US2364389A - Negative feedback amplifiers - Google Patents

Negative feedback amplifiers Download PDF

Info

Publication number
US2364389A
US2364389A US512174A US51217443A US2364389A US 2364389 A US2364389 A US 2364389A US 512174 A US512174 A US 512174A US 51217443 A US51217443 A US 51217443A US 2364389 A US2364389 A US 2364389A
Authority
US
United States
Prior art keywords
amplifier
amplifiers
parallel
negative feedback
feedback
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
US512174A
Other languages
English (en)
Inventor
Roche Alleman Holly
Elliott Thomas William
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.)
STC PLC
Original Assignee
Standard Telephone and Cables PLC
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 Standard Telephone and Cables PLC filed Critical Standard Telephone and Cables PLC
Application granted granted Critical
Publication of US2364389A publication Critical patent/US2364389A/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

Links

Images

Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J1/00—Frequency-division multiplex systems
    • H04J1/02—Details
    • H04J1/16—Monitoring arrangements
    • 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
    • H03F1/52—Circuit arrangements for protecting such amplifiers
    • H03F1/54—Circuit arrangements for protecting such amplifiers with tubes only
    • 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
    • H03F1/52—Circuit arrangements for protecting such amplifiers
    • H03F1/54—Circuit arrangements for protecting such amplifiers with tubes only
    • H03F1/542—Replacing by standby devices
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/74—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission for increasing reliability, e.g. using redundant or spare channels or apparatus

