US2235677A - Amplifier for signal transmission - Google Patents

Description

March 18, 1941. s. GUBIN 2,235,677

AMPLIFIER FOR SIGNAL TRANSMISSION I Filed Nov. 30, 1937 A 0/70 ATS/570A Zhwentor Patented Mar. 18, 1941 PATENT OFFICE AMPLIFIER FOR SIGNAL TRANSMISSION Samuel Gubin, Erlton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 30, 1937, Serial No. 177,375

14 Claims.

My invention relates to amplifiers for signal transmission and more particularly to class B amplifiers used as modulators in transmitting.

An object of my invention is to obtain a high- 5 er utilization factor in output transformers used in class B amplifiers.

A further object of my invention is to provide an amplifier circuit of the class B type for modulation that makes possible the reduction in size and simplification in design of an output coupling transformer.

A still further object of my invention is to provide, in combination with a novel class B amplifier circuit, a transformer characterized by tighter coupling and lower phase shift than heretofore possible for use with class B amplifiers.

Another object of my invention is to provide I a balanced amplifier circuit characterized by an output transformer that has no center-tapped primary winding with the attendant disadvantages.

Another object of my invention is to provide a class B amplifier circuit wherein tubes are used in series for the purpose of obtaining operation by a high voltage plate supply of twice the voltage normally used.

Still another object is to provide a bias supply at ground potential for A. (2., in combination with my improved class B amplifier wherein the U0 cathodes of certain of the tubes are operated at a high A. C. potential above ground.

The conventional class B, or push-push amplifier, has a center-tapped output transformer primary, and either half thereof is used only ..5 during each positive half cycle of the signal input wave leaving the other half idle during the concurrent negative half cycle. For a more complete understanding of the action of this type of amplifier, reference is made to L. E. Barton Patent 2,084,180, June 15, 1937, on which my invention is in the nature of an improvement.

In accordance with my invention there is no idle transformer winding. The center tap has been doneaway with and a bridge arrangement of 5 tubes is connected with a primary winding in such manner that the full winding is used for each half cycle, thereby increasing the amount of power that can be handled, by a given size transformer, as compared with the conventional to arrangement. In other words, the output circuit is undivided. When it is realized that modulating transformers in high power transmitters are enormous in size, the weight being measured in tons, the resulting saving in size, aside from .55 other advantages, can be more fully appreciated.

The broad idea of my invention is also adapted to other uses as will be set forth more clearly in the following specification and appended claims. More specific objects of the invention will also become apparent upon reading the specification 5 and claims.

The invention is illustrated in one of its modifications in the accompanying drawing in which Figure 1 is a circuit diagram in simplified form of a thermionic amplifier made in accordance with my invention,

Fig. 2 is a more detailed circuit diagram, with certain modifications, as actually built by me in accordance with my invention,

Fig. 3 is a diagram of a circuit for supplying grid bias to certain tubes shown in Fig. 1.

In the circuit illustrated in Fig. 1, four thermionic vacuum tubes I, 2, 3 and 4 preferably of the class B type, are connected in a bridge circuit resembling somewhat a single phase full wave bridge power rectifier system. An early form of bridge amplifier, using a single tube in one arm and resistors in the remaining arms is shown by Adair Patent 1,383,275. The control grids of the tubes in opposite arms are excited, or driven, in phase for each input half cycle, respectively. Specifically grids of tubes I and 3 are driven in phase and positive in direction during one half of the input signal Wave by means of an input coupling network, e. g. secondary-windings 5 and I, respectively, of an input driving transformer 9. Similarly, the grids of tubes 2 and 4 are driven in phase and in a positive direction by secondary windings 6 and 8 during the other half cycle. The primary ll of the input transformer is connected to a source of signal energy as to an audio amplifier driver tube not shown.

The grids of the class B amplifier tubes are biased, preferably negatively, as by means of cells l3, or other known equivalent, in accordance with the teachings of A. V. Loughren Patent 1,699,110. In some cases it is desirable to employ a positive bias. The operation is such that little or no plate current flows in the no signal condition. However, in this arrangement the apparatus may be greatly simplified (see Fig. 2) by using tubes of the class B zero bias type, made and used in accordance with the teachings of L. E. Barton Patent 2,084,181. Since the oathodes of tubes 3 and 4 are at a high A. C. poten- 5o tial relative to ground, the grid bias supply, as from rectified alternating current is complicated, and the use of zero bias tubes is quite desirable. The power supply for the anodes P is represented diagrammatically by a battery l5, which may be a rectified power supply source, connected across one diagonal of the bridge.

The cathodes F of tubes 3 and 4 are at high alternating potential and require insulation by some means such as indirectly heated cathode type tubes, as shown, or by means of filament type tubes heated by A. C. employing low capacity filament heating transformers, (Fig. 2).

