US2912524A - Low distortion cathode follower - Google Patents

Description

Nov'. 10, 1959 J. M. HUNT LOWDISTYORTION CATHODE FOLLOWER Filed May 14, 1956 F IG. 2

JOHN M. HUNT lNVE-NTOR ATTORN EY United States Patent O 2,912,524 LOW.DISTORTION CATHODE FOLLOWER John M. Hunt, Binghamton, N.Y., assignor to Link Avizxiftiorfi, Inc., Binghamton, N.Y., a corporation of New Application May 14, 1956, Serial No. 584,494

6 Claims. (Cl. 179-171) This invention relates to an improved cathode follower circuit to provide an output signal which is an extremely faithful replica of an applied input signal. In particular this invention relates to the use of constant current through a cathode follower to maintain linearity between changes in input and output signal levels.

In the electronic arts generally, and particularly in the communication, TV, computer, automatic control and instrumentation arts, cathode followers are extensively used. Cathode follower circuits are characterized as having high input impedance, low output impedance and a voltage gain which approaches unity. Such circuits make ideal isolation circuits orfbutfers, and are extensively used to couple low impedance loads to high impedance voltage sources, especially where the phase or polarity inversion characteristic of ordinary amplifiers is to be avoided. However, for large excursions of the input signal applied to the grid of an ordinarycathode follower, appreciable differences in grid-to-cathode'voltage result. Although the error or distortion is usually deemed insignificant in voice or speech transmission circuits, its effect on data transmission circuits and the like is regarded as serious, and it sometimes sets a limit on the accuracy of the overall system in which the cathode follower is employed. The invention establishes an environment for the cathode follower which is substantially independent of instantaneous input signal voltage.

It is well known that the linearityof cathode followers is inherently greater with high [1. tubes, and that their linearity may be greatly increased by the use of a constant current device in place of the more usual cathode resistor. This particularly applies to triode cathode followers since the ,u of many triodes at constant current is independent of plate voltage. One well-known means for establishing this environment for a cathode follower consists in utilizing a pentode as the cathode resistor, and relies on the fact that for constant bias and screen voltages, the pentode current is fairly independent of plate voltage. Similarly well known is the use of a triode to replace the normal cathode resistor as a constant current means, but some difficulty exists in this connection in insuring that the constant current triode does not run into grid current. The present invention uses a triode as a means of establishing the constant current environment without the limitations of the prior art. The present invention utilizes a triode constant current device in a manner such that greater stability is obtained with respect to tube drift than can be obtained by the use of the pentode of the prior art. In order to obtain this constant current environment one or more triodes are connected as cathode followers means to the cathode of the cathode follower in which a linear relationship is to be maintained between changes of input and output voltages.

One of the objects of the invention resides in the provision of a constant current environment for a cathode follower system wherein the linearity established between 2332,52 3- Patented Nov. 10, 1959 input and output voltage variation over a wide range of positive and negative going input signals is better than has heretofore been possible by any means established under the prior art. Another object is to provide an improved in-phase relationship between the varying input and output signals of a cathode follower system regardless of signal level.

A further object of the invention is to provide in a cathode follower system the same degree of linearity and phase relationship over a wide range of input signals for both positive and negative going input signals.

A still further object of the invention is the establishment of an environment for the cathode follower signal tube in which a greater stability withrespect to control,

electronic tube and associated circuitry is used as a.

linearizing element to improve the performance of. a cathode follower system.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts, which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims;

#For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

' Figure 1 shows a vacuum'tube circuit in which a single pentode is used in the manner of the prior art to maintain a substantially constant current through a pentode cathode follower system.

Figure 2 shows a vacuum tube circuit, where, in accordance with the invention, a single triode cathode follower is used to maintain a virtually constant current through a pentode cathode follower system.

Figure 3 shows a vacuum tube circuit wherein in accordance with the invention, a plurality of triode cathode followers are used each to assist another in maintaining constant current through a pentode cathode follower system.

