High-frequency amplifier with automatic neutrodyning circuit

High frequency amplifiers require neutrodyning to prevent the risks of self-oscillation generated by the existence of stray capacitances among the electrodes of the active component used in the amplifier. Grid tube amplifiers (such as triodes, tetrodes, pentodes etc.) are more particularly concerned. Instead of simply providing a variable inductive element, in parallel, on the stray capacitance between the input electrode and the output electrode, there is provided a star connection of three reactances between the input electrode, the output electrode and the reference electrode. Only the first reactance is variable. The others are fixed and are in a constant ratio independent of the frequency. Preferably, the variable reactance element is an inductive element, and the other two are capacitive elements. Thus, by means of this single, variable reactance element, it is possible to make a setting, at the same time, of the input or output frequency tuning of the amplifier and of the neutrodyning.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns electronic amplifiers working at high 
frequency. The term "high frequency" is used herein to mean frequencies 
which are, in principle, higher than one megahertz but, more generally, 
frequencies which are sufficiently high, in view of the stray reactances 
of the active components used in the amplifier, to entail the risk of 
self-oscillation in the amplifer. 
A risk of self-oscillation appears as soon as the stray reactances cause a 
re-injection, towards the input of the amplifer, of an excessively high 
fraction of the output signal, and as soon as this fraction is added to 
the original input signal. 
The invention is particularly applicable to amplifiers for which the main 
active component is a vacuum electronic tube (a grid tube such as a 
triode, a tetrode, a pentode, etc) and shall be described in detail with 
reference to this type of amplifier. But it can be applied more generally 
to other types of amplifiers, including transistor amplifiers. 
2. Description of the Prior Art 
The risks of self-oscillation in a triode amplifier, in an assembly where 
the cathode of the triode is grounded, are dee essentially to the 
existence of a stray capacitance between the control gate and the anode of 
the triode. Similarly, risks of undesirable self-oscillation appear in a 
transistor amplifier owing to stray junction capacitances between the base 
of the transistor, on the one hand, and the collector and emitter on the 
other hand. The same is true for field-effect transistors because of 
capacitances between the control gate and the source and drain. 
The following description shall refer solely to assemblies with grid tubes 
without said description being in any way restrictive, and the invention 
is applicable provided that the amplifier comprises an active component 
having at least three electrodes, one of which receives a high frequency 
input signal to be amplified, another gives an amplified high frequency 
signal, and a third is used notably to define a reference potential for 
the other two electrodes, at least as regards high frequency. 
FIG. 1 shows, by way of an example, a triode amplifer in which no 
precaution has been taken to neutralize risks of self-oscillation. 
The amplifier has a triode 10 with a cathode K connected to a ground M, not 
only for AC currents but also for DC current. A gate G receives the high 
frequency signal to be amplified, and an anode A gives an amplified high 
frequency signal. The gate G is connected to an input E coupled to a 
previous stage by connecting and tuning circuits (not shown) depending on 
the application envisaged. Similarly, the anode A is connected to an 
output S connected to a following stage by connecting or tuning circuits 
(not shown) depending on the application considered. By construction, 
given the proximity of the different electrodes to one another, there is a 
stray capacitance Cgk between the gate and the cathode and a stray 
capacitance Cga between the gate and the anode. 
The signal input is done by means of an input circuit which can be tuned to 
a range of frequencies to be amplified. In the example shown, the tuned 
input circuit comprises an inductive element L1 in parallel with a 
capacitive element C1. The value of the inductive element and that of the 
capacitive element can be adjusted to set the amplifer as a function of 
the frequency to be amplified. This circuit L1, C1, is connected between 
the gate and the ground by means of an uncoupling capacitive element Cd1, 
the value of which is high enough for its impedance to be negligible as 
compared with that of the other elements of this circuit at the 
frequencies considered. Thus, the circuit L1, C1 may be considered to have 
a terminal which is virtually grounded for the high frequency currents. A 
negative bias voltage -Vg is brought to the gate G through the inductive 
element L1. 
