US2285832A - Feedback amplifier - Google Patents

Feedback amplifier Download PDF

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Publication number
US2285832A
US2285832A US330671A US33067140A US2285832A US 2285832 A US2285832 A US 2285832A US 330671 A US330671 A US 330671A US 33067140 A US33067140 A US 33067140A US 2285832 A US2285832 A US 2285832A
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United States
Prior art keywords
amplifier
feedback
loop
circuit
relay
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Expired - Lifetime
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US330671A
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English (en)
Inventor
Edwin H Perkins
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AT&T Corp
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Bell Telephone Laboratories Inc
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Filing date
Publication date
Priority to BE442783D priority Critical patent/BE442783A/xx
Priority to NL63669D priority patent/NL63669C/xx
Application filed by Bell Telephone Laboratories Inc filed Critical Bell Telephone Laboratories Inc
Priority to US330671A priority patent/US2285832A/en
Priority to GB5050/41A priority patent/GB546200A/en
Priority to FR884051D priority patent/FR884051A/fr
Application granted granted Critical
Publication of US2285832A publication Critical patent/US2285832A/en
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Expired - Lifetime legal-status Critical Current

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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
    • H03F1/34—Negative-feedback-circuit arrangements with or without positive feedback
    • H03F1/36—Negative-feedback-circuit arrangements with or without positive feedback in discharge-tube amplifiers
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
    • H03F1/08—Modifications of amplifiers to reduce detrimental influences of internal impedances of amplifying elements
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B3/00—Line transmission systems
    • H04B3/02—Details
    • H04B3/04—Control of transmission; Equalising
    • H04B3/06—Control of transmission; Equalising by the transmitted signal
    • H04B3/08—Control of transmission; Equalising by the transmitted signal in negative-feedback path of line amplifier

