US2965852A - Cathode follower - Google Patents

Cathode follower Download PDF

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Publication number
US2965852A
US2965852A US464335A US46433554A US2965852A US 2965852 A US2965852 A US 2965852A US 464335 A US464335 A US 464335A US 46433554 A US46433554 A US 46433554A US 2965852 A US2965852 A US 2965852A
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United States
Prior art keywords
voltage
tube
cathode follower
output
cathode
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Expired - Lifetime
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US464335A
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English (en)
Inventor
Macdonald James Ross
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Texas Instruments Inc
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Texas Instruments Inc
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Publication date
Priority to BE542279D priority Critical patent/BE542279A/xx
Application filed by Texas Instruments Inc filed Critical Texas Instruments Inc
Priority to US464335A priority patent/US2965852A/en
Priority to GB29088/55A priority patent/GB794841A/en
Priority to DET11493A priority patent/DE1015854B/de
Priority to FR1143663D priority patent/FR1143663A/fr
Application granted granted Critical
Publication of US2965852A publication Critical patent/US2965852A/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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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
    • H03F3/50—Amplifiers in which input is applied to, or output is derived from, an impedance common to input and output circuits of the amplifying element, e.g. cathode follower
    • H03F3/52—Amplifiers in which input is applied to, or output is derived from, an impedance common to input and output circuits of the amplifying element, e.g. cathode follower with tubes only

