US2248804A - Circuit arrangement and thermionic valve for amplifying electrical oscillations - Google Patents

Circuit arrangement and thermionic valve for amplifying electrical oscillations Download PDF

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Publication number
US2248804A
US2248804A US272994A US27299439A US2248804A US 2248804 A US2248804 A US 2248804A US 272994 A US272994 A US 272994A US 27299439 A US27299439 A US 27299439A US 2248804 A US2248804 A US 2248804A
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feedback
circuit
grid
anode
amplifier
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US272994A
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English (en)
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Black Donald Harrison
Roche Alleman Holly
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International Standard Electric Corp
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International Standard Electric Corp
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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

Definitions

  • This invention relates to circuit arrangements and thermionic valves for amplifying electrical oscillations wherein negative feedback is employed to reduce distortion in accordance with known underlying principles.
  • a feedback voltage is derived from one of the auxiliary electrodes in a thermionic discharge tube having an anode, a cathode and two or more auxiliary electrodes.
  • the impedance connected between the feedback electrode and the cathode may bear the same relation to the impedance of the anode-cathode circuit as the current to the anode does to the current to the feedback electrode for the purpose hereinafter described.
  • the impedance connected between the feedback electrode and the cathode may be proportioned so that the harmonics developed therein are greater than those developed in the anode circuit.
  • This impedance may also be made dependent on frequency, or may be proportioned to control the output impedance of the amplifier.
  • a feedback of a voltage proportional to the current in the anode circuit which may also be utilised to control the output impedance,
  • a thermionic discharge tube comprising an anode, a cathode, a control grid and at least one auxiliary electrode is so arranged that the ratio of the electron currents flowing to the anode and to the said auxiliary electrode can be made substantially constant for all working values of the control grid voltage.
  • Feedback may be derived from an impedance connected to the said auxiliary electrode in such a thermionic discharge tube.
  • auxiliary electrode may be an additional grid-like electrode provided next to the anode in a valve for example, of the pentode type.
  • Feedback may alternatively or also be derived from the screen-grid circuit of a valve having an anode, a cathode, a control electrode and a screen-grid between the control grid and the anode.
  • Fig. 1 is a circuit diagram of an amplifier utilising the principles of the invention
  • Figs. 2 to 4- illustrate modifications in the output stage of the amplifier of Fig. 1;
  • Fig. 5 is a circuit diagram of an amplifier illustrating further modifications which may be made in the amplifier of Fig. 1; g I
  • Fig. 6 illustrates a thermionic valve constructed accordin to certain features of the invention and a system of connections for utilising the Valve as an amplifier in accordance with the invention
  • Fig. 7 illustrates certain modifications in the arrangement of Fi 6.
  • the feedback is taken from the screen grid circuit of the last tube and not from the amplifier output, as is usually done.
  • the advantages of flat characteristic, gain stability and the facility o-f'impedance correction are obtained with case.
  • Fig. 1 shows a three-stage amplifier embodying the principles described in the present invention.
  • Signals are led in from the transformer T1 and are applied to the grid of valve V1.
  • This valve has a plate load R3 and the screen is fed through the resistance B2, C1 being the usual decoupling condenser.
  • the bias is obtained from the resistance R4 shunted by the condenser C2.
  • From the plate of V1 the signals are led through the coupling condenser C3 to the valve V2.
  • This valve has a plate load Re, the screen being fed through R5 with the decoupling condenser C4.
  • the bias is obtained from the resistance R8 in the cathode lead, C5 being the usual decoupling condenser and R7 the leakresistance.
  • the valves used in the above amplifier may components being the same except for the resistance R13 and R14 which will be referred to later.
  • the method of obtaining feedback in this embodiment consists of the combination of feedback from the screen circuit and from the plate circuit.
  • the resistance R13 the alternating currents in both screen-grid and plate circuits will flow through this resistance and will hence be applied to the grid of the first tube.
  • the voltage generated across R14 which is in the screen-grid circuit as described with reference to Fig. 1 above.
  • the slope of the screengrid characteristic shall be the same as that of the plate characteristic.
  • This may be achieved by suitable design of the valve, or alternatively a compromise may be made by inserting in the screen-grid circuit a suitable load impedance to make the two characteristics more identical.
