US2611884A - Amplifier gas tube - Google Patents

Description

Sept. 23, 1952 w. M. WEBSTER. JR., ET AL 2,61

AMPLIFIER GAS TUBE Filed March 28, 1951 2 SHEETS--SHEE'I l .5 VOL 75 wvzA/roks MAL/4M M Wzasmi, J/a 6.60565 W fi/r/A/ Me Patented Sept. 23, 1952 AMPLIFIER GAS TUBE William Merle Webster, Jr., Princeton, and George W. Bain, Jr., New Brunswick, N. 3., assignors to Radio Corporation of America,"a corpora-v tion of Delaware Application March 28, 1951, Serial No. 217,912

This invention relates to improvements in gas tubes which are suitable for use as amplifiers in that they have continuous grid control. More particularly it relates to improvements in amplifier gas tubes, of a particular kind having very high values of transconductanceand of anode current and extremely low values of output impedance, which are described in co-pending application, Serial No. 185,745 which was filed on September 20, 1950, and was assigned to the same assignee as the present application and to novel circuits utilizing the improved tubes.

Tubes of this kind, which are referred to as "Plasmatrons, have offered very great advantages over the tubes which preceded them. However, some of them also have certain limitations. Before these limitations can be properly understood, it is necessary first to review the basic operating principles of the plasmatron. In this kind of tube there are separate discharge paths for the load current and the ionizing current. Theenergizing potential required for drawing the former, the load current; from a main cathode to a main anode, is set at well below the value required to produce ionization. However, a higher potential. is used to produce a separate ionizing, or auxiliary discharge. It ionizes the tubes gaseous filling 1 converting it into a conductive plasma consisting of positive ions and detached, free, negative electrons. The plasma (1) surrounds the main cathode and (2) fills the load current discharge path whereby (1) it neutralizes the. electron space charge surrounding the main'cathode and causes a very great increase in emission, and (2) it efiectively acts as a low impedance conductor connected between the main cathode and the main anode. Because of these efiects the load current is very large, e. g., ampere, despite the very low main anode potential, e. g., 5 volts. In'addition, the overall efficiency is very high since the great multiplication of load current which results from the use of an auxiliary discharge, can be attained even though the current-of that discharge is itself of very small magnitude.

In prior art plasmatrons the presence of the plasma influences only the direct current component of the load current. This is because the auxiliary discharge is energized with a steady direct potential whereby the density of the plasma is substantially constant as initially generated and because the application of a signal to the plasmatron control grid is its location athwart the path of the low-Velocity load current is incapable of influencing the plasmadensity. In

16 Claims. (01. 315-468) fact, as will be more clearly understood from what follows, it is because a control grid located between the main cathode and main anode is incapable of influencing the generation of plasma that it is able to retain control of the load current even though it is entirely immersed in the plasma. Incidentally, it should be noted that the control which it does have is not based oncooperation with the field of an unneutralized, negative, cathode space-charge as in the case of the control grid of a conventional hard tube. Instead its control is based on the following: the grid, when negatively biased, is surrounded by a positive-ion sheath which the input signal varies in thickness. These variations in turn vary the cross-sectional areas, and the conductivity, of

columns of conductive plasma which extend thru the grid openings between the main cathode and the main anode.

The attainment and retention of this type of load current control in a gas tube marked a great advance in the art. In most other kinds of gas tubes, e. g., thyratrons, in which the grid is unable to retain control, if indeed it ever has any control at all beyond that of firing the tube, continuous control is impossible because the ionizing discharge occurs over the very same path as the load current discharge and therefore extends through the grid opening(s). the. grid is deprived of any continuing ability to control the anode current once the gaseous filling of the tube becomes ionized. The mech'anics'of how the thyratron grid is deprivedof control is that when located in the ionizing path it has indirect effects on load current, as a result .of at! fecting the plasma density, which are opposite to, and exactly offset, its direct eiiects. In view of the foregoing description of plasmatrons .it is apparent that these mechanics do not applytoit and that this is why its grid does not lose control.

