US2149361A

US2149361A - Discharge tube amplifier

Info

Publication number: US2149361A
Application number: US510296A
Authority: US
Inventors: Rath Karl
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1931-01-21
Filing date: 1931-01-21
Publication date: 1939-03-07
Anticipated expiration: 1956-03-07
Also published as: GB398826A

Description

March 7, 1939-. K. RATH v DISCHARGE TUBE AMPLIFIER Filed Jan. 21, 1951 nuuuu'o'nuni INVENTOR Karl Raf-h LBY nrroRne r Patented Mar. 7, 19.39

PATENT OFFICE- mm m Ammrma assignments, to Radio as man. New York, N.

1., salmonl by meme Corporation of America,-

' aoomrationotneiaware I Application January 21, 1931, Serial No. 510,296

11 Claims. (Cl. 178-171) My invention relates to apparatus for and methods of operating electric amplifiers utilizing discharge devices, such as evacuated valves or radio tubes in a cascade arrangement and,

more particularly, to amplifiers having a direct connection of one amplifying unit or valve with a succeeding amplifying unit or valve in a cascade amplification system.

Amplifiers ofthis type are known as direct coupled or direct current amplifiers and are primarily designed for the amplification of direct or unilateral electric current variations. This function makes it necessary to use a direct coupling between the succeeding amplifying valves, since inductive or capacitative devices, if used as the coupling means between the valves, would suppress the direct current component of a unilateral current variation.

There is, however, another field where ampliflers with a direct coupling present meritorious advantages; namely, in the amplification of" pure alternating currents, such as speech or microphone currents, picture signal currents as used in picture telegraphy and television and the like; in short, in all such cases where a faithful amplification and reproduction of the initial current-variations is essential and vital for the successful operation of an output or translating device operated from the amplifier.-

However, direct coupled amplifiers tend to become unstable, which renders their use for amplifying current impractical.

Accordingly, an object of my invention is to provide a novel improved direct coupled amplification system.

A further object of my invention is to provide novel direct coupled amplifying circuits, which. are stablein operation, efiicient and give faithful reproduction for radio, telephone, talking picture signals in public address systems, picture telegraphy, television and the like.

. Still a further object of my invention is to provide a direct coupled amplification cascade system, which is substantially free from reaction of one amplifying unit or valve upon the preceding valve or valves insuring full stability of operation. 4

Another object of my invention is to provide means in connection with a direct coupled amplification system, using discharge valves in which the reaction of the current variations of one valve upon the biasing operating voltage on a preceding valve or valves is such that a. substantially constant biasing. potential is main- 55 tained for such preceding valve or valves and a tendency of the biasing potential to "drift" is avoided.

These and further objects and features of my invention will become more apparent as the following description proceeds, taken with reference 5 to the accompanying drawing, in which I have shown, for purposes of illustration, circuit diagrams illustrating a few embodiments of the novel idea underlying the invention. I wish it to be understood, however, that the description 10 and exemplifications according to the drawing should be regarded as illustrative only of the underlying principle and the novel underlying idea of the invention, which, as will become obvious,

is subject to many modifications and variations 16 coming within its broader scope, as set forth in the appended claims.

Figure 1 represents a well-known direct coupled amplification circuit, which I have shown for the purpose of illustration only and clearer 20 understanding of the novel features of the invention.

, Figure 2 shows a simple circuit arrangement of one form of embodiment of the invention.

Figure 3 is a similar system showing a complete operative circuit with an alternating current supply source.

Figure 4 illustrates an alternative system for practicing the invention, consisting of a structural combination of the novel parallel-feed type 30 circuit in accordance with the invention and the older series-feed or staggered type of circuit, as known in the art, and shownby Figure 1.

Figure 5 illustrates a further combined circuit arrangement comprising the system of the invention and a series-feed or staggered circuit arrangement, which forms the subject matter of my Patent No. 1,927,846, issued Sept. 26, 1933.