  • This load may consist of a resistance a portion of which is tapped off in order to provide the feedback voltage, or it may consist of one of the arrangements shown in Fig. 3 or 4.
  • the transformer T2 is now replaced by a threewinding transformer, as shown in the figure.
  • the feedback is then obtained from the screengrid by means of a potentiometer Ru and R15 which may also be used for impedance adjustment, and from the plate circuit by means of resistance R13 as previously described.
  • the load for the screen-grid circuit then consists of the third winding of the transformer T2 and the impedance of this winding may be designed to have the desired effect on the screen-grid characteristic of the tube. With this arrangement the power generated in the screen-grid circuit is conveyed through the line to the transformer T2.
  • the load in the screengrid circuit consists of the transformer T3, which then provides a second output from the amplifier and is useful in cases where monitoring etc., is desired.
  • the feedback is obtained from the resistances R13, R14 and R15 as before.
  • FIG. 5 A further modification is shown in Fig. 5, in which it will be seen that the feedback voltage is applied to the screen-grid of the first tube.
  • This arrangement has the advantage of giving somewhat improved phase shift round the feedback loop. It will be obvious that by means of a combination of Figs. 1 and 5 it is possible to obtain feedback on both control grid and screengrid and this is often the most satisfactory arrangement.
  • tetrodes or pentodes be used in the stages of the amplifier other than that from which the feedback is obtained and in the case of Fig. 5 other than that to which the feedback is applied.
  • the invention may be used for amplifiers of any number of stages and feedback may be used inside the amplifier in individual stages or round the complete circuit or any combination thereof without departing from the spirit of the invention.
  • an additional electrode for the purpose of providing the necessary feedback potential.
  • This electrode which may be of grid formation is preferably so designed and situated in the discharge tube that the electron current flowing to it is a definite fraction of the electron current flowing to the anode.
  • the auxiliary feedback grid is situated next to the anode and is so constructed that the area of the grid facing the electron stream is a certain proportion of the area of the anode facing the electron stream. This proportion may be of the order of 5% or less.
  • I is a thermionic discharge tube consisting of cathode 2 and anode I and four auxiliary electrodes 3, 4, 5 and 6.
  • the anode is connected to the high tension supply 22 via an impedance 9 which is high compared with the load impedance l9 as seen through the transformer 20.
  • the screen 4 is fed through the impedance 8 and is connected to the cathode through the condenser II which is of a magnitude sufficient to maintain the said screengrid at a substantially constant potential.
  • the input signals to the control grid 3 are applied through the transformer 2
  • the feedback electrode 6 is connected to the high tension supply 22 through the impedance 10 which is high compared with the impedances l6 and I! in series.
  • the feedback electrode 6 is also connected through the impedances l2, l and I! to the earth point IS, the impedance of condenser [2 being negligible at all relevant frequencies.
  • the resistance I1 is included in the control grid circuit of the tube as shown in the figure.
  • the sum of the impedances I6 and l! in series bears the same relationship to the output load in the anode circuit as the current to the anode I does to the current to the feedback electrode 6. If the feedback grid 6 has been so constructed that the current flowing to it is always a fixed proportion of the current flowing to the anode 1 when they are maintained at the same D. C. potential, then the voltages developed across the anode load and the impedances I6 and I! in series will be substantially equal. A portion of this voltage (determined by the relative values of i6 and I1) is fed back to the input circuit.
  • tube shown in Fig. 6 contains three grids apart from the feedback electrode, it is not intended to restrict the invention to this type of tube and it will be evident that tubes containing one, two, three or more grids apart from the feedback electrode may be employed without i,-
  • a further advantage of the method of feedback described is that the application of such feedback will tend to reduce the output impedance of the amplifier and this is usually desirable with the types of valves normally used in current practice. Furthermore, by using a combination of feedback from the special feedback grid and the anode circuit a further adjustment of output impedance is possible. Such an arrangement is shown in Fig. 7 which is similar to Fig. 6 except for the resistance 23 across which a feedback voltage dependent upon the anode current is developed in the usual manner. This may be applied to the grid circuit in series with the feedback voltage obtained from the feedback grid. The current feedback from the anode circuit tends to raise the output impedance while that from the feedback electrode does not so increase it, and it will therefore be seen that a very large amount of control is possible.