Three limitations encountered inplasmatron tubes are: that their grid-voltage versus plate- I current characteristics are often somewhat nonlinear; that they have lower input impedances than are desirable for certain purposes; and that their attainable transconductances are rather limited due to the fact that their control grids must be capable of directly controlling very large magnitude currents.

It is the object of the present invention to devise a gas amplifier tube of the kind in question with a grid-voltage versus plate-current characteristic of improved linearity.

It is a further object of the present invention As a result 3 to provide such tubes, and circuits for them, which have increased values of input impedance.

It is a further object of the present invention to provide gas tube amplifiers of the kind in question, and circuits for them, with increased values of transconductance.

These objects have been obtained by making certain modications in the tubes and by devising novel amplifier circuits for employing them to maximum advantage.

According to the principal tube modifications of the present invention the load-current control grid, which is normally included between the main anode and the main cathode, is replaced or supplemented by a constriction-modulating electrode, which is placed in the path of the auxiliary discharge. The purpose in so doing is to indirectly effect control of the load current by varying the density of the plasma without at the same time directly controlling it in an opposit and compensating manner. Since the rate at which the-density of the plasma can diminish is a function of deionization time, and therefore is limited, this type of control results in lowering the upper limit of the frequency range of operation. However, more linear e. g./I. P. characteristics result from this kind of control while at the same time the obtainable upper frequency limit is still high enough for many applications such as for audio use and in servo systems. In addition since direct control of a relatively small ionization current results in indirect control of a Very large load current, the obtainable transconductance benefits from a multiplication efiect analogous to that obtained in secondary emission electron multiplier tubes. Therefore, extremely large transconductance, such as the order of one or mor mhos have been obtained. Furthermore, for reasons which will be explained in detail below, the input impedance to the constriction electrode can be very high when it is properly biased. According to the principal features of the present invention: (1) a particular energizing circuit is used for the auxiliary discharge in combination with the use of a constriction electrode in the tube whereby it is possible for very small changes in the input signal voltage to cause surprisingly large changes in the density of the plasma; and (2) the constriction electrode is biased at an optimum point in a particular negative voltage range wherein the electron current which it collects, due to the fact that thermal velocities of electrons emitted by the auxiliary cathode exceed this negative bias, is ofiset by an opposite current flow of electrons which it gives up to the positive ion sheath surrounding this electrode. Incidentally the electrons constituting this latter current are continuously neutralizing some of the ions in the sheath but as fast as these diffuse away new replacement ions are drawn to the electrode from the plasma to replenish the sheath.

The result of the optimum bias for the constricting electrode is that the two oppositely moving currents are dynamically balanced whereby the efiective input impedance of the electrode, for small input signals, would be infinitely large except for its capitance to ground and to the other electrodes.

Since the constriction-modulating electrode can be made of very small physical size to keep down capacitance values, the actual total input impedance is very large.

In the drawing:

Figure 1 is a longitudinal sectional view of an illustrative embodiment of a tube according to 4 the present invention. The section is taken along lines I-l of Figure 2 and in a plane which is parallel to the axes of the cylindrical cathodes;

Figure 2 represents another longitudinal sectional view of this embodiment, this section being taken along the line 2-2 of Figure 1 in a plane perpendicular-to that of the section of Figure 1;

Figure 3 is a schematic circuit diagram of an amplifier system using the tube shown in Figures 1 and 2;

Figure 4 shows the anode-current versus constriction-electrode-voltage of a tube as in Figures l and 2;

Figure 5 represents a longitudinal sectional view of a modification of the tube shown in Figure 1, the section being taken like that of Figure 2;

Figure 6 is a schematic circuit diagram of a mixer system using the tube of Figure 5; and

Figure 7 shows the characteristic obtained by plotting, in rectangular coordinates, the constriction-modulating-electrode current against the voltage between this electrode and the auxiliary cathode.

