US2231374A - Negative feedback amplifier - Google Patents

Negative feedback amplifier Download PDF

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Publication number
US2231374A
US2231374A US299065A US29906539A US2231374A US 2231374 A US2231374 A US 2231374A US 299065 A US299065 A US 299065A US 29906539 A US29906539 A US 29906539A US 2231374 A US2231374 A US 2231374A
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United States
Prior art keywords
circuit
frequency
curve
amplifier
gain
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
US299065A
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English (en)
Inventor
Albert L Stillwell
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
AT&T Corp
Original Assignee
Bell Telephone Laboratories Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Bell Telephone Laboratories Inc filed Critical Bell Telephone Laboratories Inc
Priority to US299065A priority Critical patent/US2231374A/en
Priority to GB11830/40A priority patent/GB540630A/en
Application granted granted Critical
Publication of US2231374A publication Critical patent/US2231374A/en
Priority to FR870440D priority patent/FR870440A/fr
Priority to BE441790D priority patent/BE441790A/xx
Priority to NL108864A priority patent/NL69414C/xx
Priority to NL108864D priority patent/NL108864B/xx
Anticipated expiration legal-status Critical
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
    • H03F1/42—Modifications of amplifiers to extend the bandwidth
    • H03F1/48—Modifications of amplifiers to extend the bandwidth of aperiodic amplifiers
    • H03F1/50—Modifications of amplifiers to extend the bandwidth of aperiodic amplifiers with tubes only

