US2480163A - Negative feedback amplifier - Google Patents

Negative feedback amplifier Download PDF

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Publication number
US2480163A
US2480163A US588269A US58826945A US2480163A US 2480163 A US2480163 A US 2480163A US 588269 A US588269 A US 588269A US 58826945 A US58826945 A US 58826945A US 2480163 A US2480163 A US 2480163A
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United States
Prior art keywords
amplifier
voltage
feedback
phase
frequency
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Expired - Lifetime
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US588269A
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English (en)
Inventor
Romander Hugo
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STC PLC
Federal Telephone and Radio Corp
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Standard Telephone and Cables PLC
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Priority to BE464915D priority Critical patent/BE464915A/xx
Application filed by Standard Telephone and Cables PLC filed Critical Standard Telephone and Cables PLC
Priority to US588269A priority patent/US2480163A/en
Priority to GB9021/46A priority patent/GB610105A/en
Priority to FR925551D priority patent/FR925551A/fr
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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
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61K—PREPARATIONS FOR MEDICAL, DENTAL OR TOILETRY PURPOSES
    • A61K9/00—Medicinal preparations characterised by special physical form
    • A61K9/14—Particulate form, e.g. powders, Processes for size reducing of pure drugs or the resulting products, Pure drug nanoparticles
    • A61K9/16—Agglomerates; Granulates; Microbeadlets ; Microspheres; Pellets; Solid products obtained by spray drying, spray freeze drying, spray congealing,(multiple) emulsion solvent evaporation or extraction

