US2789266A

US2789266A - Bias voltage supply

Info

Publication number: US2789266A
Application number: US348255A
Authority: US
Inventors: Neal Jack Laurance
Original assignee: Individual
Current assignee: Individual
Priority date: 1953-04-13
Filing date: 1953-04-13
Publication date: 1957-04-16
Anticipated expiration: 1974-04-16

Description

April 16, 1957 J. L. NEAL BIAS VOLTAGE SUPPLY 2 Sheets-Sheet 1 Filed April 13, 1953 MAIN LOAD B W MU MH L o O a.- T5 NU T L L R m mM a MW umm am 9 a m r 7 5 L 3 l w M r 6 r w 3 4 u 3 2 m 4 m L H 7 "M MU T. N /M WK 0 T w 7 an A J v 5 April 16, 1957 J. L. NEAL 2,789,266

BIAS VOLTAGE SUPPLY Filed April 15, 1953 2 Sheets-Sheet 2 M/VENTOR J'QCK L. N E9 L United States Patent BIAS VOLTAGE SUPPLY Jack Laurance Neal, New Haven, Conn. Application April 13, 1953, Serial No. 348,255

9 Claims. (Cl. 321-16) This invention relates to a device for obtaining a negative grid voltage from a plate current supply for electron discharge tube circuits. More particularly it provides a simple arrangement that will produce negative grid voltages when it is applied to the full wave rectifiers commonly used in power supplies for electronic devices.

It is one of the principal objects of this invention to provide a simple, efi'ective device for supplying negative grid bias potential or" relatively constant magnitude which will be suitable for use as fixed grid bias voltages in thermionic tube circuits.

Another object of this invention is to provide a source of negative unidirectional potential, or of alternating potential, the magnitude of which can be made to vary with load current drawn from the full wave rectifier to which this invention is applied.

Other objects will appear hereinafter.

As is Well known in the art, many applications of electron discharge tubes require the control grids of the tubes to be maintained at a negative potential with respect to the cathodes such that no grid current will flow and no power will be consumed by the grid circuit. Heretofore various circuit arrangements have een employed in applying a constant negative bias voltage to grids of electron discharge tubes whereby this desired condition can be maintained. One common method has been the use of a separate source of electrical energy, such as a dry cell, but this has been found unsatisfactory because of the limited life of such dry cells and the consequent requirement that they be periodically replaced. Another method involves providing a separate rectifier to supply bias voltages, but the size and expense of such units makes them uneconomical for use in most electronic devices. One of the most common methods of obtaining grid bias potential, known as cathode biasing, de-

pends on the flow of plate current through a cathode resistor, such that the bias potential developed is subject to variations with the signal. These variations in potential can never be completely bypassed, with the result that distortion is introduced into the output circuit of the tube. In circuits requiring a relatively high value of negative grid bias, cathode biasing also has the disadvantages of appreciably lowering the cathode-plate potential of the tube to which it is applied.

in the present invention I claim to have developed a device to supply negative grid bias potentials substantially free of the disadvantages associated with the above mentioned methods. Moreover, as will be apparent to anyone skilled in the art, a source of negative potential suitable for use as grid bias voltage may also "be used in other circuit arrangements where, as in the case of delay voltages for automatic gain control, a source of negative potential of small magnitude is all that is required, and the current drawn therefrom is negligible.

In still other applications of thermionic tubes, such as automatic regulator circuits, it is sometimes desirable to apply a negative grid voltage'whi-ch varies with the total 'ice direct current load that the tube circuit imposes upon the power supply. As an example of this mode of operation, one may refer to voltage regulated power supplies which utilize an electron discharge tube as a variable load to compensate for fluctuations in the main current drain on the supply. In such cases the negative voltage on the grid of the variable load tube is made to vary with the main load current. By proper design, the present invention will provide a source for a ne ative unidirectional voltage suitable for such applications.

The invention will be best understood by reference to the accompanying drawings wherein like numerals of reference refer to the same or corresponding parts in the several figures.

