US2631198A - Direct current amplifier - Google Patents

Description

March 10, 1953 w. PARlsoE DIRECT CURRENT 1X1\l1l--I...IIFIERl Filed March 1l, 1950 March l0, 1953 w. PARlsoE- 2,631,198,

' DIRECT CURRENTr mvlPLrFnsRy Filed March 11, 195o z'sHEETs--SHEET 2 4' maw. 294 @Wa/M7 Patented Mar. 10, 1953 UNITEDSTATE`S PATENT OFFICE poration of Illinois' Application March I1, 1950, Serial No. 149,156

This invention relates to direct current ampli-v fiers capablev of delivering an output current of reversible direction to a load and, more particu-- larly, to bridge circuit amplifiers of this character having" electron tubes in at least two, 'se= ries arranged, bridge legs. It is an object of the invention to providev an improved amplifier of that character.

In one common form, a bridge circuit used as an amplier has four legs containing impedance devices with a power source connected across two opposed corners of' the bridge. A load clevice is connected across the other two opposed cornersv and the current therethrough may be controlled by variation of the impedancevalue of anyone or more of the impedance devices forming the fourv legs of the bridge. l

, Where a substantial voltage or power ampliiication is required, .a vacuum tube is employed as the impedance device for the variable iinpedance leg of the bridge and where greater et@ ficiency and amplification are desired each of the four bridge legs includes a tube, all as well understood in the art. A problem arises, however, in the control o i any two tubes which are arranged in seriesA between the power source connections because of the fact that such two tubes operate at different voltage levels and, accordingly, cannot be directly driven by a single D. C. signalvoltage, that is, the signalr voltage cannot be applied directly across the grid and cathode of both tubes. A

It has previouslybeen proposed to solve this problem by driving the lower voltage level tubes directly and utilizing additional tubes for driving the higher voltage level tubes. This ar'- rangement is satisfactory operationally but has the obvious disadvantage of requiring two extra tubes. The present invention is concerned with the control of both the high and low voltage` level tubes without the necessity of additional tubes. e

According to one embodiment oi the present invention, the tube operating at the lower voltage level is driven directly by the signal while the other tube, arranged in series therewithr and operating at a higher voltage level, is driven by the voltage across a biasing resistor arranged inv the plate circuit of the lower voltage level tube. The grid voltage, and hence the plate current, of the high voltage level tube is thereby varied negatively or oppositely with respect to the plate current of the lower voltage tube, that is, when the plate current of the lower voltage tube increases, the plate current of the higher voltage tube decreases. The relative values of these twoV plate currents can be maintained such that: their sumremains constant by selection .of

2 Claims. (Cl. 179-171) a biasing resistor having a proper value with respect to the characteristics of the driven or high voltage level tube. This relation, or'sorne selected Variation therefrom, is desirable in most amplier bridge circuits and the method ordea ter-mining the proper value of theA biasing re-- sistor, for the particular results desired, will be explained subsequently herein. 1

An amplifier incorporating the various fear l turesof the invention is well adaptedrto use in teleiautographic apparatus since it is capable of handling the necessary power while employing standard receiver tubes and since it isresponsive to direct current signals which iiuctuate at speeds varying from zero cycles per second'to the frequencies attending the most rapid hand-g writing. The amplifier will in fact respond vac Icurately to frequencies inV the audio range er higher. Accordingly, the Various embodiments of the invention shown in the drawings will'be described below as particularly adapted to that use, but it will be understood that the invention isAV not limited thereto.

It is another object of the invention to proivide an improved direct current bridge circuit amplifier having a large amplification factor.

It is another'object of theinvention to pro vide an improved direct current bridge circuit ampliiier having very stable characteristics about the null, or zero load'current region.

It is another object of the invention to pro' vide an improved directV current bridge circuit' amplifier havingV high power eiciency.

It is another object of the invention to provide an improved direct current bridge circuit' amplifier having the advantages specified above while being inexpensive to manufacture and reliable in operation.

