US2318061A - Automatic bias circuits - Google Patents

Description

May 4, 1943. H. J. DAILEY AUTOMATIC BIAS CIRCUITS Filed May 29, 1941 WW .n m D m 0% 3 U 4 m H 2 a m J A a a m M Patented May 4, 1943 Hampton Jennings Dailey, Bloomfield, N. J., as-

signor to Westinghouse Electric & Manufac turing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application May 29, 1941, Serial No. 395,803

1 Claim.

This invention relates to protective systems and methods and, more particularly, to thermionic valve circuits utilized in the generation or transmission of energy wherein in the absence of certain operating requirements damage may result to the valves.

The primary object of this invention is to prevent in the absence of certain operating conditions the deleterious effect which may be produced upon electron discharge devices and to this end means are provided for applying automatically bias potentials which establish static conditions in accordance with the characteristics specified for such devices.

A particular feature of this invention is that to protect electron discharge devices a safety bias potential is at all times applied to the circuit, although effectively it is counteracted by another potential which is derived from the transmission or signal currents present in the system. Consequently, if there are no transmission or signal currents in the system due to any cause whatsoever, the counteracting voltage will .be zero and the safety bias voltage becomes automatically effective in the circuit.

Another feature of this invention is that the circuit arrangement whereby the bias voltage is applied and then counteracted requires no switching mechanisms or relays of any vkind and is not dependent on mechanical. safety devices heretofore used., Furthermore, the balance'of.

potentials between the safety .bias and the derived counterbias is such that partial failure of the latter will allow a proportional effective application of the former. I-Ience,.the transitory stage between one and the other condition is radual and smooth,'permitting a balanced condition of operation of the electron tubes.

Other features and advantages will be'apparent' from the following description of the invention, pointed out in particularly by the appended claim, and taken in connection with the accompanying drawing, in which:

Figure 1 illustrates the protective circuit appacitor 2. One; half of the inductance between center tap and one, terminal is. connectedby means of conductor 3. to the grid 4 of vacuum tube 5, whereas the other half through conductor 6 to the grid 1 of vacuum tube 8. The directcurrent return path of the input circuit is completed from the center tap of the inductance l to the grounded cathodes 9 and I0 through a series network, including resistances II and I2, and a fixed voltage source represented by the battery I3. A shunt path around the resistors and the battery aforementioned may be effected by a switch M which when closed, shunts the contacts I 5 and I6 and thereby returns the mid tap connection of the inductance l directly to ground. Under normal operation, as will be described later, the switch [4 is maintained open so that the grid return path is effected through the resistors II and I2.

The output circuit of the oscillator includes the output transformer 20, of which the primary winding 2| is tuned by the condenser 22 and is divided by a center tap forming the anode return circuit through the anode supply source shown here by the battery-23. The anode 24 of tube 5 and the anode 25 of tube 8 are connected, respectively, to the free terminals of the primary winding 2|. The output voltage of the oscillator may be taken off from the secondary winding .26 of the output transformer 20.

The above circuit follows the established practice of push-pull oscillators except for the addition of the bias source l3 and the resistor 12 which are part of the control circuit about to be described. The output transformer in the embodiment illustrated in Fig. 1 includes a tertiary winding 30 across which a certain output voltage will be developed when the oscillator is in normal operation. The winding 30 is connected in series with a rectifier, shown here as a twoelement vacuum tube 3|, to the resistor l2 which is shunted by a capacity 32.

In order to simplify the illustration, the filament heater connections of the vacuum tubes have been omitted in both figures. It is well known to any one skilled in the art that the required heater current maybe obtained in any desired manner either frombatteries or from alternating current power supplies. Similarly, certain voltage sources are represented by batteries merely for the convenience of illustration and may be substituted by other suitable sources opcrating from a power line circuit.

