US2891148A - Multivibrator with cathode stabilized by a capacitor - Google Patents

Description

June 16, 1959 A, C. LUTHER, JR

MULTIVIBRATOR WITH CATHODE STABILIZED BY A CAPACITOR Filed Sept '-f--y- ,www ,wa/7M E Vl @Novem/6 I lip/iff s/f u- Vz Maen/v@ Patented June .16, 1959 MULTIVIBRATOR WITH CATHODE STABILIZED BY A CAPACITOR Application September 1, 1953, Serial No. 377,925 7 Claims. (ci. 25o- 27) This invention relates to stabilized multivibrators, and particularly to monostable and astable multivibrators wherein a cathode is stabilized by means of a capacitor connected thereto.

A monostable multivibrator comprises a pair of intercoupled electron discharge device electrode structures so arranged as to generate a substantially square output wave having apredetermined duration following the application to the multivibrator of a trigger pulse. The duration of the output square wave is determined by a timing circuit which usually consists of a timing capacitor and a timing resistor in the circuit coupling the plate of the first multivibrator electrode structure to the grid of the second multivibrator electrode structure. These electrode structures may be located within the same or separate evacuated envelopes to form one or two vacuum tubes. The first tube is normally cut-ott and the second tube is normally conducting. The trigger pulse reverses the conditions of conduction as a result ot which the second tube remains conductive for a period of time determined by the time constant of the timing circuit.

An astable multivibrator includes two timing circuits, one between an output electrode of the irst tube and an input electrode of the second tube, and another timing circuit between an output electrode of the second tube and an input electrode of the rst tube. An astable multivibrator shifts back and forth between its two conductive states at times determined by the time constants of its two timing circuits. An astable multivibrator is free running but Vit may be synchronized by the application of a synchronizing pulse wave thereto.

In my copending application Serial No. 343,623, tiled March 20, 1953, now Patent No. 2,857,512, issued October 271, 1958 on a Monostable, Multivibrator, there is` described and claimed a cathode-stabilized multivibrator including a diode connected from a cathode of oneof the tubes to a point of reference potential. The

circuit of theA copending application provides an outputr the need for a diode is eliminated and a capacitor is used to provide stabilized operation when the monostable multivibrator is `receptive to a periodic triggerpulse wave. In the case of an estable multivibrator, the cathode of one of the Ntubesis stabilized by means of a capacitor for operation at a repetition rate `determined by the timing circuits on the multivibrator. It is therefore a general 2 tubes, by tolerances of the circuit elements, and by variations in the power supply voltages.

It is a further object to provide an improved stabilized Y monostable multivibrator. l

It is a still further object to provide an improved stabilized astable multivibrator. p

In one aspect, the invention comprises a4 multivibrator circuit responsive to trigger pulses, including two electron discharge devices or tubes coupled to vprovide a recipro-conductive circuit. At least one timing circuit is employed and is connected to operate between the plate of the first tube and the grid of the second tube. The cathode of the first tube is connected thru a resistorto the negative terminal of a source of unidirectional potential and thru a storage capacitor to a point of reference potential. The storage capacitor prevents `the 'cathode potential from falling when the tube is cut-oli.

Other objects and aspects of the invention will appear from a reading of the following detailed description taken together with the appended drawing, wherein:

Fig. l is a circuit diagram of a monostable multivibrator constructed according to the teachings of this invention;

Fig. 2V is a chart showing the voltage variations with time at various identified points in the circuit of Fig. l;

Fig. 3 is a circuit diagram of a modified form of monostable multivibrator employing only one source of platecathode potential;

Fig. 4 is a chart showing the voltage variations with time at various identified points in the circuit of Fig. 3;

Fig. 5 is a circuit diagram of an astable multivibrator including the stabilizing features included in the monoisconnected to` ground thru a grid resistor 27. Tube V2 stable multivibrators shown in Figs. l and 3; and

Fig. 6 isa chart showing voltage variations with time at various identied points in the circuit of Fig. 5.