It is another object offthe invention to providean improved: direct current bridgeV circuit amplifier having an. amplifier tube in two lbridge' legs arranged in series between terminals cfa voltage source one of these tubes driving the other tube, without the necessity of additionalr control tubes,v such that the sum of the plate currents of the two tubes is maintained substantially constant. e. y f u This invention, together with further objects and advantages thereof; will best be und.er-'stoodI by reference to the following description taken connection with the accompanying drawingsand d its scope willbe pointed outv in the appended claims.. n c

In the drawingain which like parts are indi-- cated by like reference numerals?k A Fis, l isa Circuit diagram of tele-autoerapbic apparatiley in'whch a D. C.- bridse amplifier ern-1;

bodying the various features of the invention may be employed to advantage;

Fig. 2 is a circuit diagram showing a D. C. bridge amplier constructed in accordance with one embodiment of the invention;

Fig. 3 is a circuit diagram similar to Fig. 2 but illustrating another method of introducing the control signal;

Fig. 4 is a circuit diagram illustrating another form of amplifier constructed in accordance with the invention; and

Fig. 5 is a graph showing typical characteristics of an amplifier constructed in accordance with the invention.

The tele-autographic apparatus illustratedV in Fig. 1 may be similar to that described and claimed in the application of Robert Adler, Serial No. 81,'109, filed March 16, 194:9,V entitled Improvement in Follow-Up Apparatus and Systems, issued January 29, 1952, as Patent No. 2,583,720, and assigned to the same assignee as the present invention, and, accordingly, rvvill be described only briey herein.

The apparatus comprises a transmitting station 8 and a receiving station 9 connected by a transmission line I0. The transmitting station includes a stylus II with which a message or other form of intelligence may be written or drawn on a writing surface I2. The movements of the stylus I I are transmitted by a rigid arm I3 to pivotally connected links I4, I5, and I6, the latter two links I5 and I6 being pivotable about aligned, fixed axes I5a and IBa respectively. The link I5 is mechanically connected through an arm I1 -to the movable element of a variable inductance element I8, whereby pivotal movement of the link I5 will cause a variation in the inductance of the coil I8.

Similarly, the link I6 is connected through an arm I9 to a variable inductance coil 20 and I pivotal movement of the link I6 causes a change in the inductance of tha-t coil.

It will be apparent upon inspection of the arm and link arrangement that movement of the stylus along the curved X axis indicated on the writing surface I2 Will leave the link I5 stationary but will cause a pivotal movement of the link I6 and a consequent change in the inductance of the coil 20. Similarly, a movement of the stylus II along the curved Y axis indicated on the writing surface I2 will leave the link I6 stationary but will cause a pivotal movement of the link and a consequent change in the inductance of coil I8. The link mechanism including the links I3, I4, I5, and I6, shown schematically in Fig. 1, is more fully described and is claimed in a copending application of Robert Adler, entitled Improvement in Translating Apparatus and Follow-Up Systems, filed April 2, 1949, having Serial No. 85,236, issued January 29, 1952 as Patent No. 2,583,535 and assigned to the same assignee as the present invention.

The variable inductance coil I8 in combination with a fixed capacity condenser 2I forms a resonant circuit for controlling the output frequency of an oscillator 22, which will be termed herein the Y oscillator, since its output frequency is determined by the position of the stylus II along the indicated Y axis on the writing surface I2. The output of the Y oscillator 22 is carried by a conductor 23 to the transmission line I0 for transmission to the receiver station I I.

Similarly, the variable inductance coil in combination with a fixed capacity condenser 25 vforms a resonant circuit for controlling the fre-V quency of an X oscillator 26. The output frequency of the X oscillator, since it is controlled by the variable inductance coil 20, is a function of the position of the stylus II along the indicated X axis on the writing surface I2. The output of the X oscillator 26may be connected by a conductor 21 to the same transmission line I8 Which carries the signal from the Y oscillator 22.

At the receiving station, the two signals are separated by the lters 32 and 33, the Y filter 32 passing only the Y signal frequency band While the X filter 33 passes'only the X frequency band.

The Y signal passes from the Y filter 32 through an amplifier 34 and a limiter 35 to a discriminator 36.