Another tuning circuit, comprising an inductive element L2 in parallel with 
the capacitive element C2, is connected, firstly, to the anode A and, 
secondly, through an uncoupling capacitive element Cd2, having the same 
role as the capacitive element Cd1, to the ground M. The inductive element 
L2 or the capacitive element C2 can be adjusted to achieve the frequency 
tuning at the output of the amplifier. A high supply voltage Vht is 
applied to the anode through the inductive element L2. 
The circuit thus described constitutes only one embodiment. Other 
frequently used assemblies comprise a signal input at the cathode and not 
at the gate, the gate being grounded for the high frequency current by an 
uncoupling capacitive element similar to the capacitive element Cd1. 
Certain assemblies even use a signal input at the gate and an output at 
the cathode, the anode being grounded. Similar circuits could be described 
wherein the active component is a tetrode o a pentode. All these circuits 
are subject to risks of self-oscillation due to stray capacitances between 
the output and the input. The method most usually employed to neutralize 
this risk (called the neutrodyning operation) consists in placing 
inductive circuit elements in parallel on the troublesome stray 
capacitance, (the capacitance Cga in FIG. 1 because it brings a fraction 
of the output signal present at the anode directly to the gate). These 
inductive circuit elements are elements such that the association of the 
capacitive element and inductive element in parallel forms a anti-resonant 
circuit at the working frequency, namely a circuit with very high 
impedance (infinite impedance in theory), where the ohmic losses are 
overlooked. The signal fraction re-injected from the output (anode) 
towards the input (gate) will be all the more negligible as this impedance 
between the anode and the gate will be high. 
This method leads to a diagram such as the one shown in FIG. 2. In this 
figure, it is seen that a variable inductive element L3 has been added, 
series-mounted with a capacitive element Tt4, this set of elements being 
in parallel between the gate G and the anode A. The capacitive element Cd4 
is a linking capacitive element that prevents the transmission of direct 
voltage from the anode to the gate. Its impedance is negligible, at the 
frequencies considered, when compared with that of the inductive element 
L3. The inductive element L3, the value of which is such that L3.Cga.w2=1, 
forms a high impedance anti-resonant circuit with the capacitive element 
Cga, w being the pulsation corresponding to the working frequency. This 
inductive element thus achieves the desired neutrodyning effect. The 
inductive element N3 is made in the form of a variable inductive element 
to enable the neutrodyning to be set for commissioning the amplifier to 
operate at a user-specified working frequency or to enable the 
neutrodyning operation to be started again at other frequencies if the 
user wishes to make his amplifer work at different frequencies. 
The user of the amplifer has to make several settings when commissioning 
the equipment or when changing the working frequency, so as to achieve: 
efficient neutrodyning, 
frequency tuning at the input tuning circuit L1, C1, 
frequency tuning at the output tuning circuit L2, C2, 
These numerous settings entail several drawbacks. The first drawback, 
naturally, is the cost of the adjustable elements (variable inductive 
elements, variable capacitive elements, etc.) as compared with those of 
the fixed elements. The second and biggest drawback is the fact that each 
setting operation reacts on the other ones: a modification in the value of 
the inductive element L3 entails a modification in the tuning of the input 
circuit and vice-versa. 
It is well known that the frequency setting of a neutrodyned amplifer is a 
delicate operation performed by successive approximations, and that only 
specialists can perform this operation swiftly. For example, when 
installing a radio-electrical transmission amplifier, it is the installer 
who performs this operation. If the setting goes wrong later any resetting 
by the user is often done very roughly and results in lower performance 
characteristics, either because of repeated hitches requiring action by 
the installer, or even because the components, and especially the grid 
tube, are destroyed. 
An aim of the invention is to propose a high frequency amplifier assembly 
which does not have these drawbacks. 
SUMMARY OF THE INVENTION 
To achieve this aim, there is proposed a circuit structure with three 
reactance elements connected in a star connection among the three 
electrodes of the tube, or more generally, of the active component, one of 
the reactance elements connected to the first electrode, being adjustable 
and the other two reactance elements, connected to the other two 
electrodes, being fixed and essentially having, between them, a constant 
ratio among them which is independent of the frequency, in a range of 
frequencies, at which it is sought to make the amplifier work. 