Figure 1 is an embodiment of my invention when used to supply fixed negative bias to the grid of an audio amplifier tube;

Figure 2 is a symbolic block diagram showing a preferred embodiment of my invention when it is used as a source of control voltage in a voltage regulated power pp y;

Figure 3 is a schematic diagram giving the details of the preferred embodiment shown in Figure 2;

Figure 4 is a symbolic diagram of the equivalent circuit of the high voltages secondary winding of a transformer and such other parts of a full wave rectifier circuit as are necessary to explain the essence of my invention.

in all figures, 1 represents a power transformer with primary winding 2 connected to a source of single phase alternating current and a high voltage secondary winding generally represented at 4 which is centre-tapped at 7 and t ereby divided into two substantially equal sections 5 and 6 as shown. The outside terminals 8 and 9 of Winding 4 are connected to

anodes

13 and 14 or" rectifier 15. The rectifier cathode 18 feeds positive plate voltage terminal 2t), the negative terminal 21 being the centre-tap 7 which is connected to ground as shown. This arrangement is in accordance with the conventional series type full wave rectifier circuits in which the rectifying device is in series with the positive side of the direct current load. This type of circuit must be distinguished from the so called shunt connected full wave rectifiers, to which my invention is not applicable, wherein the load is in parallel with the rectifier. For the sake of completeness, the schematic diagrams 1 and 2 show low voltage secondary winding 16 which feeds the rectifier heaters 17 and a smoothing filter (also shown in Figure 2) generally represented at 19 is inserted between the rectifier cathode 18 and the positive plate voltage terminal 20. In each of the figures I have shown a full wave thermionic rectifier: it will be obvious to a person familiar with the art that other types of full wave (or indeed two half-wave) rectifiers such as selenium rectifiers, might be used instead of the high vacuum or gas filled types indicated in the drawings. In all cases, however, the outside terminals 8 and 9 of winding 4 are connected to what may conveniently be called the input elements of the rectifier, the instantaneous polarity of which determines the segment of winding 4 which is to supply the load current, and the main load on the rectifier circuit is connected between the output element of the rectifier and the centre tap 7 of winding 4.

Referring now to Figure 4 by which the basic operation of my invention may most easily be explained, the high voltage secondary winding 4 is represented by its equivalent circuit, segment 5 shown as consisting of the series combination of alternating current generator 41 (representing the elect-romotive force induced in segment 5 by actionof the primary) and the resistive and leakage inductance elements 30 which represent its internal impedance. Segment 6 of secondary winding 4 is similarily represented by alternating current generator 42 in series with impedance elements 31. The outside tenninals 8 and 9 of secondary winding 4 are connected to

anodes

13 and 14 of rectifier 15 as explained above. Cathode 18 is connected to positive direct current output terminal 20 and tap 7 is connected to ground and negative output terminal 21, with the load 46 connected between terminals 20 and 21 as shown. Connected across the tenminals 8, 9 of secondary winding 4 is a voltage divider 3 consisting of two impedance elements iii and 12 connected to bias voltage output terminal 35 as shown. As this description proceeds, it will become evident that this voltage divider, the potential developed thereon at tap 11, and the applications in which this potential can be used are of the essence of my invention.

For the purposes of the discussion which follows immediately, it will be assumed that the voltage divider 3 will provide a direct current path; the use of and efiect of series capacitive units will be discussed later. Furthermore, the term positive and negative, when used to describe the potential of :a given point in the circuit, will indicate the, polarity of that point with respect to tap 7 which, in turn, is connected to ground as discussed above.

Because of the action of the rectifier in switching the unidirectional load current from one segment of the winding 4 to the other, such that the current is flowing alternately through segment 5 when terminal 8 is positive and through segment 6 when terminal 9 is positive, there is a fluctuation in the absolute (i. e. without regard to pclarity) magnitude of the peak value of alternating potential of terminals 8 and 9 with respect to ground. The positive peak in each case is smaller than the negative peak because of the voltage drop caused by the flow of load current through the internal impedance of the respective segments of winding 4. Thus, to take a specific example, if, :at a given instant terminal 8 has attained its positive peak, terminal 9 will be at its negative peak, but the positive potential of 8 will be less than the negative potential of 9 because the drop occasioned by the internal impedance of the transformer occurs only in the segment in which the load current is flowing-4. e. segment 5. On the next half cycle the situation will be reversed so that, with respect to ground, terminal 9 is less positive than terminal 8 is negative. This fluctuatmg potential across the ends of voltage divider 3 is therefore unbalanced to the extent that the negative peaks are of greater magnitude than the positive peaks, with the actual difference in the magnitudes depending upon the internal impedance of the respective segments and the load current which flows through them.

