US1923254A - Direct coupled amplifier - Google Patents

Description

Aug. 22, 1933. w. BURSTYN r AL 1,923,254

DIRECT COUPLED AMPLIFIER Filed June 22, 1932 INVENTORS Walther Bursiyn and BY Walter TCLUSI'.%

ATTORNEY Patented Aug. 22, 1933 UNITED STATES DIRECT COUPLED AMPLIFIER Walther Burstyn and Walter Tausig, Berlin,

Germany, assignors to Radio Patents Corporation, New York, N. Y., a Corporation of New York Application June 22, 1932, Serial No. 618,707, and in Germany July 29, 1931 12 Claims.

Our invention relates to electronic valve systems and more particularly to electronic cascade amplifiers in which a direct or conductive coupling from the output electrode of one valve to the input electrode of a subsequent valve is provided.

This type of amplifier is known as direct coupled amplifier or continuous current amplifier due to its use for amplifying very slow current variations for relay operation and the like. It has the further advantage of possessing a fiat top frequency response curve comprising an extended band of frequencies, which makes its application especially suited for purposes where increased fidelity of reproduction is of primary importance such as for the amplification of sound and speech currents in public address systems, talking moving picture systems, and the like.

Direct coupled amplifiers are especially suited for amplifying television signals dueto the extended signal frequency bands encountered in television transmission.

It is an object of our invention to provide a direct coupled electronic valve circuit of simple design and of increased efliciency and dependability in operation.

In direct coupled amplifier systems hereinbefore used and described in the prior art, an increased operating voltage is usually required dependent on the number of valves connected in cascade in case it is desired to use a common voltage supply source for operating all of the amplifying valves.

Accordingly, it is another object of our invention to provide a circuit requiring a decreased operating voltage compared with systems used in the prior art.

A more specific object of our invention is to provide a direct coupled cascade system in which a self inductance coil may be used as a coupling means resulting in a simplified circuit arrangement and ease of operation.

These and further objects and aspects of our invention will become more apparent from the followingdetailed description taken with reference to the accompanying drawing, illustrating a few circuit arrangements in which the invention has been embodied. We wish it to be understood, however, that the specific examples presented should be regarded as illustrative only of the underlying principle of the invention which, as will become obvious, is subject to many variations and modifications commg within the broader scope of the invention, as expressed in the appended claims.

Figure 1 illustrates a known circuit presented for the sake of illustration and clearer understanding of the new circuit according to the invention.

Figure 2 shows a novel circuit according to invention.

Figure 3 represents a modification of the circuit according to Figure 2.

Figured relates to another modification of the fundamental circuit of the invention according to Figure 2.

Similar reference characters identify similar parts throughout the different views of the drawing.

Referring to Figure 1 of the drawing, this illustrates a known amplifying system with direct coupling, known also in the prior art as continuous current amplifier, since the output current of the last valve readily follows the slowest variations of the input potential applied to the first valve which latter may be a varying direct current obtained from a photoelectric device, thermo-couple, or the like.

The essential feature of this circuit consists in a direct conductive coupling connection of the anode of the input valve V1 to the grid of the output valve V2 in such a manner that any coupling element except resistance r is dispensed with. In order to secure proper grid bias for the output valve for insuring efiicient operation under the most favorable conditions, a balanced circuit or bridgev system is provided. One branch of the bridge is comprised of the resistors n and 1'2 and the other branch is comprised of the cathode-anode path of the valve V1 on the one hand and the resistance r on the other hand.

The grid-cathode path of the valve V2 forms one of the diagonal branches of the bridge system, the remaining diagonal branch being formed by the power source indicated by the plus and minus signs in the drawing. In such a system, the grid bias voltage of V2 will assume negative values, if the ratio of the direct current resistance of V1 to 1' becomes smaller than the ratioof n to n. For practicalreasons, it

was found necessary to operate direct current amplifiers from the alternating current house networks in view of the high anode voltage required, which can easily be secured by a power pack including the usual transformers, rectifiers, and filter elements.