It will be shown that, by an appropriate choice of the ratio between the 
two fixed impedances, it is possible to attain a strictly simultaneous 
setting of the input tuning (or output tuning as the case may be) and of 
the neutrodyning in the entire range of frequencies of the amplifier. This 
setting is done by varying the variable impedance. 
A preferred theoretical value for this ratio between two impedances will be 
defined below. However, in practice, the exact optimal theoretical value 
will perhaps not be chosen (all the more so, as it is difficult to measure 
this value) but conditions will be achieved such that the setting of the 
neutrodyning operation and of the input or output tuning can be done 
simultaneously with a single setting element. 
In principle, it will be seen to it that the ratio between the value of the 
reactance element connected to a second electrode and the value of the 
reactance element connected to a third electrode is essentially equal to 
the ratio between, on the one hand, the capacitance existing between the 
first electrode and the third electrode and, on the other hand, the 
capacitance existing between the first electrode and the second electrode. 
In certain embodiments, the variable reactance element will be a variable 
inductive element enabling the frequency setting by the user, and the 
other two reactance elements will be essentially fixed capacitive 
elements. 
In other embodiments, the variable reactance element will be a variable 
inductive element enabling the frequency setting by the user, and the 
other two reactances will be essentially fixed inductive elements. 
In other embodiments again, the variable reactance element will be a 
variable capacitive element enabling the frequency setting by the user, 
and the other two reactances will be essentially fixed inductive element. 
Should the setting element act both as a neutradyning element and as an 
element to set the input tuning, it will be assumed that the first 
electrode is the electrode receiving the input signal to be amplified, for 
example the gate of a triode in a grounded cathode assembly, the second 
electrode being the electrode giving an amplified output signal (for 
example the anode) and the third electrode being an electrode which may be 
carried to a reference potential for the signal to be amplified (for 
example, the cathode). 
The preferred value of the ratio between the fixed reactance elements 
connected to the second and third electrode is then a value Cgm/Cga, 
wherein: 
Cgm is the capacitance existing between the first electrode, receiving the 
input signal to be amplified, and the third electrode which forms the 
potential reference for the signal to be amplified. This capacitance is 
due as much to the stray capacitances as to the capacitances which may 
have been deliberately added; 
Cga is the capacitance present between the second electrode, which gives an 
amplified signal, and the first electrode; it too results as much from 
stray capacitances as from deliberately added capacitances. 
Should the setting element have to serve both for the adjustment of the 
neutrodyning and for the setting of the output tuning, it will be assumed 
that the first electrode is the electrode giving the output signal; the 
preferred value of the ratio is C2/Cga where C2 is the capacitance between 
the output electrode (first electrode) and the third electrode, while Cga 
is the capacitance between the output electrode (first electrode) and the 
input electrode (second electrode). 
In all cases, especially at high frequencies, the impedances, including the 
adjustable impedance, may be achieved without distinction by elements with 
localized constants such as capacitive elements or coils or by elements 
with distributed constant,, namely coaxial or bifilar lines, open or 
short-circuited, which may be equivalent, as the case may be and depending 
on their length, to positive or negative reactances, namely, to 
inductances or capacitance. The adjustable impedance is then formed by a 
line of this type with a short circuit (or an open circuit) of variable 
position, making it possible to vary the effective length of the line. 
Finally, as already stated, the invention applies to amplifiers for which 
the main active component may consist of other elements than a triode.

DESCRIPTION OF PREFERRED EMBODIMENTS 
A general drawing of a neutrodyned amplifer according to the invention is 
shown in FIG. 3 in the case of a triode assembly with a grounded cathode. 
It is therefore easy to explain the essential structural differences with 
respect to the drawing of FIG. 2, which is also a triode drawing with the 
cathode grounded. 
The inductive element L1 of FIGS. 1 and 2, which was used to tune the 
frequency at the amplifier input, is eliminated. The inductive element L3, 
series-mounted with the uncoupling capacitive element Cd4 of FIG. 2, is 
also eliminated and replaced by a star connection of three reactance 
elements, x'1, x'2, x'3, connected between the three electrodes of the 
tube, namely between the gate G, the cathode K and the anode A. 
