US2128553A - Variable amplifier - Google Patents

Description

Aug. 30, 1938.

|-1. BARTELS VARIABLE AMPLIFIER Filed 0013.61, 1955 w INVENTOR HAM? Men-2s ATTORNEY Patented Aug. 30, 1938 UNITED TAT S VARIABLE AlVIPLIFIER Hans Bartels, Berlin, Germany, assignorvto Tele- ,funken Gesellschaft f-iir Drahtlose Telegraphic Germany m. b. H., Berlin, Germany, a corporation of Application October 31, 1933, Serial No. 695,978 In Germany November 12, 1932 2 Claims. (0131791 171 The present invention relates to amplifiers and the like and more particularly to amplifiers incorporating variable amplification control means. I

55 An object of the present invention is to provide an arrangement for avoiding disturbances which usually occur when the amplification of a transmitter station amplifier is varied between limits.

: To control the amplification characteristics of amplifiers, it is usual to vary the grid potential of a thermionic tube. It has been found that in such arrangements while the amplification control device is being operated a variation of the anode direct current occurs in the tube. The frequency of this variation is determined by the rate at which the amplification is varied between certain limits and resultsfrom the current impulses caused by the incremental changes in the anode direct current due to bias potential variations. These current impulses, are transmitted usually through the path oftransmission and cause a disturbing noise at the receiving point. As previously inferred, the rate at which the amplification, control is varied between differ- .ent points of its range of control, will determine the frequency of the disturbing energy. Measurements have shown that the disturbing frequency range is around 1,000 cycles or more broadly in the lower or medium range of audibility. In practice, so called exponential tubes or multigrid tubes are generally used in such arrangements for the control of amplification, variation therein being accomplished by varying the direct potential of the control grid or of the auxiliary electrodes.

According to the present invention the disadvantages of the arrangements heretofore described are avoided by an arrangement associated with the amplifying system which compensates for the variations in the direct plate current with the result that impulses due to the direct anode potential variations are suppressed.

In the drawing, Figure 1 is a curve sheet utilized for purposes of explaining the invention;

Figure 2 is a circuit diagram of a preferred embodiment of the invention wherein all unnecessary parts are omitted; and 50 Figure 3 is a circuit diagram of another preferred embodiment of the invention.

As in the case of Figure 2 all sources of potential, etc., not necessary to a proper understanding of the invention, have been omitted.

The principle involved will be more readily understood by considering Figure 1. In Figure 1 there is shown a dynamic plate current-grid voltage characteristic of a control tube. At grid voltage 691, the direct anode current has the value 11. Since it is operated upon a considerably steep part of the characteristic 1), the amplifica- "tion'factor is comparatively great. A decrease of degree of amplification is obtained in the known 'mannerby an increase of the negative grid bias for instance up to the value 692 to: which corresponds' an anode direct current i2. Since the course of the characteristic is here essentially flatter, the amplification factor of the transmission station is essentially smaller as is known. As is clearly obvious from the diagram, the direct anode current likewise changes with the amplification factor in the same manner, namley i1 to 1'2 as shown in the example. As above indicated when the volume or amplification is varied rapidly between two points, the disturbing current impulse transmits a disturbing frequency through the path of transmission which action has hitherto prevented the use of such control arrangements in audio-frequency circuits with short control periods.

Figure 2 shows an embodiment of the inventive idea by way of example. The control tube I placed in the amplification circuit amplifies the impulse Ee to the value Ea. The amount of amplification of the tube is varied by an automatic or manual control of the grid bias Eg- With changes in the grid bias Eg, the above mentioned disturbing frequencies would appear in the output circuit of tube I. In order to avoid such effect, a second tube 2 is connected in parallel to the output circuit in accordance with the invention, and to which a grid potential is applied, by means of a control resistance 4, which is shunted across the source Eg. The grid of the tube 2 is connected to a point on, the resistor 4 while the cathode is connected to the negative terminal of the bias source. Thus it will be seen that the grid of the tube 2 is at a positive potential with respect to the cathode thereof, from which it follows that variations in the bias potential cause opposite changes in the two tubes; in other words, if a more negative potential is applied to the grid of tube l with respect to its cathode, the potential applied to the grid of tube 2 becomes more positive with respect to the cathode of tube 2. Thus the direct anode currents of the two tubes act oppositely in so far as the output circuit is concerned, that transmission of direct current impulses to the output circuit is suppressed.

