US2429775A - Amplifier system - Google Patents

Description

Oct. 28, 1947. c. G. sERlGHT AMPLIFIER SYSTEM 2, 1944 2 sheets-sheet 1 Filed June 2 7 www m.

oct. 28, 1947.

AMPLIFIER SYSTEM Filed-June 22, 1944 2 Sheets-Sheet Z2` ATTORNEY c. G. sERlGHT I 2,429,775

Patented Oct. 28, 1947 AMPLIFIER SYS TEM Carl G. Seright, Riverton, N. J., assignorto Radio Corporation of America, a corporation of Dela- Ware Application June 22, 1944, Serial No. 541,502

, 11 Claims.

, 1 Y My present invention relates to amplier systems, and more particularly to a, stabilized amplier system having a, plurality of reproducer I output elements.

One of the important objects of my invention is to provide a plurality of signal transmission channels with a common output circuit subject to load impedance variation; there being employed a novel and eiiective method of minimizing changes in signal output level due to said impedance variation.

Another important object of my invention is to provide a method of stabilizing the audio output level at the common output circuit of paralleled audio channels of respective lreceivers so that one or more reproducer units can be connected to the common :output circuit to listen to the output of one or more of the audio channels.

Still another object of my invention is to provide in an amplifier system a method of, and means for, utilizing negative and positive feedback of signal voltage so related as to produce a stabilized signal output level regardless of load impedance variation.

Another object of my invention is to provide a plurality of signal transmission channels, wherein each channel is adapted to be fed by signals of diierent character, each channel having a degenerative feedbackpath individual to it, andY a common positive feedback -path being provided for all the channels tojcooperate with the individual degenerative paths to provide a stabilized signal output level. Y

Other features of my invention will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically two circuit organizations whereby my invention may be carried into elect.

In the drawing:

Fig. 1 shows one embodiment of the invention,

Fig. 2 shows a modification. Y

Referring now to Fig. 1 of the accompanying drawing, wherein there is shown only so much of a pair of paralleled signal transmission channels as is essential to a proper understanding of my invention, it is to be understood that the two channels are of substantially similar construetion. However, each channel transmits signals of different character. The amplifiers I. and I of the respective transmission channels may have the signal input grids 2 and 2 thereof connected to any desired sources of signals. The signal sources are not shown, since they may be of any desired construction. For example, the channels may be fed from the respective detector loads of separate radio receivers operating in widely different frequency bands. Merely as a specic illustration of such use, one receiver could be operative in the 0.19 to 2 megacycle (me.) band, While the other is operative in the 2 to 18 mc. range. receiver might feed speech signals to one of the grids 2, while the demodulator of the other receiver would feed impulse or directional signals to the second channel. Again, the sources feeding grids 2 and 2 could be separate microphones functioning intermittently or concurrently. They could readily be electrical pickups of recordreproducing equipment.

Assuming, now, that separate receiving systems feed the detected outputs, or modulation signals, thereof respectively to said transmission channels, there will now be explained the specic construction of the channel component elements. Since the elements of the pair of channels are similar, reference will be made to the upper one of them. The lower channel components are denoted by the same numerals, except that a A prime `designation distinguishes these reference numerals from the corresponding numerals of the specifically described channel. Oficourse, as many signal channels as is desired may be fed from each demodulator, or as many separate receivers as is desired may be employed to feed a corresponding number of transmission channels having paralleled output circuits,

AAmplilier tubes I and 3, while shown as pentodes, may be of any desired and suitable types. 'Ihe signal grid 2 of tube I is coupled by condenser 4 to its modulation or other low frequency signal source. Cathode 5 is connected to ground by a bias resistor made up of two sections 6 and 1. Section I is shown as adjustable so as to provide control over the gain of the channel. Condenser 8 shunts resistor 6-1 to bypass alternating currents, and resistor I9 provides a direct current return path for grid 2 so as to apply negative bias to thev grid relative to the cathode 5. The screen I0 and plate II of tube I are connected to a point of suitable positive potential +B by respective resistors I 2 and I3. The screen and plate voltage supply leads are bypassed to ground for alternating currents by condensers I4 and I5 respectively.

I 1. The grid I6 is connected by condenser Il to Y In such case the demodulator of onethe plate end of resistor I3, while resistor I8 returns grid I6 to the grounded end of cathode resistor I9. The latter supplies negative biasing voltage thereacross for grid I6. Condenser 30 is a direct current blocking condenser in the feedback path, The screen 20 and plate 2l of the amplifier tube are.v connected to respective ends of the primary winding 22Y of output transformer 23, while the lower end of winding 22 is connected to a suitable positive potential point'` +B.