The reactance element x'1, connected between the electrode receiving the 
input signal, namely, the gate G and the common node N of the star 
connection, is a variable reactance element enabling a user to set the 
operating frequency of the amplifier. 
The reactance elements x'2 and x'3 are fixed reactance elements (or, at 
least, reactance elements for which the setting, made outside the 
installation, has not been altered when changing the working frequency). 
The values of the reactance elements, x'2 and x'3, are in a constant ratio 
(independent of the frequency) such that when it is desired to make x'l 
vary in order to set the operation of the amplifier to a chosen frequency, 
the input tuning and the neutrodyning are done simultaneously, and this is 
done in a way which depends little on whether or not the output tuning has 
been done beforehand. The preferred value to be given to this ratio shall 
be explained further below. 
The reactance element x'2 is connected between the common node N and the 
second electrode of the tube, namely the electrode (the anode A herein) 
which gives the output signal. 
The reactance element x'3 is connected between the common node N o the star 
connection and the third electrode of the tube (herein the cathode K which 
is connected to the ground). 
Cgm designates the capacitance present between the first electrode (gate G) 
and the third electrode (cathode K at ground M). This value Cgm 
essentially comprises the value of the stray gate-cathode capacitance Cgk 
which was mentioned with reference to FIG. 1 and 2, but also, as the case 
may be, the value of additional capacitive elements added n parallel 
between gate and ground such as, for example, the capacitive element C1 of 
FIGS. 1 and 2. It is found that, most often, the stray capacitance Cgk is 
high enough for it to be unnecessary to add any additional capacitance, 
such as that of the capacitive element C1, to tune the amplifier at the 
desired frequencies. 
Cga designates the stray capacitance between the first electrode (gate G) 
and the second electrode (anode A). 
The preferred theoretical ratio between the reactances x'2 and x'3 is equal 
to the ratio between the capacitances Cgm and Cga; 
EQU x'2/x'3=Cgm/Cga 
The following is a theoretical explanation which justifes the fact that, 
provided the reactance elements x'2 and x'3 are appropriately chosen, the 
assembly of FIG. 3 enables the simultaneous setting of the input tuning 
and of the neutrodyning operation throughout the range of working 
frequencies of the amplifier. The setting is done solely by variation of 
the value of the third reactance element. 
The star connection of the reactance elements x'l, x'2, x'3 is strictly 
equivalent, from an electrical point of view, to a delta connection of 
three other reactance elements, x1, x2, x3, the values of which are 
related to x'l, x'2, x'3 by the following relationships: 
EQU x1=(x'1 x'2+x'3+x'3 x'1)/ x'2 (1) 
EQU x2=(x'1 x'2+x'2 x'3+x'3 x'1)/ x'3 (2) 
EQU x3=(x'1 x'2+x'2 x'3+x'1)/ x'1 (3) 
(see figure 4) 
FIG. 5 shows a theoretical electrical diagram which is strictly equivalent 
to the diagram of FIG. 3 after a star/delta conversion. The diagram of 
FIG. 5 is not used in the present invention. It is given to facilitate the 
explanation of the choices made in the invention. 
The delta connection is close to the one used until now, for example in 
FIG. 2: the reactance element xl would be the inductive element 
L1.Cgm.w.sup.2 =1. which must be placed in parallel with the capacitance 
Cgm to achieve an input tuning by the formula L1.Cgm.w.sup.2 =1. The 
reactance element x2 would be the inductive element L3 which must be 
placed in parallel with the capacitance Cga to achieve the neutradyne 
according to the formula L3.Cga.w2=1; the reactance element x3 would be 
considered as being infinite in the diagram of FIG. 2, the output tuning 
circuit L2, C2 being taken separately and being repeated in FIG. 3. 
However, in the assembly of FIG. 2, the inductive elements L2 and L3 
should both be variable. 