Fig. 1 shows a monostable multivibrator circuit comprising first and second electron discharge devices or vacuum tubes V1 and V2. Tube V1 has a plate elec- 4trode 12, a grid electrode 22 and a cathode 15, while tube V2 has a plate electrode 29, agrid 25 and a cathode 28. A negative trigger input terminal 10 is coupled thru capacitor 11 to the plate 12 of the first multivibrator -tube V1. Plate 12 is connected thru a plate resistor 14 to a B+ terminal of a source of unidirectional potential (not shown). The negative terminal of the source is connected to ground. Cathode 15 of tube V1 is connected thru a cathode resistor 16 to the negative terminal 17 of a source of unidirectional current Ecc. The positive terminal of this source is connected to ground. Cathode 15 is also connectedr thru a storage capacitor 20 to ground.

IThe plate 12 of rst multivibrator tube V1 is connected A` thru a timing capacitor24 to the grid 25 of the second multivibrator tube V2. `The electrode structures of tubes V1 and V2 may be located in a common evacuated envelope or in separate evacuated envelopes. Grid 25 has a cathode 28 connected to the negative terminal 17,

via lead 31 to the grid 22 of iirst tube V1. The plate 29 M is returned to ground through a resistor 23.

The values of resistor 16 and storage capacitor 20 are` 4 tube V1 is cut-oil. and that tube V2 is conducting. A

object of this invention to provide an improved stabilized periodically operated multivibrator which is simpler and more Vreliable'than those previously known. `Ittisaiiother,object to provide an improved multivibrator which"`-is relatively unaiected by changes in the charge is Ideveloped and maintained on capacitor 20 so that cathode 15 is maintained at a `constant potential in the order'of` `|2 volts. `A negative trigger pulse applied to input Vterminal 1,0 passes thru capacitors 11` and 24 to grid 25 of tube V2. Tube V2 is lthereby cut-0E, causing a more positive (actually less negative) voltage on plate 29. Since plate 29 is directly connected to grid 22 of tube V1, tube V1 is rendered conductive and the potentialonV the Aplate 12of tube V1 drops. The dropV in potential is coupled thru capacitor 24 to grid 25 of-tube V2anfd `tube V2 is maintained cut-off following.` the application of the negative trigger pulsefor a period of time, determined by the time constant of the -RC timing circuit including resistor 27 and capacitor 24. The potential on grid 25 Vof tube V2 rises exponentially toward groundpotential as capacitor 24 charges up due to current flowing from B+, to ground thru resistor 14, capaci-v tor 24 and/resistor 27. When the potential on grid 25 reaches the cut-olir potential, tube yV2 starts conducting.

The'resulting potential drop on the plate 29 of tub'e V2 is Thevoltage waveforms at various identified points in the circuit are shown yin Fig. 2. It will be noted that the voltage-on the plate of tube V1 is at B+ potential when the Vtube is cut-off and the potential drops to about 158 volts when the tube is conductive. The cathode of tube V1 ismaintained at about 2 Volts above ground at all times by the action of the storage capacitor 2i). The plate of tube V2 is returned to ground thru resistor 23, and therefore the plate of tube V2 is at ground potential when tube V2 is cut-off and is at a lower potential when tube V2 is conductive. The grid of tube V1 is directly connected to the plate of tube V2, so that the voltage waveforms thereon are the same. When tube V2 is conducting, the potential on the grid of tube V2 is practically the same as the potential on the cathode, which potential'is -70 volts from source terminal 17. The negative'pulse applied from the plate of tube V1 to the grid of tubel V2 drives down the voltage on the grid of tube V2.V The voltage rises exponentially toward ground potential as timing capacitor 24 charges up by current ow'in the circuit including timing resistor 2'7.

The multivibratorfcircuit of Fig. 1 differs from prior art multivibrator circuits in the use of the storage capacitor 20 and the cathode resistor 16, and in the manner of cross-coupling the multivibrator tubes V1 and V2. Following the input trigger pulse, the multivibrator provides an output pulse having a width which does not vary from the predetermined value as the tubes age, when a new -tube is substituted in the circuit, or when the power Vsupply voltages vary. The reason why the construction provides the highly stabilized operation are set forth below.

