US2266168A - Amplifier - Google Patents

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Publication number
US2266168A
US2266168A US241805A US24180538A US2266168A US 2266168 A US2266168 A US 2266168A US 241805 A US241805 A US 241805A US 24180538 A US24180538 A US 24180538A US 2266168 A US2266168 A US 2266168A
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United States
Prior art keywords
amplifier
feedback
signal
circuit
output
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
US241805A
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English (en)
Inventor
Theodore H Crabtree
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
Priority to NL72077D priority Critical patent/NL72077C/xx
Application filed by Bell Telephone Laboratories Inc filed Critical Bell Telephone Laboratories Inc
Priority to US241805A priority patent/US2266168A/en
Priority to GB28341/39A priority patent/GB534294A/en
Priority to FR861743D priority patent/FR861743A/fr
Application granted granted Critical
Publication of US2266168A publication Critical patent/US2266168A/en
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
    • H03F1/52—Circuit arrangements for protecting such amplifiers
    • H03F1/54—Circuit arrangements for protecting such amplifiers with tubes only

Definitions

  • FIG. 1 A first figure.
  • This invention relates to amplifiers, particularly to amplifiers for sound recording systems and the object of the-invention is an amplifier capable of use with-recorders of different types.
  • one or more stages' may be used exclusively for amplifying'the energy fed back to the input of the remaining stages on which the signals to be recorded are impressed.
  • Switching means or strapping connections are provided in various portions of the circuit so that when the recorder to be driven is not of the feedback type, the feedback portion of the amplifier may be cut out or connected in such a manner as to increase the gain-of theamplifier.
  • positivefeedback at frequencies above the band of interest may be decreased or eliminated by shunting the feedback path with acondenser of such small capacity as to have only slight-elfect on transmission within the band of interest but a relatively low impedance for the 55 higher frequencies which tend to produce instability.
  • the output stage of the amplifier preferably comprises one or more self-biased pentodes of the beam tube type. 'Applicant has found that with i circuits commonly used for these tubes the average screen current increases considerably with increasing applied signalvoltage. The resulting increase in cathode current increases the grid bias and reduces the plate current. This reduction of plate current not only reduces the-output capacity of the amplifier but also results in the tube operating on a portion of its characteristic where'the harmonic distortion is greater than it is foroperation at maximum plate current.
  • An important feature of this invention consists in so regulating the potential applied to the screen'grids of the output tubes that the screen currentvaries with the applied signal in such a manner that the cathode current, which is the sum of the screenand plate currents, is substantiallythe same for all inputs up to full signal 'condition. 7
  • Fig.1 is a circuit diagram of an amplifier according to this invention.
  • Fig. 2 shows-aportion of the circuit of Fig. 1 rearranged to illustrate more'clearly thefeature ofselectively amplifying the lower frequencies
  • Fig. 3 shows-the response curves of a typical feedback recorder.
  • the amplifier shown comprises a main amplifier ll operating from recording bus-bars I2, I 3 which areenergized at a suitable level from any desired input circuit such,'for example,- as a pick-up microphone l4-and preliminary.
  • the output of the amplifier II is impressed on the driving coil l6 of a disc'recorder ll which may be of the type shown in Patent 2,161,489, granted to L. Vieth et a1. June 6, 1939.
  • the coil I6 of the recorder drives the-stylus 18 to cut arecordand simultaneously generates in the coil l9 a voltage proportional to the velocity of the stylus.
  • This voltage is fed back over the conductors 20 to the input of the negative feedback amplifier 2lthe output of which is impressed on the input of amplifier ll along with the incoming signals from the recording bus-bars l2, l3 and reduces the resonant response of the recorder to a fiat characteristic in the manner which is in general now well understood-in the art.
  • the signals from the recorder bus-bars are impressed on the primary of the transformer 22 the divided secondaries of which are shunted by resistors 23, 24 and 25, 26 respectively.
  • the portions of the secondary potential existing across resistors 24 and 26 are impressed on the grids 21 and 28 of the push-pull tubes 29 and 30 over the circuits including switch 3
  • the tubes 29 and 35 are self-biased by the in- 7 -again operating with reduced capacity and individual resistors 39, 40 and 4
  • the outputs of tubes 29 and 33 are resistance-capacity coupled in the well-known creased harmonic distortion as in the case where th'efscreen potential regulation is poor.
  • the screen grid is at the potential of thepoint 58 which is determined by the potential drop in resistor 59 due to the sum of the screen currents and the current in resistor manner to the beam tubes 45 and 46 which are p self-biased by the common cathode resistor '82, and the output of these latter tubes drives the recorder IT as already explained.
  • Internal negative feedback connections, 41 and 48 extending from the plates of tubes 45 and 45 to the oathodes of tubes 29 and 30 to reduce the noise and distortion of the amplifier and to reduce the effective output impedance of the beam tubes to a value which more nearly matches the primary impedance of the output transformer 49 in the manner well understood in the art.