In the step of the present invention, it is considered firstly, that a 
non-infinite reactance value of x3 can be added, in parallel, to the 
output tuning circuit L2, C2 although a reactance of this type does not 
appear to be necessary in principle, and although it is then possible to 
convert the delta connection of the three reactance elements xl, x2, x3, 
into a star connection of three reactance elements x'l, x'2, x'3, to 
observe the result thereof. 
The basic hypothesis, starting from the delta connection of FIG. 5, is that 
input tuning and efficient neutrondyning are achieve at the same time, 
regardless of the frequency, if it can be written (regardless of the 
frequency f (pulse w=2.pi.f)), that the value of the reactance element xl 
is a pure inductance equal to j/Cgm.w and that the value of the reactance 
element x2 is a pure inductance equal to j/Cga.w. 
For these two conditions express the fact that the circuit xl, Cgm is an 
anti-resonant circuit tuned with the frequency f and that the same is true 
for the circuit x2, Cga. The impedances of these tuned circuits are then 
infinite. 
If these equalities persist despite the variations in frequency F, it means 
hat the impedance of the anti-resonant circuit xl, Cgn will remain 
infinite as also the impedance of the anti-resonant circuit x2 Cga. 
Consequently if, in these conditions, frequency tuning is found for both 
anti-resonant circuits simultaneously, x3 can take any value without 
altering the tuning f the input circuit nor that of the neutrodyning 
circuit. Regardless of the value of x3, the impedance of x3 will not be 
brought into parallel to xl since it is insulated from it by the infinite 
impedance of the tuned circuit x2, Cga. Similarly, the impedance of x3 
will not be brought into parallel with x2 because it is insulated from it 
by the infinite impedance of the anti-resonant circuit xl, Cgm. 
If, consequently, it is possible to maintain the two above-stated equations 
permanently: 
EQU x1=j/Cgm.w (4) 
and 
EQU x2=j/Cga.w (5) 
then the tuning of the input and neutrodyning circuits could be achieved in 
a single operation, without using trial-and-error methods to obtain the 
result sought by successive approximations. 
The only way to reach the two above-mentioned equations simultaneously, 
independently of the frequency, is to keep the ratio x1/x2 constantly 
equal to the ratio Cga/Cgm. 
EQU x1/x2=Cga/Cgm (6) 
The reactance elements x1 and x2 are made conventionally in the form of 
variable inductive elements, and it is not easy to make two inductive 
elements vary simultaneously by means of a single setting slider while 
keeping the same ratio. 
According to the invention, after having introduced, in accordance with 
FIG. 5, the additional reactance element x3, which is apparently 
unnecessary, the delta/star conversion is done to achieve the diagram of 
FIG. 3 and the above equations are rewritten in keeping with the impedance 
conversion formulae recalled in FIG. 4. The result of this is firstly: 
EQU x'3/x'2=x1/x2=Cga/Cgm (7) 
and then 
EQU x'1=[(j/Cgm.w)-x'3]/[1+(Cga/Cgm)] (8) 
The equation (7) which expresses the constancy of the ratio between two 
reactance values is quite similar to the equation (6). It might therefore 
be thought that the delta/star transposition only passes on to x'2 and x'3 
the problem encountered for x1 and x2, namely the possibility of making 
two reactance values vary while keeping them constantly in the same ratio. 
The fundamental difference from the previous case lies in the fact that it 
is now possible to choose reactance values of x'3 and x'2 which are 
constant, in the ratio Cga/Cgm, and to make x'l vary as a function of the 
desired frequency f so as to maintain the equation (8). In the case of 
FIG. 5, it is not possible to keep x1 and x2 constant in the ratio Cga/Cgm 
and to make x3 vary, for the variation of x3 then has no influence on the 
tunings sought. 
In a first embodiment, shown in FIG. 6, a capacitive reactance is chosen 
for x'2 and, consequently, also for x'3. A capacitive element C'2 is thus 
connected between the second electrode (anode A) and the common node N of 
the star connection. A capacitive element C'3 is connected between the 
third electrode (cathode K or ground M) and the common node. 