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  • This invention relates to wave amplifying systems and in particular to amplifiers employing negative feedback. Specifically the invention relates to a method of preventing self-oscillation in amplifiers of this type.
  • amplifier self-os-cillation may be prevented by including within the amplier circuits, and usually in the feedback path, a network so designed that the waves which tend to produce self-oscillation are highly attenuated thus reducing the gain at these frequencies.
  • self-oscillation at all frequencies is prevented in accordance with a new principle wherein there is produced in addition to the -conventional feedback voltage, an additional voltage having such magnitude and phase that the resultant feedback voltage is always displaced 180" with respect to the input voltage.v It is immaterial whether the waves which traverse the amplifier are originally impressed thereon from an outside source or are the result of modulation or distortion components developed within the amplifier.
  • One of the primary purposes of employing negative feedback is to reduce distortion components developed within the amplifier.
  • the distortion components are most effectively reduced in the frequency range where the deviation from the desired 180 phase shift for one complete excursion around the MS path is relatively small, that is, of the order of 30 or less.
  • my circuit arrangement is very effective in reducing this type of distortion. Negative feedback may be applied almost without limit with the result that the h-um voltages are practically eliminated and still the amplifier will not oscillate at any frequency.
  • a primary object of my invention is to provide a stable negative feedback amplifier circuit arrangement which permits high gain over anextremely wide frequency range.
  • FIG. 1 is a schematic diagram illustrating a preferred embodiment of my invention
  • FIGs. 2, 3, 4 and 6 are vector diagrams which illustrate the principles of operation of my invention.
  • Fig. 5 is a polar diagram showin-g the ,variation of m8 with frequency
  • Fig. '7 is a schematic diagram illustrating a modification of Fig. l;
  • the reference character I represents a source of voltage which is to be amplii-led. This source may be of a single frequency or of a plurality of frequencies such as would occur in the output of a voice frequency amplifier. Although the source is illustrated as an alternator it could also be the secondary winding of an input transformer or the output circuit of a vacuum tube amplifier.
  • the reference character 2 represents the impedance of the source I and may be, for example, a separate resistance as shown or the plate impedance of an amplifier. The resistance 2 is connected to a junction point 3 and to the grid 4 of an amplifying tube 5.
  • the amplifying tube 5 is illustrated as a triode for simplicity although tetrodes or pentodes could be employed without departing from the principles of my invention.
  • the cathode 6 of the tube 5 is connected to ground through a biasing resistor 1.
  • the anode 8 is connected to a main amplifier 9 illustrated in block form.
  • the internal construction of the amplifier is immaterial in so far as my invention is concerned, It may b e a ⁇ single or a multistage amplifier; if the latter, the various stages may be transformer coupled, resistance-capacity coupled or by any other forms of coupling.
  • Local negative feedback circuits may exist within the amplifier 9 and the phase shift of the amplifier at various frequencies may have any values whatsoever a1- though it would be preferable to employ the best practice and make the phase shift other than the 180 required for negative feedback as small as economically desirable.
  • the amplifying tube is to be considered as part of the amplifier 9 but it is shown separately for purposes of description.
  • phase reversal of the voltages for the frequencies within the operating range must occur in order to reduce distortion in accordance with the principles of negative feedback. All of the phase reversal need not take place inthe forward or n path; some of it may be in the feedback or path.
  • phase reversal may be obtained by crisscrossing the feedback connections from the output to the input circuits in accordance with known practice.
  • the principles of myV invention apply equally well to amplifiers of this type.
  • the necessary power supplies for operation of the amplifier are not shown ⁇ but are understood to be within the amplifier 9.
  • the output impedance of the amplifier 9 is illustrated as a resistor I0.
  • a lead II connects the output of the amplifier to a load circuit, not shown.
  • a negative feedback connection comprising a blocking capacitor I3 and a resistor I4 connects with the junction point 3.
  • the gain of the amplifier at various frequencies may imply that the amplifier will oscillate at some frequency, perhaps not in the range for which the amplifier is designed but at some higher or supersonic frequency at which the phase rotation through the amplifier and feedback path is 0, 360' or a multiple thereof as above explained.
  • I provide a circuit arrangement, also connected to the junction point 3', which will produce a voltage having the proper magnitude and phase which, when combined with the feedback voltage of the main amplifier 9, will always produce a resultant feedback voltage having a 180 phase displacement with respect to the inputvoltage.
  • the circuit arrangement is so designed that it will function in the desired manner at any frequency whether this frequency be that of the input voltage or of a distortion voltage developed within the amplifier 9. This circuit arrangementA will now be described.
  • a conductor I5 leads to the input grid I6 of amplifying tube I1.
  • the cathode I8 of this tube is connected to ground through a resistor I9'.
  • TheV anode: 20 connects to what I have termed a correlativo amplifier 2
  • the tube I1 is to be considered as part of thecorrel'ative amplier but is separately shown forv purposes of description.
  • the output of the correlative amplifier is connected to an output impedance shown as a resistor 22.
  • a power supply, notl shown, is to be considered as connected to the tubes Within the 4 correlativo amplifier. If desired the power supply for both amplifiers 9 and 2I may be common.
  • correlativo amplifier 2i The characteristic feature of the correlativo amplifier 2i is that it should have substantially identical gain and phase shift characteristics with respect to frequency as does amplifier 9.
  • the feedback connection comprising capacitor I3 and resistor I4 merely impresses a predetermined fraction of the output voltage developed across resistor I@ onto the input of the main amplifier.
  • This fraction is a scalar quantity, that is, no phase shift is involved.
  • a similar feedback circuit is not shown in connection with the correlativo amplifier 2
  • the feedback connection associated with the main amplifier is such that a phase shift occurs in this connection, a corresponding circuit producing the same phase shift should be associated with the correlative amplifier.
  • correlatlve amplier may be simplified by, for example, the omission of a stage of amplification which involves nothing more than a phase shift of This phase shift could be compensated for by reversing the connections of a transformer within the amplifier or by other suitable means.