Definitions

  • This invention relates to wave translation and especially to systems involving feedback ampli.
  • Objects of the invention are to control feedback and oscillations in such systems, to increase the width of transmitted frequency band obtainable in feedback amplifiers and the lop gain ⁇ obtainable over the transmitted vfrequency band,
  • Provision of such margins ordinarily involves constructing the feedback loop or a loop so that the loop phase shift is not allowed to become zero for any frequency for which the loop gain equals or exceeds zero decibels, orin other words so that 1l is not allowed to'become zero for any frequency for ⁇ I is the fects upon amplifiers of such character caused by bie with conditional stabinty than with oom- -plete stability.
  • special problems arise in connection with getting a conditionally stable amplifier into operation and maintaining this stable operation under varying operating conditions that may aect the loop gain, as for example varying loador varying power .supply voltages.
  • the singing may persist. That is, the singing so occasioned may continue after the amplifier has been turned on or after the overload'or other abnormal operating condition has ceased.
  • a specific ,feature of the invention relates to the provision of means for automatically stopvping such oscillations due to subnormal tube -transconductance, upon their occurrence.
  • a relay responsive to changes of unidirectional space current. of one of the tubes produced .by the ⁇ building up of the oscillations Vand by their decay may introduce suiiicient loss or decrease of transmission eiiiciency in the feedback loop'to vstop the oscillations due to subnormal ltube transconductance, upon their occurafter the tube transconductance has become sufliciently high to render-the amplifier stable greater operating frequency range or a greater l amount ofnegative feedback, or both, are attainwith the loss removed, the removal of the loss being accomplished with sufficient. rapidity to prevent -oscillations from building up as the loop gain increases due tothe loss removal.
  • the relay may operate, upon the building up of oscillations, to cause a short circuit to be established across the feedback loop at some point in the loop so that the oscillations decay or cease, and the relay may operate, upon decay or cessation of ,the oscillations and consequent return of the unidirectional space current to approximately normal value and increase of tube transconductance to approximately normal operating value, to cause removal of the short circuit sufdciently rapidly to prevent oscillations from building up to the removal of the short circuit.
  • Fig. 1 is a circuit diagram of an amplifier embodying a form ofk the invention
  • Fig. 3 shows the ri-characteristic of the ampiier or polar plot ofthe loop propagation incl, Lr over a frequency range extending from far below the operating range to far above the operating range.
  • Fig. l illustrates application of the invention to a feedback amplifier -of the type employing repetition of feedback.
  • a type disclosed, for example, in H. S.Blacks above-mentioned Patent 2,102,671'and in his Patent 2,209,955, granted August 6, 1940, on copending application Serial No. 114,390, filed December 5, 1936, for Wave translation systems with the repetition taking place through an outer feedback path f from an output hybrid coil 1 'to an input hybrid coil 5.
  • the amplifier comprises tandem connected tubes
  • 55 applies the voltage from the bridge
  • 54 are the anode-cathode impedance of tube
  • 58 comprises a resistance
  • 50 may be unbalanced, if desired, with resulting advantages brought out, for example, in H. S. Black Patent 2,131,365, September 27, 1938.
  • 55 includes a stopping condenser
  • 54 comprises twoseries connected elements; first, a. feedback resistance
  • 88 are shown in the grid circuit of tube
  • 52 may be Western Electric Company type 310A tubes and tube
  • a 24-volt A battery or other suitable direct current source A with its positive pole grounded and its negative pole con? nected to the -A terminal supplies heating current to the filaments of tubes
  • a 13D-volt B battery cr other suitable direct current source B connected from ground to the -l- B terminal supplies space current for tubes and
  • supplies grid bias voltages for tube
  • 53 is supplied through grid filter resistance
  • This inductance serves, with condenser
  • 98 filter the biasing voltage for the grid of tube
  • 15 provides a by-pass aroundfbattery
  • 18 serveas a filter isolating the B battery with respect to alternating current; and condensers
  • 52 comprise resistors
  • Fig. 2 shows the gain-frequency characteristics A and ⁇ B of thelamplier of Fig. l between terminals T1 and T2 for operation without feedback and with feedback respectively, the particular amplifier for which these ⁇ characteristics were measured being amplifier for a twelve-channel cable carrier telephone system.
  • the transformer has a primary winding
  • 92 may have a capacity of the order of .l microfarad, for example.
  • 89 is a grid leak having a resistance, for example, of the order of 60,000 ohms, providing a, direct current path for biasing the grid of tube
  • 88 is shown as a coil
  • 90 becomes parallel resonant self-capacity of the transformer, at a frequency in the neighborhood of the -kilocycle peak P1 in curve A; and the resonant circuit
  • 81 renders the coupling impedance of the interupper portion of the utilized frequency range and the resonant circuit
  • 53 are coupled by an interstage plate supply circuits and screen a 12- or GO-kilocycle transmitter including the,
  • the frequency ofthe peak P1 in curve -A is determined by theresonance frequencies of transformer
  • the frequency of the peak Pz in curve B is determined by the resonance frequencies of circuit
  • a deviation equalizer 200 comprising a parallel-connected resistance 20
  • This deviation equalizer in theY outer feedback path renders the over-all gainf requency characteristic of the amplifier-practically exactly fiat over the utilized frequency range. For'example, in the 12kilocycle to 60- kilocycle transmitting amplifier for the twelvechannel cable carrier telephone system, this equalizer rendered the gain fiat within about .01 decibel over the transmission band.
  • the a loop include the input and output transformers in the general manner of Fig. 1, for example.
  • the feedback path f is a network' 205, comprising resistances 2.06, 201 and 208, and the capacity 2
  • in parallel with resistance 201 is used to improve the phase shift at high frequencies and to decrease the loss of network 205 atfrequencies above the utilized frequencyA range.
  • 5 which may have a -turns ratio of 1:1, or any desired ratio, is shown in the feedback path f.
  • the transformer improves the y longitudinal balance of the system, and enables the circuits connected to its two windings to be one symmetrical andthe other unsymmetrical (unbalanced-to-ground)
  • Fig. 3 shows the steady-state api-characteristic of the amplifier (assuming the lines or terminating impedances attached at terminals T1 Aand T2 have the normal values of 600 ohms and 135 ohms respectively).
  • the characteristic has a cross-over point at frequency f1 where it passes from the fourth quadrant into the flrstquadrant at a value of loop gain exceeding zero decibels, and with further frequency decrease the characteristic has a crossback as it passes from the first quadrant back into the fourth quadrant at frequency f2 with the loop gain still exceeding zero decibels:
  • the amplifier of Fig. 1 as so far described may be, for example, that of Fig. 15 of th above-mentioned Patent 2,209,955.
  • has its winding connected 'in the unidirectional space current circuit of tube
  • Relay 302 is adapted to operate rapidly and release slowly.
  • of relay 302 is for establishing a short circuit across the outer feedback path f of the amplifier. .
  • 2 of relay 302 is for vestablishing a short circuit across-the amplifier input terminals T to which is attached the in- 1 coming line or circuit.
  • the characteristic has four, cross-overs or cross-over points on the real axis for values of IMSI exceeding unity.
  • the inner feedback path is such that with the ⁇ outer feedback ineffective or ii'factive (for example, short-circuited by contact 3
  • 53 is 30 milliamperes.
  • the space currentof this tube builds up to 4a. value of approximately 55 milliamperes, which may be considered 'its normal singing value. This building up happens because the action of feedback is such that with full load output the space current seeks a value such that the voltage and currentlimitations of the tube will'be realized to-"' gether.
  • This circuit is so adjusted that the no load space current is somewhat less than the full load value.
  • singing occurs, because of the fact that no limiting is incorporated into the circuit, the circuit will vsing violently enough to ⁇ operate thetube at full load output.
  • this yfull load output condition is not obtained except for instantaneous peaks, and the relay 30
  • is adjusted to close its contact when a current of about 50 milliamperes flows in its winding 305 (so it will always receive sufficient current for operating it when oscillations build up in the amplifier to a steady-state amplitude), and this relay is adjusted to release its contact when the current decreases to about 35 milliamperes.
  • the contacts of relay 302 are so associated that, in opening, they open the short circuit across the feedback path ⁇ f, first, and the short circuit across The operation of starting the amplifier or turning on the tubes to ⁇ condition them for amplifying will now be described. During this operation the amplifier may be disconnected froml the incoming and outgoing lines, if desired.
  • hunting condition might thus be established if the relay were not a slow-release relay.
  • the amplifier may sing if the impedance viewed from the input terminals T1 of the hybrid coil 5 when looking toward terminals T differs too greatly from the normal value (which may be 600 ohms). For example, -ii' the impedance so viewed from terminals T1 is zero or infinity, (the values of a given by Fig. 3 do not obtain and) such improper value of impedance may cause the amplifier to sing.
  • the cycle of operation is repeated until the overload is removed.
  • Repeated operation of the relay cycle can be caused to bring up a circuit alarm if desired for example, by having relay 30I also operate a counting relay, not shown, which will introduce an alarm after any desired number of cycles of operation of relay 30
  • Such an alarm is desirable because if the amplifier were permitted to continuously repeat this relaycycle, it would indicate a circuit trouble and also the continuing cycle of operation would constitute a circuit interruption.
  • the sequence in which the contacts of relay 302 open insures that the short circuit across the feedback path ,f will always be removed before the short circuit across the terminals T is removed. Otherwise the normal full load input would effectively be too high; for with the short circuit across ,f removed, the feedback through path f may reduce the amplifier gain 15 decibels, for example, and with a short circuit established acrossthe feedback path f the amplifier which,
  • the short circuit across terminals T is advantageous in preventing any shock excitation entering the amplifier from the input line or circuit that might tend to set the amplifier singing when the short circuit across the feedback path f is first opened.
  • filters for suppressing waves of singing frequency may be used to connect terminals T and T2. of the amplifier to its attached incoming and outgoing circuits, for
  • lay 302 is advantageous as indicated above, the
  • Theamplifler using repetition of the feedback process, with the inner and outer feedback paths, is advantageous in facilitating effective short-circuiting actionl of the means for establishing the short circuit across the outer feedback path; for example, the total reduction of the insertion gain of the amplier Vto be effected by feedback may be 55 decibels, of which 40 decibels may be obtained bythe inner feedback path, leaving only 15 decibels to be effected by the outer feedback path, so the feedback voltage to be short-cir- 53 to frise sufficiently amplifier of Fig. 1, for lul, for example, by
  • a voltage limiter (not shown) may be connected between the incoming line and terminals T, to reduce danger of overload causingthe amplifier tostart singing. It may be of any suitable type, las for example, a negative feedback amplifier' with its gain-load characteristic having a shart cut-off, -as in the' case of Fig. 10 of the above-mentioned paper of H. S. Black, the overload point or cut-off value of load ybeing adjusted, for example, to a load value sufficiently low to insure that the input to terminals T cannot reach ka value so large as to cause the ,acharacteristic of Fig.
  • the total llfl was about 55'decibels but that effective in the outer path was only about ⁇ 15 decibels.
  • the amplifier, using repetition of the feedback process, ⁇ with a considerable portion of the total vnegative feedback, obtained by the inner feedback path, is also advantageous in reducing shock around the feedback loop plotted against the real part for all real ,values of frequency avoid encircling the point 1,0.
  • a Wavev translating system comprising an electric space discharge wave amplifying device, means forming with said device 'adapted to produce feedback with the loop gain exceeding zero decibels lfor a frequency at which the loop phase shift is zero and with the graph of the imaginary part ⁇ of the loop propagation plotted against the real part for all real values of frequency avoiding encircling the point'1,0, and means responsive'to a direct current component of space current. of said device for conexcitation that might occur due to removal of 75 trolling the loop propagation. 2.
  • An amplifier plifying device a feedback path therefor forming therewith a feedback loop for producing feeda feedback loop comprising a vacuum tube am stable, and means responsive to change in steady space current of said amplifying device for establishing a high admittance across the feedback loop.
  • An amplifier comprising a vacuum tube amplifying device, a feedback path therefor forming therewith a feedback loop for producing feedback that renders the a-m-plifier conditionally stable, a low impedance path for connection across said feedback loop to -form a low impedance shunt across said feedback loop, and means controlled by a direct current component back that renders the amplifier conditionally the loop phase shift changes sign at zero degrees and 'whose loop propagation has the graph of its imaginary part plotted against its real part for all real values of frequency avoid encircling the point 1,0, a circuit for supplying unidirectional space .current to said amplifying device, and means for preventing establishment of a steadystate sing.