  • the amplifier is energized from the alternating current supply 5! by means of the; usual rectifier 52 and filter network 53, the tube filament circuits (not shown) being connected to suitable taps on the winding 54 of the transformer 55. It will be noted however that the screen grids 55 and 51 of the beam tubes and 46 are connected to the point 58 between resistors 59 and 59 which comprise a bleeder circuit across the output of the filter 53. The manner in which these resistors are proportioned to give improved operation of the amplifier will now be described. 7
  • the plate current in these tubes depends on the screen voltage to a greater degree than on the plate voltage and the screen grid is therefore usually connected to the voltage source through a resistor which limits 60.
  • the potential of point 58 will be decreased due to the larger potential drop in resistor 59 and by proper choice of the values of resistors 59 and 60 the screen potential can be made to vary in sucha manner that the increase in screen current from no signal to full signal compensates for the decrease in plate current thereby keeping the cathode current and grid bias for each tube constant.
  • resistor 59 may be 5800 ohms, resistor may be 7000 ohms in which case the bleeder current i about 40 milliamperes and the total screen current of about 4 milliamperes.
  • the procedure for determining the resistor values requiredin a given case is first to determine the grid bias necessary for the desired plate current at full signal with -a given screen potential, then measure the cathode current and determine the screen potential which gives the same cathode current at no signal. 7 Having determined the difierences between the screen potentials and screen currents at full signal and no signal, respectively, the value of resistor 59 necessary to produce this variationis determined by dividing the difference in the screen :Volttgfis by the difierence in the screen currents for the twoconditions. The value of resistor 60 is then readily determined since it must be the same fraction of the total bleeder resistance that the no signal screen voltage is of the output voltage of the filter. 7
  • An amplifier regulated in' this manner not only has higher maximum output'c-apacity and lower harmonic distortion than conventional circuits butit also has a further advantage particularly in circuits using negative feedback.
  • a negative feedback amplifier When a negative feedback amplifier is energized from a rectifierwhich has poor regulation," sudden changes in load on the rectifier set up transients in the amplifier which cause blocking and moton-boating.
  • the current supplied to the output stage of an amplifier comprises a verylarge proportion of the total load on the rectifier but in this circuit the current to the output stage is substantially constant.
  • the output voltage' of the rectifier will therefore vary onlyslightly for large changes in signal level and the amplifier is consequently much more stable in its operation.
  • Fig. 3 of the drawings represents a typical undamped response characteristic of a recorder of the type indicated at IT in Fig. 1. It will be noted that such a recorder should be of the singly resonant type preferably with the resonant peak occurringslightly abovegeometric mean frequency :of the range .of interest. If, for example, the recorder is to have a fiat response for frequencies from 9000 cycles down to 30 cycles per second, this result may be obtained by using sufiicient negative feedback to reduce the recorder response to the line 6.
  • the resonant frequency of the recorder is usually fixed by other considerations within the range of about 900 to 1000 cycles so that the amount of feedback used can usually be considerably less than the value corresponding to curve 6
  • the response may be reduced only to the line 63 which is fiat over the upper portion of the desired range but definitely drooping above the limiting frequency and therefore much more stable. It will be seen, however, that reducing the amount of feedback to insure stability at high frequencies also results in a drooping characteristic-over the lower frequency range,
  • are of such capacity that their impedances for all frequencies in the transmission band are low as compared with the impedances of resistors 24, 32 and 26, 36 shunting the inputs to the tubes 29 and 30 of the main amplifier ll. Under this condition as is well understood in the art, the transmission characteristic of the coupling between the feedback amplifier 2
  • the condensers 64 and 65 are also parts of shunt paths across the inputs of tubes 29 and 30 of amplifier ll.
  • Condenser 64, resistor 66 and condenser 61 of the rectifier filter network form a path from the grid 21 of tube 29 to the grounded cathode and similarly condenser 65, resistor 68 and condenser 61 from a path across the input of tube 30.
  • Condensers 64 and 65 will ordinarily be of the order of .02 microfarad and condenser 61 of the order of 8 microfarads so that the impedance of condenser 61 is negligible as compared with the other elements of the shunt paths.
  • a signal impressed on the primary of transformer 22 will produce a potential across resistors 24 and 26 and these potentials are impressed on the shunt paths referred to above through resistors 32 and 36, respectively. Since the impedance of the condensers 64 and 65 increases with decreasing frequency the proportion of th signal potential applied to the grids of the tubes 29 and 30 will be progressively greater as the frequency decreases and the signal path will have a rising low frequency characteristic as indicated by the curve 69 in Fig. 3.
  • this increase in the efiiciency of transmission may be made of the proper magniture to compensate substantially for the drooping characteristic of the undamped recorder at thes frequencies so that as shown the flat response withnfeedbackisf extended down to the lowest frequency-of "interest. It will be clear from the foregoing explanation that this required correction at low frequenciesxmay, be obtained without impairing the feedback Ecircuit transmission characteristic by usingcoupling condensers 64 and 65 of such capacity that their impedances over thefrequencyrange in which compensation is required is greater than the impedance of resistor 66 shunting the feedback amplifier to give. increased. signal transmission at low frequencies but less thanthe impedance of resistors '32 and 24 to keep the feedback transmission substantially constant.