Referring more particularly to Figure 1, this illustrates a direct coupled amplification system 40 as hitherto known in the art, comprising in the example shown three stages, each consisting of a vacuum valve l, 2 and 3 respectively of usual construction and design, including cathodes 4, 5 and 6, grid .

electrodes

1, 8 and 9 and plate elec- 5 trodes I II, II and I 2, respectively. A high voltage potential source, supplying a voltage equal to the sum of the anode voltages of the individual valves, indicated by the plus and minus signs, is arranged in shunt to a potentiometer I5, from which are tapped suitable connections to the different electrodes of the amplifying valves. The

grid electrodes

8 and 9 of

valves

2 and 3 are directly connected to the plates of the preceding valves l and 2 respectively and resistors l3 and ll, called coupling resistors, are placed between the anode of these valves and suitable tap points.

shown at c and e on the potentiometer. The cathodes of the

valves

2 and 3 are also connected to potentiometer taps, as shown at b and d, displaced towards the negative side of the voltage drop on the potentiometer in respect to the points and e respectively, in such a manner as to provide a suitable compensating potential between points b and c and d and .e respectively, counteracting the plate voltage drop along the resistors l3 and It thus removing the effect of the high plate voltages, which would otherwise be directly appliedto the'grids 8 and 9 of

tubes

2 and 3 and render operation impossible. The input has been shown as applied directlyto the grid 1 of the first valve 6 and the negative terminal point a on the potentiometer, whereby the input currents are introduced at the terminals 18 andthe output or translating device; such as loud speaker, relay, etc. inserted in the anode circuit of the third valve is connected to the output terminals at H.

The operation of this systemis as follows:- Assuming, for instance, a variation of' the grid cathode potential of the first tube, dependent on an incoming signal, this will produce, as the case may be, an increased or decreased plate current draw through the resistor IS in the plate circuit and, accordingly, vary the potential applied to the grid 8 of the second tube, which, in its turn, will produce a similar variation of the plate current of the succeeding tube, which variation will be applied to the translating device conmission of energy through the circuit is blocked.

This is due to the fact that the plate current, such as, for instance, of tube 3 tracing the current from the plus tothe minus terminal of the high potential source, has to pass the part (1-1: of the potentiometer from which are tapped the operating potentials for the preceding valves l and 2. If, for example, the plate current of valve 3 undergoes a ehange, it will produce a corresponding change of the drop between d and a and, accordingly, shift or displace the potentials at the tap points a, b within this portion of the potentiometer, which will react on the grids; 1 of valve 8, in such a manner as to allow the grid operating potential to drift toward unstable con-. ditions. As is obvious, this disadvantage becomes the more eifective the higher the number of amplifying valves connected in cascade or, in other words, the greater the final plate current variations of the last valve or valves of the systems, thus producing a cumulative reaction on the preceding valve or valves, one of which may drift or shift its operating point to such extremes on the operating characteristic-either towards its saturation value or towards its zero value-that it becomes inoperative and causes the amplifier to entirely block the transmission of the current variations.

This disadvantageous condltioncould be prevented by making the current through the potentiometer sumciently largeso that its order of" magnitude is comparatively high as compared to the order of magnitude of the plate current variations of the valves. "As these plate currents are usually of about 1 to milliamperes for ordinary amplifying valves 'and about 20 to 30 milliamperes for power or output valves, it is necessary to have at least one ampere current flow through the potentiometer to prevent appreciable reaction effect, provided furthermore there are only a few amplifying stages-say about 2 or 3. In this way,

renders its application prohibitive for the purpose of broadcasting, radio, telephony, etc. Assuming, in the present case, an average plate voltage for each valve of about 100 volts, 2. high potential on the potentiometer of about 300 volts would be required, which will result in a resistance for the potentiometer of 300 ohms, provided the required current flow of 1 ampere, as above assumed. This will give a heat energy expenditure in the potentiometer equal to 1 300=300 watts, which is excessive, and would make .the operation too expensive and 'uneconomical, especially for the purposes above stated, such as broadcasting, television, etc. Therefore, lower potentiometer current values have to be chosen, in which case, however, the reaction and instability phenomena become appreciable and it is, therefore, the object of the present invention to provide other means for rendering the amplifier stable and dependable in operation.

The new idea underlying the invention consists broadly in that a plurality of potentiometer circuits, each including an amplifying valve, are arranged substantially in parallel relationship to a common high potential operating source and that the control or grid electrodes of the amplifying valves are connected to suitable potential points of the preceding potentiometer circuit. This is basically distinct fromthe prior circuit arrangement above described, which uses a single potentiometer arrangement, from which are tapped all the amplifying valves, in a staggered fashion. I

A simple circuit diagram embodying this idea is shown by Figure 2. The high potential which, in this case, is equal to the plate potentialrequired for one valve alone, is again shown to be supplied from a source indicated by the plus and minus signs. The valves I, 2 and 3 are all connected substantially in parallel to the potential supply, each in series with a suitable resistor or potentiometer l8, l9 and 20, respectively. Contrary to the prior arrangement, in which the plate is connected to the grid of the succeeding valve, in the novel arrangement according to the invention the grid of one valve is connected to the cathode side of the preceding valve, as shown in Figure 2, according to which the

grids

8 and 9 of

tubes

2 and 3 are connected to suitable tap points I and g respectively on the series potentiometers l8 and I 9 for the tubes i and 2. The input is again applied at terminals i6 between the grid and cathode of the first valve, a suitable tap on the potentiometer l8 being provided for adjusting the correct grid biasing potential for valve I. The

I which the loud speaker present Figure 2, the output terminals being designated in the drawing by the numeral II, to

or other translating device is to be connected.

