US1700393A - Radio frequency amplification circuits - Google Patents

Description

Jan. 29, 1929.

A. WINTHER RADIOFREQUENCY AMPLIFICATION cmcun's Filed May 13, 1925 N i mN NN T W W H i Patented Jan. 29, 1929.

UNITED STATES PATENT OFFICE.

ANTHONY WINTHER, OI KENOSHA, WISCONSIN, ASSIGNOR, BY MESNE ASSIGNMENTS,

TO.ALL-AMERICAN MOHAWK CORPORATION, OF CHICAGO, ILLINOIS, A CORPORA- 'IION OF ILLINOIS.

RADIO FREQUENCY AMPLIFICATION CIRCUITS..

A lication filed May 13, 1925. Serial No. 29,851.

This invention relates trig nprovements in radio frequency amplifyin sets and has for its principal object to provide a new and im- I proved arrangement of inductance couplings associated with successive amplifying audions. More specifically, one of the principal objects of my invention is to overcome the difficulties heretofore encountered as inherent in a vacuum tube or audions and caused by the effect of inter-electrode capacity between the grid and the plate of the tube when used in the ordinary and well-known amplifying circuits.

In audions of standard type heretofore constructed, the surface or capacity-carrying area of the plate is usually the greatest, with the areas of the rid and filamentfollowing in the order named. For instance, in one of the most popular audions now in use it is estimated that the plate has thirty-six times the surface area'of the grid, while the grid has twenty-one times the area of the filament, hence the filament area is 1/21 of the grid area and 1/756 of the plate area. As will hereinafter appear, I make use of thesedifferences in capacity areas in overcoming certain practical difiiculties among which may be mentioned the following.

Among the objectionable features encountered in audion amplifying circuits arises from the capacity effects caused by relatively strong signals or impulses which are apparently transmitted through the component parts of the successive audions and their coupling devices in the form ofstatic charges, which static charges are transmitted through the set with greater rapidity than the ordinary radio frequency currents flowing in their regular and usual channels including the ordinary coupling inductances in their respective plate and grid circuits. In other words, it has been demonstrated that extremely strong impulses such as received from nearby stations are transferred statically through coupling transformers and audions wherein the transformer coils on the one hand act first as capacity elements before the magnetic fields in the transformers have been set up. The plate and circuit elements are also effective in transferring the static charge, this static transference being obviously more rapid than the slower acting inductive operation of the coils due to the lag in the setting up of magnetic fields as suggested. I-Ience, the transformer coils or their equivalent are not as effective in selecting relatively strong signals as they are intended to, and the result is a relatively low degree of selectivity of the receiver. I

Another diliiculty arises through the relatively large capacity of the grid and plate placed in close proximity in each audion. Thus when an impulse in a plate circuit is caused by the well known controlling action of the grid, and a current passes through the plate inductance such as the usual transformer primary connected in the plate cil cuit, this inductance has a magnetic field set up therein which, as it collapses in the reversal of the controlling charge in the grid, tends to prolong the original current in the plate circuit. This prolonged current is quite strong and tends to place a new static charge on the grid separate from that being impressed on the grid circuit externally. This tendency of transference of in'ipulse from plate to grid and grid to plate sets up separate oscillations within the tube and its grid and plate circuits which soon become strong enough to dominate the incoming signals so as to exclude the same. In practice this local tube oscillation is overcome by various means, as, for instance, the use of counterbalancing capacities, or the introduction of resistance in the plate circuit or its related parts. However, such usual methods of oscillation control are objectionable, as they usually introduce losses, lower efliciency and. cause a lowering in selectivity, as any introduction of resistance causes broadness in the wave form of any circuit.

A further difiiculty of the ordinary type of amplifying circuits is that the closeness of the relatively high voltage B-battery currents in the primary coils to the secondary coils of the radio frequency transforn'ier causes a direct leakage of static charges, that might be called stray charges, between the primary and secondary coils. These stray charges cause so-called frying noises in the receiver due to minute and continuously changing impulses imposed on the grids of the successive audions.

In carrying out the present invention, I make use of the fact that the filament usually has far less capacity area than either the plate or grid. Instead of placing the primary inductance of the output circuit in juxtaposition to the plate as in prior practice, I place it next to the filament, and preferably interposed in the filament leads themselves. As will hereinafter more fully appear, this novel arrangement of the output primary is effective in overcoming the objectionable features above outlined. The transference of strong incoming signals due to capacity efi'ect is reduced on account of the fact that the transference is effective mainly between the grid and filament, instead of between the grid and plate. The effect of magnetic reaction of the primary coil tending to set up self-oscillation is also reduced to a minimum by this arrangement, while the great difference in potential heretofore required between the primary and secondary coils of coupling transformers is entirely done away with. The resulting novel form of circuit produces an increase both in efliciency and selectivity of the apparatus.

