US2138898A - Push-pull amplifier - Google Patents

Push-pull amplifier Download PDF

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Publication number
US2138898A
US2138898A US141442A US14144237A US2138898A US 2138898 A US2138898 A US 2138898A US 141442 A US141442 A US 141442A US 14144237 A US14144237 A US 14144237A US 2138898 A US2138898 A US 2138898A
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Prior art keywords
amplifier
plate
grid
tube
dissipation
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Expired - Lifetime
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US141442A
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English (en)
Inventor
Bartels Hans
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Telefunken AG
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Telefunken AG
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/26—Push-pull amplifiers; Phase-splitters therefor
    • H03F3/28—Push-pull amplifiers; Phase-splitters therefor with tubes only

Definitions

  • My present invention relates to push-pull amplifiers, and more particularly to push-pull amplifiers operating with constant grid bias potential.
  • the tube Up to an alternating (signal) current whose crest, or peak, value is equal to the quiescent current, the tube operates like an A-class amplifier, while beyond that it works as a type B amplifier as a result of a shift of the grid potential as afunction of the amplitude.
  • These amplifiers have come to be known as A prime or AB amplifiers.
  • the invention is concerned with an amplifier in which, just as inthe A-class amplifier known in the art, the operating point of the tubes is adjusted to the highest permissible plate dissipation, or practically to the said point, for all signal amplitude values and which, contradistinct to the said AB amplifier, operates with a a constant grid potential even in the presence of the highest amplitudes.
  • the terminal resistance is so proportioned that the tubes are operated dissymmetrically, or more pro cisely spealn'ng, in such a way that in each tube the loop or half-cycle of the plate current (when the tube is run under maximum load condition) corresponding to the positive grid voltage halfcycle, becomes twice as high as that of the negative grid voltage half-cycle or alternation.
  • zero plate current will fiow in the (various) tubes even in the presence of median grid (signal) potentials.
  • dissymmetric driving operation is that, contradistinct to the A amplifier, under full-load condition, thefplate. current alternation corresponding to the positive grid potential alternation will turn out to be essentially larger, or, more particularly more than twice as large as with the negative grid potential loop or alternation.
  • This feature has been known in the prior art as far as the applica'ntis aware only for amplifiers for which the working point was located at the bottom bend, or knee, of the mutual characteristic connecting the grid potential and the plate current and in which, therefore, the operating point, unlike the amplifier of the invention, is not adjusted to the highest permissible plate dissipation.
  • the plate D. C. voltage, the grid bias potential, and the terminal impedance of the amplifier should be so chosen that the working characteristic, at low medium and high amplitudes, will be as straight as possible (with the highest possible power). These relations are easily ascertainable graphically by the aid of the combined family of characteristics. Such distortions as may arise may be further minimized by means of degeneration or anti-regeneration. Since as indicated, larger powers or outputs can be obtained from smaller tubes, it is necessary, with present day tubes, to drive the grid far into the positive range of the grid potentials whenever a particularly high power is to be secured. Nonlinear distortions which are occasioned by the grid current may then be lessened by auxiliary circuit means such as, for instance, by grid current feedback schemes known in the art.
  • Fig. 1 shows characteristics used in the invention
  • Fig. 2 graphically illustrates the advantage derived by using the invention.
  • the abscissa gives the plate potentials and the ordinate the plate currents.
  • the working point is designated by P.
  • This working point lies on the hyperbola plotted bythe dashed lines which indicates the plate dissipation A0, that is to say, the admissible tube load.
  • Co-ordinated to this working point is a plate quiescent current I0 and a plate quiescent potential Eb.
  • a tube having 15 watts plate dissipation has been assumed.
  • the resistance graph being rectilinear is denoted by Ra.
  • the plate potential swings 200 V. about the plate quiescent potential of 300 V.
  • the inci dental variations of current amount to approximately 45 milliamps.
  • E 300 V
  • Ao 15 watts
  • the optimal terminal impedance becomes higher; in other words, in Equation (1) the factors will be in the neighborhood of 0.8.
  • the A-type amplifier of the invention when used with present day single-grid tubes, operates with an essentially lower terminal impedance, to be more precise, an impedance amounting to only to the optimum impedance found from the calculation.
  • the operation of this amplifier is indicated by the dashed straight-line resistance graph Rn which corresponds to a resistance of 1000 ohms. While it is true that the negative halves of the cycles are markedly cut by the abscissa axis in this mode of operation, it will be understood that, inasmuch as at that point the second tube of the push-pull arrangement begins to work, the said circumstance is unable to seriously impair conditions.
  • the grid current distortions caused by driving into the positive range may be compensated by ways and means known in the prior art.
