US2492196A - Grid biasing system - Google Patents

Description

Dec. 27, 1949 E. R. SARRATT GRID lIASING SYSTEM Filed Sept. 14, 1946 w/ 1 9. Wm w 4 e/ Wm WM a r? L 9 W 4 i 2 h a Y .n T.w J w a n 4 0 L 4 5% a z m wig? 7 1 0 /6 n4 G v. H M N T? B N .0 m w N. 1n% A m M 5 E .w/

Patented Dec. 27, 1949 UNITED STATES GRID BIASING SYSTEM Application September 14, 1946, Serial No. 697,122

9 Claims. 1

This invention relates to voltage biasing systems for biasing the control electrodes of vacuum tubes.

The principal object of this invention is to provide an effective system for applying bias voltage to the control electrode of a vacuum tube with relatively little drain of current from the source of bias voltage.

Although my invention may have use in connection with various kinds of vacuum tube circuits requiring the application of biasing voltage, it is especially applicable for use with class C type amplifiers. As is well known, the control grid of a class C type amplifier tube is biased to cut-off so that no plate current flows in the absence of excitation from a source of signals applied to the input of the amplifier. The presence of signal excitation at the control electrode or grid of such a class C amplifier has the efiect of driving the grid less negative during the positive portions of the signal cycle; and will even drive the grid somewhat positive during the peaks of these part-cycles so that the control electrode will draw substantial current.

Amplifiers of the class C type have commonly been used for handling relatively large amounts of power, for example, in radio frequency transmitters to supply the radio frequency to a transmitting antenna or the like; and their popularity for this purpose has been due to their rela- "i tively high plate circuit efiiciency. For good operation, they ordinarily are provided, however, with a substantial grid swing from the exciting driving stage, with attendant considerable current drain in the grid circuit during the positive swings, for the reasons stated above. It is often undesirable to drain so much current from the source of biasing voltage which is being used to bias the grid. In accordance with my present invention, I have provided a system adapted to supply the desired high negative grid bias to produce the condition of cut-off in the absence of signal excitation, while also minimizing excessive current drain from the biasing source in the presence of grid excitation.

I carry out my invention by the provision of a source of biasing voltage which may be connected with the control electrode or grid of the amplie. fier to produce the desired bias for the amplifier tube. In accordance with a feature of my invention, I connect with the grid circuit of the amplifier a vacuum tube of atype which is adapted to become conductive or non-conductive in accordance with voltages impressed on its electrodes; and I arrange this tube in such a wa that it is normally in a non-conductive condition, thereby causing the bias voltage source to bias the amplifier in a normal manner. But in the presence of grid circuit excitation suflicient to cause grid current, the control tube is caused to become conductive so that the grid current will readily flow through it.

A feature of my invention is the provision of resistance in series with the control tube in the grid circuit of the biased amplifier so that when the control tube becomes conductive, grid circuit current can flow through the resistor to produce the required bias on the tube. This biasing current in the presence of the grid excitation may be produced from the output of the driver stage which is supplying input to the amplifier.

For the control tube which I use in my bias supply system, I find that a tube of the thyratron type is well adapted for the purpose, especially a thyratron having two grid electrodes, that is, the usual control grid and a shield grid. Furthermore, a cathode heater type thyratron is preferred. By connecting the alternative voltage used for supplying the heater, across the control grid and shield grid, respectively, of the thyratron, so that the instantaneous voltages on these electrodes vary in accordance with the frequency of the alternating power supply, the tube is maintained in a condition of non-conductance except in the presence of exciting voltage on the biased amplifier.

The foregoing and other features of my invention will be better understood from the following detailed description and the accompanying drawing which shows an embodiment of my invention.

Referring to the drawing, there is shown an amplifier tube l comprising a cathode 2, a control grid 3 and an anode 4. The output lead 5 from the anode is shown unconnected, but it will be understood that this lead will be connected to a suitable transmission apparatus to which power is delivered according to the particular circumstances under, which the amplifier is used. The

. input of the amplifier is supplied by a driver stage comprising an amplifying tube 6 having a cathode I, a control grid 8 and an anode 9. The input to this driver, stage is supplied over a lead I!) from some signal transmission apparatus which is not shown. The anode circuit of driver tube 6 comprises a tuned circuit having a variable condenser II in parallel with inductance l2, this parallel combination being connected between the anode and a suitable source of anode voltage l3. A suitable by-pass condenser 14 is connected be- 3 tween the lower end of the resonant circuit and the grounded cathode I. The coupling from the anode circuit of the driver stage is carried to the control grid 3 of the class C amplifying tube through a coupling condenser l5.