The discriminator 33 may be of the type described and claimed ini application Serial No. 81,709 referred to above, and, accordingly, will not be described in detail herein. For the purposes of this application, it will be sufcient to say that a direct current signal voltage is proi-v duced thereby Whose magnitude is a function of the frequency of the incoming signal from the limiter 35 and of the inductance of a coil 36a` forming a part of the discriminator circuit. For every frequency within the predetermined band of Y signals, there is a corresponding value of inductance of the variable inductance coil 36a which will result in a zero signal output from the discriminator 36.

The output signal of the discriminator 36 passes through a D. C. amplifier 31 and to the rotor 38 of a D. C. motor 39 Whose eld may be The motor 39 also drives a receiver stylus 43A through a linkage system including a rigid arm 44 and links 45, 46, and 41. More specifically, the rotor 38 of the motor 39 is mechanically connected to the link 4I and the link 45, all of these elements being rotatable about a'xed axis 46a. When the rotor 38 rotates through a given angle, the link 43 will rotate through the same angle and cause the stylus 43 to move along the curved Y axis indicated on a receiver station Writing surface 48. k

In order that the operation of the circuit, shown in Fig. 1, may be understood to suiiicient degree to permit a full appreciation of the present invention, an operation of the apparatus shown in Fig. 1 will now be described. Let it be assumed that the receiver stylus 43 and the transmitter stylus II occupy corresponding positions along -their respective Y axes. Under these conditions, the variable inductance coil 36a has an inductance of such value yand the frequency of the Y signal received from the transmitter is of such value that no signal is transmitted from the discriminator 36 to the D. C. amplifier 31 Accordingly, the motor 39, the links 4I and 42 and the receiver stylus 43 remain stationary.

If now the transmitter vstylus* II is displaced along the Y axis on the Writing surface I2, the inductance of the coil I8 is changed and the out- Y put of the Y oscillator 22 will be of a diiferent frequency than before. Accordingly, the Y signal tion of the transmitter stylus H. Whenthe stylus 43has reached this corresponding position, the inductance of the coil 36a will have been changed by movement of the links 4l and 42 until it is of such Value that at the frequency of the incoming Y signal the discriminator output signal is zero, all as explained in detail in application Serial No. 81,709 referred to above.

Thus it is seen that the receiver stylus 43 will 'follow the movements of thc transmitter stylus Il along the corresponding Y axes The receiving station includes a circuit for the incoming X signal similar to the Y signal circuit described immediately above. The X signal circuits and apparatus cause the receiver stylus 43 to assume positions along the indicated X axis on the writing surface 48 which correspond to the positions of the transmitter stylus I l along the indicated X axis on the writing surface I2. Since the circuits, the apparatus, and the operation thereof are identical to the Y signal circuits apparatus and operation described above, a detail description thereof will not be given herein.

It will be apparent to those skilled in the art that the signals sent from the discriminators to the corresponding D. C. amplifiers are, for practical reasons, of too small a magnitude to drive the motor 39 of the Y signal circuit or the corresponding motor of the X signal circuit. Accordingly, a D. C. amplifier is required for each circuit. These amplifiers must be able to handle signal variations of a frequency corresponding to the rapidity of reversal of` movement of the transmitter stylus il. In the case of ordinary handwriting, variation in the D. C. signals fed to the D. C. amplifier normally fall within a range of from cycles per second to approximately cycles per second Accordingly, where the D. C. amplifier is applied to such a circuit as that shown in Fig. 1, it must be able to handle these frequencies. In other applications,` the amplifier may be called upon to respond to frequencies of a considerably higher range.

In many applications, such as that shown in Fig l, it is also desired that the amplifier have a substantial amplification factor in order that a voltage signal of substantialL magnitude may be fed to the motor 39 and the stylusf43 be driven rapidly to follow accurately the movementsV of the transmitter stylus Il. l

Still another feature which is desired in the application shown in Fig. 1 is high amplifier efficiency. In some applications, the required power output of a D. C. amplifier may be on'the order of milliwatts. In the application 0f the D. C. amplifier shown in Fig. 1, however, the desired power output is in the order of watts. Accordingly, high efficiency is desired for Various reasons, perhaps the most important of which is to permit use of standard'and hence low cost circuit elements such as, for example, amplifier tubes. Another desirable result of high eniciency in the D. C. amplifier is the reduction in operating power, this being ofv very substantial significance where the receiving station of the tele-autographic apparatus shown Fig. I is a portable unit `operating on batteries.

