may be the self
inductance of the lead in wire to the cathode is
also in the anode circuit and the anode current
being in phase with the grid voltage, causes a
voltage drop over the self-inductance of the cath
ode .lead which lags behind the applied input 0
grid voltage by an angle of substantially 90 de
grees. Between the input terminals of the valve,
there is connected the series circuit of the cath
ode-grid capacity and the self-inductance of the
cathode lead. As the cathode-grid capacity is v35
relatively small, this series circuit has a sub
stantially capacitive character. The voltage drop
over the cathode lead will therefore cause a cur
of a triode or the part of the screen grid-cathode
connection in the case of a tetrode which is in
common to the control grid-cathode connection
rent to flow through this series circuit which is
in phase with the input voltage, so that the ef
fect produced on the input circuit is that of a
that the conductivity between the control grid
and the cathode and/or the control grid-cathode
shunt resistance, i. e. of an additional damping.
In order to establish this it is supposed that in
_ capacity that is dependent upon space charge
4" phenomena are neutralized partly or wholly
whereby the detuning of the resonant input cir
regard to high frequency currents the terminals
cuit by these factors is substantially eliminated.
age having an angular velocity 01 is supplied be
tween the terminals 3 and 4. From the replace
ment diagram shown in Figure 2 of the con—
In order that the invention may be clearly un
derstood and readily carried into eifect some
50 embodiments thereof will now be described more
fully with reference to the accompanying draw
ing in which:
Fig. 1 illustrates diagrammatically a triode and
the various impedances associated with its ele—
55 ments when it is connected in a circuit;
3 and 5 are perfectly short-circuited with each
other and that a high frequency alternating volt
nections of Figure l the apparent admittance
present between the terminals 3 and 4, can be
computed. Subject to the condition that the 55
impedances Z5, and Zr are small relatively. to the
impedance of the internal electrode capacities of
V the tube it is found that
becomes very small or even negative.
It is not
necessary that the two connecting wires l and 2
should be connected to the same point of the
cathode but as an alternative they may be con
nected to different points of the cathode.
The method represented in Figure 3 has a limi
wherein S, or the mutual conductance, equals the ' tation in the fact that in connection with the
second connecting wire to the cathode the base
slope of the curve showing the total electron cur
rent ?owing through the control grid as a func
10 tion of the control grid potential.
If particularly Za and Zr are inductances La.
'and Li it is found that the apparent admittance
between the terminals 3 and 4 is represented by
from which it is clear that this admittance be
' haves as if an ohmic resistance dependent upon
20' the frequency is interconnected between the ter
minals 3 and 4.,
From" this latter equation it is apparent that
the resistance between the terminals 3 and 4 is
composed of a positive and a negative part. If
the former partis larger than the latter, Rg
25 will be a positive magnitude so that in the case
of a tuned circuit being connected between the
terminals 3 andA, as shown in Fig. 1, said cir
cuit is clamped by the resistance Rg, which de
notes the apparent shunt resistance across the
30 input circuit. In addition it is apparent from
the equation that the greater the steepness S
of the amplifying valve characteristic and the
frequency of the alternating voltage impressed
on'the grid, the larger is the said damping. For
35 this reason it was hitherto impossible to use am
has to be provided with a supplementary con- '
nection or pin. This disadvantage can be obvi 10
ated by using one of the current leads for the
?lament that heats the cathode K, partly as a
connecting wire to the cathode.
This circuit
arrangement is illustrated in Figure 4 in which
the indirectly heated cathode K is taken out of 15
the envelope of the valve not only by a conductor
l but also by way of a conductor 2 provided be
tween a point of the cathode and the heater and
of one of the cathode supply leads 3 and 4. If
the heater is supplied with alternating current 20
it is desirable that the grid circuit should be
connected to the conductor I, the anode circuit
to one of the cathode supply leads 3 and 4 in
order that the disturbances by the ?uctuating
voltage of the ?lament may be avoided as far 25
as possible.
