US1779380A - Negative impedance circuits - Google Patents

Description

Oct. 21, 1930. H. w; DUDLEY NEGATIVE IMIfEDANCE CIRCUITS Filed Au 1, 1927 15 have appeared not to be meme a, 21; 1930 UNITED STATES T T. OFFICE norms w. DUDLEY, or BROOKLYN, imw YORK, Assumes TO BELL mmnon mom'ronms, mconromm, or NEWVYORK, N. "1., A ooaronsnon or msw- YORK REGA'rivE mrmmncn omcun's This invention relates to a method and means for obtaining negative impedance and particularly to vacuum tube circuits for that purpose. I

An object of the invention is to produce two different types of negative impedance in the same electrical circuit. 1

A further object is to combine these different types of negative impedance to-pro- 1o duce advantageous results in a telephone repeater.

From time to time in the design of theoretical networks for special transmission purposes, solutions have been obtained which hysically realizable because they contained terms equivalent to negative inductances and negative capacities, as Well as negative resistances. The copending application of R. C. Mathes," Serial No. 209,752, filed August 1', 1927, discloses a circuit for obtaining a negative impedance which is the negative at all frequencies of any positive impedance which can be constructed physically. This circuit in a pre- 2:, ferred embodiment is an amplifier comprising two three-electrode vacuum tube stages, resistance-coupled, the input circuit of the first stage being short-circuited, and a 'feed back impedance connected between the out- ::o put circuit of the last tube and the input circuit of the first tube in the amplifier. In this circuit, when an electromotive force is impressed across terminals connected respectively to the input circuit of the first 3; tube between the grid thereof and the feed back impedance, andto the plate of the last tube, any desired negative resistance, negative inductance, negative ca acity, or any complex combination thereo may be ob- 40 tained across the terminals by'utllizing as the feed back impedance a similar network of positive elements having a ratio to the desired elements determined solely by the various constants of the amplifier circuit.

The copending application referred to also discloses a modification of the above described circuit necessitating only one vacuum tube in which the feed back impedance isapplied between the plate circuit and grid circuit of the same tube, a transformer of neglible distortion in phase and amplitude bein inserted between the grid circuit and the eed back impedance, the negative impedance in this case being obtained between terminals connected respectively to the plate of the tube and one terminal of the feed back impedance.

In both of the circuits of the copending application-referred to, the negativeimpedance obtained, whenutilized to balance the effect of positive impedance in an electrical circuit connected across the negative impedance terminals, has been found to be stable only when the constants of the circuits are selected so that the net impedance of the combined circuits is positive; that is the positive impedance of the circuit it faces in order to prevent singing.

In accordance with the present invention -it has'been found that either of the above described circuits besides being utilized to obtain the negative impedance having the characteristics set forth, may be utilized also at the same time to produce between two other oints in the same circuit, another negative 1m edance having different characterhe new typeof negative impedance circuit connected across these terminals, it

is stable only when the net impedance of the combined circuits is negative,that is, when the constants of the circuits are so selected negative impedances must be less than the that the negative impedance across the feed back "impedance terminals is greater than the positive impedance of the circuit it faces to prevent sing1ng.-

A practical application of the circuits of the invention is a repeater adapted for repeating a 'message in two opposite directions with the use of one repeater element. The repeater in this case is formed by balancing the two telephone lines against each other, one line being connected to the particular terminals of the amplifier circuit across which is produced one type of negative impedance and the other to the terminals thereof across which is produced the other type of negative impedance. The repeater thus formed acts for transmission in one direction effectively as a series booster, while for transmission in the opposite direction, it is equivalent'to a shunt booster.

The various objects and features of the invention will be understood from the following detailed description thereof, when read in connection with the accompanying drawing, in which the single figure shows cathodes 3 and 6 respectively 0 tubes 1 and 2 are connected together and may be heated by a common battery 9. The output circuit of tube 1 and the input circuit of tube 2 are coupled through coupling resistance 10 common to the circuits. The battery 11 supplies space current'to the plate 4 of tube 1 through coupling resistance 10, and space current to the plate 7 of tube 2 through resistance R The battery supply resistances 10 and R preferably are made non-inductive and noncapacitive. The grid potentials of tubes 1 and 2 are adjusted to any suitable values by grid batteries 12 and 13 in the respective input circuits of the tubes.

The grid 5 of tube 1 isconnected through branched paths respectively to a terminal' l l and through conductor 15 to a terminal 16.