The various D. C. amplifiers or bridge circuits, illustrated in Figs. 2', 3, and 4, are well suited for application as the D. C. amplifier 31 of the tele-autographic apparatus shown in Fig. l. Any of these three amplifiers have relatively` high eiiiciencyand can `produce a power output of several watts anda variable-polarity output voltage very accurately proportional to a signal voltage which may alternately increase and decrease with a rapidity of from zero cycles per secondv to frequencies lying in the range of. audio frequencies or even radio frequencies.

The circuit appearing in Fig. 2 employs four amplifier tubes 1I, 12', 13, and 14, each being situated inone of four legs of a bridge circuit. The cathodes 15 and 16 of the tubes 1l and 13', respectively, are connected together by a conductor 11 which is returned to ground through a pair of resistors 18 and 19. The plate 80 cf the tube 1i is connected or coupled through a resistor 3f to the cathode 82 of the tube 12. Similarly, the plate 83 of theA tube 13 is connected through `a resistor 84 to the cathode 85 of the tube 14. The plate 86 of the tube 12 and the plate 81 of the tube 14 are connected together by a conductor 88 which in turn is connected to B+, as shown. It is understood, of course, that B+ vis the positive terminal of a suitable voltage source having its negative terminal connect-i ed, in this case, to ground. A load device` 83 has its opposite ends connected to the cathodes 82 and or" the tubes 12 and 14, respectively.. The B+ voltage is, therefore, applied across two opposed corners of the bridge while the load device 89 is connected across thev other twoopposed corners of the bridge.

When the amplifiers shown in Figs. 2, 3, and 4 are applied to tele-autographic apparatussuch as that shown in Fig. l, the load 89 may be. the motor 39.

The four tubes 1l, 12, 13, and 14 may beof r many types depending on the particular application of the bridge circuit and more than one tube may be contained in a single envelope. In the application described above, it is found to be satisfactory to combine the tubes 1l and 13 in a single envelope, such as a 28D? pentode, with the screen grids 11a and 13a connected to B+' as shown, and to combine the two tubes 12 and 14 in a single envelope such as a 6SN'1 or a 12AU7. The screen grids 1Ia and 13a are indicatedv as being connected to a voltage source other than the B+ to which the plates of the tubes112r and 14 are connected since the latter; would' normally be of a higher voltage than should be applied to the screen grids HaiV and 13a, as isrwellunder.-

. stood in` the art.

A signal S is applied between the grid |00 of the tube 1l and a point lill intermediate the two resistors 18 and 19. The resistor 18 is provided-in order to maintain the grid |83` negativewith respect to the cathode 15. If the signal is expected to vary, for example, over a range of. seven Volts', from +31@ volts to -31/2 volts, a resistancevalue might be selected' such that the plate and screen currents for the two tubes 1l and 13, in passing through the resistor, cause a voltageY drop'of 5 Volts to appear thereacross, resulting in thezcathode 15 of the tube 1l being maintainedat av'olt.- age level 5 Volts'above that of thepoint 10|. Accordingly, the voltage of the grid In!! withrespect to the cathode 15 would -vary' over. the rangev 5131/2, or *1% to +81/2 volts. As will later vbecome apparent,Y the current passing through the resistor .18 will at-allV times bev-substantially constant and in any event any small deviations will have no effect other than to make the response of the circuit slightly nonlinear; The function of the resistor 13 will be explained subsequently. y y Y When the signalY voltage is zero, the voltages of. 'the grids 103 and |32 of the respective tubes -1| and 13, and hence the plate currents of these tubes, are preferably maintained at equal values. When the voltage of the grid is made more positive, to increase the current through the tube 1|, it is desired, as is well understood in the art, that an equal but opposite signal be applied to the grid |02 of the tube 13, that is, the voltage of the grid 02 should be made more negative by an equal amount to produce a corresponding decrease in the current passed by the tube 13. In the circuit shown in Fig. 2, this is accomplished by means of a conventional voltage divider.