  • the impedance connected to the auxiliary electrode may be such as to offer a low impedance to one particular frequency but a high impedance to multiples of this frequency. In this way harmonics generated in a radio frequency amplifier may be reduced without reducing the overall gain of the system to any marked extent.
  • One of the features of the method of feedback herein described is that by suitable circuit arrangements one order of harmonics may be decreased at the expense of the reduction in another order. In general, conditions are so arranged that an optimum reduction in the totalharmjonic content is obtained. It is well knownthat even order harmonics, and in particular the second harmonic, may be reduced to a low value by using valves in the so-called push-pull arrangement. By using two valves of the type described above in a push-pull circuit any second order harmonics are thereby reduced to a low level. By the adjustment of the auxiliary electrode circuit it is possible to obtain a marked reduction in the third harmonic and thus when using the push-pull arrangement very low values of second and third harmonics are obtained.
  • Wave amplifier comprising an input circuit, an output circuit and electron discharge apparatus coupled therebetween said apparatus comprising a thermionic valve having a cathode, a control electrode and, beyond said control electrode from the cathode, an output electrode and a feedback electrode, separate load impedances, each effective throughout the operating frequency range of the amplifier, being connected respectively between said output electrode and said feedback electrode, and said cathode and means for feeding back to said input circuit a voltage derived from the load impedance connected to said feedback electrode to substantially decrease the gain throughout the operating frequency range.
  • a feedback amplifier as claimed in claim 1 in which the impedance connected between the feedback electrode and the cathode is so proportioned that the harmonics in the feedback electrode circuit are greater than those developed in the anode circuit.
  • a feedback amplifier as claimed in claim 1 in which the impedance connected between the feedback electrode and the cathode is made frequency dependent in order to provide a feedback voltage which is dependent upon the frequency of the input signals.
  • a feedback amplifier as claimed in claim I further comprising means for feeding back an additional voltage proportional to the current in the anode circuit.
  • a negative feedback amplifier consisting of at least one stage the last stage of which comprises a valve having a screen-grid between the control grid and anode, and means for obtaining the feedback voltage partly from the screen grid circuit and partly from the anode circuit.
  • a negative feedback amplifier accorchng to claim 7 further comprising means for adjusting the output impedance including means for correctly proportioning the combined feedback obtained from both screen-grid and anode circuits.
  • a feedback amplifier according to claim 7 in which the impedance between the screen-grid and cathode is adjusted to make the distortion terms in the screen grid circuit approximately similar to those contained in the anode circuit.
  • a feedback amplifier according to claim 7, wherein the screen grid and anode circuits are connected through the windings of a three winding output transformer.
  • a feedback amplifier as in claim '7 further comprising means for obtaining a small auxiliary output for monitoring or other purposes from the circuit of the electrode from which the feedback voltage is obtained.
  • a feedback amplifier in accordance with claim 1 comprising a push-pull stage with feedback derived at least in part from auxiliary electrodes in the discharge tubes of said stage wherein the feedback from each discharge tube is adjusted to give the optimum reduction in the odd orders of harmonics.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Amplifiers (AREA)
US272994A 1938-05-27 1939-05-11 Circuit arrangement and thermionic valve for amplifying electrical oscillations Expired - Lifetime US2248804A (en)

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BE (1) BE434508A (no)
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GB (1) GB515271A (no)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2508416A (en) * 1946-06-26 1950-05-23 Rca Corp Stabilized high-frequency amplifier
US2843671A (en) * 1954-05-19 1958-07-15 David Bogen & Company Inc Feed back amplifiers
US2848161A (en) * 1952-10-31 1958-08-19 Rca Corp Analogue multiplication device

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
BE508233A (no) * 1951-01-12

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2508416A (en) * 1946-06-26 1950-05-23 Rca Corp Stabilized high-frequency amplifier
US2848161A (en) * 1952-10-31 1958-08-19 Rca Corp Analogue multiplication device
US2843671A (en) * 1954-05-19 1958-07-15 David Bogen & Company Inc Feed back amplifiers

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Publication number Publication date
GB515271A (en) 1939-11-30
BE434508A (no)
CH236494A (de) 1945-02-15

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