Definitions

  • the present invention relates to amplifier circuits, especia1ly of the type suited to high frequency or broad-band amplification. While not limited to any particular frequency range or use,
  • the amplifiers of the invention are especially applicable to multiplex carrier signaling or to television transmission over coaxial lines or similar means.
  • a general object of the invention is to increase the effectiveness of multistage tube circuits as amplifiers with stabilized feedback. This object is attained by means of a novel circuit arrangement which inherently increases the stability against singing at certain critical frequency ranges and which permits a higher over-all feedback gain to be realized in practice.
  • Fig. 1 isa circuit embodying my invention
  • Fig. 2 contains characteristic curves explanatory of my invention
  • Fig. 3 is a detail of the circuit of Fig. 1;
  • Fig. 4 contains additional explanatory curves.
  • the external plate circuit of the first tube comprises an inductance L1 in series with the parallel L2 C2 combination, in series with the parallel L3 C3 combination, these providing, as it were, a double trap circuit as later explained.
  • Fig. 1 A detailed description of Fig. 1 need not here be given inasmuch as it appears in full in the said West application. My invention will, however, be made clear by reference to Fig. 2.
  • abc represents an ideal e gain curve for a negative feedback amplifier, such as that of the West application referred to.
  • the portion ab represents the gain over the portion of the frequency spectrum in which transmission with fiat .and highggain is to .be present, and be is the portion over which the gain around the loop falls off. In such a circuit there will be present certain unavoidable parasitic capacities, such as that between plates and ground.
  • phase curve -b c d which gives the change in phase of the feedback voltage as a function of the frequency.
  • the connection between the two has been pointed out in U. S. patent to Bode 2,123,178, July 12, 1938, and from the form of the gain curve one can predict quite definitely the form of the phase shift curve. While over the transmitting band region ab there will ordinarily be a negative feedback in the sense that the Voltage fed back is opposite or nearly opposite in phase to the impressed signal, as one goes to frequencies above the transmitted band ah the phase of the fed back voltage is shifted and at a suificiently high frequency may be turned over to be in phase instead of out of phase with the impressed voltage.
  • the curve I) c (1' may be taken as a typical ideal phase shift curve. Toavoid instability it is necessary that the curve shall not cross the 360 degree or the zero degree line until #13 becomes a loss. As shown in Fig. 2 there is a margin of safety against singing of degrees over the portion 0 d.
  • phase curve is one which tends to fall very rapidly as one goes to frequencies just above the upper edge of the transmitting range at b and there is consequently a general tendency to exceed phase limits in this frequency range. There is also a tendency in the region of the high frequency cut-off to exceed the phase limits. More specifically, there is a tendency for the phase shift curve to follow a course such as the curve b" c e f g h, passing to regions of excessive phase shift at two places, namely, one just above the upper edge of the transmitting range and one at the still higher frequencies in the region.55
  • the purpose of my invention is to neutralize or to delay the build-up of phase shift as one goes to higher frequencies and I accomplish this by introducing circuit elements which will develop some helping phase shift as indicated by the areas A and B of Fig. 2 so that the resultant phase shift curve follows more closely the curve b c f g Z. More specifically, if at a suitable point in the p circuit, such as the coupling network between the first and second tubes, provision be made for circuit elements which at the two regions in question introduce helping phase, then there is a closer approach to the ideal condition. I have found that this can be accomplished by the insertion of two circuit elements shown in Fig.
  • the first of these consisting of an inductance L1 and the second of the parallel combination L3 C3.
  • the inductance 30 which is a coil of relatively high inductance L2 for the purpose of supplying direct current voltage to the plate and inherently possessing parasitic capacities represented by C2.
  • the impedance of L2 is so high that its distributed capacitance is considered, for the purpose of simplicity, as controlling in the frequency ranges under discussion.
  • the circuit for this coupling network may be represented as in Fig. 3.
  • the usual blocking condenser C is of such size that its importance is negligible at the frequencies here considered and the usual grid resistance 45 is so large that it does not act as an appreciable shunt for the remainder of the circuit.
  • this coupling circuit also it is contemplated that there may be a shield 46 enclosing the inductance L1, the circuit 30 and the coupling condenser C". Furthermore, the shield is to be connected at the lower end of L2.
  • the parasitic capacities across the interstage circuit at this point, such as plate-ground capacitance, etc., are represented at C1.
  • This interstage coupling constitutes a double trap circuit where by the term trap circuit as here used is meant a circuit which introduces helping phase at a predetermined portion of the frequency spectrum.
  • trap circuit as here used is meant a circuit which introduces helping phase at a predetermined portion of the frequency spectrum.
  • the manner in which these trap circuits function and perform the desired results will be better understood by reference to Fig. 4 in which the portion mn represents the gain curve for the combination of C1 and. C2 in the region where the effects of the coils are negligible. More particularly in this instance the magnitudes of the inductances and the capacities are so chosen that at a frequency below the upper edge of the transmission band, that is, below the point b of Fig.
  • a physical advantage of the arrangement of my circuit arises from the fact that L1, L2, C2 and the coupling condenser C may be placed in a shield with the shield tied to the low end of L2. This reduces the parasitic capacitance to ground from L1 and the coupling condenser C, which capacitance to ground would ordinarily add to C1. With this shielding arrangement the new and reduced parasitic capacitance of these elements would now be represented by C1 in Fig. 3. Also the capacitance of the shield to ground, represented by C1", is across C3, which itself is relatively large and is therefore able to absorb this additional capacity.
  • a multistage amplifier a plurality of vacuum tube stages in tandem with feedback connection from the output of the last to the input of the first stage, said amplifier having a tendency to produce self-oscillation at a frequency above the utilized range due to shunt parasitic capacity, a coupling network between the plate of one tube and the grid of the next tube, the said network being made up of reactive elements proportioned to resonate with said parasitic capacity in the frequency region where said tendency is otherwise most pronounced, such resonance providing a helping phase shift in said frequency region sufficient to prevent oscillation production.
  • a multistage amplifier a plurality of vacuum tube stages in tandem with negative feedback connection, said amplifier having a tendency to produce self-oscillation due to shunt parasitic capacity at a plurality of separated frequency regions one of which is near the upper edge of the utilized range and the other of which is in the vicinity of the high frequency gain cut-off, a coupling network between the plate of one tube and the grid of the next tube, said network being made up of reactive elements proportioned to produce series resonance at or near each of said frequency regions and to produce together with said parasitic capacity parallel resonance at two frequencies closely adjacent said frequency regions, such resonance effects providing a helping phase shift in each of said frequency regions sufiicient to efiectively counteract said tendency.
  • a coupling network between the plate of one tube and the grid of the next tube, said network being made up of elements of such ma itude as o g v se to amped resonance at two frequency regions, one of which is slightly above the transmitting region of the amplifier and the other of which is near the cut-off point of the amplifier, said coupling network consisting of an inductance in series with a parallel inductance-capacitance combination in series with a second inductance-capacitance combination, the inductances and capacitances being of such magnitude as to give helping phase at the regions specified.
  • a coupling network between the plate of one tube and the grid of the next tube, said network being made up of elements of such magnitude as to give rise to damped resonance at two frequency regions, one of which is slightly above the transmitting region of the amplifier and the other of which is near the cut-off point of the amplifier, said coupling network consisting of an inductance in series with a parallel inductance-capacitance combination in series with a second inductance-capacitance combination, the inductances and capacitances being of such magnitude as to give damped resonance at the regions specified.
  • a stabilized feedback amplifier for amplifying electrical waves extending over a band of frequencies and provided with a gain reducing stabilizing feedback path from a point of higher amplitude level to a point of lower amplitude level therein, said amplifier having the tendency to produce self-oscillation at a frequency near or above the uppermost frequency in said band due to parasitic capacity, a network of reactive impedances included in said amplifier certain of the impedances of which are proportioned to develop a series resonance in the vicinity of said frequency and certain of the impedances of which are proportioned to develop a shunt resonance with said parasitic capacity at a closely adjacent frequency whereby an inorement of helping phase shift is produced at said frequency of such magnitude and sign as to counteract such tendency.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Amplifiers (AREA)
US299065A 1939-10-12 1939-10-12 Negative feedback amplifier Expired - Lifetime US2231374A (en)