  • the resistors ll, 12 in the input of the feedback amplifier may be shunted by condensers l3, 14 of the order of .001 mfd. to .006 mfd. capacity.
  • condensers are of such high impedance as to have a very small effect on transmission for frequencies within the band of interest but 10w enough in impedance, for frequencies above the transmission band where the feedback is posi tive, to reduce transmission around the feed back loop to a level which does not makethe amplifier unstable. 7
  • the feedback amplifier may be readily disconnected by operating switches 33, 3
  • the switch 10 in the position shown connects the cathode resistors 39 and 42 to ground through condenser 44 thereby'by-passing theisignal currents around resistors 40, 4
  • By closing the switch 10.0n contact8l only resistor '43 is bypassed .and .the alternating potentials developed across resistors 40 and 4
  • may be used in the signal circuit.
  • switches 33 and 31 are left in the positions shown, switches 3
  • Signals applied to the transformer 22 are then fed over conductors 19, to the input of amplifier I I through switches 33 and 31.
  • this amplifier is well adapted for use as a general purpose recording amplifier and that the circuit may be readily changed to meet the requirements of recorders of different type While the invenrequired than can be obtion has beendescribed for purpose of illustration with reference to a particular circuit it will be understood that it may be modified in various ways within the scope of the following claims.
  • V 1 In an amplifying system, a source .of signals, a resonant receiving device, a signal amplifier connecting the source to the device, means for generating a voltage proportional to the response of the device, a feedback circuit including an amplifier for impressing the voltage on the input of the amplifier to make the response of the device uniform over a predetermined frequency range and a.
  • condenser coupling the feedback amplifier to the signal amplifier and formingwith the output of the feedback amplifier a shunt across the input of the signal amplifier, the capacity of the, condenser being proportioned with respect to the impedance of the input of the signal amplifier and the output of the feedback amplifier so as to give a fiat frequency transmission characteristic in the feedback circuit and a non-linear frequency transmission characteristic through the signal amplifier to compensate for the characteristic of the receiving device below the frequency range in which the response is made uniform by the feedback circuit.
  • An amplifying system comprising a source of signal currents, a receiving device, a signal amplifier having an input circuit connected to said source and an output. circuit connected to the receiving device, a second amplifier having an input circuit and an output circuit connected to the input of the signal amplifier, means. for selectively connecting the input of the second amplifier either to the receiving device to feed back to the signal amplifier a voltage proportional to the response of. the device to reduce the distortion in the output of the device or between the source of signal currents and the signal amplifierto increase the gain of the system, and means for, producing negative feedback in the signal amplifier to utilize the increased gain of the system to reduce the distortion in the signal amplifier.
  • a source of current a resonant receiving device, an amplifier having an input circuit connected to the source and an output circuit connected to the device, means associated with the device for generating electric currents of an amplitude varying with the response of the device, a negative feedback circuit connecting said means to the input of the amplifier for reducing the resonant response of the device-to a fiat characteristic over a predetermined frequency range, said circuit including reactive elementslof such value that the feedback becomesgpositive for frequencies above said range and tends-to.
  • a-source of signal currents a resonant receiving device, an amplifier having an input circuit, connected to the source and an, output circuit connected to the device,'means associated with the device for generating electric currents of anamplitude varying with the response-of the device, a circuit conducting thegenerated currents to the input of the amplifier in phase opposition to the signal currents and producing the maximum negative feedback action consistent with stable operation whereby the resonant response of the device is reduced to a substantially fiat characteristic over a frequency range extending from a frequency within the band to be transmitted up to the upperlimit of the ,band, means in said circuit for reducing transmission for frequencies above said range and, coupling elements between the circuit and the amplifier having a fiat transmission characteristic for the generated currents for selectively increasing the transmission through the amplifier for currents fromthe source of frequencies below said range and producing a fiat characteristic down to the lower limit of the band.
  • a source of signal currents bus-bars connected to said source for distributing the currents, a signal amplifier having input and output circuits, a connecting circuit between the bus-bars and the input circuit and a feedback circuitconnected to the input circuit and carrying currents varying in amplitude with the output of the amplifier, said connecting circuit having a shunt impedance and a high series impedanceconnecting a small portion of the shunt impedance across the input of the amplifier, whereby said connecting circuit presents a high impedance between the feedback circuit and the bus-bars.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Amplifiers (AREA)
  • Microwave Amplifiers (AREA)
US241805A 1938-11-22 1938-11-22 Amplifier Expired - Lifetime US2266168A (en)