  • amplifier 9 would usually comprise tubes capable of producing considerably higher output than the tubes in amplifier 2
  • the variance in power output between amplifiers 9 and 2i is permissible since amplifier ZI merely supplies a voltage of desired magnitude and phase, and the value of this voltage, as determined by the point of contact 23 of conductor 24 on resistor 22, may be the total output voltage of the amplifier, whereas in conventional feedback practice the feedback connection at contact I2 on resistor I0 in the output of amplifier 9 is usually a very small percentage of the output Voltage.
  • the essential point is that the gain and phase shift characteristics with respect to frequency, from the junction point 3 to the output of amplifiers 9 and 2I respectively, be substantially identical.
  • ampliers 9 and 2l there is one essential difference between ampliers 9 and 2l and this pertains to their manner of operation rather than to their structure.
  • the amplifier 9, being in fact a power amplifier is usually operated at high efciency with the result that the amplifying tubes of said amplifier operate over a large portion of their characteristics, and if their characteristics are not linear, distortion and modulation products are produced.
  • amplifier 2l it is not necessary that amplifier 2l operate at high efficiency.
  • the tubes of this amplifier should operate only over the linear portion of their characteristics, it being essential that no appreciable amplitude distortion occurs within the amplifier.
  • Conductor I5 also connects junction point 3 with the grid 25 of tube 29, this latter tube consistuting together with tube 2l a phase mixer 28.
  • the grid 29 of tube 2l is connected with contact 23 of resistor 22 by the conductor 2li.
  • the cathodes 30 and 3l of mixer tubes 26 and 21 are connected to ground through resistors 32 and 33 respectively, these resistors being shunted by capacitors 34 and 35.
  • the resistors 32 and 33 develop the desired negative biases for the grids of their respective tubes.
  • Resistors 36 and 31, connected to the anodes 38 and 39 of mixer tubes 26 and 21 respectively, are for isolating the output voltages of these tubes so that they will not interact one with the other but will combine to form a resultant voltage having a value which is the vector sum of the voltages developed by tubes 26 and 21.
  • This resultant voltage which appears at junction point 4U is amplified by the two-stage amplifier 4i. Resistance coupling is employed in this amplifier in order that a rninimum of phase rotation of voltages will occur therethrough. Two stages are employed in order that the phase of the voltages developed in the output of amplifier 4
  • resistors 2, I4 and 46 These resistors permit the use of a single grid on which the lcombined voltages of three separate sources may be impressed. It is assumed that the resistances are all equal and have values sufficiently great to permit the generation of voltages at each source uninfluenced by the voltages generated at the other two sources. In order to simplify the explanation of my invention I have assumed that the resistances 2, i4 and 46 are all equal so that the three voltage sources each have equal opportunities to iniiuence the net voltage on grid 4, although an increase in any of the resistors can be compensated for by a proportionate increase in the respective source voltage. Moreover, such phase rotation as may exist in practice due to tube and stray capacitance at point 3 to ground is disregarded since this effect has been found negligible over the frequency range in which self-oscillation due to feedback around the main amplifier is possible.
  • Notation or Expression Denition The voltage gain of the main amplifier without negative feedback. This is a vector quantity, having both amplitude and phase angle.
  • a reference frequency generally Within the useful frequency range of the amplifier, at which qS is 180.
  • a rfaegice frequency at which 15 is zero or a multiple In Fig. 2 I have represented by vectors the system of voltages which add vectorially to produce a net voltage V at the grid 4 of amplifying tube 5.
  • V1 the feedback voltage
  • V3 the voltage V which is the vector sum of V1, V2 and V3. That is,
  • V3 in this equation may be illustrated by reference to a conventional feedback system operating without benefit of V3.
  • the vector diagram of such a system is shown in Fig. 3, Where the amplitude and phase angle of a is the same as for Fig. 2. It is a well-known fact that if qb is 0 (or 360) and the ratio of V2 to V is greater'than unity, the conventional feedback system will become unstable; that is, it may self-oscillate.
  • One purpose of my invention is to avoid this unstable operation or self oscillation in a negative feedback amplifier.
  • a stable condition of operation may be achieved by developing a voltage represented by vector Vs having a magnitude and phase angle for all frequencies such that, when added to voltage vector V2, a resulting or net feedback voltage vector V4 will be produced which will always be substantially from V1 and have an amplitude which is a xed fraction of thev amplitude of V1 for all frequencies.
  • Vs voltage represented by vector Vs having a magnitude and phase angle for all frequencies
  • FIG. 4 This figure represents the phase and amplitude relations between the voltages impressed upon the inputs of the phase mixer 28, the voltages developed therein, and the resultant or output voltage of the phase mixer.
  • the nal (net) voltage represented by vector V and impressed on the grid 4 of tube 5, is also impressed upon the grid I6 of amplifying tube I1 and upon the grid 25 of tube 26, the latter constituting one element of the phase mixer 28.
  • the voltage impressed upon grid i6 appears, after amplification in the correlative amplifier, across resistor 22.
  • This voltage will have a phase relation identical with that appearing across resistor il] in the output of the main amplier 9, it being remembered that the correlative amplifier is so designed that its gain and phase shift-frequency characteristic is substantially the same as that of the main amplifier.
  • the contact 23 selects the desired magnitude of this voltage which is impressed on the grid 29 of tube 21, the latter constituting the second element of the phase mixer 28. This voltage is represented by vector Ve on Fig. 4.
  • V and Vc operating on the phase mixer.
  • the Voltage V is amplified and reversed in phase by the tube 26 and appears in the output circuit thereof between the point 40 and ground.
  • This amplified voltage is represented by vector V5 in Fig. 4.
  • Voltage Vc is amplified and reversed in phase by the tube 21 and also appears in the tube output circuit between the point 40 and ground.
  • This amplified voltage is represented by vector Vc.
  • the resultant of the voltage vectors V5 and Va is Va.
  • phase angle of V3' is dependent on the mag-2 nitude of Vc which in turn is determined by the position of contact 23 on resistor 22.
  • the magnitude of V3 is also dependent on the position of contact 23. This latter dependency is of particular importance for the condition where qs equals 180 as will be seen when the preferred circuit adjustments are described.
  • the voltage represented by V3 is amplified by the amplifier 4I and appears without change of phase across resistor 42.
  • Contact 43 determines the magnitude of the voltage which is impressed back on the grid 4 of tube 5, and has already been defined as vector V3.