In orderto avoid reactions of the alternating voltages of tube l upontube 2, a decoupling impedance 3 is inserted in the anode circuit of tube 2 which may be a choke coil or high ohmic resistance. Obviouslythe circuit can also be so arranged that the grid sides of tube l and 2 are connected in parallel in regard to the bias, while the anode sides are connected in opposite phase. The characteristics of the tubes must be so balanced by nature or by the use of variable resistances, that at the same variation of the grid bias, the anode current variations taking place in the opposite sense, are entirely or nearly equal to each other. simple manner by using tubes of equal type. However, it is not necessary to use identical tubes.

Another mode of execution of the subjectmatter of the invention is shown in Figure 3 and applied to a push-pull circuit. from the representation of this circuit, the grids of like tubes 5 and 6 have as regards direct voltage, the same direct grid potential applied thereto, while as regards alternating voltage, they are connected in push pull. The anode sides are connected in the same manner. The direct plate currents pass in the directions 1 and 8, i. e. in opposite directions, through the output transformer 9, so that their fields eliminate each other whereby the current impulses occurring during control and which would still cause a frequency spectrum in the anode circuit of each singletube are balanced out and cannot be noticed in the output circuit. 1

In the aforementioned examples two single tubes are always shown. Obviously it is also pos- J sible to operate with a single tube equipped with corresponding auxiliary electrodes. For this purpose tubes are to be considered, such as space charge tubes, in which, due to the current distribution within the tube, the resulting direct current in the anode and space charge circuit is practically constant. If, for instance, the space charge current decreases, the anode direct current increases and vice versa. In suitably connecting the single auxiliary electrodes the same effects can be obtained as in the before described examples of embodiments with single tubes. As suitable tube there may be considered for instance a hcxode known as such which is provided with two control grids and one anode and two auxiliary electrodes.

This can be accomplished in a- As will be seen I claim:

1. In an amplifier circuit, a multi-electrode space discharge device, means for varying the amplification characteristics of said device comprising a, circuit arrangement for varying the potential applied to at least one of the electrodes thereof, an output circuit for said device including a utilizing circuit, variations in the amplification characteristics of said device acting also to vary the plate circuit direct current of the device and means associated with the potential varying circuit arrangement and including a space discharge device parallelly disposed with respect to the output circuit of the first named space discharge device and arranged so as to cause direct current to flow in said output circuit in a sense to compensate for the variations in the said plate circuit direct current and thereby substantially prevent communication of said plate circuit direct current variations to the utilizing circuit.

2. In signalling apparatus an electronic tube having an anode, a cathode and at least one grid electrode, an input circuit for said tube connected between the grid electrode and the cathode, said input circuit including a source of biasing potential, the biasing potential applied to the grid being variable for purposes of varying the amplification characteristics of the tube, an output circuit for said tube connected between the anode of the tube and the cathode thereof, a utilizing circuit coupled to said output circuit, a second electronic tube provided with an anode, a cathode and at least one grid electrode, means for connecting said bias source across the cathode and gridelectrode of the second tube but in a sense which is opposite to the connection of the bias sourcewith respect to the cathode and grid electrode of the first electronic tube whereby a bias potential is applied to the grid of the second tube which is in effect of opposite polarity as respects the bias potential applied to the grid of the first tube, an output circuit for said second tube including a connection between the anode of the second tube and the anode of the first tube, said connection including a de-coupling impedance device and a connection including said source of bias potential between the cathode of the second tube and the cathode of the first tube.

HANS BARTELS.

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