Negative or degenerative signal voltage feedback is provided by a path 25,--23-48, of whichthe resistor I8 is common tothe output and input circuits of amplifier tube 3. The portion of the negative feedback path which is connected between plate 2| and grid I6 is enclosed ina dotted rectangle, and comprises resistor 25 and` oondenser 26 connected in series. The constants of transformer 23 and of the path 25.--2-i3 are chosen so as to-provide. degeneration to the desired extent of the signals applied to grid I6.

The secondary winding 24 of output transformer 23 has theA upper end thereof connectedv to an output lead 21while the lower end is connected to ground through a resistor 28. The latter is, unbypassed, and develops thereacross low frequency signal voltage. The connection 29.,A including direct, current blocking condenser 30 is made from the upper end of resistor 28' t0 the cathode end of resistor I9. The polarity. of winding 24 is so chosen that path 29-I9 acts as a positive signal voltage feedback path.v In other Words, this, feedback. pathV contains resistor I9 which is common to the output, and input circuits of tube. 3, and` it feeds back SignalV voltage in asense. or phase resulting in self-reinforcement., and4 oppositev to that of the negative feedback path 25.-26-I.8.

The. output connection 2.11 is common tov the high potential. terminals of' windings 24' and 24. ofv transformers 23 and', 23,. The low potential end of winding 24 is connected by lead 3I to. the ungrounded` end of resistor 28'. Hence, the resistor 23j is commento the output circuits. of both channels, andfunctions to provide positive feedbackvoltage for both. of.V amplifiers 3 and 3', the latter through condenser 3D and resistor t9".

The output loadV of both channels comprises one or more signal outlets or jacks adapted to receive respective signal reproducer devices. Thus, the common connection 32 leads from connection 2 1. to a pair of paralleljacks 33 and s4; and the latter have a. common ground return connection. Of course, as many jacks as is desired. may be shunted across the common output Circuit, of the twojchannels. The. reproducer devices` may be headphones, as wouldV be the case in aircraft, reception. where one headphone set might serve a pilot and the other setv would be that, of. the radio operator. Each ofthe headphones. would be plugged. into respective jacks 33 and 34j in such case, or one 0f them could be removed.

The function of the negative feedback path 2.5,-2-I8, and` positive feedback path 28-29--30-13 in eachA channei is to minimize changes in signalA output level due to changes in the load connected to: the common output circuit, and due to cross loading effects between the amplifiers. The receivingv systems feeding the audio channels are operable one at a time, or simultaneously.. Further, changes in the kind or number of reproducer devices plugged into the outlets or jacks affect the signal output level.

Again, rendering either transmission channel inoperative, as by turning off its filament voltage, substantially alters the load on the other circuit.

My invention provides a simple method of minimizing changes in the signal output level resulting from load impedance variations regardless of the cause` of such variations.

In orderto visualize the functioning of' my invention, consider first the effects of the feedbacks atthe load terminals Y--Y. The impedance seen by the load at Y-Y can be determined by substituting a voltage for the load, and dividing this voltage by the resulting current.

Let Eo--an alternating voltage applied Y-Y in placel of the load. Eg=the sum of the feedback voltages on grid I6.

:amplification factor of tube 3.

Ri==internal output resistance of tube 3.

Rr=resistance in ohms of positive feedback resistor 23., Y

Nzratio of output transformer 23=secondary turns+prirnary turns.

Azfraction of the` voltage established across Re whichv is. applied between cathode and grid oftube 3y in regenerative phase.

B=fraction of the voltage across the primary 22 of output transformer 23` which is applied between grid and cathode in degenerative phase.

Io=current due to voltage Eo.

at terminals The product of the current and. impedance equals the applied voltage (EnzIoRc). The total voltage acting in the output circuit is the sum of. the applied voltage and the feedback voltages, and the impedanceis the sum of the feedback resstanceRr (28) and the plate resistance of the tube reflected through transformer 23':

wherein Es=thevoltage across secondary output transformer 23.

Es= Et- I 0R11.

EgSARFIVi.

