US2025775A - Amplifier control - Google Patents

Description

Dec. 31, 1935. F. RIEBER 2,025,775

AMPLIFIER CONTROL Filed July 3, 1929 MFA/HER AMPL/F/fR.

fvPur CME B Y 4 ATTORNEY Patented Dec. 31, 1935 UNITED STATES PATENT OFFICE AMPLIFIER CONTROL 1 Frank Rieber, San Francisco, Califg assignor, by

mesne assignments, to Sound Laboratory Corporation, Ltd., a corporation of Nevada Application July 3,1929, serial No'..375,871

8 Claims. (Cl. 179 -171) This invention relates to an amplifier system, and-especially to an automatic control thereof whereby the degree of amplification 'is maintained below an undesired value.

5 Amplifier systems are now in quite general use for a variety of purposes. They can be used in transmitting and receiving signals either by wire or radio; and such signals can be telegraphic or speech signals. Furthermore, the systems can 10 be used for simple audio frequency signals, or

for amplifying the carrier frequency impulses In all such cases, it is advantageous to ensure against too great a response of the apparatus'to the signals; for otherwise serious injury may be done to the delicately adjusted apparatus; or else extremely great distortion of thesignal may result. As an instance where such great response would be annoying and harmful, attention may I be called to the photographic recording of sound for talking pictures. When the sound reaching the microphone is very loud, the light valve used in the recording system may be destroyed; and

even for moderately loud signals, the systemserves to distort them.

effects by manual controls of potentiometers or other forms of control equipment. However, even the most attentive and expert operator is yet subject to the usual physiological response delay. Furthermore, such close attention and care is required that the operator cannot maintain a uniformity of proficiency.

It is one of the objects of my invention, therefore, to make it possible to regulate the degree of response in a manner-independent of manual operation, and yet without distortion of the signals.

My control system can be easily adapted to work with the common types of electronic emission amplifiers. Such amplifiers usually include an evacuated vessel enclosing a plurality of electrodes. One of the electrodes is an electron emitting electrode, or cathode, and is usually, but not necessarily, in the form of a filament heated 50 by an electric current. The electrons thus emitted are received on an anode also enclosed in the vessel and in the form of a plate. This anode is maintained at a potential positive with respect to the filament, as by the aid of a source of potential located in a circuit externally connecting It has been proposed to guardagainst these vention;- but 1 it is to be understood that this dethe'two electrodes. The positive potential impressed on the plate serves to cause it to attract the electrons, which fiow from the cathode and constitute the "space curren 2 It has been found that the space current can 5 bevaried by large amounts by varying the po-' tential diiieren'ce, even by small'amounts, be-' tween the cathode anda point in the space between the two electrodes. I This variation in potential difference can be accomplished by the aid of a control electrode interposed between the other electrode, and usually in theform of a grid. Weak signal impulses are caused'to produce variations in potential between the cathode and the control electrode, and cause corresponding large variations in the space current. It-in this way that an amplifier effect is secured. The amplifled variations present in the"output circuit" (that is, the external circuit between the cathode and anode), can, if desired, be reamplified, bythe provision of succeeding amplifier stagea' Since such electron emission' devices are nowwell understood, there is no need for am further detailed description of'operation. Itmay, how-" ever, be !-mentioned that it is also common to feed back" some of the energy in the output cu: cult to the input circuit to produce reamplification bythe aid of the same tube, and this processis usually termed *regeneratio I 1 It is another object of my invention.- to provide an automatic amplification limiter that can be used with electronemission amplifiers. 5

It is still another'object of'myinvention to make it possible to limit the degree of regenera tion obtainable by the aid of an-electronic emission amplifier; and in fact-to make it possible automatically to reterse the regenerative efiect; when theamplification tends to become still greater. Thus a stabilizing-' result is provided that ensures against the oscillations. y a

My invention possesses-many other advantages, and has other objects which maybe made more easily apparent from a consideration of one-embodiment of my invention. For this purpose I have shown a orm in the drawing accompanying I and forming art of the present specification. I

shall now-proceed to describe this form in detail which illustrates the general principles of my indetailed description is not to be taken in a limiting sense, sincethe scope of my inventionis best de-" production of parasitic "control device or stabilizer incorporating my in- 'vention;

the signals in the amplifier output.

plication of my invention, and especially to-a-regenerative type of electron emission amplifier;

and

Fig. 3 is a graph illustrating the operation of the system shown in Fig. 2.

In Fig. 1 I show amplifier II in diagrammatic form, which is intended to operate on signal impulses and to increase the intens'itiesthereof: for example, to amplify sounds reaching a-microphone 24. I also show the outgoing circuit -I2'I'3, carrying the amplified signaling impulses and capable of impressing said amplified impulses onto a succeeding amplifier or onto a translating device 23, such as a sound recorder, or loud speaker, or the like.