According to the common practice in the operation of class C amplifiers, the control grid 3 is provided with a negative bias sufficiently high to bias the tube to cut-off. The biasing voltage is supplied from a suitable D. C. source having the positive lead [6 and the negative lead H, with a pair of resistors l8 and I9 connected there-between. This biasing voltage is carried from the midpoint 29 between resistors I8 and I9, over leads 2| and 22, through resistor 23 and over lead 24, to the grid 3, thereby applying to the grid 3 the D. C. bias voltage of point 20, and" this will be the voltage on grid 3 so long as no grid excitation is present.

For the purpose of controlling ,thebiasongrid 3, I provide in circuit with vthebias supply a-vsystem comprising a-controltube 25.. Controltube 25 is of the gas filled thyratron type, comprising a cathode. 26, a heating element. 21. for heating the cathode, acontrol grid 28, a. shieldgrid'Z'B'and an anode 39; and this thyratron controlitube isin effect connected in a.circuit across the. bias voltage supply, inasmuch as its cathode 26. is connected by conductors 3| and 2! to point.2ii.of the bias supply, andthe anode. 30 is connected through a resistor 32 to ground which is at the positive side of the bias voltage.

Voltage for the heater element 2.1. issupplied from an alternating voltage. power. source. 331 which may for example be an. ordinary sixty. cycle power supply or.v the like,. and the power supply is connected to the primary winding 34 of a power transformer 3.5,,the secondary. winding 36 of whichis provided withamid-pointconnection 31 which-is carried througha parallelarranged resistor 38 and. condenser, 39. to. the cathode 26 of the thyratron. The terminals 49j and M of the secondary winding areconnectedito the. respective ends of the heater element. 2]. Terminal 49 of the secondary winding is furthermore connected. through-a resistor 42. to the CD111- trol grid 28, and the other terminal. 41. is similarly connected through=a resistor. to the shield grid 29. There are-associatedwith .the resistors 32, 42 and 43 respective condensers: 44;, 45 and 46, the condenser 44 being connected from ground across resistor 32, the condenser 45. being connected from ground to resistor 42. at grid. 28, and the condenser 46- being connected from ground to resistor 43 at shield grid 29.

The operation of the biasing. system. illustrated and described above will be better understood from the following detailed description. thereof, wh ch assumes the amplifier to be operated. as a class C amplifier. In the. absence of excitation on the grid 3 of the amplifier, from the driver stage 5, the grid 3 will be biased tocut-ofi by the voltage across resistor l9 in the. bias supply circuit, and no grid current will. be flowing through resistor 23 in the grid return lead. .Accordingly, under this condition, the value of the voltage across resistor 19 is the voltage of the bias supply-between leads l6 and I1 minus the voltage drop across resistor l8, due to current drawn from the bias supply voltage by resistors l8 and 59 in series. This same voltage across resistor I9 is applied between the cathode 25 and the anode 39 of the thyratron 25.

In spite of the high positive voltage thus applied on anode 30 through resistor '32, the thyra- 4 tron 25 is rendered non-conductive by utilizing the alternating heater filament voltage to obtain a cut-ofi bias in the thyratron in the following manner: Since one terminal 40 of the secondary transformer winding is applied to the control grid 28, and the other terminal 4| to thcshield grid 29, the instantaneous voltages on these two grids are essentially 180 out of phase with respect to each other. Since the center tap 3? of the secondary transformer winding returns to the cathode 29 through resistor 38 shunted by condenser 39, it follows that during each half cycle when the control grid 28 goes positive, it draws current from the cathode 26, thereby causing condenser 39 to become charged with its side connected to the cathode 26 positive, and its other side negative. Resistor 38 is given a high enough resistance value so that the time constant of the condenser 39-resistor 38 combination is sufficiently great that condenser 39d0es not have time fully to discharge during the portions of the thyratron control grid excitation cycle during which this grid isinsuiilciently positive to cause grid current flow in the thyratron. Since condenser 39 holds a chargebetween successive intervals of the intermittent grid current flow, making the average voltage of'cathode 21' positive with respect to both of the grids 23' and 29, the control grid 28 goes positive with respect to the cathode for a period of time somewhat less than 130 of each cycle, and by, an amplitude which is one half of the peak voltage delivered from transformer 25 minus the voltage across condenser 39.