The full Ohms law equation can now be set down: Eor-l-nARiIoN-f- (,aBEo-nBIoRF) :IolRF--RPNZ] The assignment of polarities to the feedback voltage terms in the foregoing expression is made on the basis of their effect as they reappear in the amplifier output at X-X. These voltages are in series with Eo in the output circuit Y-Y, X-X, RF. If they act in concert with Eo to increase Io, they have the same sign as En. The polarity of the-B term voltage (enclosed in parentheses) for the condition'RF=0 is the reverse, from a parallel standpoint, of the Eo voltage, since the B voltage isV degenerative. Acting in series with En through XX, Rran'd Y-Y, this voltage therefore assists Eoin production of Io. The A term voltage is regenerative, if originated by a voltage appearing at X--X, as for example a B term voltage. The A term voltage reappearing at X-X would reinforce the originating voltage. The A term Voltage is produced by IoRF drop in RF. 1f this IORF drop is produced by va voltage Eo applied at Y-Y,

pearing at X-X produced by En applied at Y-Y.

All the voltage terms, therefore, mutually assist in the production of Io, hence all have the same sign,

Proceeding with the solution:

It is to be remembered that in the foregoing expression, both the A and the B terms are positive quantities. Losses in the output transformer 23 are neglected as they can be made negligible by suitable design.

It can be seen from 1) that increasing either the positive feedback, as by increasing the value of RF (28), or increasing the negative feedback, as by decreasing resistor 25, will result in a reduction of impedance Ro presented by the circuit to the load at Y-Y. The value of Ro can be made positive, zero, or negative, by suitable choice of values of resistor 28, A and B. A lowered value of Ro, obtainable by the use of feedback, is desirable for minimizing output variations due to load changes. Thus, if it is desired to maintain the total output into load jacks 33-34 constant as the impedance of this load is changed, values of resistor 28, A, and B are selected which result in an Ro value approximating the average value of the load.

If it is desired to maintain the output per element of load constant as the rnumber of similar load elements is varied, Values of 28, A, and B are chosen which result in an R value approaching zero. The exact circuit constants necessary to affect the value of Ru in the desired manner can either be determined by mathematical procedure of a more rigorous nature than that indicated a circuit such that the impedance presented byk each amplifier to the yother is not altered as a result of the feedback employed in each, when the load plugged into jacksV 33-34 has a selected Value.

Consider a voltage applied between points as by means of connections to output winding 24' of another amplifier. The impedance seen by this voltage consists of two components in parallel. One branch consists of the load plugged into jacks 33-34, plus resistance 28. The other branch consists of the internal output impedance of the tube 3, as modified by the ratio of transformer 23. When pentode tubes are used, this latter component is made several times the load resistance by suitable adjustment of the transformer ratio in order to obtain maximum undistorted output from the amplifier. Thus, when no feedback is employed two pentode arnplifiers paralleled as at X-X do not load each other greatly, but work eiciently into the load resistance.A In my circuit the same results are obtained while the advantages of feedback are simultaneously obtained, as set out above. Referring again to point X-X in the circuit, and assuming positive feedback through 23, 29, 30, I9 equal to the negative feedback through 25-25-48 for a particular value of load impedance, it is apparent that since these feedback effects are equal and opposite at X-X, the current through the load and also through transformer winding 24 will be the same as though no feedback to tube 3 were employed. By making the positive feedback exceed the negative feedback, the impedance seen by the second amplifier at.X--X, due to the connections to 24, can be increased over the value which would be obtained with no feedback;y This condition might be found desirable under some conditions; for example, if tubes having low internal output impedance were to be used in the amplifier.

The impedance presented by the circuit to another ampliiier paralleled at X--X varies with the load resistance plugged into jacks 33-34 in accordance with the following relation, derived in a manner similar to Equation 1.

RPN2

In the aforesaid Equation 2 Rs=resistance presented by the circuit at X-X, with feedback, but exclusive of the load branch.

Rr=load resistance plugged into jacks 33-34. Other notations are the same as given for Equation l, and the A and B terms again are to be considered positive quantities.

Equation 2 shows that for a selected value of RL, if the positive and negative feedbacks are made equal, by a proper choice of values for A, Rrand B, the circuit impedance at X-X is equal to RPN2, which is the value obtained with no feedback. The manner in which the impedance at X-X decreases with an increasing value of RL can also be found by Equation 2. A sim,- ilar representation can be made for the impedance presented by the second amplifier to the first or by a third amplifier to the first and second.

The desired characteristics for cross loading effects between amplifiers are obtained in my circuit by using a positive feedback element which is common to all the amplifiers, This element is numbered 28' in the accompanying drawing, and can either be a resistance, as shown, or a reactive or complex impedance if a frequency discrimination is to be imparted to the feedback action. The positive feedback voltage is shown applied in the cathode circuit, but it might equally well be applied in the grid circuit, or the plate circuit of the preceding tube, or at some previous point in the circuit.