The signal input secondary windings, 5, 6, 'l and 8 must be properly poled relative to each other in order that tubes in one pair of opposite arms are driven in phase, for one half cycle while those in the other pair of arms are driven in phase during the other half cycle.

The output transformer H has its primary it connected as shown across the other diagonal of the bridge, and the secondary ill to the load. In order to show how the transformer primary is utilized fully, the plate current paths are traced as follows: When the signal voltage on the grids of tubes I and 3 increases in a positive direction, current fiows, in conventional terms, from the positive end of supply source l5, through the tube 3, through the primary l9, through tube I, all in series, back to the source, negative end, the flow being indicated by arrows in full line. On the other hand, when the grids of tubes 2 and 4 are similarly excited 180 degrees later, the plate current flows in the opposite direction through the primary winding, the current path through tubes 2 and 4 being indicated by arrows in dotted lines. As shown, tubes 5 and 4 have their space current paths connected in series, the cathode of tube I being connected to the negative end of the source [5, the anode of I to the cathode of tube 4, and the anode of tube 4 to the positive end of source IS. The same is true of tubes 2 and 3.

Referring to Fig. 2, I have shown a 750 watt output modulator circuit as built by me according to the above teachings, although slightly modified. Like reference numerals correspond to similar parts in Fig. 1. Four filament type RCA 805 zero bias tubes were employed, and were energized from a 3000 volt plate supply.

The filaments were energized by power transformers the secondaries only of which are shown at 21, 28, 29 and 30. While separate windings 21 and 28 are shown, a single winding of an ordinary transformer could obviously be used for filaments of tubes l and 2 if the negative end of the anode supply is grounded. However, the windings 29 and 30 for tubes 3 and 4 are well insulated from ground and are designed to offer low capacity to ground, or primary, for high audio frequency potentials.

The input signal transformer diifers from that in Fig. 1 in that two separate transformers were used, being of a type readily available. Electrically the arrangement is similar to Fig. l. The primaries are shown connected to a driver stage using RCA type 845 tubes connected in pushpull relation.

The input and output transformers were designed in accordance with the teachings of class B amplifier practice. However, in the output transformer, since the tubes are in series, the reflected load resistance is made substantially twice the optimum load resistance per tube as determined in accordance with such class B practice.

In conventional three winding output transformers, precautions are necessary to insure that the coupling between primary halves and between primary and secondary is tight in order to reduce serious distortion in the region of plate current cut-off in class B amplifiers. This has been a problem in the usual arrangement. In my circuit however there is only one primary winding and the plate current flows through the entire winding, and it is relatively easy to obtain tight coupling between a single primary and a secondary. The result is that the characteristic class B amplifier distortion is negligible.

In many cases it is desirable to use grid bias in class B amplifiers particularly in the type having a substantial negative bias as in application of Oman, Serial No. 153,574, filed July 14, 1937, same assignee, where a voltage of around 1100 was used to bias the tubes to plate current cutoil. Since the cathodes and input circuits of tubes 3 and 4 are at a high alternating potential above ground and the usual rectified power bias source, as for example shown in said Oman application, there exists the problem of biasing the grids of tubes 3 and 4 from a bias supply at ground potential for A. C.

One way of isolating the bias apparatus from the mains, which are substantially at zero signal potential, is the use of large reactors in series with the leads connecting to the mains. This method may require expensive reactors of special design to maintain high impedance at all signal frequencies and yet to have low impedance to the mains frequency where grid regulation is required of the bias source.

A preferred arrangement for introducing bias is shown in Fig. 3. A bias supply which may consist of a battery, generator, or rectifier of A. C. is shown at 3|. Associated with the bias source is a capacity to ground, 42, which prevents the use of the bias rectifier directly in the grid circuit. A filter consisting of elements as shown in Fig. 3 performs the desired function of isolating the grid from ground for signal potentials, the terminals 23 and 24 being adapted for connection to the terminals 23 and 24 of Fig. l in place of the cell i3. A similar bias supply circuit is adapted to be connected to the terminals 23' and 24 of Fig. 1.

The inductors 35, 36, 3'! and 38, resistors 43, 44, 45, 46, 47, 48 and 50 and capacitors 39, 40, 4!, 42 and 49 which comprise this network are proportioned so as to form constant resistance networks,

where r. R 1 c for reasons given in British Patent 462,530 of 1937. This filter then presents a pure resistance at all frequencies where this relationship holds.