It will thus be evident that while at any given instant it is possible to select a point on the voltage divider 3 which will have an instantaneous potential equal to zero with respect to ground, it is not possible to select a point which will have an average or unidirectional potential equal to zero. Therefore, at any given tap 7 on the voltage divider 3, it is possible to obtain a potential having a pulsating unidirectional component that is negative with respect to ground, the magnitude of which depends primarily on the internal impedance of the transformer (which can be considered constant) and the load current. Moreover, to the extent that taps 7 and 11 are not positioned in the electrical centres of winding 4 and voltage divider 3 respectively, there will also be present at tap 11 a purely alternating potential with respect to ground. In general it will be desirable to balance out this purely alternating component by locating tap 7 as near as possible to the electrical centre of winding 4 and similarly selecting equal values for impedance elements 10 and 12 so that the potential of tap 11 varies directly withthe load current and is unidirectional (albeit pulsating) with respect to ground.

If, contrary to the assumption made above, the impedance elements 10 and 12 are capacitors as shown in v Figure 2, the potential at tap 11 will not be unidirection- :al since the capacitors block out the direct current component and allow only the alternating component to pass. If tap 7 is adjusted so as to be in the electrical centre of winding 4 and the values of impedances 16 and 12 (in this case capacitors) are made identical the potential of tap 11, because of the unbalanced positive and negative peaks discussed above, will be alternating with respect to ground and of a magnitude that varies directly with the load current. As will be obvious to any one skilled in the art, this alternating voltage corresponds to the pulsations in unidirectional voltages that would have been produced if a voltage divider providing a direct current path had been used. Indeed this same alternating voltage can be obtained from a purely resistive centre tapped voltage divider by feeding the voltage obtained at the tap through a capacitor, thereby blocking out unidirectional component and allowing only the alternating component to pass.

Referring more particularly to Figure 1 in which the voltage divider connected across winding 4 consists of two equal resistors 10 and 12, the pulsating, negative voltage thereby obtained at tap 11 is fed through the filter generally represented at 22. Because of the very low current drain, this filter 22 may be of the convenient and inexpensive resistance capacity type, shown as consisting of series resistor 23 and shunt capacitor 24. At the output terminal of the filter, the smoothed unidirectional negative voltage is applied across potentiometer 32 which is connected between 35 and 21 ground) :as shown. An audio amplifier stag generally represented at 25, consists of triode tube 26 with plate 27 connected to positive plate supply terminal 29 through output loading device 33. The cathode 23 is grounded and the audio frequency signal input 63 is impressed upon grid 29 as shown. The grid resistor 34 is connected at its upper end to grid 29 and at its lower end to the variable tap on potentiometer 32. By adjusting the position of this tap an appropriate value of negative potential will be applied to grid 29 through resistor 34, thereby maintaining the grid at a negative bias with respect to grounded cathode 28. By choosing values of resistors 10 and 12 which i are low relative to the input impedance presented by filter 22 and potentiometer 32, it is possible to reduce the direct current voltage drop occasioned by resistors 10 and 12 and thereby obtain a proportionately greater negative voltage at terminal 35. Moreover, because filter 22 has efiectively removed the pulsations in the negative voltage available at tap 11, the output voltage developed across potentiometer 32 will be essentially constant and satisfactory for use as a source of fixed bias voltage.

Figures 2 and 3 show an embodiment of my invention in which it is used as a source of control signal in a voltage regulated power supply, Figure 2 being a block symbolic diagram showing the functions of the several components schematically represented in Figure 3. In Figure 2, those parts not previously referred to consist of control tube 4%), which automatically compensates for changes in current consumed by main load 46, and the control circuit itself 36, 37, 38, and 39. As shown in Figure 3, cathode 43 of control tube is grounded and plate 44 is connected to the positive supply lead from the rectifier, either directly to cathode 18 (as in the embodiment shown) or alternatively on the main load side of filter 19. When thus connected, triode 40 serves as a secondary load on the power supply in addition to the main load 46, and the current drawn by this secondary load can be varied by changing the potential of grid 45. Automatic regulation of the voltage output of the power supply is secured by arranging that the voltage drop caused by the transformer and rectifier is kept at a relatively steady, fixed value. This is achieved by keeping the total load current flowing from rectifier cathode 18 essentially constant, irrespective of changes in current drawn by main load 46, or, in other words, reducing the internal impedance of the power supply, as it appears to. a load con nected across its output terminals and 21. To obtain this desired condition of constant total load current, it is necessary to arrange the supplementary load (i. e. triode in such a manner that it will drawn more current when main load 46 draws less and vice versa. This can be realized by arranging for the negative potential on grid to vary directly with small changes in total load current.