The impedance presented to the load at Y-Y by two or more similar amplifiers with their outputs paralleled at X-X can `be found by dividing Rp everywhere it appears in Equation 1 by the number of amplifiers in parallel. Thus, although the input to the loadfrom one of the amplifiers can be rendered largely insensible to the effect of adding Ya second amplifier in parallel, as set out above, this condition actually obtains for only oneY value of load impedance. For abnormally high load impedance, the output may be substantially decreased as the second (or third) amplifier is connected in parallel, as can be determined from Equation 2. For abnormally low load impedance, the reverse effect would be obtained. Nevertheless, the circuit constants can be so vadjusted as to maintain a more stabilized output condition than has heretofore been obtainable With circuits not incorporating the common feedback element represented by 28, and the circuit is particularly useful in cases Where one or more such amplifiers may be used individually .or simultaneously with a common load.

Merely by way of specific illustration the following list of constants is given for the system of Fig. 1:

Raar-2.2 megohms Riez-1000 Ohms R1=1000 ohms C2s=680 micromicrofarads C3a=20 microfarads 017:0.01 microfarad Transformer (2.3) impedance ratio=4021 Summing up, then, one aspect of the operation ofthe system of Fig. 1, land assuming the load at points 33, 3L. to be in the form of headphones, and for purposes of the present point considering only one of the amplifiers, we may assume first that vthe load consists of one headphone only. If, then, we add a second headphone in parallel with the rst, we cut the ohmic resistance of the load in `half so that, looking upon secondary 2li as a source of alternating current energy, the voltage drop across the load is reduced and the voltage drop across positive feedback resistor 28 is increased. We thereby attain reduced negative feedback through 25, 26, 18, and increased positive feedback across resistor 28, when energy is fed .in 4parallel to `an increased number of headphones. Of course, if the headphones were connected in series instead of in parallel, the action would be reversed in that the negative feedback would be increased and the positive feedback would be decreased-as .the number of series-connected headsets was increased. Substituting a positive parallel feedback for the negative parallel feedback, and a negative series feedback for the ,positive series feedback described elsewhere herein, would likewise reverse the action obtained.

vAnother circuit arrangement which I have devised is particularly suitable when the number of amplifiers which may be in operation in parallel is variable, and when the load impedance is variable, vand it is desired to maintain either a constant total output .per amplifier into the load, or a constant output per amplifier per element of ,load into a load consisting of a variable number of elements. Such a circuit is shown in Fig. 2, wherein one (or more) tube (or tubes) provides the common positive and negative feedbacks, while additional tubes not employing feedback within themselves are employed for additional amplifiers. With .this modified arrangement, the tube incorporating the two feedback paths is always left .in the circuit regardless of any other combination of outputs, and may serve as one of .the amplifiers and also as a variable resistor which is governed by the load impedance, or may fulll the latter function only. The essential features to be noted about this circuit are:

l(A) The impedance presented Aby the output circuit of each of the amplifiers not employing feedback to the other amplifier output circuit, is equal to RPN2, and this is also the impedance presented by the feedback tube 3 to the other amplifiers when the load has a selected Value for d which the positive and negative feedback effects are made equivalent. A typical output pentode has an internal output resistance of 70,000 ohms, and a rated load resistance of 7000 ohms. Thus, several such tubes can be operated in parallel without serious loss in energy available to the load from any of the tubes.

(B) The impedance presented to the other ampliers by the tube incorporating the feedback is that given by the aforesaid Equation 2. This impedance can be seen rto vary in an inverse manner as the load impedance is varied. The feedback tube 3, therefore, alters its impedance in a manner tending to use up the excess energy from all sources when the load impedance is increased, and conversely tends to apply additional energy to the load when the load 'impedance is reduced regardless of the original source of the signal or signals.

A preliminary set 0f values for A, RF, and B can be arrived at by substituting actual circuit constants in Equations 1 and 2. Final circuit values are more easily determined by laboratory experimental procedure.