Even during the time intervalswhen the control grid 28 becomes positive, tube 25 is still maintained nonconducting because of the fact that the shield grid28is negative with respect to the cathode during these same, times. and-by an amplitude which is one-halfthe peak voltage delivered from the transformer plus the voltage across condenser 39, Accordingly, the net effect of these alternations of voltage on the thyratron grids 28 and 29 is to keep the thyratron in a nonconducting condition, because during the halfcycle when the shield grid is positive, the control grid has gone more negative due to the voltage on condenser 39, and accordingly-the tube is kept at cut-oil. The shield grid 29, moreover, does not draw any current, due to the greater'effect of the control grid 28 on the space charge.

When excitation is applied to the grid 30f the class C amplifier stage, I from its driver, a negativepotential is suddenly applied to the cathode 29 of the thyratron 25 by reason of the fact that the grid 3 draws current during the positive peaks of the excitation cycle, and this current flows through resistor 23 in the direction to make the thyratron cathode 26 more negative. Although the grids 28 and 29 are connected to the cathode circuit of the thyratron through resistors 42 and 43 respectively, these grids do not follow the negative voltage change asv rapidly as the cathode 26 because of the time retarding action of the resistor 42condenser 45 time-delay circuit and the resistor 43-condenser-46-time-delay circuit; and the condensers 45 and 45 will be made of large enough capacity. relative to the respective resistors 42 and 43 to delay the change of voltage at these grids sufficiently. Accordingly, the control and shieldgrid of the thyratron will assume a potential which is more positive relative to the cathode 26 than prior to the plac ing of excitation on the class ,C amplifiergrid,

This more positive potential on the thyratron grids will be suflicient to ionize the gas in the tube thereby making the thyratron conductive. When the thyratron thus becomes conductivait provides a relatively low resistance path for current flow between its cathode and anode, that is, between resistors 23 and 32.

Since the resistance of the thyratron becomes very low when it is conductive, the principal resistance in the grid return lead of control grid 3 of the class C amplifier i will be the resistors 23 and 32, and these resistors will carry substantial unidirectional current owing to the positive grid swings of grid 3. Accordingly, the bias on grid 3 is then in effect supplied by the grid rectifica tion of voltage through resistor 23, supplied from the driver stage 6.

When the excitation is removed from grid 3, the unidirectional current through resistor 23 is discontinued, thereby causing cathode 21 of the thyratron to become less negative. This causes deionization of the thyratron so that it again becomes nonconductive, thereby allowing the biasing resistors l8 and 19 to regain control of the bias voltage which is applied to grid 3. The shunting of resistor 32 by condenser 44 serves to accelerate the deionization of the thyratron upon cessation of the excitation and to raise the potential of the cathode 26 at which tube becomes nonconducting.

The actual values selected for the various circuit elements are not especially critical, as it will merely be required that the values of the various circuit elements will be mutually related to each other to produce the cooperation and sequence of operations which have been described. Although all the circuit values are not necessarily critical, the following is an example of a set of values which has been found satisfactory in a system of this type.

Control tube 25-type 2050 thyratron Biasing voltage supply across leads I 6-ll300 volts D. C.

Secondary voltage of transformer 35-6.3 volts Resistor 18-750 k.

Resistor Iii-500 k.

Resistor 38-150 k.

Resistor 42-50 k.

Resistor 43-10 k.

Resistor 32-750 ohms Resistor 23-500 ohms Condenser 39-.5 microfarad Condenser 32--.004'7 microfarad Condenser 44-2 microfarads Condenser 45.0022 microfarad Condenser 41-0.1 microfarad It will be recognized that by my invention 1 have provided an effective grid biasing system for an amplifier, which is especially useful for class C type amplifiers in which relatively little current or power is drawn from the established biasing supply, and in which the current drawn by the grid circuit of the class C amplifier during excitation is furnished from power derived from the excitation source rather than from the source of bias supply.

My novel biasing system is not limited to the specific arrangement shown, which is described for illustrative purposes rather than by way of limitation, and it will be understood that modifications may be made to fit individual circumstances. While a thyratron tube has been described and illustrated as the control tube in the system, some other type of tube capable of performing a similar function might be used.

I claim:

1. A biasing system for application to a control grid of a vacuum tube which is subject to application of an excitation voltage, said system comprising a source of fixed grid bias voltage, a resistor in circuit between the source and the control grid, and a control tube connected across the source, said control tube being of the trigger type and maintained normally in a nonconducting condition, and means responsive to the application of an excitation signal on said control grid for rendering said control tube conductive, and means responsive to the excitation for developing a unidirectional voltage across the resistor, whereby the control tube forms a conductive path in series with the resistor and bypassing the bias voltage source.