Definitions

  • This invention relates to an improved type of cathode follower for connecting a high impedance source to a low impedance load. More specifically, this invention relates to a cathode follower circuit and a method wherein the voltage dllference between the input voltage and the output voltage is amplified and fed back in such a manner as to reduce the output impedance of the cathode follower and, in addition, reduce distortion in the output voltage.
  • One of these devices is a transformer in which the primary side is impedance-matched to the source and the secondary side is impedance-matched to the load by winding each side with the number of turns of wire required to provide the necessary impedance transformation ratio.
  • the number of turns on the secondary then is smaller than the number of turns on the primary and is in a proportion to the primary turns as the square root of the ratio of the impedance of the source to the impedance of the load.
  • Such a device is known as a step-down transformer and, according to transformer theory, the voltage in the secondary is to the voltage in the primary as the number of turns in the secondary is to the number of turns in the primary. Consequently, in a step-down transformer, the voltage in the secondary is reduced in direct proportion to the ratio of the number of secondary turns to the number of primary turns and thus, such a device is undesirable for use in connecting a high impedance source to a low impedance load.
  • transformers In addition to voltage reduction from the primary to the secondary, transformers have a limited frequency response and, in applications where it is desired to feed back part of the output voltage to reduce distortion, vinterstage transformers tend to limit the amount of negative feedback which may beremployed.
  • cathode follower Another meanswell-known in the art for connecting the voltage developed across a high impedance source to a low impedance load is the cathode follower, discussions of which can be found in Applied Electronics by Gray, Second edition, pp. 428-435; Radio Engineering by Terman, pp. 308-311; and Electronic Engineering Principles by Ryder, pp. 179-183.
  • the plate load impedance is eliminated and voltage is developed across the cathode resistor and thus follows the voltage applied to the grid.
  • the cathode follower is useful for supplying a low impedance load and hence finds extensive application as an impedance transformer between a source having a high impedance and a load having a low impedance.
  • the cathode follower avoids the voltage loss inherent in the step-down transformer and has the added advantages of excellent frequency response, high input impedance and low distortion. Further, even though it lacks voltage amplification, the cathode follower can provide power amplification because the ratio of its input impedance to its output impedance is very large.
  • the cathode follower As an impedance transformer, there arecertain limitations to its usewhich must be considered.-
  • One limitation on the cathode follower is that, for a given; AC. input voltage, the cathode follower output begins to be clipped on negative peaks when the output load impedance is sufiiclently low that the peak A.C. current through the output load equals the quiescent current in; the cathode follower load resistance. This follows from a consideration of the fact that if the load impedance is sufiiciently low, the current through the tube flows mainly through the load impedance rather than the cathode load resistance.
  • the cur rent through the load resistance is limited to the quiescent" current through the cathode load resistance until the tube reaches its cutoff point and the negative peak volt-; ages are then clipped at that level.
  • Another limitation is that, on positive A.C. voltage peaks, the cathode follower can deliver positive current peaks far larger than its quiescent cathode current, but the output voltage begins to be distorted when the current required by the load produces an internal voltage drop across the effective iniernal resistance l/g which is appreciable compared to the input voltage.
  • the present invention has been conceived to overcome some of the limitations on the use of the cathode follower as an impedance trans former and thus constitutes an improved type of cathode follower.
  • This invention consists essentially of a tube with a resistor connected in the cathode path to ground or to a fixed negative potential as in the ordinary cathode follower and an additional tube connected either in parallel or in series with the first tube.
  • the input voltage is applied to the grid of the first tube thereby producing an output voltage which is very nearly equal to but smaller than that of the input voltage.
  • the difference between the input and output voltages is then fed to a differential amplifier and this amplified error voltage applied to the grid of either the parallel or series connected tube and with such a phase sense that the error at the output of the first tube is reduced.
  • the feedback is thus negative feedback and augments the cathode follower, or in other words, reduces its output impedance and the distortion in its output voltage.
  • a negative feed-back path is provded for the au mented cathode follower driver which is essentially outside of the main amplification path of any circuit in which it might be used. With such a separate active error feed-back path, greater overall feedback than might otherwise be possible may be used to greatly reduce distortion throughout the circuit.