.A resistor |03 is connected at one end to the grid |02 of the tube 13 and at its other end to the plate 00 of the tube 1|, as shown. The grid |02 is connected also through another resistor |04 to ground, as shown. The two resistors |03 and |04 constitute a voltage divider having imposed thereacross the voltage between the plate 80 of the tube 1| and ground. 'W'hen a change in signal voltage makes the grid |00 of the tube 1| more positive, the current through the tube 1| increases and the additional current must pass through the load resi-Stor 89 as will subsequently become more apparent. The voltage drop across this resistor and the increased voltage drop across the resistor 8| cause the plate 80 to become more negative, and since this voltage appears across the voltage divider resistors |03 and |04 the Yvoltage across the resistor |04 will be smaller and the voltage of the grid |02 will be lower. The current through the tube 13 will, accordingly, be reduced, and by proper selection of the resistors 03 and |04, as is well understood in the art, the Voltage of the grid |02 can be made to vary equally with but in opposition to the voltage of the grid |00 whereby when the plate current of the tube 1| is increased by one milliampere, the plate current of the tube 13will be decreased by one milliampere and vice versa.

If desired, the voltages of the grids |00 and |02 need not vary equally and oppositely but the voltage of thegrid |02 can be made to vary in accordance with an opposite or negative linear function of the voltage change of the grid |00.

It will' be immediately apparent that the voltage vof the vgrid |02 can approach but cannot reach ground voltage. Accordingly, in order to permit the voltage of the grid |02 to'be maintained negative with respect to the cathode 13 (say volts with zero signal voltage as suggested above) the resistor 19 is connected between the point'llll and ground. This resistor provides a substantially constant, positive bias to the point |0| and the voltage of the grid |02 can be made to vary about the voltage level of the point |0|, as does the voltage of the grid |00. I

With the plate currents of the tubes 1| and 13 varying inversely and preferably by equal amounts with any given change in signal voltage. as described immediately above, it will be apparentthat any change in signal voltage will cause a change in the current passing through the load 89. However, a greater change in current through the load resistor can be obtained for a given change in signal voltage by properly varying the grid voltages and hence the plate currents of the other two tubes 12 and 14. Under balanced conditions, with zero signal voltage, for example, the plate currents of all four tubes will preferably be equal and no current will pass through the load 89. When the plate current of the tube 1 |is increased by a signal variation, the current passed by thetube 12 should be diminf 8 ished while the current carried Vby the tube 14 should be increased as this will make'the bridge circuit more sensistive than if the plate currents of only the tubes 1| and 13 are varied.

`It may be desirable under certain circumstances to make the plate current of the tube 12 equal at all times to the plate current of the tube 13, and the current of the tube 14 equal to that of the tube 1|. In other cases, it may be desired that the currents of the tubes 12 and 14 be made to vary more widely than the currents of Vthe tubes,1| and 13'for a given change in signal voltage. It is with the proper control of the various grids, operating at substantially diiering voltage levels, that the present invention is primarily concerned.

It is apparent upon inspection of Fig. 2 that the signal S cannot be applied directly across the grid and cathode of both the tube 1| and the tube 12 since the tube 12 operates at a much higher voltage level than the tube 1|. An obvious solution to this problem would be to provide another tube, controlled by the signal voltage and arranged to drive the grid |03 of the tube 12.

In the embodiments of the invention illustrated in Figs. 2, 3, and e, the grid of the tube 12 is driven by the tube 1| without the necessity of an additional tube for that purpose. Y

The grid |03 of the tube 12 is connected by a. conductor |04 to the plate 80 of the tube 1|, and, accordingly, its voltage will always be the same as that of the plate 80. However, the cathode 82 of the tube 12 is connected to the plate 80 of the tube 1| through a biasing resistor 8|. It will be immediately apparent that with any current passing through the tube 1| and hence the biasing resistor 8|, the grid 03 of the tube 12 will be negative with respect to the cathode 82 of the same tube. It will also be apparent that as the plate current of the tube 1| increases, the negative bias of the grid |03 becomes greater with a resultant reduction of plate current in the tube 12. Similarly, as the plate Ycurrent of the tube 1| decreases, the negative .bias .of the grid |03 becomes less and the plate current of the tube 12 increases.

