US1925568A - Neutralizing system - Google Patents

Description

Sept. 5, 1933. w. VAN B. ROBERTS 1,925,568

NEUTRALIZ'ING SYSTEM Filed Jan. 7, 1927 T 1 1 I I W 1 1 INVENTOR WALTER VAN B. ROBERT TORNEY Patented Sept. 5, 1933 UNITED STATES PATENT OFFICE Q 1,925,568 a I I I NEUTRALIZING SYSTEM Walter van B. Roberts, Bronxville, N. Y., as-

signor to Radio Corporation of America, a cor-- poration of Delaware i I Application January 7, 1927. Serial No 159,532 '3 Claims. (01. 179171) This invention relates to amplifiers, detectors, repeaters, or other equivalent arrangements. In general it relates to arrangements wherein energy is introduced, at one set ofterminals and 5 taken off at another set of terminals in a changed condition. The change may be either of current voltageor power characteristics. The particular device relates to amplifier circuits wherein an inherent capacity coupling exists which produces undesirable results and means are provided for counterbalancingor neutralizing this capacitycoupling. s

It will perhaps be appreciated in the present state of the art of vacuum tube circuits that whenever an undesirable feed-back coupling or such a coupling as to'progressively increase its effect of any oscillationslon the input circuit from the output circuit of an amplifying device in such a way as to produce spontaneous oscillations; that a magnetic coupling between the input circuit and the output circuit may be used which will react on the input circuit so as to' cut down feed-back or reactive effect to a degree .below the limits at which spontaneous oscillations are effective. Various arrangements have heretofore been effective and particularly disclosed and described in various patents.

The patent to Hartley 1,183,875 dated May 23, 1916, shows one type of arrangement wherein a counter-feed-back is usedto reduce the amount of direct feed-back. In the same way: this invention uses a counter-feed-back. coupling. The present invention is novel in the factthat it is rigorously a Wheatstones bridge. On account of this possibility of perfect balance, it is possible to operate a regenerative detector after a blockingstage of amplification and use. a very critical amount of regeneration without stirring up current'in the amplifier input and thus throwin the detector in and out of oscillation.

In analyzing the undesirable oscillations used in an amplifier or-equivalent arrangements, two general features must be taken into considerationr First, the arrangement by itselfoftentimes has a tendency to produce spontaneous self-sustained oscillations'which will continue of their own accord if suflicient coupling is present. Secondly, undesirable oscillations may be introduced into the output connections of the device which will react through the arrangement and into any other means which may be connected to the input connections. With the present arrangement this second difiiculty has been over-' come so that both spontaneous.self-sustained oscillations and reactive oscillations'on the output introduced between the filament and plate. 'The circuit are prevented from effecting the input circuit to. such a degree as to effect any means connected tothe input circuit.

It is understood how a'capacity coupling oftentimesexists between the grid and plate of a three 6 or more element vacuum tube which produces undesirable effects betweenthe input and output circuits. In this invention this capacity coupling is counter-balanced or neutralized by a proportional capacity coupling between the plate and 5 cathode of the vacuum device. This capacity coupling bears a relation to the natural capacity coupling between the plate and grid determined by the magnetic couplings between the two circuits. 1

Further and more definite objects will be apparent in connection with the'following specification,.claimsand drawing of which Fig. 1 represents a simple adaptation of my arrangement,

Fig. 2 shows the equivalent bridge for the purpose of analyzing the theoretical'balance of the system, and

Fig. 3 shows one way a series of such amplifiers I may be coupled together to produce several stages of amplification.

Fig. 4 shows a variation in construction.

Fig. 5 shows still another arrangement 'Fig. 6 showshow a plurality of tubes may be used with a common battery.

Referring more particularly to Fig. 1, l represents the tuning arrangement shown as acondenser but might as well beany other equivalent arrangement, 2 and 3 make up the corresponding element which is shown as an inductive coil with an associated adjustable tap 4 directly connected to the battery 5 and the output coil 6. The grid and lfilament of the vacuum tube 7 as indicated as F and G are directly connected to the opposite terminals of the coils 2 and 3.

Assuming that the capacity 8 represented by dotted lines is a natural inherent capacity between the grid and plate elements of the vacuum tube 7 then a corresponding capacity 9 may be size of this second capacity 9 will be determined by the degree of coupling between the grid and plate circuits through the coupling coils 2 and 3. It can be readily seen that if the tap 4 is adjusted to the midpoint between the grid and the filament and the external capacity 9 is the same as the natural capacity 8, then any current which happens to be flowing in the output'coil 6 due to voltage applied to coil 6 will divide equally at the tap 4; part flowingthrough the capacity 8 and part flowing through the capacity 9. Of course, this will not be true if current is flowing between the filament and any other electrode so as to upset the balance but this will be true for all oscillations that are produced externally as from circuit 10 into the coil 6 regardless of the oscillations produced in the coil 6 through the operation and functioning in the proper manner of the vacuum tube 7.

In this way it can be seen that extraneous, undesirable oscillations are prevented from affecting any circuit which happens to be coupled to the input circuit of the tube 7. In addition to this it can also be seen that the coupling between section 3 of the coil 2 which includes the output circuit will produce a counterfeed-back into the.

grid circuit so that spontaneous sustained oscillations of the tube itself will be prevented. In this way both types of oscillations may be eliminated from the circuit.