The cathode 3 of tube 1 is connected directly to a terminal 17. A terminal 18 is connected through a battery 19 to a point in the output circuit of tube 2 between the anode 7 thereof and the resistance R The battery 19 is poled so as to oppose the effect of space current battery 11 and its voltage is made equal to the voltage of space current battery 11 less the potential drop in resistance R L represents a telephone line having a characteristic impedance represented by the box Z, the terminals 21 and 22 of which line are adapted to be connected across the terminals 14 and 17 of the amplifier circuit, and L represents a second telephone line having a characteristic impedance represented by above described, it will be assumed that the line L, is coupled in suitable manner by its terminals 21 and 22 to the terminals 14 and 17 of the amplifier, and that line L is connected by its terminals 23 and 24 to the terminals 16 and 18 of the amplifier circuit. 7

To simplify the explanation of the operation of the circuit, it will be assumed that the impedance 2 of line L is zero and that an electromotive force 0 is applied by the line L to terminals 16 and-18 of the amplifier circuit in such a manner as to drive a current I in the direction indicated by the solid arrow to the left of the figure through conductor 15, impedance Z of telephone'line L and through R, and R (the internal resistance of the space path of tube 2) in parallel. It is also assumed that the impedance Z of line L connected across the terminals 21 and 22 of the amplifier has a finite value, no electromotive force bei g applied acrom these terminals. The resu tant drop of potential in impedance Z due to the current flowing therein will be impressed on the in? put circuit of the amplifier across grid 5 and cathode 3 of tube 1. This voltage will be amplified in the amplifier circuit, and the voltage set up thereby in tube 2 of the amplifier will cause a, current to flow in the outputcircuit of that tube in the direction away from the anode 7. Because of the high impedance of resistance R thereto, most of this current, which will be desig nated at I,,,, is transmitted through battery 19, the impedance 2 of line L conductor 15, and impedance Z of telephone line L in the same direction, as indicated by the dotted arrow, as the original current I due to the voltage 6 impressed across the terminals 16 and 18.

The effect, therefore, is to produce a cur rent flowing through the positive impedance in the circuit (which is where R, is the A. C. plate circuit resistance of tube 2), which current is in phase with that due to the voltage e alone applied across the terminals 16 and 18. When the total current is greater than the latter current the offeet is one of inserting across these terminals an impedance 2', which, as will be explained later, may be made any desired negative impedance. The value of the impedance 2', as is shown in the copending application of R. C. Mathes referred to, depends solely on the values of the constants. of the amplifier circuit, and may be expressed in terms of the resistance R the impedance Z, the net volt- IOU age amplification M from the grid of tube 1 v of negative, impedance for a may be obtained at terminals 16 and 18 by suitably selecting the relative values of the elements given in the equations. p I

It has been determined, however, that to 1 prevent singing effects in the circuit described, it is necessary that the circuit elements be so selected that the negative of the impedance 2' is in general less than the positive impedance 2: in the circuit which it faces, in other words, so that the net impedance of the combined circuits is positive.

The portion of the circuit of the drawing which has been described above is identical with that disclosed and claimed in the copending. application of R. C. Mathes referred to, supra.

Now in accordance with this invention means are provided for obtaining'between planation of the operation ofthis part of this of the drawing.

portion. of the circuit in obtaining this different type of negative impedance, it. will be assumed that an electromotive force e is ap plied by the telephone line L across the terminals 14 and 17 of the amplifier circuit, and

that the impedance'Z of the line L equalszero. It is assumed also that the impedance 2 of line L connected across the terminals 16 and 18 of the amplifier has a finite value, no electromotive force in this case being applied across these terminals.

It will be assumed that the electromotive force 2' drives a current I, through the cir cuit including R and R in parallel, and impedance z of telephone line L in the direc tion indicated by the solid arrow to the right The resultant drop in potential in impedance 2 will cause an increase in the negative potential of grid 5 of tube 1,

resulting in a voltage being set up in the last tube of the amplifier, tube 2, which will cause a current to flow in the output circuit of that tube in the direction away from its anode. Because of the high impedance of resistance R thereto, most of this current, which may be designated as 1 will flow through the battery 19 and the impedance z of telephone line L as indicated by the dotted arrow, in the direction opposite to that of the original current I, due'to the voltage 6 impressed on the amplifier by telephone line L Now if the constants of the amplifier circuit are so selected that the current I driven through the impedance 2 by the voltage set up in tube 2 of the amplifier is made larger than the current I," due to the voltage e impressed across the terminals 14 and 17, the resultant current in impedance 2 of telephone line L will flow in the-direction opposite to that due to the impressed electromotive force 6. This means that in effect a negative impedance Z is'produced across the terminals 14 and 17, facing the impressed E. M. F. This negative impedance Z, it has been found, has such characteristics that it must in general be greater in magnitude than the positive impedance Z it faces to prevent singing in the circuit. 'lVhen the constants of the circuit. are so selected that the current I is less than the current I, the resultant current flowing through the impedance 2 is decreased so that the effect is one of multiplying an impedance by a value greater than unity.

