US2247442A - Neutralizing arrangement for amplifiers - Google Patents

Description

July 1, 1941. E. LABIN NEUTRALIZING ARRANGEMENT FOR AMPLIFIERS 2 Sheets-Sheet 1 Filed Feb. 2, 194Q vvvvv EM 2 z" INVENTOR E. L AB/N er W ATTORNEY July 1, 1941. LAB|N 2,247,442

NEUTRALI ZING ARRANGEMENT FOR AMPLIFIERS Filed Feb. 2, 1940 2 Sheets-Sheet 2 /N I/EN TOR E. LAB/N A TTORNEV Patented July 1, 1941 hdihdiz rein NEUTRALIZING ARRANGEMENT FOR ADWLIFIER-S Application February 2, 1940, Serial No. 316,932 In France February 14, 1939 7 Claims.

operation of such a stage that there should only i.

be slight over-tensions in the plat circuit, and, consequently, for a given capacity very low impedances and a low output of the stage. Moreover, for ultra-short waves the filament and grid connections of the stage have reactances which are not negligible on account of their length and their dimensions in view of the wave-lengths employed. These dimensions, which are inevitably too large, cause various reactions which are all the more important the lower the plate impedance. These reactions modify the operation of the stage in various ways at the frequency of tuning, and for the side bands.

Moreover, the amplification of wide frequency bands to a level sufiicient to modulate a transmitter of considerable power leads to the use of grid modulation on account of the great dimculties involved in the use of plate modulation. However, it is well known that complete grid modulation is accompanied by very considerable non-lincarities. This latter drawback is mainly due to the fact that if the neutralization employed is not perfect, the power coming from the driving stage is modulated in such a way that it is opposed to the modulation of the whole of the transmitter.

Consequently, it is the main object of th invention to provide means for reducing, and even practically eliminating, these dificulties and drawbacks.

In particular, the present invention has the object of providing transmitter stage circuits modulated by a wide frequency band operating with ultra-short waves which have mainly a low effective plate capacity, reactances of grid and Z1 filament connections forming an integral part of the circuits in such a way that the reactions in these circuits are easily adjustable, and in which a modulation of the power of energization of the last stage is in the same direction as the output modulation which permits a substantially linear total modulation.

In accordance with one of the characteristics of the invention, a transmitter stage for ultrashort waves modulated by a wide band is arranged in such a way that the Width of the band transmitted is ensured by the compensation of the positive reactions of the stage by the adjustment of the ratio of the input and output impedances of the circuit, this stage preferably consisting of a symmetrically mounted valve circuit.

In accordance with another characteristic of the invention, said stage is neutralized so as to obtain a low effective capacity of its plat circuit, the variations of impedance of the neutralizingmeans in the frequency band transmitted coming into play in the compensation of the positive reactions of the stage.

In accordance with another characteristic of the invention, this stage is energized by its filament circuit, the modulation being applied in the same direction as the output energy on the terminals of the plate circuit, and said stage being neutralized by means of an inductance or grid impedance of predetermined value with respect to the characteristic capacitances of said stage.

In accordance with another characteristic of the invention, the neutralizing grid impedance consists of a quarter wave-length transmission line terminated in a variable impedance, or inductance, connected in such a way with respect to the grids that in association with the grid impedance of the stage it supplies the necessary neutralizing reactance, said quarter wavelength transmissicn line being arranged so as to reduce the capacitance with respect to the earth of said grid circuit.

In accordance with other characteristics of the invention, a transmitter system comprises an energized stage incorporating the previous characteristics, and an energization stage connected by a quarter wave-length transmission line, the impedance of the input circuit being adjusted by the characteristic impedance of this transmission line and the coupling resistances between said. driving stage and said driven stage and the tuning of the input circuit of said driven stage and of the coupling circuit being simultaneously adjusted by means of a single impedance, for instance a shunt capacitance placed at the input of said transmission line.

In accordance with still other characteristics of the invention, the heating current of the driven stage is brought by means of conductors arranged inside the transmission line connecting the two stages, or in the case of the stage with symmetrical mounting of two valves the two valves of the driven stage to the stage of energization, these transmission lines being each extended by another quarter wave-length transmission line of identical or difierent characteristic impedance from that of the first lines.

The invention will b explained in detail in the following description based on the attached drawings, in which:

Fig. 1 shows schematically the mounting of an amplifier stage of a transmitter system incorporating characteristics of the invention;

Fig. 2 gives the equivalent diagram of one of the valves of Fig. 1, and the diagram of Fig. 3 shows the composition of a T-bridged quadripole in the general case; and

Fig. 4 shows schematically a transmitter system incorporating characteristics of the invention.