The capacitances of C'2 and C'3 have the ratio: 
EQU C'3/C'2=Cgm/Cga (9) 
so that their reactances are in the reverse ratio. 
And depending on the formula (8), the value of the reactance element x'l 
should assume the value: 
EQU x'1=[(j/Cgm. w)-1(jC'3. w)]/[1+(Cga/Cgm)] 
EQU or x'=j [(1/Cgm. w)+(1/C'3. w)/[1+(Cga/Cgm)] (10) 
The reactance of x'l should therefore be a positive reactance and it will 
therefore be achieved by a variable inductive element, the value of which 
will vary between two limits, which will be chosen as a function of the 
following numerical values: 
the value of the stray capacitance Cga and of the capacitance Cgm; 
value chosen for C'3; 
limit values w0 and w1 of the range of frequencies in which it is desired 
to make the amplifier work 
When the inductance is made to vary between these limits, a direct 
correspondence will be found between the inductance value and the 
frequency value for which the frequency tuning is made. Only one setting 
will be necessary. 
In stating that the capacitances of C'3 and C'2 are fixed, it is meant that 
they are not designed to be set by the user when he seeks to tune his 
amplifier (especially to change operating frequency). However, it will be 
understood that at least one of the two capacitive elements can be 
adjusted once and for all when the amplifier is installed (or even when 
the active component of the amplifier is replaced) given the uncertainties 
weighing on the value of the capacitances Cga and Cgm. 
In another embodiment, which can be seen in FIG. 7, the reactance elements 
x'2 and x'3 are made in the form of inductive elements L'2 and L'3 and the 
reactance element x'l is made in the form of a variable capacitive element 
C'l. 
The value of the reactance element x1 is deduced from the formula (8) with 
x'3 =jL'3.w. 
EQU x'1=[(j/Cgm.w)-jL'3. w)]/[1+(Cga/Cgm)] (11) 
The value of the inductive element L'3 is chosen, as a function of the 
range of frequency values desired for the amplifier, in such a way that 
L'3.w is always substantially greater than 1/Cgm.w in this range. Thus, 
x'l has a negative reactance throughout this range and may be made in the 
form of a variable capacitive element C'l as shown in FIG. 7. 
But it can be seen, from the formula (11), that it is also possible to 
choose an inductive element L'3 such that for the entire range of 
frequencies desired, L'3.w is smaller than 1/Cgm.w. An assembly is ten 
achieved such as the one shown in FIG. 8, for these three reactance 
elements x'l, x'2, x'3 must now be made by three inductive elements L'l, 
L'2, L'3, the first one being a variable inductive element. 
Finally, for x'l, it is also possible to choose an impedance with both an 
inductive characteristic and a capacitive characteristic, but, of course, 
in order not to lose the main advantage of the invention, namely the 
possibility of setting with a single variable element, a variable 
capacitive element and a variable inductive element will not be used at 
the same time. A line with distributed constants will be used which, for a 
given frequency, will have either a positive or negative reactance 
depending on its length. The frequency will be tuned by adjusting this 
length. 
In FIGS. 7 and 8, uncoupling capacitive elements, Cd6 and Cd5 respectively, 
have been added in series with the inductive elements, L'l and L'2, to 
prevent the transmission of DC potentials between the anode and the gate 
by means of the inductive elements. These uncoupling capacitive elements 
have a capacitance value which is sufficiently high to form null impedance 
short-circuits at the working frequencies considered. Their value, 
therefore, ddoes not come into play in the above mathematical formulae. 
We shall now explain how the invention can be transposed to cases where it 
is sought to make a simultaneous setting of the output tuning (and no 
longer the input tuning) and the neutrodyning. This may be valuable, for 
example, in the course of amplifiers with a wide input band wherein 
practically only the output must be tuned to a working frequency. 
The diagram of FIG. 2 (the diagram of the standard neutrodyned amplifier) 
is again taken as the starting point: firstly, the variable output tuning 
inductive element L2 is eliminated and secondly, the neutrodyning variable 
inductive element L3 is eliminated. Then, between the gate electrodes G, 
cathode electrodes K and anode electrodes A, a delta connection of three 
reactance elements, x1, x2, x3, is interposed as in FIG. 5: x1 between 
gate and cathode, x2 between gate and anode, x3 between anode and cathode. 