  • Figure 1 is a schematic circuit diagram for an augmented cathode follower wherein a tube is connected in parallel to the cathode follower;
  • Figure 2 is a curve of the theoretical and experimental ratio of the output voltage with total load R to the output voltage with only cathode resistance lead R plotted against the load resistance for the parallel augmented cathode fol ower driver circuit of Fi ure 1;
  • Figure 3 is a curve of the theoretical and experimental ratio of the error voltage normalized with respect to the input voltage plotted against load resistance for the parallel augmented cathode follower circuit of Figure 1;
  • Figure 4 is a schematic circuit diagram for an augmented cathode follower wherein a tube is connected in series with the cathode follower;
  • Figure 5 is a plot of the intermodulation distortion against R.M.S. output voltage for an ordinar cathode follower driver, a series augmented cathode follower driver, a parallel augmented cathode follower driver all driving an output tube grid and a parallel augmented cathode follower driver with no added load.
  • a parallel augmented cathode follower driver circuit is shown schematically in Figure 1.
  • the input voltage e is fed by lead 10 to the grid of tube V1 which in this diagram is shown as one-half of a double triode tube.
  • Tube V2 the other half of the double triode tube, is connected in parallel with tube V1 by means of lead 13 connecting the plates of the two tubes.
  • the D.C. voltage to the plates of tubes V1 and V2 is supplied by a voltage source to the common plate connection, lead 13.
  • Cathode resistor 11 is connected in series with tube V1 by a lead 14 and in series with tube V2 by a lead 15.
  • the output voltage e is developed across resistor 11 and applied to a low impedance load by lead 12.
  • the input voltage 2 is fed through resistor 17 to the grid of tube V3 by lead 16 while the output voltage e is applied to the grid of tube V4 by lead 18.
  • the cathodes of tubes V3 and V4 are connected together by a common lead 20 and in series with resistor 19.
  • the plate of tube V3 is connected directly to a D.C. voltage source while the plate of tube V4 is connected to a second D.C. voltage source through pate load resistor 21.
  • the circuit consisting of tube V3, tube V4, and resistors 19 and 21 provides for amplification of the difference between the input voltage e and the output voltage a in a manner described in Vacuum-tube Amplifiers, Radiation Laboratory Series, volume 18, section 11-10.
  • Tube V3 is connected as a cathode follower and thus, the input voltage e to its grid is transmitted with very little drop to the cathode of tube V4.
  • Tube V4 acts as an amplifier with its grid biassed negatively with respect to its cathode.
  • the difference between the input voltage 2 and the output voltage e is amplified by the voltage drop gain across resistor 21 to give the amplified error signal (g e g e or, its approximate equivalent, g(e -e)
  • the amplified error signal (g e g e or, its approximate equivalent, g(e -e)
  • This error voltage provides a high stability and reduced distortion when fed back to the cathode follower V2.
  • this active error feed-back path of th au mented c"thode fol ower is Outside the main amplification path of any circuit into which it might be connected and thus allows greater overall negative feedback to be used in the entire circuit with no amplification of the distortion from stages ahead of the augmented cathode follower dri er.
  • the error voltage from tube V4 follows a path throu h lead 22, condenser 26 and lead 25 to the grid of tube V2.
  • Condenser 26 provides an A.C. byp ss around resistor 23 and, since resistor 24 is of co siderab e ma nitude compared with the reactance of condenser 26 at fre uencies of interest.
  • the error volta e feeds to the grid of tube V2 w th verv little or no voltage drop through resistor 24.
  • the grid of tube V2 is provided with a negative D.C. bias equal to that of the negative D.C. bias on the grid of tube V1 bv means of a voltage divider consisting of resistors 23 and 24.
  • the voltage through resistor 21 is applied to t e voltage divider and the voltage drop across resistor 23 reduces the voltage to the proper D.C. level for the grid of tube V2.
  • g voltage gain from grid to plate of V4.
  • g voltage gain from grid of V3 to plate of V4.
  • r rlalate resistance of tubes V1 and V2
  • identi- R the cathode resistance of tubes V1 and V2 (resistor 11).
  • the cathode follower of this invention is essentially augmented by the multiplication factor g.
  • the internal resistance r, for the augmented cathode follower is approximately equal to:
  • FIG. 4 Another means for augmenting a cathode follower is shown by the series connection of Figure 4.
  • the input voltage e is fed to the grid of tube V5 throu h lead 40.
  • the cathode of tube V5 is connected to a fixed negative potential through resistor 41.
  • tube V5 is connected in series with tube V9 by the connection 50 between the plate of tube V5 and the cathode of tube V9.
  • the output voltage e developed across resistor 41 is shown as being fed to an output tube V10 by lead 42.
  • the difference between the input voltage e and the output voltage E; is amplified in a differential amplifier. This is accomplished by connecting the output voltage e to the grid of tube V7 through lead 43.
