US1978208A - Distortionless amplifier - Google Patents

Description

Oct. 23, 1934, w. A. KNOOP DISTORTIONLESS AMPLIFIER Filed April 18, 1931 Fla. /1

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INVENTOR W. A. KNOOP BY A TTORNE V Patented Oct. 23, 1934 UNITED STATES PATENT OFFICE Application April 18, 1931, Serial No. 531,106

5 Claims.

This invention relates to repeating devices arranged to be operated by distortionless amplifiers of the space dischargetype.

An object of this invention is to compensate for distortion introduced in the signals by a vacuum tube amplifier, whereby the grid potentialplate current characteristic is made to approximate a straight line.

, In an amplifying and repeating system it is desirable to have as nearly as possible a distortionless or straight-line amplifying characteristic in order that the original signals be faithfully retransmitted. The response of the sys tem toincoming signals of positive polarity should "be of the same order as the response to incoming signals of negative polarities. When substantial, departure from this condition exists, the amplified signals are, of course, distorted. An amplifying system having a'curved characteristic produces anamplified signal which is not symmetrical with respect to a normal no-signal output. In general, the positive impulses of an incoming signal are amplified more than the negative impulses and the result of such unsymmetrical amplification has been found to be a time-distortion of the component impulses of the incoming signal. For example, when a repeating relay is employed to retransmit such time-distorted signals, the retransmitted signal "differs materially in length of component impulses from theoriginal incoming signal. Timedistortion of this sort is not permissible in the art of faithful amplification and repeating of signals and has contributed to the needfor dis- "tortionless amplifying systems in communicapensator'tubearranged inparallel, andthe out: put voltage or .the. compensator tube is superimf Heretofore various methods of and means for compensating for the distortion introduced in signals by vacuum tube amplifiers have been employed, such asthe known push-pull arrangement of vacuum tubes, and the compensating tube arrangement disclosed in U. syPatent 1,856,373, May 3, 1932, toE. TgBurton. In the push-pull arrangement the distortion introduced in the positive signals by one tube is equalized by the. distortion introducedflin the negative signals by the other tube. so that the ,output signals of positive and negative polarities are symmetrical. In

- the compensating tube arrangements of the" above patentthe input voltage is impressed on.

the grid circuits of .an amplifier and a composed on the signal in theinput orthe output circuit or the amplifying tube 'to make the input voltage-output current characteristic of the system symmetrical on both sides of the zero point of the operating range of the input voltage.

According to the present invention two similar vacuum tubes are arranged to have their output circuits interconnected in opposite relation through separate windings of an impulse repeating device in such a manner that the normal space currents of both vacuum tubes are bal-. anced. The two tubes have common sources of normal grid voltage, normal plate voltage and filament heating energy. Hence, when the normal space current effects are once balanced, this balance is substantially maintained when the normal voltage sources change, for example, when plateor grid batteries deteriorate, or when the filament heating energy may vary, as the tubes have similar characteristics. In addition,,a suit; able resistance is connected in the common output connection of the two tubes, thereby providing a resistance coupling of said tubes. The effect ofthis resistance will be discussed in detail below. The impulses to be repeated are impressed on only one tube so that the amplifier tube, which is normally balanced by what may be termed a balancingtube, is in a condition to produce an amplifier output current free of any distortion which might be introduced by such amplifier tube, tooperate the repeating device. A better understanding of the inventionwill be had from thefollowing detailed description and appended claims when taken in conjunction with the accompanying drawing of which:

Fig. 1 showsa preferredembodiment of the invention; and I Fig. 2 shows several graphs of the grid potentialeplate current characteristics of the amplifier circuitshown in Fig. 1. Referring to Fig. 1', theamplifier tubell is arranged twithitsinput circuit connected to the conductors 19, which designate any system over' whichan incoming signal, which itgisdesired to amplifyandfretransmit, is received. The plate circuit of tube 11 extends through the right hand windings of a three-position polar relay 12,'meter 13,1resistance element 14, battery 15, and back to its filament and plate. The balancing'tube 16 has its plate circuitarranged to extend through the left 'hand windings of relay 12, meter 13, resistance element 14, battery 15 andback to its filament, and, plate. The plate circuits are respectively connected through the windings of relay l z in opposite directions so as to produce; a difierential efiect in the relay. With equal turns in-thewindingsbf*relay 12, and with equal currents in the plate circuits of tubes 11 and 16 flowing therethrough, this differential effect results in a condition of no resultant flux produced in the relay core and the relay armature remains in a neutral position.

Battery 17 and potentiometer 18 serve to maintain equal normal negative biases on the grids of tubes 11 and 16, respectively. Whereas the tubes 11 and 16 have been assumed similar, two commercial tubes are seldom found which are identical in their characteristics, and the same normal grid bias applied to two such tubes would not in general result in equal normal space currents. Potentiometer 18, as shown, provides an adjustable normal grid bias for the balancing tube 16 so that any unbalance in the direct current components of the normal space currents of both tubes will be avoided and the relay armature will normally be maintained in its central position. For purposes of illustration, it has been assumed in Fig. 1 that tube 16 requires less normal grid bias than tube 11 to produce the same space current. In practice, however, the potentiometer 18 might be arranged to affect the voltage of the grid of tube 11, or the tubes could be exchanged in position to permit the operation of the potentiometer. Any difference in space currents will be indicated on the meter 13, which is a differential meter, well known in the art, indicating the inequality ofcurrents flowing therethrough.