  • relay apparatus including contacts for establishing a low impedance shunt across said feedback loop, a reof space current of saidl amplifier for controlling connection of said low impedance path across said feedback loop.
  • Anamplifler comprising a vacuum. tube amplifying device,'a feedback path therefor forming therewith a feedback vloop for producing feedback that renders the amplifier conditionally stable, means for forming a, low impedance shunt across said feedback loop, and means controlled by direct space current of said amplifier for removing said shunt.
  • An amplifier comprising a vacuum tube am ⁇ p lifying device, a feedback path therefor forming therewith a feedback loop for producing feedback that renders the amplifier conditionally stable
  • the amplifier for preventing steady-state oscillations around said loop from being produced by transient oscillations caused by said overloading.
  • An amplifier comprising a vacuum tube amplifying device, a ⁇ feedback path therefor forming 'therewith a feedback loop for producing feedback that renders the amplifier. conditionally stable, contacts for establishing a low impedance shunt lay winding connected in said unidirectional space current circuit of said amplifying device, and means operated by energization of said winding upon energization of said amplifying device for'controlling operation of said contacts.
  • An amplifier and an incoming circuit therefor said amplifier comprising a vacuum tube amplifying device, a feedback path therefor forming therewith a feedback loop for producing feedback that renders the amplifier conditionally stable, and means for preventing establishment of a steady-state sing varound the feedback loop upon occurrence of overload ofthe amplifier, compris-y ing'means responsive to change in direct space current of said amplifying device for establishing a lowimpedance shunt across the feedback loop and introducing a large transmission loss -between the incoming circuit and the amplifying device.
  • An amplifier' comprising a vacuum tube amplifying device, a feedback path therefor forming therewith a feedback loop for .producing feedback that renders stable, and means responsive .to-change of steady space value causedby building up of self-sustained oscillations around the feedback loop for establishing a low impedance shunt across said feedback loop and responsive-to restoration of said unidirectional space current to its normal-value for removing said shunt.
  • An amplifier comprising a vacuum ltubev amplifying device, a feedback path therefor form'- ing therewith a dynamically stablefeedback loop having loop gain so great over a frequency band so wide that the loop phase shift is zero at a frequency for which the loop gain exceeds zero deci- 'across the loop, a plate circuit for said amplifying device, and means comprising a winding'in said plate circuit for operating said contacts upon the building up of self-sustained oscillations around the loop.
  • amplifier comprising a vacuum tube amplifymg device, a feedbackpath therefor forming therewith a feedback loop for producing feedback and with the graph of the imaginary part of the loop propagation plotted against the real part for all real values of frequency avoiding encircling the point 1,0 with loop gain exceeding zero decibels at -a frequency'of zero loop phase shift, a circuit for supplying unidirectional space current to said amplifying device, and means for prearound the feedback loop comprising relay apparatus including contacts for establishing a low ,venting establishment 'of a steady-state' sing ceeds zero decibels for two frequencies at which bels, a relay responsive to increase of a direct current component of space current of said amplifying device produced by building u-p of selfsustained oscillations around the loop upon energization of said amplifying device to condition said device for amplifying, and means controlled by said relay for placing a low impedance shunt across said-path.
  • An amplifier comprising a vacuum tube amplifying device, va feedback path therefor forming therewith a dynamically stable feedback loop having loop gain decrease so rapidly with change of frequency at one side of the operating frequency range of the amplifier that before the decreasing loop gain reaches zero decibels the loop phase changes sign at Zerodegrees, a switch.
  • thel amplifier conditionally 3 current of said amplifier from its normal decrease of said space current to its normal value upon cessation of said oscillations.
  • An amplifier comprising an electronic wave amplifying device, a feedback path from the output of saiddevice to the input of said device adapted to form with said device a dynamically stable.
  • feedback loop having loop propagation whose graph of imaginary part versus real part for all real values of frequency avoids encircling the point 1,0 and' having loop gain so great ovei ⁇ a frequency band extending so far upwardly from a frequency at which the loop phase shift is 180 degrees that before the loop gain decreases to zero decibels with frequency -increaseabove said band the loop phase shift decreases to zero degrees, means introducing transmission loss -in said loop when the gain of said amplifying device has a subnormal value and means responsive to a steady current component of space current of said device after increase of its gain above said subnormal value for removing said loss.
  • An amplifier comprising a vacuum tube amplifying device, a feedback path therefor forming therewith a feedback loop for producing feedback that renders the amplifier conditionally stable, a space current supply circuit for said amplifying device, said amplifying element increasing its unidirectional space current in said space current supply circuit above the normaloperating value when self-sustained oscillations build up around the feedback loop upon energization of said amplifying device, a relay operable to closed position when energized for short-circuiting said feedback pathL and a relay having a winding in said spacecurrent supply circuit for energizing said rst-mentioned relay in response to said increase of space current and for deenergizing saidl first-mentioned relay to remove said short circuit in response to decrease of said means.
  • An amplifier and an incoming circuit therefor said ampliner comprising a vacuum tube amplifying device, a feedback path therefor forming therewith a feedback loop for producing feedback that renders the amplifier conditionally stable,l means for preventing establishment of a steady-state sing around the feedback loop upon occurrence of overload of the amplifier, comprising'means responsive to change of unidirectional space current of said ampliiier from its normal value for establishing a low impedance shunt across the feedback loop and introducing a large transmission loss between the incoming circuit when the gain of said device is normal and tends v to produce steady state oscillation of said amplifier when the gain of said device is subnormal andvmeans forV counteracting said tendency comprising means to control propagation of said one ⁇ of said paths andjmeans responsive to a direct current component of space current ofsaid device for controlling said propagation.l control EDwiN H. Pnnms.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Amplifiers (AREA)
US330671A 1940-04-20 1940-04-20 Feedback amplifier Expired - Lifetime US2285832A (en)

