US2658117A - High impedance power supply - Google Patents

Description

Nov. 3, 1953 D. E. SUNSTEIN ET AL 2,658,117

HIGH IMPEDANCE POWER SUPPLY Filed Nov. 16, 1949 JNVENT RS flue/7 a mu 5m? .OHl/ID a. wave/0 Paten'ted Nov. 3, 1953 Davidlil. S'unstein, vCynwyd, and 'rallen CrMlinster, -H atboro, -Pa., assignors to Philco Corporation,

Philadelphia, 'vania Pa., a corporation of Pennsyl- Application November 16, 1949, Serial No. 127,61 4

4 Claims.

invention herein described and claimed is high relative to that of the load impedance.

As is known, high-impedance-power supplies may be employed to advantage where it is desired "to maintain the amplitude of the alternatfing load current substantially constant irrespective of variations in the load impedance. Highimpedance power supplies may also be advantageously. employedwhere it is desired that, in response to a varying control voltage, the load currentvaries linearly desp' e the fact that the load may be inherently non-linear.

The A.-C. impedance of a power supply may, of course, be increased by the addition of a series resistor ofhigh value. Theinsertion of a series resistor has the disadvantage, however, of simultaneously increasing the D.-C. resistanceoi the supply, thus making it necessary to provide a higher D.-C.. supply voltage than would otherwise be required to deliver the desired amount of current to the load. i

In accordance with the presentv invention,- a

high-impedance A.-C. power supply is provided which requires butrelatively low D.-C. supply voltage to deliver the desired amount of current to the load.

. .We have knowledge of one prior art circuit capable of functioning assuch a highrimpedance power supply without requiring a high supply voltagemsiich a circuit is shown and described in the textbook Vacuum Tube Amplifiers edited by George Valley, Jr. and Henry Wallman, and published in 1948 by McGraw-Hill Book Company,. Inc. as volumel8 of the Radiation Laboratory Series. On pages 433-434 of that book there is described and illustrated (Figure 11.18) a constant-current device functioning as a pentode. load resistor.

The present invention constitutes an improvement over the prior art circuit shownand described in the above textbook; and to facilitate anunderstanding .of the present invention, a brief description of that prior art circuit will first be'given.

. In the prior art circuit referred to above, a pen: tode is shown having acathode-loaded triode connected between. the source of direct-current plate voltage, and the anode of the pentode. The cathode-loaded triode ,is biased positively by a battery connected between thegrid of the triode: and the pentode-anode end of the cathode load resistor.

As will be readily understood by those skilled in .the art, the positivelybiased cathode-loaded triode has a very high impedance to alternating current but a relatively lowimpedanceto direct current- It-will be seen then that, if theoutput loadimpedanceof the pentode be connectedebetween the pentode-anode and ground, the impedance of the power supply will-comprise the high impedance of the pentode in parallel with the high alternating-current impedance of the cathodeloaded triode. Thus,while the A.-Q. impedance of the power supply will be high, a, relatively low value of DL-C. plate voltage will be sufiicient, to deliver a relatively high value of current to the load.

The fact that a relatively'low va1ue,;,of ;.D.-C. plate voltage may ;be employedis advantageous from a cost standpoint, since; it ispossible to use standard types of low voltage tubes as well as other less expensivecircuit elements Utilization; of the prior; art: circuitarrangement has however, been impeded bygthe necessity of using either a battery or a, floating power supplyto provide the constant potential difierence required to obtain between the grid .;of .the cathode-loaded triodeand the pentode-anode end of; the cathode load resisto r. Neither-of these forms of -constant potentiahds satisfactoryefor centain applications. For;;example, while a highi p a e o er-supplrm y b6; able for: use in an. airborne radar-installation, tosupply current to the sweep circuits ofa cathode-ray display'tube, the use of a .battery as asourcez-of constant ,pQtentialmay, be undesirable because of aging and weight considerations. A floating powensupply'requirestheeuse of either a battery or a high-voltage power transformer and hence may be likewise objectionable as too bulky and heavy. R v 1 The present invention provides novel electronic means for providing a constant potential, difference between the grid of thecathode-loadedtriode and the pentode-anodeend of the cathode load resistor, neither a battery nor a transformer being required. 7 s 1:

It is theprimary'object-of this invention to provide a novel and improvedpower supply whose A.-C. impedance is high relative to the load impedance. I

A, more specific object of this invention is to provide an improved form of-low-voltage'highimpedance power supply which isless bulky and less weighty and which requires less maintenance than prior art low-voltage high-impedance power supp1ies. "1 These and other objects, advantages and features of the present invention will become more clearly evident from the following description when considered in connection with the accompanying single figure of drawing showing a pre-- ferred form of high-impedance power supply embodying the present invention.