This makes it necessary to use an input tube From this it follows that immediately after the switching-on of the amplifier, a positive grid bias equal to the full anode voltage, will be applied to the grid of the valve V2, resulting in a high excess load being imposed upon the output valve during the starting period. As is obvious, this may injure or destroy the valve unless special precaution is taken to prevent overloading. One means to obtain this effect consists in the provision of a thermostatic switch closing the anode circuit only after the elapse of a few seconds. Such a switch has the disadvantage of being independable in operation and it would not function properly immediately following the disconnection of the circuit.

Furthermore, it is known to provide a compensation circuit for applying a special grid bias during the starting period. This, however, complicates the circuit arrangement and entails an additional voltage drop, thus necessitating a further increase of the anode voltage and the cost of the apparatus. Also, the provision of a special auxiliary rectifying tube for producing starting grid bias has similar drawbacks.

As a further means for decreasing excess load of the output valve duringthe starting period, it has been suggested to increase the inner resistance of the battery eliminator supplying the amplifier sufficiently so that the excess load is kept .within moderate limits. It is obvious that this method does not constitute a favorable solution of the problem.

In Figure 2, we have illustrated a novel circuit in accordance with the invention for overcoming the aforementioned disadvantages which besides assures increased efficiency of operation. According to the new circuit, the grid of the output valve V2 is directly and conductively connected to the cathode of the input valve V1 contrary to the usual connection to the anode, as according to the prior art illustrated in Figure l.

The drop of the signal voltage supplied by the input valve is produced by resistance r inserted in the cathode lead of V1. This circuit also constitutes a bridge circuit in principle. The resistance r and the direct current resistance of V1 constitute one branch of the bridge and the resistances n and n, the latter being shunted by the cathode-anode path of V2 forming the other branch of the bridge system. The cathode-grid path of V2 again forms the one diagonal branch of the system.

If no anode current is flowing through V1, the grid of V2 will receive a negative bias equal to the voltage drop of T1, thus avoiding the danger of excess load of V2 during the starting. The grid bias voltage of V2 will assume its proper operating value when the anode current through V1 begins to flow, provided all the circuit elements are properly designed. The resistance w shunted by a capacity 0 in the cathode lead of valve V1 serves to provide the proper grid biasing potential for the valve input V1 in a manner well known.

In a circuit just described a choke coil is substituted for the resistance r in accordance with our invention, thus greatly simplifying both the design and operation of the amplifier. As is known, choke coils in contrast to transformers may be designed with sufficient impedance to enable their use for audio frequency amplification. As no further coupling elements in such a direct choke coupling are used, the amplifier will operate highly independent of frequency.

In Figure 3 we have illustrated an amplification system utilizing a choke coil as hereinbefore described. This circuit merely diifers from Figure 2 in that choke coil d has been substituted for resistance r. A further simplification is obtained by omitting-the resistance r2, in which case the value of resistance n has to be increased. Thus, one arm of the bridge system is comprised by the cathode-anode path of V2 in place of the combination of cathode-anode path with resistance 12 connected in parallel thereto. This, as will be understood, makes it necessary to readjust resistance 11 to restore proper balance of the system.

It is obvious that the resistance 12 according to Figures 2 and 4 may also be dispensed with, with 11 properly modified-as pointed out. It is understood that both a resistance 1' and choke coil d in combination may be provided in the cathode lead of V2, thus combining Figures 2 and 3. The resistance 12 which, together with T1 forms a separate direct current circuit from the positive pole of the anode potential source over the cathode of V2 to the negative pole of the source, in some cases has the effect of stabilizing the cathode potential of V2, thus insuring increased balance of the circuit.

The resistance n has to be chosen in such a way that its voltage drop will secure proper operating voltage bias for valve V2. We have also shown in this figure a grid bias resistor w shunted by capacity a for securing proper operating'bias of the grid of the input valve V1 in a manner similar as described in Figure 2. One of the main advantages of the choke coil coupling, as illustrated, resides in the fact of a considerable decrease of the anode voltage required, as compared with the resistance coupling illustrated in Figure 2.