My invention may best be understood by reference to the accompanying drawing, which shows in diagram form an improved circuit constructed in accordance therewith. The single figure illustrates the main parts of a receiving set comprising two stages of radio-frequency amplification of improved form, as used with a standard audion detector having its output coupled with an audio transformer in the usual manner.

Referring in detail to the parts of the circuit illustrated, an antenna 1 is connected with a grid circuit inductance 2, a part 2 of which forms a common antenna inductance having a conductor 7 leading to ground 7*. The first audion A is provided with a grid 4, plate 5, and filament 6 of the usual form. A variable condenser 3 is connected across the grid inductance 2 for tuning the sa me in the usual manner. The parts herein before. described may be called one of the standard arrangements of the tuned radio f requency amplifying unit. My circuit differs from standard or well known arrangements by connecting the plate 5 through conductors 8 and 9 directly (without effective inductance) to the plus side of a source of high potential which, in the form shown, comprises a B-battery 10 of the usual form. A further difference to be noted in this circuit is the provision of a coil 11 formed by winding the two filament loads 12 and 13 of the first audion A in parallel relation, these leads being wound in the same direction and insulated from each other. At the lower end of the coil 11, that is to say, the end of the coil away from the filament 6, the filament lead 13 is continued by lead 13 to the plus side of a source of filament potential of relatively low value, in the form shown, this source comprising an A battery 14. The minus side of the A battery 14 is connected through leads 12 and '12 to the other filament lead 12 through the lower end of the coil 11 as shown. From the diagram it will be seen that this coil 11 forms the primary of an intermediate transformer 15 coupled to the secondary 16 forming the grid inductance of the second audion B. The grid circuit of this second audion is of the standard form including a grid 16 and variable condenser 17 to tune the secondary or grid circuit inductance 16. The plate and filament parts of the circuit, however, are the same as described in connection with the first audion A, the plate 18 of the second audion, B, being connected through common lead 9 direct to the plus side of B battery 10, and the filament 19 of second tube B being provided with leads 20 and 21 formed in a coil 22 similar to the coil 11 associated with the first tube A. These filament leads are connected at the lower ends through leads Q0" and 21.?to common A battery wires 13 and 12 as shown. The coil 22 forms the primary of second transformer 25 coupled with grid inductances 26 of the grid circuit of detector audion C. The grid circuit of this last audion C is tuned by variable condenser 27 in the usual manner. Other details of the detector audion and its output need not be described as they are arranged in standard form, well known to those skilled in the art.

From the above description it will be observed that my invention is particularly concerned with the transformers 15 and 25 and the provision of primaries therefor disposed on the minus side of the plate battery 10 and preferably in the filament leads of their respective audions. from standard tuned radio frequency circuits will now be observed. By providing the primary of the transformers as described there is no coil or effective inductance between the plates of audions A and B, and their respective sources of potential, but the plates .5 and 18 are. connected directly to the plus side of the high potential source or battery 10. It will also he noted that the filament current in flowing through the conductors of coils 11 and 22 neutralizes itself in coming and going, thus producing no magnetic field as a result of the filament current itself. The radio frequency current drawn through these coils by the controlling action of the grids is drawn alike through both wires since the filaments 6 and 19 simply produce a terminal for these radio frequency currents. From a standpoint of radio frequency currents, both conductors in these coils, as for instance, coil 11, draw on the negative end of the high potential battery 10, because the low voltage battery l l virtually short-circuits these two wires. In other words, both wires 12" and 13 are connected to the negative end of battery 10, the wire 13 being connected directly and the wire 12 being connected indirectly through the battery 14.

The following differences It is now generally believed that currents flow from the negative end of the battery and not from the, positive end. The positive end, however, seems to be the highest in potential and the negative end, as shown in this figure, is neutral or connected to ground. It is therefore apparent that since coils 11 and 22 act as the primaries of the radio frequency transformers 15 and 25 and they are connected to a negative end of the battery 10 and also to ground as explained, they are at a relatively low potential relative to ground and the minus-A battery of lead 12". Thus, the primary circuits of the coupling transformers are at a much lower potential than primaries of standard hook-ups in which the primary potentials are usually very high.