The gas tube [0 shown in Figures 1 and 2 comprises a gas tight envelope II which, in this illustrative embodiment, is of substantially rectangular shape. The load current in this tube originates from a main indirectly heated cathode I2 and is received at a main anode 13. An auxiliary indirectly heated cathode 14 serves as a source of electrons for the ionizing discharge. A slotted shield electrode I5 is placed around the auxiliary cathode l4. It forms the auxiliary discharge into a stream which has a narrow constriction Where it emerges from the shield and is directed at the main. cathode [2. The presence of a constriction in the ionizing stream reduces its current thereby raising the impedance of the auxiliary discharge by making it work hard. In other words by using the shield electrode a higher are. drop is required to start and to sustain the auxiliary discharge. The are drop automatically adjusts itself so that a suflicient number of ionizing collisions for a self-sustained discharge will occur, despite the small magnitude of the ionizing current, because of the increased average velocity of the electrons constituting this current. In this arrangement these electrons attain ionizing velocities after they have traversed only a fraction of. the distance between the slot I! in the shield electrode l 5 and the nearest electrode which is polarized to attract them, i. e., the main cathode, therefore, since they are capable of, having ionizing collisions in traversing all of the remainder of this distance, the efliciency of the auxiliary discharge as a means for generating plasma is greatly increased. If the shield 15 were not employed, the auxiliary current would be very large and the arc drop between the auxiliary cathode l4 and the group of electrodes servin as a composite auxiliary anode, the main cathode l2 and the main anode [3 herein, would be only slightly above the ionizing potential for the gaseous filling of the tube. Because of this the ionizing efliciency of the auxiliary discharge, and therefore the efiiciency of the entire tube, would be poor. Hence the shield is rarely omitted in plasmatron practice.

According to the present invention the shield electrode is always used and according to its principal feature a constriction-modulating electrode 1-6 is used in combination with it to cooperate with its slot 11. The modulating electrode 16 in the example shown herein comprises a pair of parallel wires or rods [8 connected together at their tops and bottoms by conductive spacers H! to provide a unipotential electrode structure having a slot 2d substantially coinciding with the slot IT. The electrode I6 is supported between the top and bottom of the envelope. H by rods 2! and 22, the former of which is sealed partway through the bottom of the envelope to act merely as a support element whereas the latter extends through the top of the envelope to serve as a terminal pin.

When the electrode 16 is negatively biased the direct effect of applying a signal. voltage to it is to vary the thickness of a positive ion sheath which surrounds it and hence the indirect effect is to eifectively vary the size of slot 28.

Each of the two cathodes, i. e., the main cathode l2 and the auxiliary cathode i4 is supported on individual rods 23 and 24 each of which extends through the top of the envelope I I to serve as a terminal pin for polarizing the cathode and for conducting an electrical current to and/or from one end of its internal filamentary heater. In addition these cathodes are provided with terminal pins 25 and 26, also sealed through the top of envelope ll, each of which serves for conducting the same current from and/or to the other end of one of the heaters. The main anode I3 is supported by two rods 2'! which are fused part way through the envelope H and another rod 28 which is fused entirely through it whereby it may be used as an external terminal. The shield is supported on a rod 29 which extends,

like the rods 23. 24, and 28, through the envelope so that it may also serve as a terminal pin.

Tube It may be processed in any number of ways well known in the art to provide a gaseous filling within its envelope prior to sealing off. Any suitable gas or mixture of gases may be utilized. The gas pressure for any particular embodiment will be in accordance with its specific electrode geometry and spacings and must be such to favor the formation of a self sustaining ionizing discharge. A number of plasmatron tubes have been found to operate satisfactorily with a filling of helium of a pressure of approximately 750 microns. However, as is well known other gases and other pressures may be used, c. g. pressures which lie within the range of between approximately 100 microns and several millimeters of mercury.