The grid |05 of the tube 14 is similarly connected by a conductor |00 to the plate 83 of the tube 13. The biasing resistor 84 maintains the grid |05 negative with respect to the cathode 85 and as the plate current of the tube 13 increases the current in the resistor 84 increases and the grid |05 is made more negative with respect to the cathode 85, thereby reducing the plate current of the tube 14. Similarly, as the plate current of the tube 13 decreases, the plate current of the tube 14 increases.

The circuit shown in Fig. 3 is identical to that shown in Fig. 2 with the exception of the means for applying inverse signals to the grids |00 and |02 of the tubes 1| and13, respectively. In Fig. 3 a signal S is applied across a resistor l|0 having its two endsconnected to the grids |00 and |02 of the tubes i 1| and 13 respectively. The centerV point of the resistor ||0 isl connected to ground, as shown. Connecting the center point to some point on the bridge circuit of suitable voltage levelcau'ses the potential of the ends of the resistor ||0, and hence the grids |00 and |02v to vary equallyand inversely. Connection ofv the resistor to ground as shown in Fig. 3 causes thecathodes A15 and 16 to be maintained positive with respect to the respective grids |00 and |02 by virtue of the voltage drop across the resistor 18 connectngthese cathodes to ground. It will be immediately apparent that a signal of One polarity will cause the grid I to become more `positive and the grid |02 to become more negative by equal amounts While a signal of the reverse polarity will cause the grid |02 to become more positive and the grid to become more negative by equal amounts.

This circuit for applying equal inverse signal voltages to the grids |03 and |02 has the advantage of simplicity as compared to the circuit shown in Fig. 2. On the other hand, the arrangement shown in Fig. 3 utilizes only one-half of the available signal voltage and, accordingly, is not as satisfactory where only a small signal voltage is available.

The actual ohmic values of the resistors 8| and 84 with respect to the characteristics of the tubes employed in the circuits shown in Figs. 2 and 3 have a controlling influence upon the characteristics of the bridge circuit. Frequently a linear performance of the circuit over a maximum range is desired for various reasons Well recognized in the art. This can be obtained by using resistors 8| and 84 having ohmic values equal to Where gm is the transconductance of the respective tubes 12 and 14. With resistors 8| and 84 having such values, an increase of say 2 milliamperes in the plate current of the tube 1| Will be ac-companied by a decrease'cf 2 milliamperes in the plate current of the tube 12. The plateV current of the tube 13 in Figs. 2 and 3 Will also be reduced by 2 milliarnperes, b-y virtue of the voltage divider ID3-|64 of Fig. 2 or the resistor |`|0 of Fig. 3, and, as a result of the reduced voltage drop -across resistor 84, the plate current of the tube 14 will be increased by the same amount.

The current through the load 89 will be equal to the difference in the plate currents of the tubes 1| and 12, or of the tubes 13 and 14. The load current is, therefore, a. linear function of the plate current of the tube 1| and, accordingly, a linear function of the signal voltage S. As the current through the tubes 1| and 14 continues to increase and the current through the tubes 12 and 13 continues to decrease, the voltage drop across the load resistor will continue to follow this function until the plate current through the tubes 12 and 13 has reached zero. At this point, with a properly designed circuit, the plate current through the tubes 1| and 14 will have reached a near maximum value. Any further change in the signal voltage in the same direction will cause relatively little change in the current through the load 89 since the current through the tubes 12 and 13 has already become zero.

This characteristic of the circuit may be visualized more clearly in the solid curve designated gm in Fig. 5, in which the load current I is plotted against the voltage V appearing on the grid Hill of the. tube 1|. It will be seen in Fig. 5 that, by making the value of the resistors 8| and 84 equal to the reciprocal of the transconductance of the respective tubes 12 and 14, a substantially linear output is obtained over almost the entire op" erating range of the circuit.