As discussed previously, when the tap 7 is at the electrical centre of winding 4 and the elements 10 and 12 are of equal impedance, the potential at tap 11 (pulsating and unidirectional when 3 provides a direct current path, and alternating when it includes series capacitive elements) has a magnitude that bears a direct relationship to the total load current being drawn from the rectifier cathode. By analysis and experience I have found that this relationship is substantially linear over that part of the transformer and rectifier characteristics usually used in power supply circuits. It is clear, therefore, that the magnitude of this voltage, be it pulsating and unidirectional or alternating, is a direct indication of the load on the rectifier and is therefore useful as a control signal. Indeed, when a relatively high impedance voltage divider consisting of two equal resistors 10. and 12 is used, the pulsating negative potential available at tap 11 can be applied directly to the grid 45 of control tube 40, and a certain amount of voltage regulation obtained thereby. However, I have found that with such an arrangement 'a given change in total load current causes so small an increment in control voltage relative to the total control voltage available at tap 11 that it is not sufiicient to provide the desired change in plate current drawn by the control tube 4%. This disadvantage may beovercome by using equal

capacitive elements

10 and 1 2 as shown, thereby securing an alternating signal voltage at tap 11 This voltage is then fed into a base clipper arrangement (a device Well known in the art) which consists of an amplifier tube so arranged that only a given portion of the negative peaks of the signal voltage applied to its grid will etfect its plate current. Consequently the signal developed across the load in the plate circuit of such a tube will be more an amplification of changes in signal voltage (since these appear as changes in the peak potential applied to the grid) than it will be of the absolute value of signal itself. The base clipper 36 forms part of the grid circuit of amplifier 37, and the output of amplifier 3'7, which is a measure of the changes in signal voltage taken from tap 11, is rectified by control signal rectifier 38, and applied to the grid of control tube it} through filter 39, as best shown in Figure 2.

Referring now to the details of this arrangement as shown in Figure 3, equal capacitors 10 and 12 are connected in series between terminals 8 and 9 of secondary winding 4. The voltage at tap 11 is fed directly to the grid 43 of the triode section of tube 47 and grid 48 is also connected, by means of grid leak resistor 52, to the variable tap 62 on potentiometer 53. The lower end of potentiometer 53 is grounded as shown, the upper end being connected. to a source of positive potential (in this case rectifier cathode 18) through a filter consisting of capacitor 54 and resistor 55. The cathode of tube 47 is grounded and the plate 49 is connected to one end of the primary 5% of transformer 57, the other end being connected to rectifier cathode 18 through resistor 56. Secondary winding 59 of transformer 57 is connected between diode plate 5 of tube 47 and grid 45 of control tube 29. Resistor as, which acts as a diode load resistor, is connected between grid 45 and ground. A capacitor 61, hunted across resistor 6%; provides filtering action for the unidirectional voltage which is developed across resistor 6t and applied to grid 45. In this particular embodiment I have shown a single tube 47 which includes a triode and diode having a common cathode. It is obvious that these functions can be fulfilled by two separate tubes, or, alternatively, a triode tube may be used for the clipper-amplifier circuit, and a crystal diode used as a rectifier.

In the absence of signal from tap 11, the relatively high positive potential at the upper end of potentiometer 53 will cause grid 48 to. assume a positive potential and draw grid current, the magnitudes of which depend upon the characteristics of tube 47, the grid leak resistor and the position of variable tap 62 on potentiometer 53. Because grid leak resistor 52 has a value of several mego'hans, even small amount of grid current will cause a considerable voltage drop to be developed across it, so that grid 43 will attain a potential which is only slightly positive. Moreover, it is not possible for signal voltage from tap 11 to appreciably increase the positive potential of grid because in so doing the grid current would increase, causing a corresponding increase of potential. drop across grid leak resistor 52 and thereby keeping the grid at approximately the same potential. However, when the signal voltage reaches a negative value suflicient to overcome the slightly positive potential assumed by the grid, the grid current ceases, the signal voltage gains control of the grid potential, and therefore, the plate current. Thus it will be seen that depending upon the position of the variable tap on potentiometer 53, a given portion of each negative peak of the signal voltage is amplified and reproduced in the plate circuit of tube 47. Because a given increment in the signal voltage from tap 1-1 is greater relative to. a portion of each negative peak than it is to the amplitude of the full negative half cycle, changes. in input signal are accentuated in the triode plate circuit of tube $7. This signal receives further amplification by audio transformer 57 and is rectified, filtered and appears as a unidirectional negative potential on grid 45 of control tube 49.