  • the input voltage e is connected into the grid of a cathode follower tube V8 and the voltage drop across resistor 45 in the cathode circuit of tube V8 is fed to the grid of tube V6 by lead 46.
  • Tubes V6 and V7 are identical halves of a double triode tube, have a common cathode connection 47 and are connected in series with lead 48 to a fixed negative potential. The difference between the halves of the double triode lies in the fact that tube V6 is a cathode follower while tube V7 is an amplifier.
  • the difference between the voltages e and 2;; is amplified by the voltage drop across the plate load resistance 49.
  • the gain through tube V7 is equal to (g e --g e which is essentially equal to g(e e )
  • This amplified error voltage is then fed by lead 51 to the grid of tube V9.
  • the series augmented cathode follower is likewise provided with error feedback to increase its stability and reduce distortion and its active error feedback path is essentially out of the main amplification path of any circuit into which it might be connected.
  • the error voltage is applied to the grid of tube V9 by lead 51 and acts to augment the cathode follower tube V5 in the following manner.
  • a low resistance load such as the grid of the output tube V10 when driven positive
  • the voltage drop across the tube V5 increases.
  • any internal resistance drop will be compensated by an increase in the plate voltage of tube V5 due to the error voltage (2 supplied to the grid of tube V9 by the amplified error voltage.
  • This compensation effect continues as the load increases until the grid of tube V9 arrives at the zero bias condition for the tube. Compensation is limited at this point since there can be no further voltage drop across tube V9.
  • the output impedance of the series augmented cathode follower is considerably higher, being in the order of 50-70 ohms as against 5 ohms for the parallel circuit.
  • the series augmented cathode follower considerably augments the range of loads over which an undistorted output voltage can be maintained when compared with an ordinary cathode follower, the reduction being in the order of 200 to 400 ohms to 60 ohms.
  • the intermodulation distortion increases at a lesser rate from that point on until the diode line of the tube is reached and there the distortion increases sharply.
  • the diode line is reached when the grid of the output tube is sufficiently positive that it loses control of plate current and the tube then begins to act as a diode.
  • the curve for the series augmented cathode follower driver (SACFD) shows considerable distortion over the range between approximately 30 volts R.M.S. and the diode line but still the distortion is considerably lower than in an ordinary cathode follower driver.
  • the curve for the parallel augmented cathode follower driver (PACFD) shows an intermodulation distortion of less than 0.2% in the range of 30 to 50 volts R.M.S.
  • the present invention has been described by means of two schematic circuit diagrams showing a parallel augmented cathode follower driver and a series augmented cathode follower driver together with specific voltages, resistance values, tube types and bias conditions to illustrate the invention as actually reduced to practice.
  • these circuits may be modified at will without departing from the purpose of this invention which is to disclose an improved type of cathode follower with any extremely low output impedance and with a minimum of distortion in the output voltage. Therefore, any changes or modifications to the circuit shown and described in this invention which can be made without departing from the objects stated herein are claimed as within the scope of this invention.
  • an improved circuit for reducing output impedance and output voltage distortion which comprises a vacuum tube having a grid, plate and cathode, means connecting said vacuum tube as a single stage cathode follower, said last named means including a load impedance connected to the cathode of said vacuum tube, a cathode follower means having an input and an output, means amplifying the difierence etween the input voltage at the grid of said vacuum tube and the output voltage generated across said load impedance to provide active error voltage, means feeding said active error voltage to the input of said cathode follower means, and means electrically connecting the output of said cathode follower means to augment the current flow ing through said load impedance.
  • an improved circuit for reducing output impedance and output voltage distortion as defined in claim 1, said cathode follower means comprising a second vacuum tube having a grid, plate and cathode connected in parallel with said first mentioned vacuum tube plate-to-plate and cathode-to-cathodc with said active error voltage fed to the grid of said second vacuum tube.
  • said cathode follower means comprising a second vacuum tube having a grid, plate and cathode connect-2d in series in the plate circuit of said first mentioned vacuum tube with the cathode of said second vacuum tube connected to the plate of said first mentioned vacuum tube and with said active error voltage fed to the grid of said second vacuum tube.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Amplifiers (AREA)
  • Supply Devices, Intensifiers, Converters, And Telemotors (AREA)
US464335A 1954-10-25 1954-10-25 Cathode follower Expired - Lifetime US2965852A (en)