Priority Applications (6)

Application Number Priority Date Filing Date Title
US299065A US2231374A (en) 1939-10-12 1939-10-12 Negative feedback amplifier
GB11830/40A GB540630A (en) 1939-10-12 1940-07-18 Negative feedback amplifier
FR870440D FR870440A (fr) 1939-10-12 1941-02-27 Amplificateurs d'ondes électriques
BE441790D BE441790A (ru) 1939-10-12 1941-06-19
NL108864A NL69414C (ru) 1939-10-12 1942-12-10
NL108864D NL108864B (ru) 1939-10-12 1942-12-10

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US299065A US2231374A (en) 1939-10-12 1939-10-12 Negative feedback amplifier

Publications (1)

Publication Number Publication Date
US2231374A true US2231374A (en) 1941-02-11

Family

ID=23153168

Family Applications (1)

Application Number Title Priority Date Filing Date
US299065A Expired - Lifetime US2231374A (en) 1939-10-12 1939-10-12 Negative feedback amplifier

Country Status (5)

Country Link
US (1) US2231374A (ru)
BE (1) BE441790A (ru)
FR (1) FR870440A (ru)
GB (1) GB540630A (ru)
NL (2) NL108864B (ru)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2428363A (en) * 1944-07-26 1947-10-07 Bell Telephone Labor Inc Negative feed-back amplifier
US2475547A (en) * 1943-12-14 1949-07-05 Automatic Elect Lab Thermionic valve amplifier
US2658958A (en) * 1949-07-16 1953-11-10 Wilcox Gay Corp Negative feedback frequency response compensation amplifier system
US2692377A (en) * 1946-01-16 1954-10-19 Jr George A Brettell Position plotter
US2865986A (en) * 1953-08-03 1958-12-23 Louis W Parker High power frequency discriminator
US3443096A (en) * 1964-02-29 1969-05-06 Hitachi Ltd Circuit arrangement for use in radiation measurements and the like purposes

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2475547A (en) * 1943-12-14 1949-07-05 Automatic Elect Lab Thermionic valve amplifier
US2428363A (en) * 1944-07-26 1947-10-07 Bell Telephone Labor Inc Negative feed-back amplifier
US2692377A (en) * 1946-01-16 1954-10-19 Jr George A Brettell Position plotter
US2658958A (en) * 1949-07-16 1953-11-10 Wilcox Gay Corp Negative feedback frequency response compensation amplifier system
US2865986A (en) * 1953-08-03 1958-12-23 Louis W Parker High power frequency discriminator
US3443096A (en) * 1964-02-29 1969-05-06 Hitachi Ltd Circuit arrangement for use in radiation measurements and the like purposes

Also Published As

Publication number Publication date
GB540630A (en) 1941-10-23
NL69414C (ru) 1952-02-15
BE441790A (ru) 1941-07-31
FR870440A (fr) 1942-03-11
NL108864B (ru)

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