From the foregoing it will be evidentthat the negative voltage on grid 45 of control tube 40 will. adjust itself automatically to compensate for changes in the total? load current, the mean value of Which can be adjusted over a narrow range by the position of the variable tap 62 on potentiometer 53. Consequently, if load 46 suddenly draws less current, the total load current will tend to ecrease, thereby causing the signal voltage at tap 11 to decrease. This in turn reduces the negative voltage applied to grid 45 and causes control tube 40 to draw more current and restore the total load current to approximately its previous value. Moreover, because of the clipper amplifier arrangement, a small change in voltage from tap 11 will cause a relatively larger change in the rectified negative voltage applied to grid 45 so that a very high degree of regulation is possible.

What I claim as my invention is:

1. In a power supply circuit which uses a single series connected full wave rectifier and a power transformer with a centre tapped secondary winding whose outside terminals are connected to the input of the rectifier, a main load circuit connected in series between the output of the rectifier and the centre tap, a source of D. C. potential having a polarity relative to the centre tap opposite to the D. C. potential at the output of the rectifier, said source being obtained from the junction of two equal impedance providing a D. C. path connected in series between the outside terminals of the secondary winding.

2. In a power supply circuit which uses a single series connected full wave rectifier and a power transformer with a centre-tapped secondary winding whose outside terminals are connected to the anodes of the rectifier; a main load circuit connected between the cathode of the rectifier and the centre tap, a source of D. C. potential which is negative with respect to the centre tap, said source being obtained at the junction of two equal resistances connected in series between the anodes of the rectifier.

3. In a circuit for supplying plate and grid potentials to electron discharge tubes; a single series connected full wave rectifier, a power transformer having a centre tapped secondary winding whose outside terminals are connected to the anodes of the rectifier, a load circuit connected between the cathode of the rectifier and the centre tap, two equal resistances connected in series between the anodes of the rectifier, voltage divider means connected between the junction of the resistances and the centre tap, means whereby at least part of the potential across said voltage divider means is used as grid bias in the electron discharge tube circuit.

4. In a circuit for supplying plate and grid potentials to electron discharge tubes: a single series connected full wave rectifier, a power transformer having a centre tapped secondary Winding whose outside terminals are connected to the anodes of the recitfier, connection means whereby the plate voltage for the electron discharge tubes is obtained from between the cathode of the rectifier and the centre tap, two equal resistances connected in series between the anodes of the rectifier, a voltage divider connected between the junction of the resistances and the centre tap, and filter and conductor means whereby at least part of the voltage developed across said voltage divider is used as grid bias in the electron discharge tube circuit.

5. In a voltage regulater power supply circuit: a single full wave rectifier, a power transformer having a centre tapped secondary winding whose outside terminals are connected to the input of the rectifier, a load connected in series between the output of the rectifier and the centre tap, two equal impedance elements connected in series between the outside terminals of the winding and means whereby the potential difference between the junction of the impedances and the centre tap is used as a source of control signal for an element in the circuit which controls the output voltage of the supply.

6. In a voltage regulated power supply circuit using a single full wave rectifier, a centre tapped transformer winding whose outside terminals are connected to the input elements of the rectifier, the plate and cathode of a control tube connected across the output of the rectifier in parallel with the load, two equal impedances providing a D. C. path connected in series between the input elementsof the rectifier, and means whereby at least part of the potential difference between the junction of the inipedances and the centre tap is applied to a grid of the control tube.

7. The circuit as claimed in claim 6 in which the full wave rectifier is a full Wave thermionic rectifier and the input elements are the anodes thereof.

8. In a voltage regulated power supply circuit using a single full wave rectifier, a centre tapped transformer winding whose outside terminals are connected to the anodes of the full wave rectifier, a control tube having a plate and cathode connected in parallel with the load between the cathode of the rectifier and the centre tap, two equal capacitors connected in series between the anodes, means whereby the difierence in potential between the junction of the capacitors and the centre tap is applied to the input of an amplifier, a half wave rectifier whereby the signal at the output of the amplifier is converted into a negative unidirectional potential, and filter means whereby the negative unidirectional potential is applied to the grid of the control tube so as to regulate the plate current therein.

9. The circuit as claimed in claim 8 in which the diffcrence in potential between the junction and the centre tap is applied to the input of the amplifier through a base clipper circuit whereby only a given position of the negative peaks of the potential appears in the output of the amplifier.

References Cited in the file of this patent UNITED STATES PATENTS 1,968,875 Cooper Aug. 7, 1934 2,052,413 Lord Aug. 25, 1936 2,310,112 Palmer Feb. 2, 1943 2,315,619 Hutcheson Apr. 6, 1943