`AsV is set forth on page 190 of vol. 19, Waveforms of the. M LT.: Radiation Laboratory Series, the duration of the output square wave of a monostable multivibrator is in accordance with the formula:

. Y Eu -Ei T 1s proportional to lnEuwEe where E1 is the initial voltage from which the timing waveform begins, Eu is the ultimate Voltage which the timing waveform would reach if it were permitted to doY so, and Ec is thercritical voltage or cut-off voltage of tube V2 at which the transition occurs to terminate the 22, but this voltagedifference is so small'compared -with output square wave. In the circuit of Fig. 1, Eur is fixed at zero volts for ground potential by reason of timing- T is proportional to lagi-l E c IfE, and E,3 are maintained at constant values despite changes 3in-the conductivity of the tubes with age and despite differencesl betweentubes which may be sub;-v

with the period between trigger pulses.

2E.,c that even large changes in the voltage difference have a negligible effect on the voltage E1. This advantageous condition results from the construction wherein a cathode resistor 16 is employed between cathode 15 and l*the negative terminal 17 of the source of voltage Ecc. Resistor 16 is effective when tube V1 is conducting.

Storage capacitor 20 is charged to +2 volts when tube V1 is conducting, and the time constant of the circuit including capacitor 20 and resistor 16 is long compared Storage capacitor 20 maintains cathode 15 at a constant potential of about 2 volts above ground and prevents the cathode of tube V1 from going negative when tube V1 is cut-off. Therefore, only a moderate voltage drop developed across resistor 23 by the action of tube V2 is needed to cut tube V1 off.

The value of Ec in the circuit of Fig. l is equal to Ecc plus the voltage difference between the cathode 28 and grid 25 of tube V2 when the tube just begins conducting. This voltage difference is small compared with E'ce and it usually varies only plus or minus one volt in different tubes of the same type. Therefore, tube changes have a negligible effect on the value of Ec. Furthermore, small changes in the voltage Ec will cause changes inthe time at which tube V2 shifts from the non-conductive condition to the conductive condition. The effect of these changes are minimized by making the timing. Waveform steep where it intersects the cutaoff Vvoltage Ec soi` that the changes have very little effect on the time at which the transition takes place. This is done by making E., a large percentage of Ei. In Fig. 1, E is about 50% of Ei when plate resistor 14 equals cathode resistor 16. The effects of tube changes on E, and Ec, and consequently on the duration of the output square wave can be further minimized by the use of tubes V1 and V2 which have a high amplification factor.

Changes in the negative bias voltage -Ecc from the terminal 17 of the source have very little effect on the duration of the output square wave because the changesin Ecc change both E1 and Ec in such a manner that the ratio Ei/Ec in the formula above remains practically constant. Changes in the B+ voltage have no effect on tube V2. Changes in the B+ voltage will normally be a small percentage of the total Voltage of the B+ voltage Y plus the E6c applied across the circuit of tube V1. The effect of cathode resistor 16 is to stabilize the current which ows thru tube V1 thereby maintaining at a relatively constant value the voltage change produced on the plate 12 when the tube goes from the cut off to the conducting condition.

The circuit of Fig. 1 imposes no limitations of any consequence on the sizes of plate resistor 1'4 and plate 4resistor 23. Therefore, these resistors may be made as small as is necessary to provide rthe desired speed of operation Iin the transitions between the two conditions, of the circuit.

Fig. 1 of the drawing includes the values of circuit components given by way of example to illustratea circuit operating in the desired stable manner with a trigger pulse wave having 900 ypulses per second. The negative going output pulses had a duration of 950 microseconds, leaving a periodV of micro-seconds between output pulses.

Fig. 3 shows a modified circuit differing fromthat shown in Fig.1 in that a source of4 negative potential is; 1 not required. VCorresponding circuit elements in Fig.,3 are given the same numeral they bear in Fig., 1.y Itwill scribed in connection with Fig. 1. `is;` even more stable in the` face of variationsinthe B+ `supply voltage.

, cancel out.

`trodes of the tubes in the circuit of Fig. 3.

essaies This intermediate voltage reference is ground.