Before describing the operation of the protective circuit of Fig. 1, a descriptionof Fig. 2 may be in order, since essentially the operation in both circuits is similar, except for certain circuit mod plifiers of this type are generally known as non linear amplifiers and may find application -'in transmitters or in industrial systems where linearity is not required for the intended operation of the amplifier. The sourci o'r supplying the input voltage to the input circuit of the tube is merely illustrated by means of a block diagram and is marked as the exciter. This may be-a preceding amplifier or an oscillator, or any suitable source of voltage which the amplifier is called upon to amplify; Identical elements are designateol with-similar reference characters inboth 'Th input circuit-ofthe tube 35 includes the grid 35,-a grid resistor l l and a gridcurrent path between cathodeand grid comprising in series the potentiometer 38 and the bias-source represerited by the battery 13. One terminal of latter and also-tlie-cathodeSi! of the tube grounded. A low impedance path for the mission currents supplied to the amplifier tained through the 'by-pass condenser it; between the cathode 39'and the "ground potential side of grid resistor H. The exciter may be capacitivelycoupled to the input circuit through the coupling condenser M.

The output circuit of the tube 35 includes the anode 42, the primary winding as of the output transformer 45 and-the anode potential source shownby the battery 23. -The primary win-cling E3 'is' also tuned to the transmissionsignal-irequency bymeansof the -condensert. The output ofthe amplifier may be obtained for a utilization' circuit' not shown here from the secondary winding 4'! of the transformer :35:

damage of the tubes. In prior systems relays operating on grid current fiow or on excessive anode current were utilized as protective means whereby upon condition of no excitation the plate voltage source was disconnected from the vacuum tubes. In the system in accordance h this invention, no mechanical switching devices are employed instead a fixed bias source is maintained in the input circuit ready at all times to supply a bias voltage of sufiicient magnitude to prevent excessive current drain of the tubes should grid excitation cease for any reason at all. The battery E3 of Fig. 1 or of Fig. 2 is the source of the protective bias voltage. Under normal operation, there is no need for protective bias and as a matter of fact normal operation could not be obtained in the presence of such bias voltage. To eliminate the effect of the bias voltage under normal operation, a portion of the output circuit oi the oscillator or the amplifier is utilized to furnish a voltage of such magnitudeand in suchpolarity which will effectively counteract the fixed bias voltage, so that as far as theop'eration of the system is concerned, there is efiectively no initial bias voltage present. Assuming static conditions in'the operation of the oscillator of Fig. 1, the'oscillato'r may be started by short circuiting' the terminals l5 and [6 with the switch l4, whereby the grids 4 and 1 will be at cathode potential. The change in plate current so obtained will start"theoscillato r into operation so that in the output circuit there appears a certain oscillatory energy which, in the 'form of a voltage at the frequency of oscillation The protective circuit herein shovm-is-capaci-" tively coupled to theoutpuvofthe amplifier by means of thecondehser 43 which is connected to the cathode 49 of therectifier tube Eiland to the anode 62 of the-amplifier tube 351;The load resistance for the rectifier tubebii isithepotentiometer 88 which cohnects-to' 'tlie cathode through conductor 5! and-to'the anodeEZ of the rectifier 59-, through-conductor 53. The potentiometer 33 is alsdshunted'bythe capacitorfi i.

The variable arm of the potentiometer 38 connects to thegrid circuit resistor 31.

Considering the'op'eration of the protective circuit for either oscillator or the amplifier it should be borne in mind that grid current flowing through the resistors I l is the only means' whereby a bias is developed on the grid'electrodes of the tubes. Under normal operation, this bias is sufiicient to prevent excessive current drain whichat zero bias condition would seriously dam age the'vacuum tubes."-'I'l1e value of'resistors i i is-so chosen that with'pr'oper" excitation of the determined by the tuned 'circuitsappear s' also across'the terminals of the tertiary winding3ll causing a current flowin the circuit'including the rectifier 3|, resistor I2 and condenser 32. The rectifier 3| is so connected in th'eci'rcuit that the polarity of the voltage appearing across the resistor l2 is'in opposition to the voltagesup plied by the battery l3. When'the' voltage de! rived from the protective "circuit; that 'is 'iroin the rectifier 3|, across the resistor 12 i's'o'f the same magnitude as the voltage of the battery 13,