This leads to the diagram of FIG. 9, which however, is not a part of the 
present invention. 
This diagram is then converted by replacing the reactance elements, x1, x2, 
x3, by a star connection of three reactance elements, x'l, x'2, x'3, with 
the same mathematical relationships as above. These relationship are 
recalled in FIG. 4. This leads to the diagram of FIG. 10. 
According to the invention, the reactance elements x'l and x'3 are made in 
the form of fixed reactance elements (or, if necessary, reactance elements 
which can be set once and for all according to the characteristics of the 
tube and the desired frequency range) and the reactance element x'2 is 
made in the form of a variable reactance element which can be set by the 
use according to the working frequency. 
The output tuning and neutrodyning are achieved simultaneously on condition 
that the following are had simultaneously. 
EQU x3=j/C2.w (12) 
and 
EQU x2=j/Cga.w (13) 
wherein C2 represents the total capacitance between the anode and the 
cathode. This expresses the fact that there is an anti-resonant circuit 
tuned in output and an anti-resonant circuit tuned between the anode and 
the gate. The reasoning is the same as the on applied to the simultaneous 
input tuning and neutrodyning. 
The only way to arrive at these two equations simultaneously, despite the 
frequency variations, is to keep the ratio x3/x2 constantly equal to 
Cga/C2. 
By making the delta/star conversion of the reactance of x1, x2, x3, these 
two equations are re-written s follows: 
EQU x'3/x'=Cga/C2 (14) 
EQU '2=[(j/C 2. w-x'3]/[1+(Cga/C2)] (15) 
It is quite possible to choose the reactances x'l and x'3 as being constant 
and to adjust the tuning by acting solely on the value of the reactance 
element x'2. 
As for the input tuning, several embodiments are possible, among which: 
(a) x' and x'3 are capacitive elements, and x'2 is then a variable 
inductive element. 
(b) x'l and x'3 are inductive elements (in series, here necessary, with 
uncoupling capacitive elements of almost null reactance) and x'2 is then 
either a variable inductive element or a variable capactive element 
depending on the sign of (1/C2.w)-x'3 in the frequency range considered. 
The reactance elements x'l, x'2 and x'3 may naturally also be lines with 
distributed constants. 
Without entering into the details of all the possible embodiments of 
amplifiers, it will be understood that the invention can be applied 
whenever a neutrodyning and an input or output tuning problem has to be 
resolved. 
For example, the invention can be used in the case of a triode assembly 
with gate at the ground, the input signal being applied to the cathode and 
tee output signal being given by the anode. 
In this case, the reactance element x'l, which is variable if the 
neutrodyning is done simultaneously with the input tuning, is connected 
between the common node of the star connection and the cathode. The 
reactance element x'2, which is variable if it is sought to do the 
neutrodyning at the same time as the output tuning, is connected between 
the common node and the gate, and the fixed reactance element x'3 is 
connected between the common node and the gate. The computation of the 
reactances then brings out the fact that the ratio of reactances x'2/x'3 
(the example of neutrodyning with input tuning) is preferably equal to Cka 
where Ckm is the total capacitance (stray capacitance and external 
capacitances if any) existing between cathode and gate, and Cka is the 
total capacitance (in principle a stray capacitance) between anode and 
cathode. The term Cgm/Cga of the equations (9) 25 and (10) should 
therefore be replaced by Ckm/Cka. In the case of neutrodyning done with 
output tuning, the capacitance Cga should be replaced by the capacitance 
Cka in the equations 14 and 15 to obtain the equations defining the 
relative values of the three reactance elements in star connection. 
Other assemblies again are possible, for example with tetrodes or pentodes 
and, in this case, the three reactance elements are mounted between the 
electrode receiving the input signal to be amplified, the electrode giving 
the output signal and the electrode defining the reference potential 
(ground) for these two signals. The other electrodes modify, if necessary, 
the distribution of the stray capacitances, but do not modify the 
principle of the invention.