The circuit illustrated and described above forms'a novel form of 21 type repeater in which the impedances Z and ,2, respectively, of two telephone lines are balanced against each other. For proper operation, the constants ofthe amplifier circuit and that of the telephone line are selected so that its net impedance is negative for Z +Z and ositive for 2+z'. Accordingly, this pro uces the effect that for transmission from line L .line L the signaling currents have the same direction with the amplifier in the circuit as with it but, whereas for transmission from line L to lineL the signaling current in-the receivingline L is opposite in direction to that which would be obtained ifathe repeater were omitted. Therefore, for transmission in one direction the amplifier arrangement operates as a series booster,

while for transmission in the opposite direction, the amplifier is equivalent to a shunt booster.

The theory on which the operation of the above .described circuit is based will be clear from the following analysis, in which the characters used in the equations are the same as those referred to in the previous description. i

The fundamental equations for the circults shown in the drawing are:

where and From Equation (4) it is seen that R is I always a positive resistance. K is a positive real number when From Equation (1) it is seen that 2 can be made the negative of any positive lmpedance Z by making Z .R K K similarly from Equation (2) Z, Z0 if 2 KZO so that by adding the resistance in series with Z, the impedance is made Z as desired.

From the foregoing, it is seen that any de sired negative impedance may be obtained as a driving point impedance from either of two points in the circuit of the drawing. This negative impedance may be used as another impedance would be so long as the system retains approximately linear currentvoltage relations, i. e., the tubes are not overloaded and the circuit does not sing.

)Ve will now consider the repeater application in which Z and z are the positive impedances of telephone lines. Here the conditions for all repeater gains, i. e.,

z= 2; Z Z (8) will be examined. Substituting from Equation (8) in Equations (1) and (2), both equations reduce to Kz=z+R 9) The resistance can of course be added on K=1 and Z=z+R (10) in this case are easily calculated since'the same current must flow through both Z and .2. With an electromotive force 0 in Z, the current is while with an electromotive force e in impedance 2, the current 1s so that changing M changes the gains in the two directions giving an unsymmetrical repeater.

If M=2 then i=i', so that this is the condition for equal amplification in the two directions. The Equation 12 shows, however, that a resistance in series with z or Z can be used to control the gain. As this does not change. M the gain in the two directions is changed by the same amount.

It is to be understood, that in accordance with the invention, any suitable means other than the means shown may be utilized for separating the A. C. and D. C. paths in the output circuit of the tubes for obtaining the two types of negative impedance, for example, condensers, repeating coils or inductances may be used for this purpose.

The telephone repeater formed by balancing two telephone lines against each other, as described above, utilizes a resistance coupled amplifier of two stages for obtaining the two impedances of different characteristics facing the telephone lines. It is apparent, however, that the type of circuit disclosed in Fig. 2 of the Mathes application referred to, in which only one vacuum tube is used, the impedance feed back being applied between the plate and grid circuits of the same tube through a transformer of negligible distortion in phase and amplitude, may be utilized also for forming the novel form of repeater described above. I f such an amplifier is used, one of the telephone lines should be connected across the feed back terminals and the other line connected between the plate of the tube and the primary winding of the transformer.

The system above illustrated and described should be considered merely as typical and not as limiting the invention, the scope of which is defined in the appended claims.

What is claimed is:

1. The method of obtaining the negative of a given reactive impedance in a circuit comprising the given impedance and a source of electromotive force comprising deriving a voltage from the circuit, proportioned with respect to the value of said reactive impedance, amplifying the voltage and applying the amplified voltage to the original circuit in out of phase relation to the source of volt from the drop of potential in said impedance, amplifying the derived voltage and applying the amplified voltage to the circuit in out of phase relation to the voltage of said source, the'amplified voltage being of suflicient magnitude to produce in the circuit current of greater amplitude than would be produced by the said source of voltage.

3. The method of securing the negative of a desired reactive impedance between a pair of terminals in a circuit comprising constructing a positive impedance similar in characteristics to the impedance the negative of whichis desired, deriving a voltage from the drop of potential in said positive impedance'in circuit with a source of voltage, am

plifying the derived voltage and applying the amplified voltage to said circuit in series with said pair of terminals and said positive impedance in out of phase relation to the voltage of said source of voltage, the amplified voltage being of suflicient magnitude to produce in the circuit current of greater amplitude than would be produced by the said source of voltage acting throughsaid positive impedance.