- It has been found, however, that when the incoming signals received over conductors 19, which diagrammatically represent any system over which a signal is received, are impressed on the grid of amplifier 11, the grid potential-plate current characteristic of the amplifier circuit is sufficiently curved to produce undesirable distortion in the signals in the output circuit. This is substantiallyovercome by inserting in the path common to both plate circuits the resistance element 14, thereby causing the volt-ampere output characteristic of the'system to approach considerably closer to the trace of a straight line. The effect of this resistance can be understood from the following discussion.

When an incoming signal on system 19 alters the normal grid potential of tube 11 so as to cause a decrease in space current in this tube, the current flowing through the right hand windings of the relay and the total current through resistance 14 are decreased. There is, therefore, a decreased voltagedrop in the external plate circuit of tube 11 and the voltage of battery 15 effective on the plate of tube 11 is increased. This increased plate voltage tends to producean increasedspace current in tube 11 in' accordance with the plate a coupling effectbetween tube 11 and tube 16. In

the case assumed above, the plate voltage of tube 16 is likewise increased but not however, by the,

same amount as in tube 11 as its plate voltage is effected only by the change of voltage'drop across resistance 14 whereas the plate voltage of tube 11 is also affected by the change of voltage drop in the impedance of the relay winding in the plate circuit of tube 11. The increase of plate voltage of tube 16 results ina change in its space current in i accordance with the plate voltage plate current plate potential change in tube 11 is dependent upon a value of grid voltage other than the normal bias which is maintained on tube 16. However, the net effect of the resistance 14 has been found to be a flattening of the amplifying characteristic of tube 11 and an optimum value of this resistance was found as indicated below. The

changes in the circuits of Fig. 1, when an incoming signal in system 19 causes an increase in space current of tube 11 will not be discussed as they may be readily understood from the foregoing discussion. Of course, the net change of current through the windings of relay 12 produces an effective flux which throws the relay to one or the other ofits transmitting positions depending upon the incoming signal and a substantially un distorted signal is retransmitted. I

In Fig. 2 is shown a chart wherein several characteristics of milliamperes effective to operate the relay 12 plotted against the grid potentials for different values of resistance used in the element 14. The characteristic assumes a shape of graph A when the resistance element 14 has a value of 1000 ohms; graph B when it has a zero value; and graph C when it has a value of 400 ohms. These graphs are the resultof actual tests and therefore it is readily seen that with a resistance of 400 ohms used for element 14, the distortion is completely eliminated from the output signals particularly over the operating range of the input 15 voltages used in telegraph communication.

What is claimed is: p

1. In a signal impulse repeating system, a source of incoming signal current impulses, a cir-; cuit arrangement havingan input and output circuit, said input circuit being responsive to said impulses, a'second circuit arrangement having an input and an output circuit, saidinput circuit being operative independently of said impulses for correcting distortion of impulsesinthe output circuit of the first mentioned circuit arrangement, the output circuits of said circuit arrangements having a common path, aplurality of relaying devices in said circuit arrangements, an output voltage source in said path and common to said devices, an impulse receiving device for repeating impulses received from said relaying devices, characterized'in this that an impedance element is arrangedin said common path for causing said repeating device to, operate to either of two'positions every time a signal current impulse is impressed on the'input circuit of the first mentioned circuit arrangement whereby the incoming signal" current 'impulses'are repeated in .the form in which they wereoriginally transmitted.

,2 ..In va signal impulse repeating system, a source of incoming signal current impulses of positive and negative polarity, a circuit arrangement havingran" input and an output circuit, said input circuit being responsive to said impulses, a second circuit arrangement having an input and an output circuit, said input'circuit being operative independently of said impulses for correcting distortion of impulsesin the outputcircuit of the fi'rst mentioned-circuit arrangement,

the output circuits of said circuit arrangements having a common path, a plurality of space discharge devices interconnecting said input circuits and said output circuits respectively, a voltage source common to said space discharge devices and included in said path, an impulse repeating device for repeating impulses received from the output circuits of both of said discharge devices,

windings on said repeating device arranged to permit the current in the output circuit of the first mentioned circuit arrangement to control said repeating device in response to an incoming signal impulse of positive polarity and to permit the current in the output circuit of the second mentioned circuit arrangement to control said repeating device in response to an incoming signal impulse of negative polarity, and an impedance element in said common path whereby the currents that are permitted to how through said windings are so regulated that the incoming signal current impulses are repeated in the form in which they were originally transmitted.

3. A signal impulse repeating system according to claim 2, wherein said impedance element is a non-inductive resistance arranged in said common path to increase and decrease the voltage drop across said second mentioned circuit arrangement in response to incoming signal impulses of positive and negative polarities, respectively.

4. In an impulse repeating system, an amplifier circuit comprising a plurality of space discharge devices, each device comprising a filament,

grid and plate element, said devices having common sources of heating energy for said filaments, of normal grid potential and of normal plate potential, repeating means interconnecting the plate circuits of said devices wherein a balance of effects of the normal output currents of said devices is maintained, a source of signal impulses of positive and negative polarity, means for impressing said signal impulses on only one of said devices to operate said repeating means, and a resistance element connected in a common path in the plate circuits of said space discharge devices for, respectively increasing and decreasing the potential drop across the plate circuit of the other of said space discharge devices in response to impulses of positive and negative polarity.

5. In a signal impulse repeating system, an amplifier circuit comprising a plurality of space discharge devices, each device comprising anode, cathode and impedance control elements, said devices connected with their output circuits in pushpull relationship and having common sources of anode potential, cathode heating energy and normal impedance control element potential, means to produce equal normal space currents in said devices, additional means to impress signal impulses on the impedance control element of one of said devices, a common connection path in the anode-cathode circuits of said devices, and an impedance arranged in said common connection path whereby the system produces undistorted amplified signal impulses.

WILLIAM A. KNOOP.

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