Figure 3 shows a circuit which is suitable for using the gas tube shown in Figures 1 and 2. In this figure three direct potentials sources are shown, a low-potential load-current-energizing source 30; a high potential auxiliary-dischargeenergizing source 3!; and a biasing-source 32. The source 30 is used for maintaining the small non-ionizing potential difference between the main cathode l2 and the main anode l3. It may consist of a number of series-parallel connected 1 /2 volts dry cells, a storage battery, or any similar fairly-high-current, low-voltage source. This is because of the fact that although it must not provide much voltage it must be capable of delivering to an output circuit 33 a very substantial signal-bearing current, i. e., a current which may easily have an average value of the order of one to several amperes. Conversely the source 3! should be capable of providing a relatively much higher potential but need not be capable of providing a particularly large continuous average current. This is because it serves to energize the eflicient, low-current ionizing-discharge. Finally the source 32 is only called upon to provide a low voltage and that with little current since it merely provides a small negative bias between the constriction-modulating electrode [6 and the auxiliary cathode M. In the circuit of Figure 3 the main cathode is at the direct potential of ground and the main anode is nearly so since the source 3!] establishes only a small voltage difference between these elements. As to alternating currents they are both grounded since the source 30 may be assumed to have a low internal alternating current impedance whetherv it is a battery with Or without a by-pass condenser or an equivalent low voltage power supply. Connected in series with the source 30, the main cathode l2 and the main anode [3 there is the primary of an output transformer 33 for coupling the varying component of the output current of tube It to a utilization device (not shown) such as a loud speaker. In this arrangement the main anode and main cathode serve togetheras a composite anode for attracting and receiving electrons from the auxiliary cathode M to establish the auxiliary discharge. Accordingly, the source 3| is connected between the auxiliary cathode l4 and ground, i. e., the voltage reference for the composite anode, in such polarity that the auxiliary cathode is sufiiciently negative with respect thereto to produce the required ionizing discharge.

As is well known, it is necessary to use a current limiting means in series with a, gaseous dis charge to prevent such a great current flow as to damage the gas tube in which it occurs and/ or the circuit or source feeding it. The means usually employed is a series resistor of relatively very large value with respect to the internal impedance of the gas tube. In such an arrangement small current changes can produce marked changes in the cathode-to-anode voltage impressed across the tube which therefore tendsto act as a constant current device. According to the present invention, the series resistor employed between the source 3| and the auxiliary cathode M has a relatively very small value with respect to the impedance of the auxiliary discharge, e. g., the value of the resistor 34 shown herein may be as little as 75 to 100 ohms. Two features of the present invention are that it is possible to. use a small limiting resistor because of the arrangement'of tube l0 and that it is advantageous to do so as will be explained. The reason why it is even possible to use such a small limiting resistor without drawing excessive current is that the constriction employed in the ionizing discharge herein so greatly increases the impedance'of the auxiliary discharge that it alone almost suffices adequately to limit its own current. The reason why it is advantageous is that by choosing a source 3! which provides a voltage only a little larger than that required to sustain a, going arc in the tube Iii, while at the same time choosing a resistor 34 which is small with respectto the arc impedance a new type of load current control becomes possible.

. Under these conditions most of the total IR drop it will be possible to effect large changes in its auxiliary discharge current by varying the arc impedance. According to the present invention, the arc impedance is controlled by modulating the constriction in the auxiliary discharge, This is accomplished by applying a signal voltage to the constriction modulating electrode [6 to dynamically vary its negative direct-potential bias. This markedly changes the arc impedance by effectively changing the width of the slot 26, through varying the sheath thickness as explained above, thereby changing both the voltage drop across the arc and the current through it. Though each change in the voltage drop across the arc has an opposite effect on the plasma density to that ofthe accompanying change in the current through the arc, the magnitude of the latter change is by far the greater. For example, as a negative signal-voltage increment is added to the negative bias of the constriction-modulating electrode, the plasma sheath about it will thicken; the arm impedance will increase; the arc drop will increase; and the arc current will decrease. The increase in the arc drop will cause the ionizing electrons to move with higher velocities so that on the average each of them will have more ionizing collisions. At the same time, however, the reduced auxiliary current will tend to lessen the number of ionizing collisions, and it is this effect which will predominate. Therefore, the overall effect of such a signal increment will be to cause a. reduction in the plasma density and in the load current which it supports. A characteristic curve obtained in testing a tube of the type shown in Figures 1 and 2 is shown in Figure 4. It indicates that the load current decreased in a fairly linear manner from about (1) ampere down to substantially zero current as the constrictionmodulating electrode bias was varied from zero volts to 1 volt. V