Referring to the figures, the electrical system shown in Figure 1 comprises a pentode vacuum tube V connected as a resistance coupled cathode follower system in which a pentode tube is used as the cathode follower tube, and includes in its cathode circuit the resistor R together with the pentode tube V and its associated circuitry. Substantially constant screen voltage is applied to the screen 13 of the cathode follower tube, taken by way of example, from the battery source B Input to the cathode follower system is between terminals 10 and 11 which are connected to control grid 12 and ground respectively. The plate 15 is connected to B+ and the suppressor grid 23 is connected to the cathode 14 in conventional manner. The output signal. is developed across the cathode impedance and is applied across the very high resistance load resistor R appearing at the output terminal 25.- For the pentode tube V employed in the circuit of Figure 1, it is necessary that three parameters be held constant, or the effects of their variation eliminated. These are: the plate-to-cathode voltage; screen-to-cathode voltage; and the plate current. By the use of a vacuum tube having an unusually high plate resistance, the effect on grid-to-cathode voltage of unusually wide excursions in plate voltage can be made very small. The use of a dry cell provides constant voltage to the screen 1.3 independently of cathode follower operating level. The problem of maintaining constant current in the cathode follower V is more difiicult, particularly in view of the fact that the voltage drop across the cathode follower load resistor R varies with input level. In the prior art depicted by the circuit of Figure 1, this current stabilization has been attempted by the use of a pentode V Referring now to the pentode V its plate 16 is connected to the lower end of the cathode resistor R whilst its cathode 17 is returned to the negative source of voltage through a conventional bias resistor R The suppressor 13 is returned to cathode 17 in conventional manner, whilst the screen grid 21 is taken to the output connection 22 of a fixed potential divider comprised of resistors R and R The control grid 19 is connected at 20 to the lower end of cathode resistor R Figure 1, therefore, represents a circuit of the prior art in which the problem of maintaining constant current in the cathode follower tube regardless of instantaneous input signal has been attempted by the use of high resistance pentode tube as a constant current device. One drawback to the use of this device is that the screen resistors R and R together with the cathode bias resistor R establish the plate current level at which V operates, and since this establishes the current through V the linearity of the system is critically dependent on the stability of the screen potential established by the resistors R and R Figure 2 shows an electrical circuit according to the invention in which V represents a pentode cathode follower tube identical with V of Figure 1. Circuit elements common to Figures 1 and 2 are identified by similar numbers. Substantially constant screen voltage is again conveniently taken from a battery although any floating constant voltage source may be used, and R represents the cathode resistor. Signal input is applied between control grid 10 and ground 11 in conventional manner, the output being developed across the cathode impedance and applied across the high resistance level resistor R The output voltage appears at terminal 25. The lower vacuum tube V represents a triode connected according to the invention in such a manner as to control the flow of current through V Since the lower end of the cathode resistor R is necessarily negative with respect to cathode follower output, this resistor may be returned through a battery to the cathode 28 of V the control grid 26, which is connected to the cathode 14 of V 13-]- voltage being applied in normal manner to the plate 27 of V It is well known that for an input of e volts at the control grid of V an output of approximately i.e. Ewe, will be developed across the cathode circuit impedance. For large increases of input signal this relationship diverges by an amount which increases with signal input, resulting in a progressively smaller voltage being developed across the cathode impedance. In the invention, this output voltage is applied in combination with a battery to the grid 26 of V in such a manner that the voltage at the control grid 26 of V is the resultant of the cathode output of V and the battery B Thus any decrease in output from V causes a reduction in the efiective grid voltage of V and an increase in the current drawn by V thereby increasing the voltage drop across R Since R the battery B and R are efiectively in series, the drop in voltage across R is compensated by the gain across R The grid voltage on V is thus restored, and the current through V is maintained at constant level. The second cathode follower tube and battery combination provides, within the limitations characteristic of uncorrected cathode followers, an approximately constant voltage drop across resistor R thereby assuring virtually constant current through tube V Thus the second cathode follower V functions as a corrective circuit to linearize the first cathode follower. Moreover, it has obviated the danger of a change in the current level through V due to possible drifting of the screen voltage associated with the current controlling pentode of the prior art. It has, in addition, the merit of a greater inherent stability than the pentode of the prior art, inasmuch as the triode constant current tube has a much greater stability with respect to tube drift, since a given change of grid-to-cathode voltage causes a smaller percentage change in the drop across its cathode resistor.

Figure 3 shows an electrical circuit according to the invention in which the degree of correction afforded by the second cathode follower V of Figure 2 has been further improved by the use of a third cathode follower V Again, since the lower end of R is necessarily negative with respect to cathode follower output, it may be connected in series with a battery to the cathode 30 of V the grid 29 of which is returned to the cathode 28 of V B+ voltage is applied in conventional manner to the plate 31 of V Referring to Figure 3, any reduction due to nonlinearity in the voltage developed across R will result in lowering the voltage at the control grid 29 of V thus increasing the current through V; and its cathode resistor R This increase in voltage across the cathode resistor R of V is effectively in series with the battery B and R and thus tends to restore the voltage to ground of the cathode 28 of V thereby maintaining virtually constant current through V In this manner error in the second cathode follower V is virtually eliminated with the result that the environment of the first cathode follower is established almost entirely by the battery volt ages and is to a very high degree unaffected by the input signal.

By cascading cathode followers in this manner, it is possible to reduce to an undetectable level the error be tween cathode follower output and input voltage provided that the first cathode follower operates into an extremely high impedance load. In actual practice, since this load must of necessity have a finite impedance, an irreducible error arises from load current. The non-linearity of transconductance of conventional vacuum tubes, therefore, introduces an irreducible error in the linearity of the complete system, regardless of the number of stages cascaded. It is apparent, therefore, that either embodiment of the invention establishes a constant current en vironment for the cathode follower system wherein the linearity established between input and output voltage variations over a wide range of positive and negative input signals is better than has heretofore been possible by any means established under the prior art. Moreover, a better phase relationship has been established between the varying input and output signals regardless of signal level, the same degree of linearity and phase correspondence being maintained over a wide range of both posi tive and negative-going input signals. The invention provides, moreover, an environment for the cathode follower system which has a greater inherent freedom from control tube drift than is possible in the prior art. The invention thus provides a novel circuit arrangement wherein at least one electronic tube and associated circuitry is used as a linearizing element to improve the performance of a cathode follower system.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustra tive and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secure by Letters Patent is: v