Priority Applications (4)

Application Number Priority Date Filing Date Title
NL72077D NL72077C (enrdf_load_stackoverflow) 1938-11-22
US241805A US2266168A (en) 1938-11-22 1938-11-22 Amplifier
GB28341/39A GB534294A (en) 1938-11-22 1939-10-20 Improvements in thermionic tube amplifiers
FR861743D FR861743A (fr) 1938-11-22 1939-11-18 Amplificateurs à tubes thermioniques

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US241805A US2266168A (en) 1938-11-22 1938-11-22 Amplifier

Publications (1)

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US2266168A true US2266168A (en) 1941-12-16

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US241805A Expired - Lifetime US2266168A (en) 1938-11-22 1938-11-22 Amplifier

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US (1) US2266168A (enrdf_load_stackoverflow)
FR (1) FR861743A (enrdf_load_stackoverflow)
GB (1) GB534294A (enrdf_load_stackoverflow)
NL (1) NL72077C (enrdf_load_stackoverflow)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2425486A (en) * 1945-04-19 1947-08-12 Dictaphone Corp Electric phonograph
US2481533A (en) * 1944-06-06 1949-09-13 Rca Corp Audio amplifier circuits for radio transmitters
US2522567A (en) * 1946-06-21 1950-09-19 Emory G Cook Amplifier feed-back system for magnetic motors
US2652458A (en) * 1949-01-13 1953-09-15 Bendix Aviat Corp Amplifier with positive and negative feedback
US2892157A (en) * 1955-10-21 1959-06-23 Itt Modulators
US3148537A (en) * 1956-11-26 1964-09-15 North American Aviation Inc Dyna-electronic compensator

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2481533A (en) * 1944-06-06 1949-09-13 Rca Corp Audio amplifier circuits for radio transmitters
US2425486A (en) * 1945-04-19 1947-08-12 Dictaphone Corp Electric phonograph
US2522567A (en) * 1946-06-21 1950-09-19 Emory G Cook Amplifier feed-back system for magnetic motors
US2652458A (en) * 1949-01-13 1953-09-15 Bendix Aviat Corp Amplifier with positive and negative feedback
US2892157A (en) * 1955-10-21 1959-06-23 Itt Modulators
US3148537A (en) * 1956-11-26 1964-09-15 North American Aviation Inc Dyna-electronic compensator

Also Published As

Publication number Publication date
NL72077C (enrdf_load_stackoverflow)
FR861743A (fr) 1941-02-15
GB534294A (en) 1941-03-04

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