In the circuit of Figure 3 the basing source is shown to be connected between the auxiliary cathode l4 and the constriction modulating electrode I6 by a circuit including the secondary of an input transformer 35. The dotted line 39 is intended to show the shield electrode [5 may be biased from the same source as the constriction modulating electrode or that it simply may be left floating. When the shield electrode l5 is left floating it will become self biasedby collecting thermal electrons from the auxiliary cathode l4 until it has charged in a negative direction to a point sufficiently below the potential thereof to cease to attract electrons therefrom. Where the shield electrode is biased by the use of an external source of potential, it is not necessary to bias it at the same potential as the constriction modulating electrode.

Figure 5 is a modification of the tube shown in Figs. 1 and 2 where two load current modulating means are employed; (1) a constriction modulating electrode of the kind described in detail above, and a load current control grid 36 which may be of the kind(s) described in the above-mentioned copending application, Serial No. 185,745. The result is a dual input tube which may be useful in avariety of applications such as a very low output impedance mixer.

Figure 6 shows a schematic circuit diagram for a mixer circuit which is suitable for using the tube of Figure 5. It is substantially the same circuit as that shown in Figure 3 except that the control grid 36 is represented in this circuit along with a second signal input circuit which circuit comprises a transformer 37 and a biasing source 38 connected in a manner which is selfexplanatory as shown.

While it is preferable for the limiting resistor 34 to have a value which is but a fraction of the impedance of the ionizing discharge, it may have difierent relative values with the following results: (1) it may have a value which up to a certain point approaches or even exceeds that of the arc impedance provided it is employed for applications in which lower values of transconductance are adequate. Up to that certain point increases in the value of resistor 34 will result in progressively lower transconductances since the disparity between the change in one direction in the plasma density which is caused by a given change in the ionizing current and the opposite change in plasma density caused by the accompanying change in the arc drop will become progressively smaller as the value of resistor 34 is increased. (2,) It may have even higher values in a range in which the disparity between the two effects on plasma density which are mentioned above will have diminished to 0 and then reappeared in the reversed sense. This is a range in which the changes in plasma density produced by changes in the arc drop will have become predominate over the others. In this range a characteristic curve plotted in rectangular coordinates as in Fig. 4 will slope in the other direction, and the transconductances represented by it will be negative. (3) It may have a value near zero or be omitted entirely, providing the arc is made to work so hard, for example, by the use of an extremely narrow constriction, that its impedance alone is capable of 'sufiiciently limiting the current. Of course, the resistor 34 may in any instance be eliminated if the source 3! has enough internal impedance to provide the limiting effect or if it be replaced by some suitable equivalent such as the cathode-anode circuit of a discharge device.

What is claimed is:

1. A gas amplifier tube comprising: a sealed envelope containing a gaseous filling; means for carrying a load current through the tube comprising a main cathode and a main anode in cooperative spaced relationship; means including an auxiliary cathode for producing an ionizing discharge to provide a conductive plasma between said main cathode and anode; means adjacent said auxiliary cathode for forming the ionizing discharge into a directed stream having a constricted portion of smaller cross-sectional area than the rest; and means for constrictionmodulating said stream to thereby modulate the plasma density.-

2. A gas tube as in claim 1 in which the means for forming the ionizing discharge into a stream comprises a slotted shield between the auxiliary cathode and said first-mentioned means with its slot in alignment with the path along which said stream is to be directed.

3. A gas tube as in claim 2 in which said constriction-modulating means comprises a slotted electrode having a small total area as compared to said shield and positioned with its slot in substantial registry with that of the shield.