Definitions

  • the present invention relates to electric negative feedback amplifiers, and concerns particularly arrangements for preventing instability during switching operations.
  • the principal object of the present invention is, therefore, to overcome this difficulty by providing a switching arrangement which does not cause the parallel connected amplifiers to be unstable, and which at the same time does not entail any appreciable sacrifice in the normal performance of individual amplifiers which would otherwise have to be made for the purpose of avoiding instability during parallel operation.
  • an arrangement for'connecting between two sections of an electric signal transmission line two similar negative feedback amplifiers in parallel without producing instability, each of the amplie bombs having input and output transformers comprising means for causing the frequency of series resonance of the input and/or output transformer of one amplifier to differ by a predetermined amount from the frequency of parallel resonance of the input and/or output transformer of the sists in an arrangement for obtaining stable operation of two similar parallel connected neg ative feedback amplifiers inserted at a point in an electric signal transmission line comprising circuit means connected in tandem with one of the amplifiers, and adapted to prevent either amplifier from modifying the feedback loop of the other in such manner as to render it unstable.
  • the invention lies in an amplifying arrangement for an electric signal transmission line comprising a first negative feedback amplifier connected to the line and a second similar amplifier adapted to be connected in parallel with the first through circuit elements, the amplifiers and elements being designed so that the polar curve representing the transmission round the 11,6 path of either amplifier" does not enclose the point (1,0), whether the amplifiers are operated singly or in so parallel.
  • the invention provides an amplifying arrangement for anelectrio signal transmission line comprising two similar negative feedback amplifiers connected in 25 parallel thereto, means being provided for rendering dissimilar the transmission characteristics of the s loops of the two amplifiers in such a manner that the parallel combination is stable.
  • Figs. 2, 3, 4, 6 and 7 show schematic circuit diagrams used to explain the eflect of the transformers onthe feedback loop of the amplifier
  • Figs. 5, 8, 9 and 10 show curves to illustrate the transmission conditions in the feedback loop
  • FIG. 11 shows a block schematic diagram of an arrangement according to the invention. 7
  • a negative feedback amplifier will be regarded as comprising a forward amplitying path of which the. voltage transfer. ratio is denoted by 1., and a feedback path of which the voltage transfer ratio is denoted by ,8.
  • Both a, and B are in general complex quantities and take account of the phase change as well as the corresponding galn or loss.
  • the loop comprising the forward and feedback paths is known as the feedback loop, and s represents the voltage transfer ratio in magnitude and phase for one transit round the loop.
  • the feedback 455 loop is often called the as long and the gain the n? gain.
  • FIG. 1 A negative feedback amplifier of a type used on co-axial cables is shown diagrammatically in Fig. 1. This figure only shows the details necessary for an understanding .of the invention, and it will be understood that the amplifier may be otherwise designed in any appropriate way.
  • the amplifier comprises an input transformer IT which connects the line (represented by the impedance Z) to the control grid of the first thermionic amplifying valve V1, and an output transformer OT for coupling the anode of the last valve V2 to the line.
  • the anode of V1 may be coupled in any suitable way, including further amplifying stages if necessary, to the control grid of V2; this coupling is indicated in Fig. 1 by the dotted line connecting this anode and control grid. If the amplifier should be a single stage amplifier, then the anode of V1 would be connected directly to the output transformer OT. It will be understood, of course, that V1 and V2 may all represent two or more valves connected in parallel, and may have any number of additional grids (not shown), appropriately polarised.
  • a feedback resistance R is connected, as shown.
  • the secondary wind ng of the input transformer IT is connected between the control grid of the first valve V1 and the high potential terminal of E. This is an ordinary current feedback arrangement, by which a potential proportional to the output current is fed back to the control grid of the first stage.
  • the phase shift for transmission once round the 5 loop should ideally be 180 or an odd multiple thereof at all frequencies. In practice, of course, this can never be achieved, and it is well known that the amplifier may become unstable if the phase shift reaches zero or a multiple of 360 at any frequency.
  • the condition for stability in this case is that the 1 -8 gain must be zer or less at the frequency at which the phase shift is 360.
  • Detailed information on the stabil ty conditions and design of these amplifiers may b found in the papers in the Bell S stem Techn cal Journal by H. Nyquist (Jan. 1932 pa es 126 to and H. W.
  • Bode July, 1940 pages 421 to In order to meet the stability reou rernents it is necessary to control the amplifier perf rmanc not only over the working frequency ra but also outside this range. For example. it is nephews sary to ensure that as the freouenc is raised above the working range, the :8 gain becomes zero or negative before the, phase shift has changed to 360 or a multiple thereof. The components which influence the up characteristics of the loop must therefore be proportioned accordingly. and the input and output transformers play an important part in this connection.
  • the desired characteristics in the neighbourhood of the upper cut-off frequency may be substantially secured by suitably choosing the frequency at which the leakage reactance of each transformer resonates with its self capacity.
  • Fig. 2 is shown the equivalent circuit of the output transformer from the terminals I, 2 as seen from the anode circuit of the last valve V2.
  • Fig. 2 is similar to part of Fig. 101! shown on page 189 of Radio Engineering by 1". E. Terman. second edition, 1937.
  • n is the impedance transformat on ratio
  • Fig. 4 shows the network presented to the output current i1 (Fig. 1).
  • Co is the output capacity of the last valve stage
  • Fig. shows the curves relating log 1' (curve A) and e (curve B) to log a, for a typical amplifier. It will be seen that r is practically equal to l at low frequencies, rises slightly and then falls rather steeply at mid-frequencies and then rises again to a practically constant value less than 1 at high frequencies. The final constant value is due to the fact that the network reduces at high frequencies to the two condensers C1 and C2, the impedance of L: having risen to such an extent that its effect in shunting C1 is negligible.
  • the transformer OT When another amplifier is connected to the line in parallel with that shown in Fig. 1, the transformer OT will have another similar transformer bridged across its secondary winding at the terminals 3 and t. Since the second amplifier-is the same as the first, the bridging impedance can be determined from Fig. 2. Fig. 6
  • Fig. 9 shows the 14,8 gain expressed on a decibel scale (curve A) and phase shift (curve B) related to 10g f, where j is the. frequency, for the conditions of Fig. 7. It will be seen that the phase shift has become 360 or more over a short mid-frequency range, and there is a net gain round the loop over part of this range. The amplifier will therefore be expected to sing.
  • Fig. 10 is similar to Fig. 9 and shows the curves which would be obtained for either amplifier alone, that is, when not shunted by another amplifier. It will be seen that the phase shift does not reach 360 until well after the gain has become negative so that there is a very good margin of stability.
  • the parallel connection of the amplifiers may not always result in actual singing, but evenso, the state of the circuit is one of conditional stability in which although the point (1,0) is not enclosed, the configuration of the polar diagram is such that some small change in s such as might be due to ageing, replacement of valves and the like would cause the point to be enclosed, and thelamplifier's would sing. Such a condition cannot be regarded as satisfactory.
  • the instability caused by connecting the second amplifier in parallel with the first is prevented by separating the resonance frequencies, whose coincidence is the cause of the trouble, by an amount calcu-' lated to produce a sumcient margin of stability.
  • One way in which this may be done is to connect the second amplifier at each end through an impedance network which slightly modifies the impedance presented to the transformer of the first amplifier so that it resonates at a different frequency.
  • A1 is the ampli her which is normally connected in the circuit and A2 is a spare amplifier which is to replace it for a short period.
  • Two networks N1 and N are connected in tandem with the amplifier A2 at the input and output, respectively.
  • S1 and S are intended to represent any suitable switching or jacking system for connecting the amplifiers in parallel. After the switches have been closed, the amplifier A1 may be removed by other jacks or switches not shown, so that the amplifier A: carries the tramc in the meantime. N1 and N2 should therefore be designed to introduce a very small loss. If the amplifier A1 is defective or othewise expected to be switched out for a long period, another normal amplifier may be switched in parallel with A2, which may afterwards be removed by opening the switches S1 and S2, and it will then be available for use again. v
  • the networks N1 and Ni will generally be composed of reactive elements, and in one preferred arrangement each consists of a single inductance connected in series with the conductor corresponding to the central conductor of the co-axial cable; or ina balanced system, two equal inductances would be used, one connected in series with each wire.
  • the effect is to increase one of the inductances L3 in Fig. '1, (that is. effectively to increase the leakage inductance of the corresponding transformer) so that the two resonance frequencies are separated. It will be seen that this will increase Lo in shunt with R: for A1 and L3 in series with R3 for A1.
  • the inductance connected in series with the central conductor was about 4% microhenrys.
  • the means for preventing instability are connected externally to the spare amplifier and accordingly it does not require the provision of any special amplifier for switching purposes. This is an important consideration in the interests of standardisation.
  • An arrangement i'or connecting between two sections of an electric signal transmission line two similar negative feedback amplifiers in parallel without producing instability, each of the amplifiers having input and output transformers, comprising means for causing the frequency ofseries resonance of one of said transformers of one amplifier to differ by a predeter-' mined amount from the frequency of parallel resonance of the corresponding transformer of the other amplifier.
  • An amplifying arrangement for an electric signal transmission line comprising two similar negative feedback amplifiers connected in parallel thereto, means being provided for rendering dissimilar the transmission characteristics of the p13 loops of the two amplifiers in such manner that the parallel combination is stable.