Referring to Figs. 1 and 2, two valves V and V are mounted symmetrically with their filaments F and F connected by an impedance Z, their grids G and G by an impedance Z4 and their plates P and P by an output transformer T, a condenser being connected to the terminals of the primary of this transformer. The two valves V and V' are energized by their filament as indicated at E, the neutralization being ensured by the impe-dances Z4 and the modulation being app-lied between the middle point of the grids MG and earth (not shown). In accordance with the drawings, it is clear that the output power taken between the plates P and P is in phase with the driving power, and consequently these two powers are modulated in the same direction, which permits a complete modulation to be obtained with great facility. The possibility of neutralizing by means of a suitable impedance Z4 between the grids G and G may be shown from the diagram of Fig. 2 which shows the equivalent diagram of one of the valves of Fig. 1.

In this diagram the earth M represents the plane of symmetry of the real symmetrical mounting. The capacities C1, C2, C3 are the filament-grid, grid-plate and plate-filament capacities respectively of the valve, including the external capacities. The grid-earth impedance Z4 shown in series with a resistance r is the neutralizing impedance. The natural inductances of the filaments and of the plates form part of the outside circuits. The natural inductance of the grid circuit forms a part of the impedance Z4.

The whole constitutes a bridged-T quadripole closed on the terminal impedances Z, Z. The general case of the bridged-T network is shown in Fig. 3 in which the series resistances are R1. R2, while R3 represents the bridge resistance of network and R4 the shunt resistance. The terminal imped-ances Z and Z are respectively connected to the terminals l-2 and 3-4 of the quadripole. In order that the arrangement of Fig. 2 may be neutralized it is necessary and sufficient that in the equivalent bridged-T network of Fig. 3 the output current I be zero, whatever the input current I; When this condition is realized it is verified whatever the values of the terminal impedances Z and Z.

The determination of the output current is readily made by applying Kirchhoffs Law to the network of Fig. 3, by which means the following equations are obtained:

In the above equations, E represents any given electromotive force acting through the terminal impedance Z.

Solving the Equations A, B and C simultaneously for the value of I, the result is where A is the discriminant of the system of equations.

The condition of equilibrium is very easily written? this being the condition that reduces I to Zero regardless of the value of E and hence makes I independent of the input current I.

Moreover, the bridged-T network supplies the impedances Z12 seen from terminals I and 2 and Z34 seen from terminals 3 and 4. These impedances themselves are also independent of the terminal impedances when the Condition 1 is ful filled. Thus, the impedances Z12 and Z34 are obtained: Z.Z=R1

In stating by definition for the circuit of Fig. 2:

F= 7 -I- -I- C3 and 7 these relations 1 and 2 become in which mo designates the pulsation of the carrier current.

It will be seen from the Relations 1' and 2' that the arrangement of Fig. 2 may be neutralized by means of an inductive reactance Z4 between the grids, the value of which reactance is such that it is tuned to the frequency of the carrier current with a definite capacity from the three characteristic capacities of the arrangement by the relations 3.

It will, moreover, be seen that the efiective capacities between the input and output terminals are not the capacities C1 and C2 but greater capacities Ci and C'z defined by the following relations:

In other words, neutralizing, instead of at least doubling the effective capacity of the plate circuit, as in ordinary neutro'dyne circuits, only multiplies the capacities by the factor 'Y which is usually less than 2.

In an embodiment of the present application the factor has been found equal to 1.5 instead of 2. The result is that for a given band width, that is to say, for a given Q factor of the plate circuit the permissible impedance of the plate circuit in the arrangement according to the invention is less than 33 per cent higher than in the usual circuits.

The grid impedance having been fixed by the condition of neutralizing the dynamic operation of the arrangement depends not only upon the driving potential and the plate impedance, but also on the negative feedback which is developed on the terminals of the input impedance, the circuit of the plate current being closed by this impedance, and by the positive feedback on the terminals of the grid impedance, the input and output potentials developing each a grid potential of such direction that it increases the output power by the fact thatthe neutralizing reactance is positive.

The negative feedback in the filament circuit always exists to a certain extent, even in ordinary circuits in which it is inevitable, but in these circuits it is not possible to countenbalan'ce it by the positive feedback on the grids, or at least it is very difficult to arrive at this for a very wide band.

One of the characteristics of the invention consists in the fact that in order to obtain a good operation of the transmitter stage the input, out- This relation .5 defines the positive reaction.