Interposed between amplifier II and circuit |2I3, is the amplification control device I4. This is in the form of a four armed Wheatstone bridge. The amplifier output connects across one diagonal thereof, and circuit. |2|3 across the other. The bridge I4 is shown as formed of a series of four impedances, such as resistors I5, I5, I1, I I. If the ratio of impedance I5 to impedance I5 is the same as that of impedance I8 to impedance II, it can readily be shownthat the bridge is balanced; that-is, that any potential diiference impressed by the amplifier system.

across points I9, 20 will cause no potential differ ence to exist between points 2|, 22, to which the circuit I2, I3 connects. Thus under such circumstances, no matter how great the amplification secured by the amplifier I I, this amplfied-signal is not made available in circuit I2-I3 However, if the prescribed ratios change, the bridge becomes unbalanced, and as the ratios departmore and more from thebalanced condition,

a greater eirec't is produced in circuit I2--I3. The device I4 is so constructed and arranged that these ratios change. automatically and continuously in response to variations inthe intensity of How this eflect is secured willnow be described.

Two opposite resistance arms such-as |6, I8, of the bridge I 4 are made from material such that any increase in temperature causes a very large increase in resistance. Such material may be iron wire, or manganin, or the like. These-arms IS, I 8, can be enveloped in tubes to conserve heat generated by the flow of current therethrough, so that the greater the current flow, the greater the temperature becomes, and consequently, the

greater the resistance. The other two arms I5, I! can be made from ordinary conducting wire.

It is apparent that the resistance can be so chosen that any desiredcontrolling effect is secured. It the resistances I6, I8, are made from fine wire, they can respond more quickly to the variations in potential across points I9, 20 and the circuit |2-I3 is correspondingly caused to respond quickly. On the other hand, the use of larger wire for these resistances causes a heat lag that may produce substantial lag in response in the circuit I2-I3. This speed of response can also be modified by compassing the resistances I6, I8 in material of any desired heat conserving or insulating quality, to control the rate of heat dissipation from these resistances.

The various resistances are of course so designed that during normal operation of the system (that is, while no unusually loud signals are transmitted by amplifier II), the bridge is unbalanced, and an E. M. F. exists across points 2|, '22. -As'thefiiitensity0f the signal increases, resistances 16,18 increase, and this increase :serves to *bring the bridge more "nearly to balanced condition. Thereis a corresponding reductionin 'theratio between the E. M. F. across 5 points 2I,-2-2,"and' that across points I9, 2II. Thus even a very greatly increased signal efiect serves only slightly tofincrease the E. M. F. across points 2|, 22. In factthe constants of the design can be chosen actually to reduce the E. M. F. across lo these-points upon too great an increase in the signal intensity in the output side of amplifier "I I. The limit'to which such 'a signal reduction can be carrie'djis of course also determined by the radiation characteristic'of the resistors I8, I8.

In Fig. 2 I show my invention used in conjunc- 'tion with an'electron emission amplifier 25 which is connected up" to provide a feed-back effect.

"In this 'case,;I show a conventional microphone circuitincluding a microphone 26,-battery 21, and primary winding 28,0f a transformer 29. This circuit is shown merely asan example of one way in which signaling impulses are passed to the amplifier tube 25.

When the microphone 26 is caused to operate 26 in response to sounds reaching it, the resistance of circuit26-2l-28 varies in accordance with the sound waves, the currentvaries correspondiriglyfand an F. isinduced across secondary coil. '30.

ondary '30 connects through a biasing battery 3| to the cathode $32} and the upper end connects through a variable potentiometer resistance 33 to35 the controlelectrode 34. The. potentiometer resistance can. bevaried by movement of a variable contact device 35. The output circuit of amplifier includesiplate or anode 36, primary coil 31 oi a transtormer 38, B battery 39, and filament-.40 or cathode 22. I

The variations in' space current of amplifier 25, due to variations in E. M. F. appliedto the input of amplifier 25cause corresponding varia tions in E. M.'F.'in the secondary coil 40 of transformer 33. These varying E. M. F.'s can be impressed upon succeeding stages of amplifiers,

such as indicated at 4|, and finally upon a translating device 142, such as a recorder or loud speaker. The'secondary coil 4|) for this purpose'co is connected by conductors 43, 44 to the amplifier,4|.

The E. M. F. impressed upon the input side of amplifier 25 is to some extent dependent upon the output E. MLF. thereof. Thus for weak sig- 55,

nals, the E. M. F. impressed across the input circuit is increased by the system I have evolved; and for strong signals, the E. M. F. is reduced thereby. There is, therefore, a stabilizing effect, tending to maintain the intensity of the output E. M. F. substantially constant.

