US2151070A - Amplification control circuit - Google Patents

Description

March 21, 1939. BARTELS AMPLIFICATION CONTROL CIRCUIT Filed Jan. 6, 1937 LESS POSITIVE 2 1 NEGATIVE ems ON 6 Hg '1 POST IVE BIAS GqDECREASES TO mam {I III 1 1 NEGATIVE BIAS 7 INVENTOR HANS BARTELS ATTORNEY' Patented Mar. 21, 1939 PATENT OFFICE AMPLIFIGATION CONTROL omoorr Hans Bartels, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application January 6, 1937, Serial No. 119,257

In Germany November 21, 1935 1 Claim.

My present invention relates to gain control circuits, and more particularly to amplification regulation networks.

In the past there was described an arrange- 5 ment for controlling the degree of amplification in transmission systems utilizing the principle of potential displacement of the grid, or of anotherelectrode, and in which the variation of the plate current of the controlled tube, as regards lo the outputcircuit, is compensated during the control performance in that a single tube with corresponding auxiliary electrodes is used in which'due to the current distribution'within the tube, the resultant direct current flowing in the 16 output circuit remains practically constant in spite of variations of the, bias potentials of the auxiliary electrodes. 7

The present invention relates to a special construction of the above circuit arrangement, with go the use of electron tubes having five, or a greater number of, electrodes, especially hexodes, whereby a variation of the steady plate current, and hence the appearance of D. C. impulses during control performance, is avoided in that the direct controlfpotentials are simultaneously applied to two grids of the tube to be controlled, whereby the value and direction of this applied control potential are to be so chosen that the steady plate current remains practically constant.

30 In hexodes of the type used in receivers employing .automatic volume control it is already known to vary simultaneously the grid bias of the control grid as well as of a further grid, usually one situated near the anode, when fading is being 35 controlled. However, care was never taken that the steady plate current remain entirely or almost constant. In these last named hexodes there is bythe way no ground for such measure.

. In the drawing:

40 Figs. 1 and 2 show graphically the operation of the circuit,

Fig. 3 is an amplifier circuit embodying the invention.

. The functioning of the circuit arrangement will now be elucidated by reference to Figure 1. The curves e 4, e" 4 etc., characterize the dependence of the plate current upon the bias of the control grid at various biasing potentials of the grid situated next the anode. The points A, B, C, D designate points on the individual characteristics which correspond to the same plate current. These working points with different slopes, and also different amplifications, have different grid biasing potentions E'g1, E" 1, etc., of the control 55 grid corresponding thereto. Now, if in order to Jet vary the amplification without varying the steady plate current a transition is to take place in accordance with the invention, from point A of the one characteristic to point B of the other characteristic, it is necessary that besides the change of the grid potential from e' 4 to 6" 4, also the bias of the control grid be brought at the same time from the value E'gl to the value E 1.

Fig. 3 shows an example of the circuit arrangement. The impulses applied to the input termil0 nals I and 2 are amplified by the tube 3, and applied to the output terminals 4 and 5 across transformer 6. The grid G2 next to the control grid G1 has applied to it a part of the potential of the plate current source 1, shown as a battery for the sake of simplicity. The grid G3 has plate potential with the exception of the low voltage drop in the primary winding of the transformer 6. The grid G4, also utilized for the control of the amplification, is connected to the cathode across the direct potential control source Egr. The direct current potential control source Egr consists of a rectifier 9, a condenser 80 and a resistor H having taps. It detects a part of the output energy. A part of the impulses of the direct control potential is also applied to the control grid G1 through an interposed bias battery Egl. Since, in general, the biasing potentials Egl and Eg4 should not be displaced by values with the same proportion in order to vary the amplification at constant direct plate current, it will be of advantage to connect in parallel to the tap point of the control voltage source a non-linear resistor (for instance dry rectifier l2) in series with a variable resistor l3, so as to obtain in this way the non-linear relationship. The source 8 positively biases grid G4.

According to Fig. 3, the non-linear relationship is achieved in that the displacement potential of the grid next to the anode is bridged by a non-linear resistor. Obviously, it is also possible to bridge the control potential applied to the other grid, by means of a correspondingly dimensioned non-linear resistor. The non-linear resistance path between cathode and grid G4 is intended to insure an adaptation, or balance, of the necessary amount of variation of the grid biasing voltages of both grids. If, for instance, the grid biasing voltage of grid G4 is reduced from zero V. to -1 V., then the grid biasing potential of grid G1 would have to be decreased from -10 V. to around -8 V. in order that the same steady plate current may be obtained. When the grid G4 is changed from 0 to 2 V., then the change of the biasing voltage of grid G1 necessary for a constant plate feed current will be from 10 V. to 6.5 V. rather than the anticipated change of the biasing voltage from 10 to -6 V. in the presence of linear conditions. Now, inasmuch as it is not practicable to alter the grid voltage to suit different cases, the non-linear path has been provided between the cathode and thegrid G4.

Fig. 2 shows the dependence of the voltage Eg4 upon Egl. The constant plate currents in, ia2, iag serve as parameters. Fig. 2 can be readily developed from Fig. 1. In Figs. 1 and 2 the same values of E 1 are plotted along the abscissa. Fig. 2 is developed from Fig. 1 by measuring the values of Eg4 corresponding to a number of points along the steady current line passing through A, B, C, D in Fig. 1. These values of Eg4 are, then, plotted with reference to the ordinate of Fig. 2; this produces the curve 2'31. In a similar manner the other two curves of Fig. 2 are obtained, proceed ing from difierent current abscissae of Fig. 1. For a definite plate current the respective values of E 1 and E 1 are plotted. This circuit organization is to be used as an expandor and compressor (compandor) scheme; hence, it is of practical value in all transmission equipment in which a constriction or an expansion of the volume range is desired.

What is claimed is:

In combination with a source of electrical waves, an electron discharge tube including at least a cathode an output electrode, a wave input electrode, an auxiliary electrode, and means maintaining said'auxiliary electrode ata positive potential with respect to said input electrode, an

' output circuit connected to said output electrode,

means for varying the direct current potentials of the inp'ut'and auxiliary electrodes in polarity opposing sense, and a path having a non-linear resistance characteristic connected between said cathode and auxiliary electrode for maintaining the potential relations of the input and auxiliary electrodes such that the average current fiow in the output circuit is substantially steady.

HANS BARTELS.

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