If we substitute equivalent elements of the circuit shown in Fig. 1 into an equivalent bridge arrangement it will result in a diagram somewhat similar to that shown in Fig. 2 where F, G and P indicate filament, grid and plate, respectively and the tap is indicated at 4, separating the part 3 from'the coil 2. The inherent capacity between grid and plate is indicated at 8 and the introduced counter-balancing capacity at 9. Now if oscillations are introduced on the coil 2, 3 through the operation of the tube they will necessarily be reproduced in the coil 11 but on account of the counter-feed-back qualities of a coupling between section 3 and the coil 2 no self-sustained oscillations of thetube will be permitted and also any oscillations which might be introduced on the coil 11 will be balanced out between the terminals G and F on account of the-inherent symmetry of the system.

This arrangement may also be adapted for use with a multistage amplifier arrangement such as is shown in Fig. 3. Where 1, 2, and 3 are, respectively, similar elements as are shown in Fig. 1,.

but instead of having a direct connection from the plate'to the taps 4, a large capacity condenser 12 is used. These condensers 12 prevent the fiow of .high potential directly from the plate connections to the taps 4 from the filament circuits which would, in this way, cause a short circuit of the direct current flow. Also choke coils 13 may be used to prevent short circuit of the oscillating current produced in the vacuum tubes. In this way oscillations may be introduced in the coils 2, 3 transferred through the coupling 14- to another set of coils 2, 3 amplified again and taken off through the coupling 15. The high potential battery may be connected to the terminals 16 and 1'7 as indicated by plus B and minus B.

Another way of expressing the method of neutralization might be as follows:--If full signal strength is applied between the grid and filament to coils 2 and 3 and the rotor of the tuning condenser is at filament potential, eliminating hand capacity .effects'then at the same time current in the output circuit will have no effect upon the a potential dilference between the grid and the rate means. The space current, that is the current which flows in the ordinary sense between the filament and plate of the vacuum tube will, through direct magnetic coupling of the coil 2, counteract the effect of the capacitative coupling between grid and plate while the current pro duced by extraneous application of voltage will be counteracted through the two circuits leading to imaginary condenser 9 connected between the plate and filament thereby maintaining perfect balancing. If the magnetic coupling is so chosen as to prevent undesirable oscillations at the highest frequency of the range used, then it will also prevent oscillations at any other frequency and the neutralization for external sources of potential will be automatically taken care of on account of the symmetry of the arrangement.

However, it is not intended to be limited to the used of a connection tap 4 located exactly in the middle of the coil 2 but it might as well be at any convenient point along the coil, provided, the condenser 9 were adjusted to compensate for the difference produced by the unbalance. It is even advantageous in practice to use a tapquite close to the filament end of the coil or a separate feedback coil of a few turns compared tothe number of turns of the grid coil. Other equivalents might as well be substituted for those shown to the extent which such devices are well known in the art. In Fig. 4 the B battery may be connected direct to filament, so as to be available for more than one tube. Electrically this figure is the-same as Fig. 1 with the addition of coil 5. If the arrangement of Figure 5 is used, the input coupling coil 10 may be coupled to any part of coil 2, as no current is produced in any part of coil 2 by voltage introduced into the load circuit- Fig. 6 is another way of making this possible. Fig. 5 also shows the steady current supplied to the platethrough a choke coil 13, whose apparent capacityto high frequency currents is considered to be part of condenser l.

It is advisable to make capacity 4 much larger than capacity 3, as this reduces the feed back of voltage generated by the tube, and also allows the use of a very'high resistance at'Z, in fact, makes the addition of resistance 7 unnecessary in most practical work. r

In Fig. 6 the application of the invention is to to receiver employing a one way repeater followed by a regenerative or oscillating detector. More than one stage .of amplification may be used if desired. When the filament is lighted, the interpedances between plate and gricland between plate and filament. A blocking condenser 8.. is inserted to keep the constant potential of the plate 011 the grid. 1% is an auxiliary by-pass condenser and 15 represents the primary to thedetector in series with counter feedback coil 5.

This invention enjoys the following advantages: great stability, the rotor of the input tuning condenser is at filament, potential, anytype of load or output circuit may be used, and it is comparatively free from possibility of parasitic oscillations. e l

Having described my invention, I claim:

1. In a wave repeating system, a space dis charge device for repeatingpotential variations comprising an input and an output electrode between which two. electrodes appreciable capacity exists and a-filamentary cathode, a pair of condensers in series connected between the input electrode and said filamentary cathode, an output circuit for utilizing the variation repeated by said device, said output circuit being connected across the output electrode and the common point of said two condensers, a counterbalancing coupling between the filamentary cathode and the output electrode and a source of current in series with a choke coil connected between the filamentary cathode and the output electrode.

2. In a wave repeating system a space discharge device for repeating potential variations having a filamentary cathode and an input and an output electrode between which appreciable capacity exists, a tuned input circuit adapted to have potential variations impressedthereon, an

output circuit for utilizing said potential variations, a pair of reactances in series connected between the input electrode and said filamentary cathode, said output circuit being connected between the. output electrode and a point intermediate said two reactances, a capacitive counterbalancing coupling between the filamentary cathode and the output electrode and a source of current connected between the filamentary cathode and the output electrode.

3. In a wave repeating system, a space discharge device for repeating potential variations having a filamentary cathode and an input and an output electrode between which appreciable capacity exists, a tuned input circuit adapted to i have potential variations impressed thereon, an outputcircuit for utilizing said potential variations, a pair of reactances in series connected between the input electrode and said filamentary cathode, said output circuit being connected between the output electrode and a point intermediate said tWo reactances, a capacitive counter-balancing coupling between the filamentary cathode and the output electrode, a source of current connected between the filamentary cathode and the output electrode and an impedance in series with a blocking condenser connected across said input and output electrodes.

WALTER VAN B. ROBERTS.

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