If a linear performance of the amplifier or bridge circuit is not essential, it is possible to 10 obtain an output'which vis greater with respect t0 the signal voltage'than is available with the re' sistors 8| and 84 being equal to the reciprocal of the transconductance of the respective tubes 12 and 14. This is accomplished by making these resistors of somewhat higher resistance value. The characteristic curve resulting from such an arrangement is shown as a dotted curve in Fig. 5 and is indicated by the expression 1 R gm The steeper portion of rthis curve, namely, that portion near the indicated vertical coordinate, is made steep by the fact that a given change in signal voltage produces a proportionate change in the plate current in the tubes 1| and 13, but produces corresponding changes in the plate currents of the tubes 12 and 14 which are greater than the changes appearing in the tubes 1| and 13. More specifically, if the plate current through the tube 1| is increased by two milliamperes, for example, and the plate current in the tube 13 is reduced by two milliamperes, the plate current in the tube 14 may be increased by three or perhaps four milliamperes, for example, while the plate current through the tube 12 is decreased by three or four milliamperes. The net result will, of course, be a current I through the load 89 which is greater 'than that which would have appeared ifthe plate currents ofthe tubes 12 and 14 had varied equally with the plate current of the tubes 1| and 13, the latter being the case when the resistors 8| and 84 are equal to the reciprocal of the transconductance of the respective tubes 12 and 14.

The increased response of the tubes 12 and 14, caused by the biasing resistors being greater than the reciprocal of the transconductance of the respective tubes 12 and 14, results in the plate current of one of the tubes 12 and 14 reaching a maximum desired value and the plate current of the other of these tubes becoming zero, before the tubes 1| and 13 have reached the condition of maximum or zero plate current. This point is indicated by the hump in vthe dotted curve in Fig. 5. It Will be readily understood that as the signal voltage is Varied beyond the value correspending to this hump, no substantial change will appear in the plate currents of the tubes 12 and 14 and, accordingly, the further change in the current through the load 89 is the result of the respective changes in the plate currents in the tubes 1| and 13 only. Accordingly. the curve attens considerably to a slope which is less than that of the solid curve shown in Fig. 5.

The use of biasing resistors 8| and 84 having resistance values greater than the reciprocal of the transconductance of the corresponding tubes 12 and 14 results, then, in a nonlinear response. but causes a greater amplification over at leastpart of the operating range, especially in the operating range adjacent the zero or neutral signal voltage, This arrangement is then more satisfactory Where a linear operating `characteristic is not essential and greater amplification is desired especially over the operating range surrounding the zero or neutral signal voltage.

The curves shown in Fig. 5 are somewhat idealized as applied to the circuit shown in Fig. y2

since the voltage divider ID3-|04 used to control* the grid |02 of the tube 13 in that circuit results in a nonlinear performance of the circuit under some conditions.

Another type of bridge circuit is shown in Fig.

4,' this circuit employing Voltage sources in two legs of the bridge and vamplifier tubes in the other two legs. The left-hand portion of the bridge is identical to that appearing in Fig. 1 and the circuit portions have, accordingly, been assigned the same reference numerals. However, each of the bridge legs making up the right-hand side of theY circuit is a voltage source comprising, in the particular embodiment illustrated, a secondary winding |20 of a transformer, a rectifier |2 and a condenser |22, connected as shown. For most purposes, primary windings |20a, corresponding to the. secondary windings |20, would constitute a single'winding which may be4 connected to any suitable source of alternating current having fixed frequency and voltage. With a substantially constant voltage induced in the windings |20, a substantially constant D. C. voltage will appear across the condensers |22.

The operation of this circuit will be immediately apparent to those skilled in the art in view of the description of Figs. 2 and 3 appearing above. As the voltage of the grid l of the tube is raised, the plate current through the tube 1| and hence through the biasing resistor 8 I. will increase. The increased negative bias of the grid |03 of the tube 12 caused by the increased current through the biasing resistor 8| reduces the current-passing through the tube '12. This increase of plate current in the tube 'H and the corresponding decrease of plate current in the tube 12 results, of course, in a current flowing through the load 89.