  • the desired adjustment of contact 43 on resistor 42 may most readily be made by applying a voltage from source l having a frequency for which the phase angle qs is zero, that is, a critical frequency. It is preferable that the particular critical frequency selected be the next higher frequency above the mid-frequency. When employing this frequency, the voltage V3 should be of such magnitude as to reduce the output of the main amplifier to that value which corresponds to aVi (for the critical frequency) divided by the numerical value of l-a at mid-frequency.
  • the net gain of the feedback amplier should be a (measured at a particular frequency) divided by the numerical value of l-a measured at mid-frequency.
  • a suitable voltmeter may be placed across resistor l and the position of contact 43 varied until the output voltage corresponds to that indicated above.
  • the vector Di represents the distortion voltage at an assumed frequency as it first appears on, or is applied'to, the input of the tube 5 at the junction point 3, it being remembered that distortion voltages are rst generated within the ampiirler 9.
  • the result of this applied distortion voltage will be a feedback voltage D2 and a phase mixer voltage D3 resulting in a net voltage D.
  • B has no inherent phase angle, so that for the distortion frequency assumed the phase angle o1 is the phase angle of u.
  • Vectors D1 and D2 although representing voltages between the junction point 3 and ground, are directly proportional to, and therefore representative of, the distortion voltage first appearing across output resistor Il] without feedback and to this voltage as modified by feedback, respectively.
  • the resultant distortion voltage is therefore proportional to the vector sum of D1 and D2 or D. It will be seen, therefore, that in accordance with my invention the distortion has been changed in the ratio of vector D1 to vector D.
  • the resultant distortion voltage is proportional to vector D, that is, the sum of vectors D1 and D2.
  • D the sum of vectors D1 and D2.
  • the locus of the ends of the vectors representative of the final distortion is a circle shown by the dotted circle 41 having its center at the extremity of vector D1, and having a radius equal to D2.
  • phase shifting network in the amplier circuits such that the phase angle of up will be substantially 180 at the desired frequency.
  • Another advantage of the circuit arrangements of my invention is that they may be applied to any existing amplifier whose operation it is desired to improve. All that is required is that a polar diagram showing the variation of ,u with frequency be made of the existing amplifier, and a correlative amplier be constructed which has a ,a-frequency characteristic represented by substantially the same diagram.
  • constitute an active network which may be connected to the circuits of the main amplifier at a single point, namely the junction point 3.
  • FIG. 7 Another manner in which the applied voltage, the feedback voltage, and the phase mixer voltage may be caused to act independently is shown in Fig. 7.
  • I have dispensed with the isolating resistors I4 and 4B and have submitted a multigrid tube 48 for the triode 5 of Fig. l.
  • Resistor 2 may now have la relatively low value without affecting the operation of my invention.
  • I have illustrated the tube 48 as having three control grids, 49, 5i] and 5I, one for each of the voltages which is to independently exert control on the anode current of the amplifying tube d8. A voltage representing the resultant of the combined effects of these voltages will be found to exist across the resistor 52 connected between the cathode 53 and ground.
  • the desired potential to be applied to tubes l'l and 26 may be selected by contacts 54 and 55 respectively on resistor 52. It will be noticed that no phase change has been assumed between the-voltages applied to the grids of the tube 48 and those voltages obtained from the resistor 52. This will be substantially true for the range of frequencies of greater interest. Resistors 56 and 51 are grid leaks for establishing an appropriate D. C. potential on the grids 5
  • a vectorfrequency characteristic is understood to be the vector quotient of the output voltage divided by the applied input voltage over a frequency range.
  • the vector-frequency characteristic takes account of the phase angle relationship as well as the amplitude relationship between the output Voltage and applied input voltage.
  • a feedback amplifier having an input circuit and an output circuit and a feedback connection therebetween, a correlative amplilier having an input circuit and an output circuit
  • a phase mixer having two input circuits and an output circuit, means connecting the input circuits of said feedback land correlative amplifiers and one of the input circuits of said phase mixer together at a common point, means connecting the output circuit of said correlative amplifier to the other input circuit of said phase mixer, means connecting the output of said phase mixer to said first connecting means, a source of potential applied to said common point, and a load circuit connected to the output circuit of said feedback amplifier.
  • a feedback amplifier having an input and an output circuit and a feedback connection therebetween, a correlative amplifier having an input and an output circuit, a phase mixer having input and output circuits, means connecting the input circuits of the correlative .amplifier and phase mixer to the input circuit of the feedback amplifier, means connecting the output circuit of the correlative amplifier to the input circuit of the phase mixer, means connecting the output circuit of the phase mixer to the input circuit of the feedback amplifier, a source of applied voltage having a range of frequencies connected to the input circuit of th'e feedback amplifier and voltage adjusting means for adjusting the magnitude and phase of the voltage developed by the phase mixer to a Value such that the final voltage effective in the input circuit of the feedback amplifier will have the same phase as the applied voltage from said source over said range of frequencies.
  • the in' put circuit of the feedback amplifier comprises a multigrid tube, the applied voltage being confnected to a grid cf said tube, said feedback connection being connected toa grid of said tube; and the output of the phase mixer being connected to a grid of said tube.
  • a negative feedback amplifier comprising an input circuit and an output circuit and. al feedback connection therebetween for provid-ing a 35 2,244,249
  • said additional circuit comprising a phase mixer circuit having two input circuits and an output circuit and a correlative amplifier having an input and output circuit, said input circuit of said correlative amplifier and one of said input circuits of said phase mixer being connected to the input of said amplifier, the output circuit of said correlative amplifier being connected to the other input circuit of said phase mixer, and said output circuit of said phase mixer being connected to said input circuit of said amplifier.
  • a negative feedback amplier according to claim 5, further comprising adjustable means connecting said correlative amplifier to one of the input circuits of said phase mixer for adjusting the magnitude and phase of the voltage applied thereto such that the output Voltage of said phase mixer is zero for frequencies for which the phase shift of said feedback voltage is HUGO ROMAN'DER.