  • This amplifier distinguishes itself very favorably from the standard type of A amplifier by the fact that there is a substantial reduction of the terminal impedance combined with a simultaneous increase of the positive range of the grid potentials, contrary to what is true of present day dimension rules according to which resistance Ra with a view to preventing overloading must be chosen larger if the positive grid voltage range is included. (See straight dot-dash graph Ra, Fig. 1.) In this new working scheme, the plate D. C., even when the amplitudes are of but average value, will no longer stay constant, but will experience an essential rise so that, where large I amplitudes are involved, it may attain a value twice as high.
  • Fig. 2 shows the inter-relation between the non-linear distortion (blur factor) and the power output.
  • Graph I refers to the usual A-class amplifier whose terminal impedance (referred to an individual tube) is 4200 ohms u while graph II illustrates the situation for the new A-type amplifier having a terminal impedance of about 1000 ohms (also referred to an individual tube or 500 ohms referred to one-half of or 2000 ohms referred tothe entire primary winding of the output transformer).
  • the grid is driven far into the positive grid potential range, considerable grid currents will arise, and the grid resistance becomes comparatively low. This fact must be taken into consideration when choosing the inner resistance of the driver tube.
  • the inner resistance of the driver tube R1 driver must be substantially lower than the minimum grid resistance Rg min.
  • the customary way of furnishing the grid bias potential by tapping a resistance included in the plate-filament circuit and thus traversed by plate current may here not be employed.
  • a distinct grid voltage source of supply is required, or else additional or auxiliary means must be provided in order that a practically stable grid bias potential may be obtained even with a markedly fluctuating plate current.
  • the plate potential source of supply must be suitably designed to suit the new operating conditions here disclosed. Inasmuch as plate currents of appreciable size are flowing, the inner resistance of the D. C. voltage source must be as low as feasible. Where amplifiers fed with A. C.
  • the said end is insurable by the use of a gaseous amplifier, or else by the insertion of stabilizing means of a kind known in the art, such as choke coils whose impedance varies as a function of the D. 0. load, or equivalent means.
  • Method of operating a push-pull amplifier which consists in adjusting the fixed bias relative to the plate voltage so that the non-signalling plate dissipation is the maximum permissible, and secondly adjusting the load resistance to the lowest value which does not cause the plate dissipation to increase appreciably in the presence of signal input up to the magnitude of input that causes the distortion to reach a predetermined value.
  • the method of increasing the maximum output of a push-pull vacuum tube amplifier which comprises choosing operating voltages such as to produce substantially the maximum allowable dissipation in the absence of signals and a zero signal current per tube sufficiently low so that the instantaneous current per tube may be increased by more than twice the zero signal current per tube, and choosing the load resistance sufficiently low so that at maximum signal input to the amplifier the instantaneous current per tube increases by more than twice the zero signal current per tube, whereby each tube is driven to substantially the maximum allowable average dissipation in the presence of maximum input to the amplifier as well as during the zero signal condition.
  • the method of operating a push-pull amplifier including a pair of tubes having an output load resistance and fixed energizing supply voltages which comprises choosing said voltages to produce maximum allowable dissipation in the absence of signal input, and choosing said load resistance to cause substantially maximum allowable dissipation in the presence of maximum signal input.
  • the method of operating a push-pull vacuum tube amplifier having fixed operating voltages and a resistance load which comprises choosing said voltages to bring the operating point of said amplifier on the linear portion of its characteristic but below the middle portion thereof, and also to cause maximum allowable dissipation in said amplifier in the absence of signal input, whereby on weak signals the amplifier operates class A with a dissipation that decreases with increasing signal input for a range of signal inputs, and also choosing said load resistance so low that on strong signals the dissipation again increases and approaches said maximum allowable value.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Amplifiers (AREA)
US141442A 1936-02-28 1937-05-08 Push-pull amplifier Expired - Lifetime US2138898A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
DE207615X 1936-02-28

Publications (1)

Publication Number Publication Date
US2138898A true US2138898A (en) 1938-12-06

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Family Applications (1)

Application Number Title Priority Date Filing Date
US141442A Expired - Lifetime US2138898A (en) 1936-02-28 1937-05-08 Push-pull amplifier

Country Status (5)

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US (1) US2138898A (en:Method)
BE (1) BE420254A (en:Method)
CH (1) CH207615A (en:Method)
FR (1) FR818319A (en:Method)
GB (1) GB493136A (en:Method)

Also Published As

Publication number Publication date
CH207615A (de) 1939-11-15
BE420254A (en:Method)
GB493136A (en) 1938-10-03
FR818319A (fr) 1937-09-23

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