Referring now to the drawing, the preferred embodiment of the new and improved power supply comprises the elements contained within the dashed-line rectangle and includes a pentode II), a triode II, a triode I2, and a voltage-regulator tube I3. In the preferred embodiment, pentode Ill operates as a triode, its screen grid being connected directly to its anode, and its suppressor grid being connected to its cathode. While these connections reduce the inherently high impedance of the pentode, the tube I is reestablished as high-impedance device by virtue of the cathode load resistor I4. This arrangement is advantageous in that it permits the eil'ective plate impedance of tube I0 to be controlled, over a wide range, through selection of the value of the cathode load resistor I4. For, as is well understood by those skilled in the art, the effective plate impedance is related to the cathode load impedance in the manner indicated by the equation Zp=T1J+ +1) Re where Z =the effective plate impedance r =the internal plate resistance of the tube Rk=cathode load impedance, and

,l=the amplification factor of the tube.

The anode of tube It is connected to a suitable source of plate-supply voltage, B++, by way of the cathode-loaded triode II, resistor I constituting the cathode load.

The grid of triode I I is, in accordance with the present invention, maintained at a substantially fixed potential with respect to the anode of tube ID by novel means which include the voltageregulator tube I3 and the cathode-follower tube I2.

The voltage-regulator tube I3 may be a conventional two-electrode gas-filled tube having the well-known property of maintaining a substantially constant voltage across its electrodes despite variations in the amount of current flowing therethrough. The anode of the voltageregulator tube I3 is connected directly to the grid of triode H, and is also connected, by way of resistor IE, to the source of plate-supply voltage, B++. The cathode of the voltage-regulator tube I3 is connected to the cathode of the cathodefollower tube I2. The grid of the cathode-follower tube I2 is connected directly to the anode of tube Ill. The plate of cathode-follower tube I2 is connected to a source of plate-supply voltage, B+, whose potential may be lower than that of the previously-mentioned source of plate voltage, B++. Resistor II constitutes the cathode load of tube I2.

Since tube I2 is operating as a cathode follower, the positive potential of its cathode follows cophasally any variations which may occur in the potential of the anode of tube I0. And since the voltage across the regulator tube i3 remains substantially constant, the voltage on the grid of tube I I will also vary cophasally with the voltage on the anode of tube ID, the voltage difierence between the grid of tube II and the anode of tube Ill therefore remaining substantially constant. Stated another way, the potential of the grid of tube II is maintained positive with respect to that of the anode of tube In by an amount substantially equal to the voltage drop across the regulator tube l3, the voltage difference between the grid and the cathode of tube I2 being negligible. And the potential of the grid of tube I I, relative to that of the cathode of tube II, is preferably such as to permit tube II to conduct on the linear part of its grid-voltage-plate current characteristic.

By the arrangement described above, the anode of tube I0 is supplied with D.-C. plate voltage through means offering high impedance to alternating current but low impedance to direct current. The alternating-current impedance of the cathode-loaded tube I I is given approximately by the same equation as that given above, namely,

where the values of Zp, r a and R1; are those pertaining to the cathode-loaded triode II.' Thus. by suitable choice of Br and ,lL, the A.-C. impedance of the cathode-loaded triode II may be made as large as desired.

It will be seen, then, that if the useful output load I9 be connected between the anode of pentode I0 and ground, the internal impedance of the power supply comprises the high A.-C. impedance of tube ID in parallel with the high A.-C. impedance of the arrangement comprised of tubes II, I2 and I3 and their associated elements.

The function of the cathode-follower tube I2 will become clear by considering what the effect would be if the voltage regulator tube I3 were to be connected directly between the grid of tube .II and the anode of tube II]. If that were done,

the resistor I6 and the regulator tube I3 would be serially connected in shunt with the cathodeloaded tube II, and since the A.-C. impedance of the resistor It and regulator tube I3 is relatively low, the impedance of the power supply would be low rather than high. In the arrangement proposed by the present invention, the high A.C. input impedance of the cathode-follower tube I2 is effectively in series with the relatively low A.-C. impedance of the regulator tube I3 and resistor I6, thus providing a high A.-C. impedance in shunt with the cathode-loaded tube II.