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US588269A 1945-04-14 1945-04-14 Negative feedback amplifier Expired - Lifetime US2480163A (en)

Priority Applications (4)

Application Number Priority Date Filing Date Title
BE464915D BE464915A (en(2012)) 1945-04-14
US588269A US2480163A (en) 1945-04-14 1945-04-14 Negative feedback amplifier
GB9021/46A GB610105A (en) 1945-04-14 1946-03-23 Negative feedback amplifier
FR925551D FR925551A (fr) 1945-04-14 1946-04-12 Perfectionnements aux systèmes amplificateurs d'ondes

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2647173A (en) * 1947-11-17 1953-07-28 Gen Electric Multiple feedback system
US2724807A (en) * 1950-02-16 1955-11-22 Harold B Rex Frequency-selective systems
US2915628A (en) * 1953-07-03 1959-12-01 Honeywell Regulator Co Electrical control apparatus
US2934713A (en) * 1954-09-17 1960-04-26 Itt Anode-follower amplifier
US3225298A (en) * 1962-07-30 1965-12-21 Hewlett Packard Co Impedance to voltage converter including a positive feedback path for supplying impedance testing current
US3317851A (en) * 1963-07-18 1967-05-02 Julie Res Lab Inc Frequency and amplification stabilized high power amplifier
US3525037A (en) * 1967-11-14 1970-08-18 Ampex Method and apparatus for measuring subsurface electrical impedance utilizing first and second successively transmitted signals at different frequencies

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1994486A (en) * 1933-11-11 1935-03-19 Bell Telephone Labor Inc Vacuum tube circuit
US2227048A (en) * 1938-07-09 1940-12-31 Bell Telephone Labor Inc Negative feedback amplifier
US2244249A (en) * 1938-12-31 1941-06-03 Radio Patents Corp Wave translation system

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1994486A (en) * 1933-11-11 1935-03-19 Bell Telephone Labor Inc Vacuum tube circuit
US2227048A (en) * 1938-07-09 1940-12-31 Bell Telephone Labor Inc Negative feedback amplifier
US2244249A (en) * 1938-12-31 1941-06-03 Radio Patents Corp Wave translation system

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2647173A (en) * 1947-11-17 1953-07-28 Gen Electric Multiple feedback system
US2724807A (en) * 1950-02-16 1955-11-22 Harold B Rex Frequency-selective systems
US2915628A (en) * 1953-07-03 1959-12-01 Honeywell Regulator Co Electrical control apparatus
US2934713A (en) * 1954-09-17 1960-04-26 Itt Anode-follower amplifier
US3225298A (en) * 1962-07-30 1965-12-21 Hewlett Packard Co Impedance to voltage converter including a positive feedback path for supplying impedance testing current
US3317851A (en) * 1963-07-18 1967-05-02 Julie Res Lab Inc Frequency and amplification stabilized high power amplifier
US3525037A (en) * 1967-11-14 1970-08-18 Ampex Method and apparatus for measuring subsurface electrical impedance utilizing first and second successively transmitted signals at different frequencies

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FR925551A (fr) 1947-09-08
BE464915A (en(2012))

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