From the above it will be seen that my invention effects a reduction in distortion as well as a reduction in the size of the modulation transformer or reactor used in a class B amplifier combination. While it may appear that the number of tubes has been increased as an offset to the above advantage in size economy, such is not the case. For a given output, say 750 watts, two pairs of parallel connected tubes of the type disclosed above, or two tubes of double the power rating would ordinarily be used. So what I have done is to arrange this given tube complement in such manner as to obtain new and unusual results as above set forth. The reduction in size of the modulation transformer for a given power output, as a result of my invention, becomes very important in transmitters where the transformer is of the large coil cooled type like those used in A. C. power stations. It will further be seen that I have solved the problem of grid bias supply in the case of class B amplifiers having their inputv electrodes operated at a potential that is high above ground for A. C. or signal potentials.

While my invention has been illustrated and described for purposes of convenience in its present preferred application to audio frequency amplifiers of the class B type, it is not intended that my invention shall be limited thereto but may be applied to other amplifiers, e. g. those operating at radio frequency, and wherein it is desired to simplify the apparatus and reduce the distortion. While my invention has been disclosed in connection with thermionic vacuum tube devices of the conventional type as amplifiers it is not intended that I shall be limited to such devices. While I have disclosed an output coupling transformer, I can use an output inductor or a resistor, which may be the load itself. This is made possible by the fact that the output is undivided as distinguished from the conventional divided output circuit, as illustrated by Colpitts 1,128,292. Although I have shown four tubes as constituting the arms of a bridge, a lesser number of tubes can be employed and still maintain undivided output circuit condition.

I claim as my invention:

1. A signal amplifier of the balanced type comprising, in combination, two pairs of thermionic tubes connected in bridge relation, an input coupling. network arranged to control in phase a pair of tubes in opposite arms of the bridge during a half cycle of input signal energy and the other pair of tubes during the other half cycle of energy, and an output transformer connected across a diagonal of said bridge.

2. The invention as set forth in claim 1 characterized in that said tubes are of the class B type operated without bias.

3. The invention as set forth in claim 1 wherein one of the tubes has its input electrodes operated at a high A. C. potential above ground, and a bias supply at substantially A. C. ground potential for said input electrodes.

4. The invention as set forth in claim 1 wherein said tubes are operated at substantially no plate current in the no input signal condition.

5. A signal amplifier of the balanced type comprising, in combination, a source of signals, four thermionic tubes arranged in the four arms of a bridge, respectively, a high voltage supply of space current connected across one diagonal of said bridge, an output coupling means connected across the other diagonal of said bridge, signal means for causing space current to flow alternately through said coupling means, in series through one pair of tubes in opposite arms of the bridge and through the coupling means in one direction, and serially through the other pair of tubes and said coupling means in the reverse direction.

6. The invention as set forth in claim 5 wherein two of said tubes have their anodes connected to the positive end of said voltage supply and the two other of said tubes have their cathodes connected to the negative end of said supply.

7. The invention as set forth in claim 5 wherein two of said tubes have their anodes connected to the positive end of said voltage supply and their cathodes connected to the end terminals of said coupling means, respectively.

8. A bridge signal amplifier of the balanced type comprising in combination, a plurality of thermionic tubes disposed in arms of said bridge, an output coupling device connected to terminals across a diagonal of said bridge, said terminals being at high A. C. potential above ground, one of said tubes having its cathode connected to one of said diagonal terminals, a bias supply at ground potential for A. C., and means connecting said bias supply to the input electrodes of said tube while maintaining said electrodes at high A. C. potential above ground.

9. A balanced type amplifier comprising a pair of thermionic devices connected in series and having cathode, control and anode electrodes, an input coupling network for exciting said devices in phase as signal frequency potentials, and an undivided output load circuit comprising an inductor connected at one end to the anode of one of said devices and at the other end to the cathode of another of said devices.

10. The invention as set forth in claim 9 wherein said devices are included in the arms of a Wheatstone bridge.

11. A signal amplifier comprising in combination, a plurality of thermionic tubes arranged to operate as a balanced amplifier, an anode supply source connected to supply space current to said tubes in series relation, and an undivided output coupling circuit with end terminals connected to electrodes of said tubes in series therewith and characterized by the absence of any intermediate terminal connection tosaid tubes or supply source, whereby full utilization of said coupling means is effected.

12. A class B amplifier comprising in combination, a plurality of amplifying devices included respectively in the arms of a bridge, an anode supply connected across one diagonal of said bridge, output coupling means connected across the other diagonal, and an input coupling network arranged to control in like manner the tubes in one pair of opposite arms of said bridge but in reverse phase to the control of the tubes in the other pair of arms for causing alternating flow of anode currents in said output coupling means.

13. The invention as set forth in claim 11 wherein said output circuit comprises an inductor the end terminals of which are directly connected to the output electrodes of said tubes but with no space current flowing in said circuit in the no signal input condition.

14. The invention as set forth in claim 9 wherein said inductor is the primary of an audio frequency transformer, characterized in that the coupling between primary and secondary is substantially tighter than in the case of a divided primary winding.

SAMUEL GU'BIN.

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