Landscapes

  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Amplifiers (AREA)
  • Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
US512174A 1942-12-24 1943-11-29 Negative feedback amplifiers Expired - Lifetime US2364389A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB268701X 1942-12-24

Publications (1)

Publication Number Publication Date
US2364389A true US2364389A (en) 1944-12-05

Family

ID=10250498

Family Applications (1)

Application Number Title Priority Date Filing Date
US512174A Expired - Lifetime US2364389A (en) 1942-12-24 1943-11-29 Negative feedback amplifiers

Country Status (6)

Country Link
US (1) US2364389A (en:Method)
BE (1) BE467630A (en:Method)
CH (1) CH268701A (en:Method)
FR (1) FR939157A (en:Method)
GB (1) GB562392A (en:Method)
NL (1) NL74579C (en:Method)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2429775A (en) * 1944-06-22 1947-10-28 Rca Corp Amplifier system
US2429613A (en) * 1943-10-19 1947-10-28 Standard Telephones Cables Ltd Pulse multiplex communication system

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2429613A (en) * 1943-10-19 1947-10-28 Standard Telephones Cables Ltd Pulse multiplex communication system
US2429775A (en) * 1944-06-22 1947-10-28 Rca Corp Amplifier system

Also Published As

Publication number Publication date
NL74579C (en:Method)
FR939157A (fr) 1948-11-05
CH268701A (fr) 1950-05-31
BE467630A (en:Method)
GB562392A (en) 1944-06-29

Similar Documents

Publication Publication Date Title
US12155352B2 (en) Power amplifier arrangement
US2412995A (en) Amplifier of electromagnetic energy
US2364389A (en) Negative feedback amplifiers
US2751442A (en) Distortionless feedback amplifier
US3202927A (en) Continuous operable negative feedback amplifier
US3586881A (en) Transistor hybrid circuit
GB553847A (en) Electric wave amplification
US2978579A (en) Signal mixers
US2393709A (en) Distortion reduction on modulated amplifiers
US2282381A (en) Amplifier
US3348163A (en) Redundant amplifier circuit
US2611833A (en) Electric amplifier with negative feedback
GB810109A (en) Improvements in or relating to amplifier circuit arrangements
US3105125A (en) Power separation filter
US3242442A (en) Feedback oscillator with plural forward transmission paths
US2266168A (en) Amplifier
US2556219A (en) Negative feedback circuit for parallel-connected thermionic amplifiers
US2397625A (en) Thermionic feed-back amplifier
US2098950A (en) Vacuum tube circuit
US2210997A (en) Feedback amplifier circuit
US4061984A (en) Transistor power amplifier for transmitting systems
US2312748A (en) Volume control circuit
US2230256A (en) Wave amplifying system
US2631201A (en) Signal amplifier
US2269693A (en) Wide range amplifier circuits