the two voltages will cancel and effectively there is zero bias onthe grids' l and'l. ""At this some the grid 'current'fiowing'inthe input circuit 0f the oscillator between the grids 4" 'and'1;. through grid'circuit, the grid currentwill' be of 'suf fieient magnitude to develop therequired bias voltage across these resistors. In the oscillator. the ex citation of the input'circuit is obtained by feed back from the output'circuit'through'the inherent grid-anode capacitance of the vacuum tubes. In the amplifier the excitation is provided from an outside source. 'Whatever'may be the source of gridexcitation; it is necessarythat' it should be maintained because failure thereof results in the resistor .H Will bethes0le meaIiS"01"piovid ing a bias forthe'grids depending upon thwalue of the resistor and the excitation voltage on the grid electrodes. 7

The value of the resistor -l2 is so "chosei'i that when the oscillator isoperating' at its rated out put the rectified voltage of the tertiary winding 30 shall maintain therethrougha current of such magnitude which will produce a-voltagesubstantially equal in magnitude to thevoltage of the battery I3; 4 A 7. Similarly, in Fig. 2; the amplifier'when' properly excited will have an outp'ut'voltage of 'sufiic'ierit magnitude which, when rectifiedby the diode?!) due to the capacitive couplingthrough the con denser 48, causes a unidirectional current fiow in the potentiometer 38 of suflicient -niagni tud;e' 'to' cancel out the negative voltage of the "battery I3 at the point wherethe arm of the potentiometer-- is set. It is to be noted here that in starting the amplifier in' operation, there is no heed as it is furnished from a-sepamte s urte. ssum ing staue'conaiuon "or-tire amplifier, tlie vaiiie of the negative voltage supplied by the battery I3 is so chosen that it will not prevent operation of the tube as an amplifier, although it will prevent excessive plate current which may damage the tube. Consequently, when the exciter supplies voltage to the input circuit, the amplifier will operate at a lower value of amplification because of the bias voltage present. Whatever the amplification may be, a certain amount of voltage will be obtained across the potentiometer 38 which in proportion counteracts the bias of the battery I3. This also increases the amplification so that the voltage across the potentiometer 38 is progressively increased as amplification of the tube increases, unitl a point is reached when this voltage will be equal to the battery voltage or may even be higher. The sliding arm of the potentiometer, however, permits a regulation so that such portion of the potentiometer be selected where the two voltages cancel each other and there is zero effective bias on the grid 36. This is the normal operating condition at which time the operating bias is derived solely from grid current flow through the resistor l I.

It is evident from the above that if for any reason at all oscillation or amplification ceases, the counter-voltage becomes automatically zero, since it depends upon the operation of the system and the protective bias for the grid circuits is automatically applied from the battery I3.

I claim as my invention:

A protective system for oscillation circuits ineluding, in combination, a pair of electron discharge devices containing anode, cathode and at least one grid electrode, respectively, an input circuit including said grid and cathode electrodes, an output circuit including said anode and cathode electrodes, both said circuits being tunable to the frequency of oscillations, a resistance in said input circuit across which under normal operation a bias voltage is being developed due to grid current flow, a second resistance and a source of potential in series with said first resistance, said resistances and said source of potential forming a current path between grid electrodes and cathodes in said input circuit, a secondary circuit coupled to said output circuit including a rectifier and said second resistance, a condenser in parallel with said resistance whereby said rectifier produces a unidirectional voltage across said resistance, the elements of said secondary circuit being so proportioned that the voltage produced by said rectifier across said second resistance at normal oscillatory energy in said output circuit is substantially equal in magnitude to the Voltage of said source, said rectifier being so connected in said secondary circuit that the voltage developed across said second resistance is of a polarity opposite to the polarity of said source in said input circuit whereby under normal operation the effect'of said two voltages is substantially canceled and upon failure of operation in the absence of oscillatory energy the voltage of said source is the effective control potential in said.input circuit, and means for initiating oscillatory current fiow comprising switching means for momentarily short circuiting said gridcathode path formed by said resistances and said source of potential.

HAMPTON JENNINGS DAILEY.

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