The operation of this new system is as followsz-With no current variations being applied to the input terminal ii, a certain zero or rest ing operating plate. current will ilow through all of the valves shunted across the plate current supply. The resistance of the series potentiometers ll, l8 and 20 is preferably of the order of the resistance of the valves, in which case the maximum potential drop change will-take place on the potentiometer, dependent on a change of which is exactly 3 volts below the potential of the cathode of valve I; that is, at a potential of 97 volts, so that, assuming equal potential drop through all the valves, thegrid '8 of the tube 2 will be at a 3 volt negative potential in respect to its cathode 5, whichit is assumed, in the present example, corresponds to the proper and favorable grid biasing operating potential on the rectilinear portion of the. grid voltage-plate current operating characteristic curve of the valve. The same conditions hold for the valve 3, which receives its operating grid bias from potentiometer is of the preceding valve 2. Supposing now that the input currents supplied at terminals l6 increase the grid potential of valve 1, then, as is obvious, this valve will drawan increased current and the drop across the potentiometer l8 will increase;

that is, the potential of the tap point will also be increased and, accordingly, the .grid potential of valve 2 also becomes more positive, whereby this valve will also draw an increased current, which is greatly amplified as compared to the current through the valve I. The valve 2 will act in the same manner on valve 3. As all the valves are connected in shunt to the operating voltage potential source, it is clearly seen that the currents from one valve do not aflect a preceding valve, as in the case of Figure -1 and, accordingly,

it will be obvious the tendency of reaction and instability is overcome. Furthermore, a basic difference consists in that the potential values on the potentiometers l8, I9 and 20 of the individual valves are being displaced in the same direction; that is, either towards points of increasing or decreasing potential, so that the relative potential diiference applied between the grids remains substantially constant and 'a drift .of the operating potential bias is prevented, which makes for complete stabilization and dependability of the circuit system. On the other hand, returning to Figure 1, from a simple theoretical analysis that the potential points determining the grid-cathode bias of the tubes are displaced in oppositedirections, depending on changes of succeeding plate cu'rrents, which has the contrary effect to the system according to the invention shown by Figure 2; namely, of tending to cumulatively increase the tendency to become unstable and drift of the entire circuit system.

\ Referring to Figure 3, I have shown a complete wiring circuit of an amplifying system, which is essentially identical to Figure12 and contains an e arrangement for direct operation from an alternating current source, as shown at 2|, such as, for instance, directly from an alternating current house net work. The valves I, 2 and 3, in this case, are assumed to be equipped with filaments 4, B and l for alternating current heating by direct connection to the secondary windings 2|, 22 and 28, respectively of an input transformer 24 supplied from the source 25. The mid tap point of the secondaries 2i, 22 and 23, in

a well-known manner, serve as cathode connecting terminals. The remaining parts of the circuit are identical to Figure 2. The high voltage plate supply is provided through a rectifier 21 connected to a further secondary winding 26 of the transformer 24 and a filter or smoothing device of usual design, comprising

parallel capacities

28 and 28 and a series inductor 303 It is, of course, obvious that, in the place of direct byalternating current or indirectly heated cathodes may be used, of known construction, such as is shown in the following Figures 4 and 5. It is, furthermore, possible to the cathodes, so-called heating provide direct current heated cathodes, such as by connecting the open ends of the filaments, according to Figure 2, to a suitable point on the potentiometers l8, l9 and 20, respectively.