Priority Applications (5)

Application Number Priority Date Filing Date Title
BE542279D BE542279A (de) 1954-10-25
US464335A US2965852A (en) 1954-10-25 1954-10-25 Cathode follower
GB29088/55A GB794841A (en) 1954-10-25 1955-10-12 Improved cathode follower
DET11493A DE1015854B (de) 1954-10-25 1955-10-22 Kathodenverstaerkerschaltung mit verringertem Ausgangswiderstand
FR1143663D FR1143663A (fr) 1954-10-25 1955-10-25 Montage à charge cathodique perfectionné

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US464335A US2965852A (en) 1954-10-25 1954-10-25 Cathode follower

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US2965852A true US2965852A (en) 1960-12-20

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US464335A Expired - Lifetime US2965852A (en) 1954-10-25 1954-10-25 Cathode follower

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US (1) US2965852A (de)
BE (1) BE542279A (de)
DE (1) DE1015854B (de)
FR (1) FR1143663A (de)
GB (1) GB794841A (de)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2994832A (en) * 1958-04-08 1961-08-01 Bell Telephone Labor Inc Transistor amplifier
USD882397S1 (en) 2013-09-09 2020-04-28 Kraft Foods Group Brands Llc Lid

Citations (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB620140A (en) * 1946-03-20 1949-03-21 British Thomson Houston Co Ltd Improvements relating to d.c. amplifiers
US2592193A (en) * 1949-03-03 1952-04-08 Us Sec War Means for reducing amplitude distortion in cathode-follower amplifiers
US2628266A (en) * 1949-01-22 1953-02-10 Rca Corp Analysis of signal transfer devices
US2647174A (en) * 1950-09-23 1953-07-28 Du Mont Allen B Lab Inc Adjustable beam-trace-positioning amplifier
US2685000A (en) * 1949-04-29 1954-07-27 Rca Corp Stabilized direct current amplifier
US2709205A (en) * 1949-07-06 1955-05-24 Southern Instr Ltd Direct coupled thermionic valve amplifiers
US2714136A (en) * 1951-02-27 1955-07-26 Gen Precision Lab Inc Stabilized direct-coupled amplifier
US2730573A (en) * 1948-12-01 1956-01-10 Sperry Gyroscope Co Ltd Feed-back amplifier systems and servo mechanisms that are adapted to respond to input changes at very low frequencies
US2737547A (en) * 1952-10-01 1956-03-06 Hughes Aircraft Co Cathode follower circuits
US2763733A (en) * 1952-03-21 1956-09-18 Wallace H Coulter Amplifier having series-connected output tubes
US2795654A (en) * 1954-03-02 1957-06-11 James R Macdonald High impedance electronic circuit
US2801296A (en) * 1954-02-09 1957-07-30 Bell Telephone Labor Inc D.-c. summing amplifier drift correction
US2810025A (en) * 1954-07-15 1957-10-15 Hughes Aircraft Co Direct-coupled feedback amplifier
US2896027A (en) * 1953-10-19 1959-07-21 Melpar Inc Reflex amplifiers

Patent Citations (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB620140A (en) * 1946-03-20 1949-03-21 British Thomson Houston Co Ltd Improvements relating to d.c. amplifiers
US2730573A (en) * 1948-12-01 1956-01-10 Sperry Gyroscope Co Ltd Feed-back amplifier systems and servo mechanisms that are adapted to respond to input changes at very low frequencies
US2628266A (en) * 1949-01-22 1953-02-10 Rca Corp Analysis of signal transfer devices
US2592193A (en) * 1949-03-03 1952-04-08 Us Sec War Means for reducing amplitude distortion in cathode-follower amplifiers
US2685000A (en) * 1949-04-29 1954-07-27 Rca Corp Stabilized direct current amplifier
US2709205A (en) * 1949-07-06 1955-05-24 Southern Instr Ltd Direct coupled thermionic valve amplifiers
US2647174A (en) * 1950-09-23 1953-07-28 Du Mont Allen B Lab Inc Adjustable beam-trace-positioning amplifier
US2714136A (en) * 1951-02-27 1955-07-26 Gen Precision Lab Inc Stabilized direct-coupled amplifier
US2763733A (en) * 1952-03-21 1956-09-18 Wallace H Coulter Amplifier having series-connected output tubes
US2737547A (en) * 1952-10-01 1956-03-06 Hughes Aircraft Co Cathode follower circuits
US2896027A (en) * 1953-10-19 1959-07-21 Melpar Inc Reflex amplifiers
US2801296A (en) * 1954-02-09 1957-07-30 Bell Telephone Labor Inc D.-c. summing amplifier drift correction
US2795654A (en) * 1954-03-02 1957-06-11 James R Macdonald High impedance electronic circuit
US2810025A (en) * 1954-07-15 1957-10-15 Hughes Aircraft Co Direct-coupled feedback amplifier

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2994832A (en) * 1958-04-08 1961-08-01 Bell Telephone Labor Inc Transistor amplifier
USD882397S1 (en) 2013-09-09 2020-04-28 Kraft Foods Group Brands Llc Lid
USD885181S1 (en) 2013-09-09 2020-05-26 Kraft Foods Group Brands Llc Lid

Also Published As

Publication number Publication date
GB794841A (en) 1958-05-14
DE1015854B (de) 1957-09-19
BE542279A (de)
FR1143663A (fr) 1957-10-03

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