The provision of the cathode-follower tube I2 creates no additional plate-voltage supply problem, since the plate potential, B+, required for the cathode-follower tube I2 may be obtained from the source of plate voltage, B++, by way of a suitable voltage divider.

It will be understood that, in a practical arrangement of this type, provision must be made for exciting, in the power supply, the alternating current signals which it is desired to deliver to the load.

This is preferably accomplished by providing a source of alternating current control signals, which may here be schematically represented by box I8 and which is usually coupled to the control grid of tube It]. The current through load I9 will then vary linearly in response to the variations of the aforesaid control signals and its amplitude will be substantially independent of variations in the load impedance.

Alternatively, the A.-C. control signals may, of course, be generated within the power supply itself by incorporating tube ID in a suitable regenerative circuit.

It will be understood that the choice of tube characteristics, plate voltage, and other circuit parameters will depend upon the specific values of current output, or degree of linearity, which it is desired to obtain. Determination of these parameters is well within the skill of those versed in the art.

By way of example, in a particular case it was desired to supply a total currrent swing of 2 milliamperes to a slightly non-linear load in response to a 200 volt swing in control voltage. In that case, we employed a triode-connected 6AG5 pentode for tube I0, the two halves of a 2051 double triode for tubes II and I2, and a 5651 voltage-regulator tube for tube 13. The platesupply voltages B++ and 3+ were respectively 600 volts and 450 volts positive with respect to ground. Resistor I4 had a value of 100,000 ohms; resistor l5, 70,000 ohms; resistor I6, 86,000 ohms; and resistor 40,000 ohms.

While apparatus constructed according to our invention has been described with reference to a single embodiment it will be understood that alternative arrangements will suggest themselves to those skilled in the art. We, therefore, desire our inventive concept to be limited only by the scope of the appended claims.

We claim:

1. A high-impedance alternative-current power supply comprising a first vacuum tube having at least anode, cathode and control grid electrodes; means for applying a unidirectional plate-supply voltage to the anode of said first tube, said means including a second vacuum tube having at least anode, cathode and control grid electrodes, said cathode of said second tube being connected to said anode of said first tube by way of a resistor; means for maintaining said control grid of said second tube at a predetermined potential with respect to the anode of said first tube, said lastnamed means comprising a voltage-regulator tube having anode and cathode electrodes and a cathode-follower circuit, said circuit including a tube having at least anode, cathode and control grid electrodes, the anode of said regulator tube being connected to the control grid of said second tube, and to the anode of said second tube by way of a resistor, the cathode of said regulator tube being connected to the cathode of said tube in said cathode-follower circuit, and the control grid of said cathode-follower tube being connected to the anode of said first tube; means for applying an alternating-current signal to the control grid of said first tube; and means for taking an output load current from the anode of said first tube.

2. Apparatus according to claim 1 characterized in that said first tube has pentode electrodes, its screen grid being connected to said anode and its suppressor grid being connected to said cathode.

3. In a high impedance alternating-current supply, a first vacuum tube having at least anode, cathode, and control grid electrodes, a second vacuum tube having at least anode, cathode and control grid electrodes, the cathode of said first tube being connected to the anode of said second tube via a resistor; and a cathode follower circuit comprising a third vacuum tube having at least anode, cathode and control grid electrodes and a cathode load resistor connected to said lastnamed cathode, said last-named grid constituting the input electrode of said cathode follower circuit and being connected to the anode of said second tube and said cathode of said third tube constituting the output electrode of said cathode follower circuit and being connected to the grid of said first tube, thereby to reproduce variations in the anode potential of said second tube cophasally at said last-named grid.

4. Apparatus according to claim 3 and further including means for maintaining a constant potential difference between said output elec-' trode and the grid of said first tube.

DAVID E. SUNSTEIN. ALLEN C. MUNSTER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,206,123 Rinia July 2, 1940 2,210,393 Braden Aug. 6, 1940 2,326,614 Bowman Aug. 10, 1943 2,398,916 Brewer Apr. 23, 1946 2,434,939 Levy Jan. 27, 1948 2,456,638 Kenyon Dec. 21, 1948 2,519,377 Jenkins Aug. 22, 1950

Images