The circuit in accordance with the invention presents a further important advantage, especially when an alternating current supply arrangement is used, as illustrated in Figures 3, 4

and 5. As is well known, the direct currents as supplied from such an arrangement, in most cases, contain an alternating current ripple, ,which produces a disturbing hum in the output device connected to the amplifier, such as loud speaker. In order to suppress this hum to an inappreciable value, it'would benecessary to use very bulky and expensive smoothing arrangements, such as smoothing filters, as illustrated in Figures 3, 4 and 5. It was, therefore, found necessary to provide further means, in addition to the usual filter devices, for minimizing or suppressing the eil'ect of the alternating current hum. For this purpose, it has been proposed to provide a neutralizing or compensation arrangement, which consists in that a neutralizing or counteracting potential is applied 'to the grid of one or a number of the amplifying valves, whifih counteracts the variations producing the hum note in the output of the amplifier. It is a special advantage of the circuit of my invention thatit does not need such additional means and that it inherently acts to suppress any transmission or amplification of ripple or hum notes occurring in the direct current potential supplied from the rectifying system. .This is due to the fact that, with a change of the anode current, the drop in the individual potentiometer shunt circuits comprised by the valves and their series potentiometers undergoes a change in the same direction, so that the relative potential diiference between grid and cathodes of the valves remain substantially constant. Thishas already been pointed out above and is also the reason for the stability of the system.

Referring to Figure 2, it will be seen that, with a variation of the plate potential, the potential drop through the. circuit comprised by .valve l and potentiometer I 8 and the circuitcomprised by valve2 and potentiometer i9 will vary by substantially the same amount and the potentials between grid and cathode of valve 2 will be shifted in the same direction, thus maintaining a constant relative value and preventing the variations of the high-potential, due to a ripple or hum component, from aflectingyalve 2 and being amplified and transmitted to valve 3 and to the output or translating device.

The new amplification system in accordance with the invention, such as described hereinbefore, is subject to many variations and modifica tions, as is obvious, embodying the underlying inventive principle. One. of these modifications consists in a combination with a series feed system above referred to and as known in the art, according to Figure 1.

Figure 4 constitutes a combination of the circuits of Figuresl and 2, providing a common potentiometer I5 for the first valve and the two succeeding valves, the latter being arranged accordingtothe parallel feed arrangement of the invention. Accordingly, only one plate potential tap point; namely,for valve 1 is provided at c, the

valves

2 and 3 being operated in parallel relationship by'the same plate potential. The grid 8 of the second valve is directly connected to the anode of the first valve, which includes a coupling resistor H in its plate circuit connected to the tap point e on the potentiometer. The cathodes 5 and 6 of the

parallel valves

2 and 3 are joined together, each including suitable cathode lead series potentiometer resistors l9 and 20 and the common cathode terminal connected to the tap point b on the potentiometer at a position towards the negative terminal relative to the plate potential tap point 0. The

input is again applied to the grid and cathode of the first valve I, which is given a. suitable bias by shifting the tap point a of the cathode 4 in a positive sense on the potentiometer and the output is again supplied from the terminal l1 connected in the common anode lead for the parallel

connected valves

2 and 3. I have furthermore shown in this figure indirectly heated cathodes 4, 5 and 6, provided in the well-known manner with

separate heaters

32, 33 and 34, respectively, operated in parallel from the secondary winding 3| of an input transformer 24 supplied from an alternating current supply source 25. The high plate potential is provided in a manner similar to Figure 3 through the further secondary winding 26 of the input transformer 24, rectifier 21 and smoothing arrangement.

If, in a system according to Figures 4 and 5, similar valves are used, the two last valves operating in parallel are equivalent to a power valve of twice the capacity, producing high current variations necessary for operating the translating device, such as a loud speaker. It is, of course, also possible to connect the first two valves in parallel arrangement and provide the third valve in series thereto, or any other mixed combination may be provided, as the particular case may require.

Referring to Figure 5, this represents a combination of a parallel feed direct coupled amplification system, as shown by Figure 2, with a series feed system, as forms the subject matter .of Patent 1,927,846, issued Sept. 26, 1933, consisting substantially of a Wheatstone-bridge coupling arrangement between one valve and a sucgrid bias potential variations in accordance with the unbalancing of the bridge circuit produced by input current variations applied to the input valve. In its preferred embodiment, this system comprises at least two potentiometers l5 and 35, according'to Figure 5, connected across the high potential supply terminals, whereby the plate electrodes and the cathode electrodes oi succeeding valves are connected to proper points,

each on one of said potentiometers in a stag-' gered fashion, in such a manner as to provide the proper grid bias operating potentials for the valves, which, as in the usual circuit, have their plates directly coupled to the grid electrodes of a succeeding valve. This is shown in Figure 5,

according to which the anode It ofvalve l is.

tial level difference :c, 11 between the points i and of the potentiometers and i5 respectively corresponds to the proper operating grid biasing voltage required for the valve 8. The common plate terminal of the tubes 2 and3 is'connected to a proper tap it on the potentiometer 35. The input is again applied atHi, whereby a proper grid bias for the input valve l is procured by suitable taps l and h on the potentiometers 35 and I5, respectively. The remaining elements and connections are identical to Figure 4. Although, as has been pointed out, a series feed arrangement has a tendency to become unstable, it is readily understood that a'combined system, as per Figures 4 and 5, with only two effective series stages, will give sufilcient stability and dependable results in cases where only a few am-- plifylng stages are required.