4. The method of effectively increasing the eflective value of a given reactive impedance, comprising derivinga voltage from a c1rcu1t lncludmg the given reactlve impedance anda source of potential, proportioned with respect to the value of said given impedance, amplifying the derived ,voltage, and applying the amplified voltage to said circuit in out-of-phase relation to the voltage of said source, the amplification being of a degree to produce in'the circuit a smaller current flow than would be produced 'by said source of voltage acting through the given impedance.- 7 W 5. A combination for obtaining a negative impedance of certain definite characteristics between the terminals of an electrical circuit including a source of electromotive force, said combination comprising an amplifier circuit consisting of a space discharge device having an input circuit and an output circuit, an impedance network in said inputcircuit, the terminals of said network being connected across the terminals of said electrical circuit and across said source of electromotive force independently of said amplifier circuit, and means for feeding. back the amplified '1tages from said output circuit .of said' de vice to the input circuit thereof across said 66 impedance networkin phase with the e'lec-l .work, said terminals of said electrical circuit tromotive force impressed thereon, the characteristics of said negative impedance thus] obtained between said terminals of said electrical circuit being such that said negative impedance is stable only when the net impedance of'the combined circuits is negative.

6. Means for obtaining a negative impedance between two terminals of an electrical circuit comprising, a three-electrode space discharge device having an input circuit and an output circuit, means comprisin an im-- pedance network having terminals %or feeding back the amplified voltages produced in the output circuit of said space discharge de- Woe to the input circuitthereof, means for producing a 180 phase shift of the fed back Voltages, and a conductive'connection independent of said device from each of said electrical circuit terminals to a different terminalv of said impedance network. '7. Means for obtaining a negative impedance between two terminals of an electrical circuit comprising an impedance network connected between said terminals by connections of negligible impedance, an amplifier consistingof an even number of vacuum tube stages and having an input circuit and an output circuit, said impedance network being connected directly in said output circuit comprising an amplifier consisting of an even number ofspacedischarge devices resistance coupled, the outputof the last space discharge device in said amplifier being coupled to the input circuit of the first device in said amplifier through an impedance netbeing connected across said. impedance network, independently of said amplifier, the negative impedance obtained between said terminals having such characteristics that it no is stable only whenits value, which is determined solely by the constants of said ampli- -fier,exceeds that of the positive impedance in said electrical circuit which it faces.

9. Means for obtaining a negative impedance of certain definite characteristics between two terminals of an electrical circuit comprising an amplifier consisting of an even number of space discharge devices, re-

sistance coupled, each ofsaid space discharge devices having an input and an output-circuit and having a cathode, an anode and a. control electrode, an impedance network coupling the output circuit of the last space discharge device in said amplifier-to the inm put circuit of the first space discharge device in said 'amplifier, a connection from one of said terminals to said input circuit of said first discharge device between the cathode 'thereof'and one end of said impedance net-- no work, and a second connection-from the other of said terminals to said input circuit of said first device between the cathode thereof and negative impedance having different charact-eristics, one of said lines being connected between the first mentioned two terminals and the other of said lines being connected between the other of said terminals, said certain characteristics of the first mentioned negative impedance being such that it is stable only when it is greater than the positive impedance of the line which it faces, and said different characteristics of the last mentioned negative impedance being such that it is stable only when it is less than the positive impedance of the line it faces. g

11. A telephone repeater for repeating signaling currents in either direction between two lines comprising a single amplifier circuit, providing between two terminals thereof connected across one of said lines a negative impedance which for stability necessitates that it should be greater than the positive impedance of the line it faces, and providing between two other terminals thereof a negative impedance which for stability necessitates that it should be less than the positive impedance of the line it faces.

12. A repeater for repeating signaling currents in either direction between two telephone lines comprising an amplifier and having such characteristics that for transmission between the lines in one direction the current has the same direction in the receiving line with the amplifier in as with it out and for transmission in the opposite direction, the current in the receiving line is opposite in direction to that which would be obtained if the amplifier were omitted and the two lines were directly connected to each other.

13. A repeater circuit comprising a single amplifier for repeating signaling currents in either direction between two lines, said amplifier being arranged to give any desired ratio between the amplifications of the signaling currents in the two direct-ions of transmission.

14. A repeater circuit comprising a single amplifier for repeating signaling currents in I either direction between two lines, and includcuit proportioned with respect to the value of said impedance, amplifying the derived voltage, and applying the amplified voltage to said circuit in out-of-phase relation to the voltage of said source, the amplification of said voltage bein of a degree to produce in said circuit the e ect of said impedance multiplied by a real negative factor.

In witness whereof, I hereunto subscribe my name this 28 da of July, A. D'. 1927.

Hal ER W. DUDLEY.

comprising deriving a voltage from said cir-

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