The actual driving potential between filament earth is VFM given by the following relation which defines the negative reaction:

in which EX represents the driving potential in the absence of plate current, I1 the component of the plate current on the fundamental frequency, and ZFM the impedance between filament and earth at this frequency.

Taking into consideration the Relations 5 and 6 it is easy to compute the effective grid-filament and plate-filament potentials which are given by the following formulae:

From the Formulae '7 it is possible to deduce the complete operation of the valve.

In fact, in designating by a the coefiicient of amplification of the tube it is possible to admit that the plate current depends, as a first approximation, only upon the sum If, in accordance with certain characteristics of the invention, the coefficient of I1 be cancelled in the preceding sum, the positive and negative feedback will balance each other, and the plate current does not depend upon the frequency if the driving potential remains constant. The condition thus obtained is explained by the relation:

which is only approximate, but which shows that by suitably adjusting the ratio of the input and output impedances for an impedance between grids of a given nature, it is possible to ensure to a very great extent the impedance of the plate current with respect to the frequency of modulation and, consequently, a wide transmission band for a sufficiently damped plate impedance.

For the frequency at which the stage is neutralized the coefficients-mi and m do not depend on the terminal impedances of the quadripole or on the grid impedance, while for the other frequencies the said coeilicients vary so that the variation of the ratio of the plate and filament impedance with frequency has to follow a predetermined law depending mainly upon the law of variation of the impedance between the grids.

In accordance with one characteristic of the invention the impedance between grids must be at the carrier current frequency, an inductance of which the value is given by the Formula 1. In fact, for power transmitters employing large dimension tubes for ultra-short waves, the natural inductance of the grid connections is already greater than the value necessary toneutralize the stage, so that the outside circuit must be capacitive. In accordance with one characteristic of the invention, there is inserted between the grids a variable capacity adjusted to the suitable value to neutralize the stage.

lhis variable capacity must, of course, be shunted by choke self-in-ductances, or resistances, so as to permit a correct bias for the grids. If the power supplied by the stage is high the amount of space occupied by the grid condenser and the associated circuit becomes considerable owing to the fact that the condenser is'traversed by a high high-frequency current substantially equal to the sum-of the circulation currents of the plate and filament circuits.

When a stage mounted in the manner indicated is modulated by variation of the grid potential with very wide frequency bands of the type of those employed in television, the large amount of space occupied by the grid circuit involves a high capacity to earth which loads the modulator for th e high-est modulation frequencies.

In order to reduce this harmful capacity to earth while permitting an easier mechanical arrangement in accordance with one characteristic of the invention the capacity between the grids is replaced by a transmission line having an electrical length equal to a quarter or an odd multiple of a quarter of the carrier wave-length. terminated by an adjustable impedance or inductance.

The polarization can then be applied between the middle point of said grid inductance and earth, and it will no longer be necessary to provide a circuit for the direct current.

The low variation of the inductance necessary to adjust the capacitance between grids to its exact value may be obtained in various ways, for example, by causing a disc of solid copper to rotate inside the solenoid constituting the self inductance. When the plane of the disc is parallel to the planes of the turns the inductance is minimum, when the plane of the disc is normal to the previous position the inductance is maximum. Another adjustment is also possible by variation of the characteristic impedance of the transmission line constituting the quarter-wave line.

The capacity of such a unit to earth is substantially in proportion to the surface of the straight section of the transmission line, owing to the fact that the inductance may occupy a relatively small amount of space. The two tubes composing this transmission line will thus be u brought nearer to the distance permitted by the high frequency potential to which they are subjected. With equal high frequency powers it is thus possible to obtain a reduction of about 30 per cent of the capacity to earth.

From a grid circuit composed either as just described, or by a simple variable capacity, a sumciently damped driving circuit of easily adjustable impedance should be obtained, in accordance with other features of the invention. Moreover, a

the invention provides means for ensuring the supply of the filaments with heating current, without interfering with the high frequency circult.

In accordance with certain characteristics of the invention consequently, the filaments of a symmetrical stage are energized through a transmission line having an electrical length equal to one quarter of the wave-length of the carrier wave, calculated from the point of coupling of the preceding stage to the accessible filament terminals, or slightly beyond these terminals. The coupling to the preceding stage is obtained through dissipating elements in series, such for example as water cooled resistances and the tuning of the circuits is obtained by means of a single variable condenser inserted between the coupling terminals.

'Moreover, the heating current supply of the filament is obtained by, means of conductors located inside hollow conductors constituting the quarter-wave transmission line, and in order to insulate the coupling terminals of the heating current sources from high frequency, the whole of the driving line is extended by a length again equal to a quarter of a wave-length of the carrier wave as far as the source of current itself.