4. A gas tube as in claim 1 and including a control grid between said main cathode and main anode.

5. A gas tube comprising: a sealed envelope containing a gaseous filling; a group of load circuit electrodes within the envelope and including at least one main cathode and a main anode having, respectively, electron-emitting and electron receiving surfaces which define opposite ends of a predetermined load current path; means including an auxiliary cathode for producing an ionizing discharge of electrons along a path within said envelope which does not coincide for all of its length with all of said load current path but includes a portion in such close proximity thereto, for example, coincidental therewith or crosswise or adjacent thereto, that said ionizing discharge bathes the main cathode and extends over all of said current path; a slotted shield adjacent said auxiliary cathode for forming the ionizing electron current into a directed stream including a constricted portion of smaller cross-sectional area than the rest; a slotted electrode positioned with its slot in substantial registry with that of the shield; and terminal means for applying a signal to said slotted electrode to constriction-modulate said stream.

6. A gas tube as in claim 5 and including a control grid positioned athwart said load current path.

'7. A gas amplifier tube comprising: a sealed envelope containing a gaseous filling; means for carrying a load current thru the tube comprising a main cathode and a main anode in cooperative spaced relationship; means including an auxiliary cathode for producing an ionizing discharge to provide a conductive plasma between said main cathode and main anode; means adjacent said auxiliary cathode for forming the ionizing discharge into a directed stream including a constricted portion; means for constriction-modulating said stream to thereby modulate the plasma density; and a control grid between said main cathode and said main anode.

8. A gas tube comprising a sealed envelope containing, a gaseous filling, within the envelope an auxiliary cathode, a main cathode, and an anode positioned in said order, a slotted shield between said auxiliary cathode and said main cathode with its slot in alignment with a straight line therebetween, and a slotted electrode having a small total area as compared to that of the shield and positioned with its slot in substantial registry with that of the shield.

9. A tube as in claim 8 and further comprising a control grid between said main cathode and said main anode.

10. A tube as in claim 8 in which the main cathode is cylindrical and is adapted to emit radially and the main anode surrounds it in most radial directions except those towards said auxiliary cathode.

11. An amplifier circuit comprising a tube as in claim 1 and a source of potential connected between said auxiliary and main cathodes, said source providing a potential somewhat greater, but not very much greater, than the sustaining potential of the ionizing discharge, and said source being connected therebetween over a resistor having a smaller value than the efiective resistance of the ionizing discharge, whereby during the operation-of the tube the average voltage drop between the auxiliary cathode and the main cathode is greater than that across said resistor.

12. An amplifier circuit comprising a gas tube as in claim 5, a source of potential connected between said auxiliary and main cathodes, said source providing a potential greater than the sustaining potential of the ionizing discharge, said source being connected therebetween over a resistor, and a source of potential biasing said slotted electrode at a negative potential with respect to said auxiliary cathode.

13. An amplifier circuit as in claim 12 whereby in the operation of said circuit a sheath of positive ions will surround said slotted electrode and in which said negative biasing potential has a predetermined value at which the current of electrons which the slotted electrode receives from the auxiliary cathode is in substantial dynamic balance with the current of electrons which it gives up to said sheath.

14. An amplifier circuit as in claim 13 in which the potential provided by said first-mentioned source is somewhat greater than the sustaining potential of the ionizing discharge but not much greater and said resistor has a value smaller than the effective resistance of the ionizing discharge.

15. An amplifier circuit as in claim 14 in which said resistor has a value which is but a small fraction of said effective resistance.

16. An amplifier circuit comprising a tube as in claim 5 and a source of potential biasing said electrode at a negative potential with respect to said auxiliary cathode whereby in the operation of said circuit a sheath of positive ions will surround said slotted electrodes, said negative potential having a predetermined value at which the current of electrons which the slotted electrode receives is in substantial dynamic balance with the current of electrons which it gives up to said sheath.

WILLIAM MERLE WEBSTER, JR. GEORGE W. BAIN, JR.

No references cited.

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