Some of the advantages of the circuit just described may now be pointed out as follows; Since there is a relatively high capacity between grid and plate in commercial forms of audions, there will be a tendency to pass static charges through the audion in the manner heretofore described in connection with standard amplifying circuits, but since in my improved circuit it is necessary to pass the charge to second grid 16 from plate 5, the static effect is considerably minimized since there is no direct induction coupling between the circuits of plate 5 and grid 16 as in the case of standard circuits. In other words, there is no closely coupled coil or inductance in the plate circuit connected with plate 5 thru a secondary coil as in the usual transformer arrangement. Any transference of static charges from audion to audion must take place between the grid and filament, as for instance the grid 4 and filament 6 of tube A, and this static effect is considerably reduced by reason of the fact that the grid to filament capacity in commercial tubes is usually much less than the grid to plate capacity, owing to the smaller area of the filament in the audion, as at the outset of this specification. Therefore, the effect of static charges as a factor in the reducing selectivity is considerably minimized as compared with circuits of standard arrangement.

My improved circuit furthermore, provides means for reducing the tendency of selfoscillation in the audion. This is due mainly to the fact that there are no coils or inductances on the high potential sides of the plate circuits. It will be noted that the plates are connected directly to the positive terminal of the -B-battery 10 and as soon as the grids reverse in charge the plate circuit current stops instantly, and there is no high voltage reactance such as is commonly caused by the plate inductance when its magnetic field collapses. Since the capacity between the grid and filament is much less than between plate and grid, any tendency on the part of the filament to charge the grid is very small and when a reactance does occur in the coil 11 usual primary coils included in the plate cir cuit. Furthermore, the high charges dissipate themselves much more readily through the adjacent ground connections than through the plate, since the latter is effectively isolated by thehigh resistance plate battery 10.

Thus, in my improved circuit the tendency of the tubes toward self-oscillation is greatly decreased as compared with standard hook ups. A much larger number of radio frequency amplifying stages may therefore be employed Without experiencing the usual diflicul'ties due to self-oscillation usually encountered in standard arrangement.

Another advantage is the arrangement of the primaries of the radio frequenc transformers which are both connected to t 1e negative end of the plate battery as already described and are also virtually grounded. Therefore, there is no high voltage in the primaries 11 and 22 having a tendency to leak into the secondary coils 16 and 26 respectively, therefore eliminating one of the main causes of so-called frying noises and disturbances in the tubes. In other words, the primaries of the transformers 15 and 25 are practically at zero potential and therefore do not impart stray currents to their respective secondaries.

It will be understood that my invention may also be utilized in connection with the usual alternative form of transformers, commonly termed auto-transformers, in which a part of a single winding is common to both primary and secondary circuits, as for ex ample in the form of inductances 2 and 2 shown herein for the aerial connections. In such a transformer one of the primary windings connected to the filament leads, such as 12 or 13 of audion A, would be common with and form a part of the secondary winding 16.

Although I have shown anddescribed my invention as applied to a two-stage radio frequency amplifying hobk-up especially adapt ed for receiving, it will be understood that its use is not limited to radio receiving sets of this character but may find a wide application wherever amplifying devices are employed, as, for instance, in radio frequency transmitting sets and audio frequency amplifiers such as used either in radio sets or in long distance telephone service. I

I claim:

1. In combination with an audion having a filament, a source of potential therefor, and an inter-stage output transformer in which the sole effective primary windin consists of a conductor that carries current between the filament and its source of potential.

2. In combination with an audion having a filament, a source of potential therefor, and an inter-stage output transformer in which the sole effective primary winding consists of Lil two parallel wires wound together, and each being interposed in the filament leads of said audion amplifier.

3. In combination with an audion having a filament, an inter-stage amplifying transformer in which the sole effective primary winding consists of two parallel wires wound together, and each being interposed in the filament leads of said audion amplifier, but' with the filament current arranged to flow through said parallel wires ino "posite directions. i

4. In combination with an" audion having filament and plate circuits, an output transformer in which the primary winding is common with the filament circuit and non-inducsaid audion and its source of plate potential.

In combination with an audion having.

filament and plate circuits provided with realtively low and high potential sources, said potential sources having a common point of connection, said filament circuit including a winding consisting of two parallel wires arranged to conduct current in opposite directions therethrough, and an output circuit having! the sole magnetic coupling connection with said filament circuit windings.

Signed at Kenosha, 1s., this 9th day of May, 1925.

ANTHONY WINTHER.

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