Similarly, the signal S may reduce the plate current of the tube 1| in Fig. 3 below the value corresponding to balanced condition and the plate current of the tube 'i2 will increase by an equal amount. Current will then flow from right -to left through load 89.

g The resistance value of the biasing resistor 8| in Fig. 4 may, of course, be made equal to the reciprocal of the .transconductance of the tubey 12, as in the circuits of Figs, 2 and 3, to obtain linear operation over a maximum range. Also,v the use of a resistor 8| having greater resistance` than the value 9m of the tube 12 will result in greater sensitivity of the circuit, that is, a greater amplification factor, but results in a nonlinear output.

While particular embodiments of the invention have been shown, it will be understood, of course, that the invention is not limited thereto since many modifications may be made, and it is, therefore, contemplated to cover by the appended claims any such modications as fall within the true spirit and scope of the invention.

The invention having thus been described, what is claimed and desired to be secured by Letters Patent is:

l. A direct current bridge circuit amplier comprising four amplifier tubes, one each in the four legs of said bridge, the cathodes of the first and second tubes being connected together and the plates of the third and fourth said tubes being connected together, a direct current source of potential connected between the plates of said third and fourth tubes and the cathodes of said rst and second tubes, the plate of said first tube being'coupled to the cathode of said third tube and the plate of said second tube being coupled to said cathode of said fourth tube, a load connecting the cathodes of said third and fourth tubes, a voltage divider connected at one end tothe plate' of said rst tube and terminating at its other end at a point whose voltage is below that of the cathode of said rst tube, a tap on said Voltage divider connected to the grid of said second tube for controlling the voltage thereof, theposition of said tap being selected whereby the voltage variations of said grid of said second tube are opposite to and equal in amount to the variations in the grid voltage of said first tube, means for varying the grid voltage of said third tube in accordance with an opposite linear function of the plate current of said first tube including a resistor arranged in the plate circuit of said first tube to carry the plate current thereof and an electrical connection between the grid of said third tube and the plate of said rst tube, and means for varying the grid voltage of said fourth tube in accordance with an opposite linear function of said second tube including a resistor arranged in the plate circuit of said second tube to carry the plate current thereof and an electrical connection between the grid of said fourth tube and the plate of said second tube, said resistors each having a resistance value substantially equal to the reciprocal of the transconductance of the adjoining one of said third and fourth tubes.

2. A direct current bridge circuit amplier comprising four amplier tubes, one each in the four legs of said bridge, the cathodes of the rst and second tubes being connected together and the plates of theV third and fourth said tubes being connected together, a direct current source of potential connected between the plates of said third and fourth tubes'and the cathodes of saidy rst and second tubes, the plate of said rst tube being coupled to the cathode of said third tube and the plate of saidl second tube being coupled to said cathodev of said fourth tube, a load connecting the cathodes of said third and fourth tubes, a voltage divider connected at one end to the plate of said first tube and terminating at its other end at a point whose Voltage is below that of the cathode of said rst tube, a tap on said voltage divider connected to the grid of said second tube for controlling the voltage thereof, the position of `said tap being selected whereby the voltage Variations of said grid of said second tube are opposite to and equal in amount to the variations in the grid voltage of Said first tube, means for varying the grid voltf age of vsaid third tube lin accordance with an opposite linear function of the plate current of said first tube including a resistor arranged in the plate circuit of said first tube to carry the plate current thereof and an Yelectrical connec# tion between the grid of said third tube and the plate of said first tube, and means for varying the grid voltage of said fourth tube in accordance with an opposite linear function of said second tube including a resistor arranged in the plate circuit of said second tube to carry the plate current thereof and an electrical connec tion between the grid of said fourth tube and the plate of said second tube.

WILBERT PARISOE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,310,342 AllZt Feb. 9, 1943 '2,329,073 Mitchell el', a1 Sept. '7, 1943 2,424,893 Mansford July 29, 1947

Images