1. A low-distortion cathode follower circuit, comprising in combination; a pentode having a control grid connected to receive an input signal which varies with respect to a ground potential,- a cathode, a screen grid, and a plate connected to a suitable constant potential supply; a triode having a grid coupled directly to the cathode of said pentode, a plate connected directly to the plate of said pentode, and a cathode coupled through a constant potential supply and a first resistor to the cathode of said pentode and coupled through a second resistor to ground; and, a load coupled between the cathode of said pentode and ground to provide an output potential substantially equal to the input signal.

2. A low-distortion cathode follower circuit, comprising in combination; a pentode having a control grid connected to an input terminal to receive an input signal which varies with respect to a ground potential, a cathode connected to an output terminal, a screen grid, a suppressor grid connected to said cathode, and a plate connected to a suitable constant potential supply; a triode having a control grid coupled directly to the cathode of said pentode, a plate connected directly to the plate of said pentode, and a cathode coupled through a constant potential supply and a first resistor to the cathode of said pentode and coupled through a second resistor to ground; and, aload impedance coupled between the cathode of said pentode and ground to provide an output potential substantially equal to the input signal.

3. A low-distortion cathode follower circuit, comprising in combination; a pentode having acontrol grid connected to an input terminal to receive an input signal which is variable with respect to a ground potential, a cathode connected to an output terminal, a screen grid connected through a constant biasing potential to said cathode, a suppressor grid connected to said cathode, and a plate connected to asuitable constant potential supply; a triode having a grid coupled directly to the cathode of said pentode, a plate connected directly to the plate of said pentode, and a cathode coupled through a constant potential supply and a first resistor to the cathode of said pentode and coupled through a load resistor to ground; a load impedance coupled between the cathode of said pentode and ground to provid'ean output potential; said pentode providing, in response to an input signal, an output signal on its cathode equal to the input signal plus an error due to distortion; said input signal plus error constituting the input signal provided on the grid of said triode and causing a change in the potential drop across the load resistor of said triode tending to correct for the error signal by changing the sum of the potentials appearing across the series connection of the first and load resistors by an amount to substantially compensate the error.

4. A low-distortion cathode follower circuit comprising; a pentode and a triode, input means connecting to the control grid of said pentode to receive an input signal which varies with respect to ground potential, output means connecting the cathode of said pentode across a load to ground, means connecting the plate of said pentode to the plate of said triode and to a constant potential, means connecting the cathode of said pentode directly to the grid of said triode, means connecting the cathode of said pentode to the cathode of said triode through a first resistor and a second constant potential source in series, means connecting the cathode of said triode through a second resistor to ground, said first resistor and said second constant potential and said second resistor providing a series connection between the cathode of said pentode and ground, whereby the potential appearing across said second resistor provides a correction for error potentials appearing across said series connection between the cathode of said pentode and ground.

5. A low-distortion cathode follower circuit comprising, a pentode and a triode, input means connecting to the control grid of said pentode to receive an input signal which varies with respect to ground potential, output means connecting the cathode of said pentode across a load to ground, means connecting the plate of said pentode to the plate of said triode and to a constant potential, means coupling the cathode of said pentode directly to the grid of said triode, means connecting the cathode of said pentode to the cathode of said triode through a first resistor and a second constant potential source in series, means connecting the cathode of said triode through a second resistor to ground, said firstresistor and said constant potential and said second resistor providing a series connection between the cathode of said pentode and ground, said second resistor providing means for introducing a corrective potential to the potential appearing across said series connection, said corrective potential resulting from cathode current flow that varies in the opposite direction to the variation in the error potential appearing on the grid of said triode and thus provides a corresponding corrective decrease or increase in the potential appearing across said second resistor and accordingly in the sum of the potentials in said series connection.

6. A circuit according to claim 1 having a second triode including a plate, a cathode and a control grid; means coupling the plate of said second triode directly to the plate of said first triode, means coupling the grid of said second triode directly to the cathode of said first triode, means including a resistor and a constant potential supply coupling the cathode of said second triode to the cathode of said first triode, and means coupling the cathode of said second triode directly to ground.

References Cited in the file of this patent UNITED STATES PATENTS 2,579,633 Wadzinski Dec. 25, 1951 2,586,803 Fleming Feb. 26, 1952 2,662,938 Goldstine Dec. 15, 1953 2,708,687 Schlesinger May 17, 1955 2,734,944 Green et al Feb. 14, 1956 2,770,684 Thomas Nov. 13, 1956 2,845,574 Shapiro July 29, 1958 OTHER REFERENCES Text, Vacuum Tube Amplifiers, by Walley & Wallman, Radiation Laboratory Series 18, 1948, pages 430-

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