1. In a cascade amplification system, comprising a first valve and a second valve to becontrolled by said first valve, each of said valves having a cathode, a control electrode and a plate and being connected to, a common potential source substantially in parallel relationship, individual potentiometer means in series with each of the cathode leads of said valves and a direct current connection from the control electrode of said second valve to a. tap point on the potentiometer in the cathode lead of said first valve.

2. In a cascade amplification system, comprising a first valve, a second valve to be controlled by said first valve, said valves having cathode, grid and plate electrodes, a common plate potential supply source, individual series potentiometer means in each of.the cathode leads of said valves and a direct conductive coupling connection from the grid electrode of said second valve to a tap point on the potentiometer of said first valve.

3. In a cascade amplifier comprising a plurality of vacuum tubes having cathode, anode and control electrodes, 9. common anode current supply source, a. plurality of circuits connected to said source and arranged in parallel relationship with respect to the source, each circuit including a potentiometer and one of said tubes, said tubes being arranged at the positive ends of said circuits, conductive coupling connections from the control electrodes of said tubes to a point on the potentiometer circuit including the preceding tube and means for applying input voltage vari- 8,149,861 a I I 5 8.In-aca scadeampiifierasdescribedinclaim.

ations to the control electrode of'the first of said 4. In a cascade amplifier, the combination of a plurality of amplifying valves each having cathode, anode and grid electrodes, a common anode current supply source, a plurality of circuits connected to said source and arranged in parallel relationship with respect to the source, each including a potentiometer and one of said valves, said valves being arranged at the positive ends of said circuits, direct coupling connections from the control electrodes of said valves to a point on the cathode side of the potentiometer circuit including the preceding valve, means for applying input voltage variations to the grid of the'first valve and further means for connecting a translating device in the common anode supply lead for said valves.

5. In a cascade amplifier comprising a plurality of amplifying units each having cathode, grid,

and plate electrode; 'a common plate potential supply source for said units, the anodes of said units being connected to the positive pole of said source and the cathodes of said units being connected to the negative pole of said source; in-

dividual biasing resistance means in each of the cathode supply leads of said units; and a direct conductive coupling connection from a tap point on said biasing means to the grid of the succeeding amplifying unit. a

6. In a cascade amplifier comprising a plurality of thermionic amplifying units each having cathode, grid and plate electrodes; a common plate potential supply source; individual biasing potentiometers connected to the cathode of said am y ng units to form individual branch circuits therewith connected across said source-in substantial parallel relationship; and a direct conductive coupling connection from the grid of said amplifying units to a .point on the potentiometer in the preceding branch circuit including the preceding amplifying circuit.

7. In a cascade amplifier as described in claim 6. means to render the potentials of the cathode ofaaldimitsindependentoi'thecathcdeheatlng 6 in which the cathodes of the, alternating current heated type are used with a separate heating transformer secondary winding for each individual cathode.

9. In a cascade amplifier as described in claim 6 in which units with indirectly heated cathodes are provided for said amplifying units.

10. An amplifier comprising a first amplifying unit; a second amplifying unit, each having a cathode, anode and control electrode; a commonanode potential source; biasing impedance means connected to the cathodes of said units to form independent branch circuits together wlthsaid units connected across said source in substantial parallel relationship; and a direct coupling connection from the control electrode of said second unit to an intermediate point of said impedance means, said impedance means being designed to secure proper biasing potential for the control electrode of said second unit relative to its cathode. a v

11. An amplifying device comprising in combination, first, second and third amplifying tubes each having a filament, a control grid and an anode, a transformer having separate secondary windings connected to said filaments, a source of positive potential, a connection between said anodes and the positive terminal of said source, a resistor connected to the midpoint of the secondary winding of the first'tube filament and to the negative terminal of said potential source, connections between the grids of the first and second tubes and spaced apart points on said resistor, a second resistor connected to the midpointof the secondary winding of the second tube filament and the negative terminal of said po-' tial source.

' KARL RL'I'H.