To secure this result, a Wheatstone bridge device 45 is included, of the same characteristics as device I4 of Fig. 1. The arms 41, 48 have a high temperature coefllcient so that their resistances increase rapidly with increase in current fiow, while the other two arms, 49, 50 are made from the usual kind of resistance wire. The output circuit of amplifier-25 is caused to affect the E. M. F. across the diagonal points 5|, 52, as by connecting thesepoints respectively to conductors 43, 44. p

When the ratio of resistance 49 to resistance .48 is the same as that of resistance 41 to resistance 50, then the E. M. F. across the other diagonal points 53, 54, is reduced to zero; if the resistances 41, 48 are less than required by these ratios, the points 53, 54 have a potential difi'erence; and if the resistances 41, 48 are greater than required by these ratios, then the points 53, 54 have a potential difference of opposite sign. Thus this potential difierence can be used to produce automatically either a regenerative or a reverse feed-back efi'ect, depending upon the strength of the signal.

To secure this result, points 53, 54 can be connected to afiect the potential drop across potentio'meter resistance 33, any desired portion of which can be included in the input circuit of amplifier 25. Thus points 53, 54 can be connected to the primary coil 55 of a transformer 56, the secondary coil 51 of which is' connected across resistance 33. It is apparent that by varying the position of tap 35 on resistance 33, any desired portion of the potential drop in resistance 33 can be included in the input circuit. The direction of this drop is chosen to be in phase with the E. M. F. across coil 30, when the signals are weak; and of course they are automatically caused to be out of phase when the signals are strong, due to the automatic selection of the E. M. F. across points 53, 54 as just explained.

Athough rapidly varying currents flow through the circuits, yet the resistances 41, 48 can. be readily caused to respond to them. p

The controlling effect of the variable tap 35 is best illustrated in connection with Fig. 3. The ordinates of curves A, B, C represent output E. M. R's across points 52. The abscissae represent input E. M. F.s, say that existing across coil 3!). Curve A shows a rising character-- istic in general, the greater the input E. M. F. the greater the output E. M. F. Curve B shows a flat characteristic over a wide range, indicating no variation of output asthe input E. M. F. varies; while curve C shows a characteristic that first rises to a maximum and then declines upon further increase in the inputE. M. F. All these characteristics can be obtained by varying the position of contact 35.

It is apparent that I have provided a system that serves to prevent the production of undesirably intense signals, and yet without any danger of producing distortion.

I claim:

1. In an amplifier system for electric signal impulses, an amplifier, a Wheatstone bridge having resistance arms, means for impressing all the signaling impulses from the amplifier across a diagonal of the bridge, and an outgoing circuit connected across the other diagonal, the resistance of at least one of the arms varying directly with the current resulting from the intensity of the impressed signaling impulses, whereby the potential difference across said other diagonal is controlled in accordance with the said intensity.

2. In an amplifier system for electric signal impulses, an amplifier, a Wheatstone bridge having resistance arms, a pair of opposite resistance arms having a characteristic such that variations in current fiow therein cause substantial variations in resistance in said arms, means for impressing all the signaling impulses from the amplifier across a diagonal of thebridge, and an outgoing circuit connected across the other diagonal.

3. The combination as set forth in claim 2, in

which the resistance characteristic of said pair 5 of opposite arms are so chosen that as the signal intensity increases, the ratio of the electromotive force passed by the bridge to the outgoing circuit, to the signaling electromotive force is reduced.

4. In a feed-back amplifier system for signaling impulses, an amplifier, an input and an output circuit therefor, a Wheatstone bridge having four resistance arms, one diagonal of the bridge being connected so as to be afiected by the electromotive force in the output circuit,

and the other diagonal being connected to a circuit coupled to the input circuit so as to affect the input circuit electromotive force at least one of the arms being constructed so that an increase in current flow therein causes a substantially large increase in resistance.

' 5. The combination as set forth in claim 4, in which the arms of the bridge are so proportioned that for weak signals the electromotive force across the said other diagonal serves to whereby the potentiometer may variably affect the input circuit, at least one of the arms being constructed so that an increase in current fiow therein causes a substantially large increase in, resistance.

7. The combination as set forth in claim 6, in

, which the arms of the bridge are so proportioned that for weak signals theelectromotive force across the said other diagonal serves to produce positive feed-back, and for strong signals, negative feed-back.

8. In a feed-back amplifier system for'signab ing impulses, an amplifier, an input and an output circuit therefor, and means for interconnecting the input and output circuits to secure a feed-back, said means including a Wheatstone bridge across one diagonal of which the output circuit is connected, and a coupling circuit connected across the other diagonal of the bridge .56 and inductively coupled to the input circuit, at least one of the impedances of the bridge in creasing in value with an increased current flow therethrough, the bridge impedances cooperating to cause a reversal of potential across the seconxl 00 diagonal with respect to the first diagonal when a potential difference exceeding a predetermined maximum potential difference is impressed across the first diagonal, whereby the energy fed by the coupling circuit to the input circuit reo5 verses in direction when the energy in the output circuit increases beyond a definite value.

FRANK RIEBER.

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