Figure 4 shows a circuit in accordance with the invention similar to Figure 3 for use with direct current heater type valves in which, as is known, the valves are operated in series directly from a direct current supply system. In this case resistances n and T2 are connected in series with the cathode heater circuit as illustrated. Otherwise, the circuit is identical to the circuit shown in Figure 3.

It is understood that the circuit as described may be modified in many ways without departing from the spirit of the invention. This, for instance, a common bulb may be used enclosing the electrode elements of both valves V1 and V2 with the cathode of V1 directly connected to the grid of V2 inside the common evacuated vessel, as is known in multiple tubes.

What we claim is:

1. In an electrical system, a first discharge valve; a second discharge valve, said valves having cathode, anode, and control electrodes; a source of energy; a direct conductive coupling connection from the cathode of said first valve to the control electrode of said second valve; means including circuit connections for connecting the anode circuits of said valves in parallel across said source; and a choke coil inserted in the cathode lead of said first valve.

having cathode grid and anode electrodes; a

source of anode potential supply; a direct conductive coupling connection from the cathode of said first valve to the grid of said second valve; means including circuit connections for connecting the anode of circuits of said valves in parallel across saidsource; impedance means inserted in the cathode leads of said valves comprising a choke coil in the cathode lead of said \first valve for maintaining predeterminedxoperating potentials of said electrodes and for producing coupling potential for said second valve. 4.\In a system as described in claim 3, a resistairce shunted by a bypass condenser in the cathode 'lead of said first valve for producing grid bias potential for said first valve.

5. In a system as described in claim 3, at least one further direct current circuit extending from the positive pole of said source over the cathode of said second valve, to the negative pole of said source. I

6. In an electrical system, a first thermionic valve; a secon thermionic valve, said valves a source of anodepotential supply; means-for directly and ,conduetively connecting the oath-X ode of said first valve to the control electrode;

of said second valve; \means including circuit connection from the anode of each of said valves to the positive" terminal of said source;

"cathodes are used.

further means including circuit connections from the cathode electrodes of said valves to the opposite terminal of said source; an inductance means inserted in a cathode lead of said first valve; and an impedance means inserted in the cathode lead of said second valve for maintaining predetermined operating potential for said electrodes and for producing coupling potential between said valves.

7. In a system as described in claim 6, a biasing resistor in the cathode lead of said first valve shunted by a bypass condenser for producing grid biasing potential for said first valve.

8. In a system as described in claim 6, at least one further direct current circuit extending from the positive pole of said source over the cathode of said second valve to the negative pole of said source.

9. In a system as described in claim 6 in which said valves are provided with cathodes heated indirectly from a separate heater.

10. In an electrical system; a first thermionic tube; a second thermionic tube; said tubes having cathode, anode and grid electrodes; a source of anode potential supply; connections from the anodes of said tubes to the positive pole of said source; a resistance and a choke coil in series connected between the cathode of said first tube and the negative pole of said source; a further resistance connecting the cathode of said second tube with the negative pole of said source; and a direct conductive coupling connection from the cathode of said first tube to the grid of said second tube.

11. In an electrical system as claimed in claim 10 with means for rendering the cathode potential of said tubes independent of the cathode heating current.

12. In an electrical system as claimed in claim '10 in which the tubes with indirectly heated WALTHER BURSTY'N. WALTER TAUSIG.

DIISCLAI Me: R

1,923,254.'++Walther Barstyvnand Walter Tausig, Berlin, Germany. DIRECT COU15LED AMPLIFIER.

Patent dated August 22, 1933.

Disclaimer filed May 11,

1938, by the assignee, Radio Corporation of America. Hereb enters this disclaimer to claims 4, 6, 7, and 10 in the said specification.

[ fiiciat Gazette June 14, 1938.]

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