2. A grid biasing system for application to a control grid of -a vacuum tube which is sub ject to application of an excitation voltage, said system comprising a source of fixed grid bias voltage, a resistor in circuit withthe control grid, and a control tube connected in circuit with the resistor, and forming a shunt across the bias source, said control-tube being a thyratron maintained normally in a non-conducting condition, and means responsive to the application of the excitation voltage on said control grid for rendering said thyratron conductive, and means responsive to the excitation voltage for developing a unidirectional voltage across the resistor, whereby the thyratron forms a conductive path in series with the resistor and bypassing the bias source, the bias on said control grid then being due to the unidirectional voltage through the resister.

3. A biasing system for a control grid of a vacuum tube which is subject to application of an excitation voltage, said system comprising a source of grid bias voltage, a resistor in the grid return lead, a gaseous control tube connected effectively across the bias source, said control tube having an anode, a cathode, a heating filament for the cathode, a control grid and a shield grid, a source of alternating power voltage for supplying voltage to the heating filament, and a connection from one side of the alternating power voltage source to the control grid of the control tube and a connection from the other side of the alternating voltage source to the shield grid, whereby the instantaneous voltages on the control grid and shield grid of the control tube are maintained 180 out of phase with each other, and a connection for applying voltage to the anode of the control tube, the voltages on the electrodes of the control tube being proportioned to maintain the control tube normally in a nonconducting condition, means responsive to excitation on the control grid of the vacuum tube for applying a unidirectional voltage to said cathode to make the control tube conductive, whereby the control tube forms a conductive path across the bias voltage source, and a unidirectional current flow in said resistor to furnish bias for the control grid of the vacuum tube.

4. A biasing system according to claim 3 in which the vacuum tube is a class C' amplifier tube adapted to have the excitation voltage make its control grid intermittently positive.

5. A biasing system for applying-negative bias to a control grid of a class C amplifier tube to which is applied a signal excitation voltage of sufficient amplitude to drive said grid positive during positive peaks of the excitation voltage,

said system comprising a source of grid bias voltage, a connection from said grid tpgotigh a t eesetive 2.91% 9.2 w i e h s v e ee rce e h fe e twe part es a e ihede e cathod h at r. a eee... e. shie d. grid, and an. ehq e, t e med? e s; ehe e bein @2 eee ee ress the r es (chase eeu ce w th the enqeeat the nee t ve ide and the eeth e t h nese he 9 the eet e. entree i alternatin newe e t ter 92 teme ing vc eee t2 beat alte nate newer eltese eeer e ha n e 1 9? ts ki nnected o e ih r rq cq tm d. a i it h r; Si t the h ietmn sh e d rid ered m d i t onnec ed m Meta Paral e a ranged. condens r and reei ter-te he. hyret e re ed he by-the n et e ehew vol ages en e h -a e ont ol grid and shield grid are maintained 180 out Of new; w t Gae eth r wel the b en o the eeit ti n e, main ain h ret n n9 sees ue ive. ut'w en he exc tat n ta e i Pres nt a ni r tiona rentowsh u h es st?! n e p ee th e e nd mak s he thy tetwn ath de sufim etw more si i to mele t hy 'e iqn co d ctive 9 that the ia iqltese is b -nes si-b the brra n and current throughthe resistor.

A, in e e em a cor n o m, 5 in w h a P l l n a ed. res stor and co dense re eeen ete betwe n he thr tron. ncode and the bias olt ge Q 6-,

A b asin y em eecerdiee to laim 5 i hi h a Parall arra ed T QF end c d n e b a l i rnish d by t ni-d r t onal between t e th ratr zi n de nd eqsiti e side or, the the v n a e sourc a d ositiv i e cenhcc d to he cathode of the class Cemnli er t be- 3. A biasing syste n eccording to. claim 5 in which a resistor'is connected between one side c h eate and h th ratr ntr l rid. and another resistor; is connected between the other side. of. the heater and the thyratron shield rid.

9, A biasing system according to claim 5 in which a perallel arranged resistor and condenser are connected between'the thyratron anode and the positive; side. of the bias voltage source, and e nes tor o nected e ee one Side of e heater and the thyratron control grid, and another resistor is connected between the other side c h lee e and he h e r h e d. and e q n nser is; n c d etw en he br n contigol grid and the positive side of the voltage source and, a condenser is connected between the thyratron shield grid and the positive side of the vq e leme- R- R A REFERENCES. CI

The following references are of record in the file of; this patent:

UN/ITEP rem IPA'IENTS.

N m e Neme D 2 ,129;088 George. Sept. 6,1938

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