Priority Applications (5)

Application Number Priority Date Filing Date Title
BE442783D BE442783A (es) 1940-04-20
NL63669D NL63669C (es) 1940-04-20
US330671A US2285832A (en) 1940-04-20 1940-04-20 Feedback amplifier
GB5050/41A GB546200A (en) 1940-04-20 1941-04-18 Thermionic amplifiers employing negative feedback
FR884051D FR884051A (fr) 1940-04-20 1941-06-30 Systèmes de transmission d'ondes électriques

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US330671A US2285832A (en) 1940-04-20 1940-04-20 Feedback amplifier

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US2285832A true US2285832A (en) 1942-06-09

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US330671A Expired - Lifetime US2285832A (en) 1940-04-20 1940-04-20 Feedback amplifier

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BE (1) BE442783A (es)
FR (1) FR884051A (es)
GB (1) GB546200A (es)
NL (1) NL63669C (es)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2582498A (en) * 1949-08-30 1952-01-15 Bell Telephone Labor Inc Negative impedance repeater and loading system
US4199730A (en) * 1949-09-28 1980-04-22 The United States Of America As Represented By The Secretary Of The Army Double peaked amplifier

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2582498A (en) * 1949-08-30 1952-01-15 Bell Telephone Labor Inc Negative impedance repeater and loading system
US4199730A (en) * 1949-09-28 1980-04-22 The United States Of America As Represented By The Secretary Of The Army Double peaked amplifier

Also Published As

Publication number Publication date
BE442783A (es)
GB546200A (en) 1942-07-01
FR884051A (fr) 1943-08-02
NL63669C (es)

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