If the common connecting conductor to the
cathode includes in addition to an inductance
111‘ an ohmic resistance Rf it is found that the
admittance between the terminals 3 and 4 of 30
Fig. 1 can be designated approximately by the
Zi”=jwC,,;+jwC0a—f-S{—jwC,,~R+w’C,,;L;—w2L,, W}
plifying valves with very great steepness of char
acteristic in high frequency ampli?ers, since
what could be won in ampli?cation by the greater
steepness was invalidated by the greater damp
40 ing. It is thus desirable that the apparent re
sistance'between the grid and the cathode should
be rendered a maximum which according to the
invention is ensured by minimizing the induct
from which it is apparent that the coupling in
troduces an apparent grid-cathode impedance
which can be represented in part by a negative
capacity, that is to say, by an impedance with
ance L: which, as is shown in Fig. 2 when the
45 impedance Zr is replaced therein by aninduct
ode capacity with decreasing negative grid volt
age which is due to space charge effects, since 45
this capacity and the negative capacity brought
about by the coupling due to the resistance R:
ance Lf, couples the anode circuit to the grid
circuit, in which case, as is immediately apparent
from the above equation, the resistance Rg, be
comes very large or may even assume a negative
50 value so that a tuned circuit connected between
the grid and the cathode is but little damped or
even negatively damped. .
According to the invention, the inductance can
be made very low byvconnecting the anode circuit
55 and the grid circuit each separately to the oath
ode and by so arranging the connecting wires
necessary for this purpose that the coupling be
tween both is small. The arrangement of such
an amplifying valve with two connectingwires
60 from the base to the indirectly heated cathode
is diagrammatically shown in Fig. 3. This ?g
ure shows an indirectly heated triode comprising
7 an anode A, a grid G and an indirectly heated
cathode K. For the sake of clearness the heater
65 is omitted.
Two wires I and 2 are ?xed to a
point P of the cathode and are connected to
separate pins in the base as diagrammatically
shown. The two connecting wires are spaced as
far apart as possible in order to minimize the
70 magneticcoupling between them. The grid cir
cuit is connected to one of the connecting wires,
say I, and the anode circuit to the other con
necting wire 2. Thus it is ensured that the in
ductance designated by Li becomes very small
75 and consequently the apparent conductivity also
the same frequency dependencygas a capacity 40'
but with opposite phase. Such a coupling of the
anode and the grid circuit permits of partly or
wholly neutralizing the increase of the grid-cath
are dependent in the same sense upon the slope
S. Such a coupling by means of an ohmic re
sistance included in' the connecting conductor 50
to the cathode, is particularly useful when a
plurality of amplifying stages are interconnected
by oscillatory circuits, the tuning of the circuits
being effected by means of condensers which are
mechanically coupled to each other. If in such
an ampli?er the grid’biases of the amplifying
valves are varied differently, for example for
volume control, the circuits will thus be caused
to be out of tune relatively to each other. As
described hereinbefore this can be obviated by 60
means of ohmic resistances included in the oath
ode leads, as by a correct choice of these resist
ances the capacity, brought about by the ampli
fying valves, in parallel with the oscillatory cir
cuits is made independent of the negative grid 65
When the amplifying valve shown in Figure 1
serves as a detector, the impedance Za inthe
anode circuit behaves, insofar as high frequency
currents are contemplated, essentially in a ca
pacitative manner. In this case Za can be rep
resented by
and for the admitance between the terminals 3
and 4 in Figure 1 is found
C a
from which it is apparent that the latter term
to a tetrode having a screen grid SG having an
impedance ZSG in the screen circuit and which
corresponds to the impedance Za of Fig. l. The
10 brings about a very high damping.
This damp—
ing can be neutralized by also choosing for Z:
a capacity C: so that
screen grid valves, in which case the anode cir
cuit of the triode has to be replaced by the
screen grid circuit of the tetrode. Fig. 6 shows
such an embodiment of the invention as applied
1 _i_ _C_” a
This case in which the anode and the grid cir
cuits are coupled by a capacity Cf in the com
mon cathode lead is shown in Figure 5. The
operation of this circuit arrangement will be clear
20 from the above considerations so that further
consideration can be dispensed with.
In the speci?cation as well as in the drawing
the invention is only mentioned as applied to
indirectly heated triodes.
It will, however, be
obvious that the invention can also be applied
with advantage to multigrid valves, for example
usual useful load ‘circuit impedance ZL is con
nected to the anode and the cathode and heater
connections I , 3 and 4 are similar to those shown 10
in Fig. 4.
What we claim is:
In combination, an electron discharge tube
having a cathode, a heater for said cathode, a
control grid, a screen grid and an anode, a con
nection within said tube between said heater and
one end of said cathode, a circuit including an
impedance connecting said screen grid and said
heater, an input circuit connecting said control
grid and being directly connected to the other 20
end of said cathode and an output circuit con
nected to said anode and directly to said one end
of said cathode.
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