The modification shown in Fig. 2 follows the numbering of Fig. 1. The feedback amplifier tube 3 is provided with the positive feedback element Z3 and the negative feedback path 25-26. It will be seen that the electrical circuits associated directly with tube 3 are exactly the same as those in Fig. 1. Tube 3 is provided with the same circuits as in Fig. l, save that the degenerative path 2526 is omitted,'and condense-r '30 by-passes cathode resistor i9 to ground. The third amplifier 3 is provided with circuits exactly similar to those of tube 3. Therefore, the same numerals are used for the circuits of tube 3, except that double prime designations are utilized in the latter case. Leads 40 may connect to additional signal amplifier output channels.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my 'invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

-l. In combination with at least two signal transmission channels each having input .terminals adapted to be connected to a respectively different source of signals, a common output circuit for said channels, separate means for each channel providing negative signal voltage feedback, and a common positive signal voltage feedback path from said common output circuit to each of said channels.

2. In combination with at least two signal transmission channels each having input terminals adapted to be connected to a respectively different source of signals, a common output circuit for said channels, separate means for each channel providing negative signal voltage feedback, a common positive signal voltage feedback path from said common output circuit to each of said channels, and said common positive feedback path including an impedance in said common output circuit across which is developed the positive feedback voltage.

3. In combination with a pair of amplifier channels, each of said channels having input terminals upon which may be applied signals of different character, a common output circuit for said channels including a load whose impedance is adapted to vary, separate negative signal 9 voltage feedback means in each of said channels, an impedance in said common output circuit adapted to develop a signal voltage whose magnitude depends on the load impedance, and means for applying said last named signal voltage in regenerative phase to each of said channels therevby to overcome the effect of the variable load impedance on the signal output level at said common output circuit.

4. In combination with a plurality of audio frequency amplifiers, separate signal sources each feeding a respective one of the ampliers, acommon output circuit connected to the output terminals of all of said ampliers, said output circuit including a load whose impedance is adapted to vary, and separate degenerative and regenerative feedback means operatively associated with at least one of said amplifiers to overcome the eiect of said variable load impedance.

5. In combination with at least two audio frequency signal transmission channels each having input terminals adapted to be connected to a respectively diierent source of audio signals, a common audio output circuit for said channels, a plurality of reproducer outlet jacks in said output circuit, means for at least one channel providing negative signal voltage feedback, and a positive signal voltage feedback path from said common output circuit to at least said one channel.

6. In combination with at least two signal transmission channels each having input terminals adapted to be connected to a respectively different source of signals, a common output circuit for said channels, means for one channel providing negative signal voltage feedback, and a positive signal voltage feedback path from said common output circuit to said channel, and said positive feedback path including a resistive impedance in said common output circuit across which is developed the positive feedback voltage.

7. In combination with a pair of amplier channels, each of said channels having input terminals upon which may be applied signals of different character, a common output circuit for said channels including a load whose impedance is adapted to vary, negative signal voltage feedback means in at least one of Said channels, a resistive impedance in said common output circuit adapted to develop a signal voltage whose magnitude depends on the load impedance, and means for applying said last named signal voltage in regenerative phase to said channel thereby to overcome the effect of the variable load impedance on the signal output level at said common output circuit.

8. In combination with at least three signal transmission channels each having input terminals adapted to be connected to a respectively different source of signals, a common output circuit for said channels, means in one channel providing negative signal voltage feedback, and a positive signal voltage feedback path from said common output circuit to said one channel.

9. In combination with at least three signal transmission channels each having input terminals adapted to be connected to a respectively diiierent source of signals, a common output circuit for said channels, means in one channel providing negative signal voltage feedback, a positive signal voltage feedback path from said common output circuit to said one channel, and said positive feedback path including an impedance in said common output lcircuit across which is developed the positive feedback voltage.

10. In combination with at least three amplier channels, each of said channels having input terminals upon which may be applied signals of diiferent character, a common output circuit for said channels including a load whose impedance is adapted to vary, negative signal voltage feedback means in one of said channels, a resistor in said common output circuit adapted to develop aY signal voltage whose magnitude depends on the load impedance, and means for applying said last named signal voltage in regenerative phase to said one channel thereby to overcome the effect of the variable load impedance on the signal output level at said common output circuit.

ll. In combination with three audio frequency signal transmission channels each having input terminals adapted to be connected to a respectively diiferent source of audio signals, a common audio output circuit for all of said channels, a plurality of reproducer outlet jacks in said output circuit, means for one channel providing negative signal voltage feedback, and a positive signal voltage feedback path from said common output circuit to the one channel.

CARL G. SERIGHT.

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

UNITED STATES PATENTS Number Name Date 2,365,575 Maxwell Dec. 19, 1944 2,220,770 Mayer NOV. 5, 1940 2,250,996 Mayer July 29, 1941 2,364,389 Roche et al. Dec. 5, 1944 2,131,366 Black Sept. 27, 1938

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