An embodiment of a device of this kind is shown schematically in Fig. 4, in which I and I designate the valves driven through the quarterwave lines 2 and 2, 'of which the characteristic impedance Z may be adjusted by means of the variable condenser E.

The coupling isobtained on the terminals A and A through the impedances 3 and 3 which comprise dissipating elements shown in the form oi. resistances. the condensers 4 and .4 of high capacity merely serving to insulate the high potential from the driving stage,

The quarter-wave transmission line 2-2 is extended through another quarter-wave line E-5'.short-circuited at its end as indicated at I. and earthed to this short circuit, this line 55' forming a choke inductance for the high frequency. r.

The conductors carrying the-heating current ofthe valves l and. l are located inside the conductors 2+2 and 55', and in this way are completely screened from the'point of view of high frequency currents which are only circulating outside the conductors.

The tuning condenser E is located between the terminals A and A at a position of low impedance, that is to say, of relatively low alternating potential. The tuning of the coupling circuit is carried out by means of the condenser 6 by short-circuiting the terminals B and B at the other end of the quarter-wave line 2-2. In this way the valves I and l' are placed out of circuit. Then the short circuit is removed, followed by again adjusting the same condenser E. The filament circuit is thus tuned with the minimum of capacity compatible with the valves employed.

Small condensers ii] are inserted in the filament connections as shown.

The modulation is applied between the grids and earth through a resistance ll between the middle point of the inductance I 2 and a transmission line l3 preferably a quarter wave-length long connecting the ends of the inductance l2 to the grids of the Valves I and l. The plates of the valves i and I are connected to the load through transmission lines M, a shunt capacity it being placed in shunt between the conductors of these lines I4.

With such an arrangement the input circuit can easily be tuned to the natural capacities of the valves I and I by means of a condenser 6 of low Value, but nevertheless supporting relatively great alternating potentials and the impedance from the valves l and I, can easily be adjusted by adjusting the characteristic impedance Zo of the lines 2-2', the damping of the input circuit of the valves being obtained by means of the resistances 3 and 3'. Moreover, the adaptation of the load to the driving stage is accomplished by means of the same condenser E and an over-tension of the plate circuit of the driving stage does not interfere with the filament circuit of the driven stage l-l'. Finally, the condensers 4 and 4' insulate the sources and the heating conductors of the filaments of the valves and I from the high frequency standpoint.

The special arrangements which have just been described are only givenby way of illustration, and it should be understood that the invention may be employed in other embodiments and may comprise numerous variants and adaptations without departing from its scope.

What is claimed is:

1. In an amplifier, a space discharge tube hav ing an anode, a cathode and at least one grid, inherent capacitances between pairs of said electrodes, an input circuit connected between said. cathode and ground, an output circuit connected between said anode and ground, an inductance between said grid and ground of a value determined by the said inherent capaci'tances, whereby said interelectrode capacitances' are substantial y neutralized.

2. In combination. a snace discharge tube having an anode, a cathode and a grid, inherent capacitances between pairs of said electrodes, a circuit connected between ground and one of said electrodes, a second circuit connected between ground a second of said electrodes, and an in ductance connected between ground and the third electrode, said inductance being so proportioned with respect to said inherent capacitances as to substantially prevent reaction at a given operating frequency between the first two circuits above mentioned.

3. In combination, a space discharge tube having an anode, a cathode and a grid, inherent capacitances between the pairs of said electrodes, an input circuit connected between one of said electrodes and ground, an output circuit connected between a second of said electrodes and ground, and an inductance forming with the three inherent capacitances a bridged-T network, the value of said inductance being such as substantially to balance the network thereby neutralizing the reaction between said input and output circuits.

4. In an amplifier, a space discharge tube having an anode, a cathode and a grid, inherent capacitive reactances R1, R2 and R3 between the respective pairs of said electrodes, and an inductive reactance R4 substantially of the value determined by the relation connected between one of said electrodes and ground to reduce reaction between the remaining two electrodes.

5. A system in accordance with the next preceding claim in which the inductive reactance R4 is connected between the grid and ground.

6. In combination, a space discharge tube having an anode, a cathode and a grid, inherent capacitive reactances R1, R2 and R3 between the respective pairs of said electrodes, an inductive reactance R4 substantially satisfying the relation connected between said grid and ground, an input circuit connected between said cathode and ground, and an output circuit connected between said anode and ground.

'7. A system in accordance with the next preceding claim, in which the relationship is substantially satisfied at a predetermined work- :ing frequency.

EMILE LABIN.

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