The multivibrator circuit of Fig. 3 operates in the highly -stabilized manner exactly the same as that de- The circuit of Fig. 3

This is because all voltage levels are proportional to the one B+ voltage and changes tend to The circuit of Fig. 3 has one practical disadvantage compared with the circuit of Fig. 1 in that `the storage capacitor 20 in Fig. 3 normally carries a .charge equal to 70 volts, whereas the storage capacitor .20 in Fig. l carries a voltage charge of only 2 volts. .Therefore, the capacitor 20 in Fig. 3 must be capable of carrying a larger voltage charge.

Fig. 4 shows the voltage waveforms on identified elec- It will be noted that the waveforms of Fig. 4 are similar to those shown in Fig. 2, the difference being due to the use in the circuit of Fig. 3 of a single high voltage power source.

. Fig. 5 shows an astable or free running multivibrator circuit which is the same as the circuit shown in Fig. 1

except that the grid resistor 23 in Fig. 1 is replaced "by an inductor 35 and a capacitor 36 connected in shunt .to constitute a ringing circuit. A damping diode 37 is also` connected in shunt with inductor 35 and capacitor 'determined by capacitor 24 and resistor 27, tube V2 is again rendered conductive, and a surge of plate current passes` from ground thru the ringing circuit and thru ltube V2 to the source of -70 volts. This surge of plate lcurrent starts an oscillation in the ringing circuit having aperiod determined by the values of inductor 35 and capacitor 36. The initial half-cycle of the resulting oscillationis a negative going wave as shown in Fig. 6. The following positive going half-cycle of the oscillation is prevented by conduction thru the damping diode 37. Therefore, after a negative half-cycle of oscillation in the ringing circuit, the potential on the grid 22 of tube V1 and on the plate 29 of tube V2 returns to ground potential. During this negative half-cycle, tube V1 is cut-oft' and tube V2 is conductive. Capacitor 20 maintains the cathode of tube V1 at a constant potential of about 2 volts above ground. At the end of the halfcycle of oscillation in the ringing circuit, the grid 22 of tube V1 returns to ground potential and tube V1 starts to conduct. In the meantime, when tube V1 was cutol, timing capacitor 24 was recharged so that after tube V1 becomes conductive, a new timing cycle is initiated. It is thus apparent that the ringing circuit 35, 36 determines the period of time that tube V1 is cut-oit and the timing circuit 24, Z7 determines the time during which tube V2 is cut-olf. A trigger pulse wave could be applied thru capacitor 24 to the grid 25 of tube V2 to synchronize the operation of the free running multivibrator with the trigger pulse wave.

Fig. 5 includes the values of circuit components which were found to provide the desired stabilized operation with a period of 31.75 micro-seconds and with positive pulses on the plate of tube V1 having a duration of 2.5 micro-seconds. For the values of circuit elements shown, it was found that tubes could age to the point where they provided only one-half their normal emission of electrons and yet the frequency varied only plus or minus 2% from the value for which the circuit was designed.

All of the circuits shown in Figs. 1, 3 and 5 are relatively unaifected in their operation by changes in power supply voltage, changes in the vacuum tubes and changes' in or manufacturing tolerances of most of the circuit elements. The stabilized operation results from the use of a relatively large degenerative cathode resistor 16 in the circuit of tube V1, the use of storage capacitor 20 connected to the cathode 15 of tube V1 to maintain the potential thereon substantially constant, and the manner of coupling the plate of tube V2 to the r grid of tube V1 so that a moderate voltage transition on the plate of tube V2 causes tube VZ to render tube V1 cut-off.

What is claimed is: 1. A stabilized multivibrator comprising first and second electron current devices each having an emitting .,electrode, a control electrode, and a collecting electrode, a timing capacitor coupled between said collecting electrode of said rst device and said control electrode of said second device, a direct connection between said collecting electrode of said second device and said control electrode of said first device, a point of reference potential, means -for applying a unidirectional potential that is positive with respect to said point of reference potential solely to said collecting electrode of said rst device, means for applying a synchronizing pulse wave to said multivibrator `to control `the frequency of operation thereof, a irst resistor connected at one end to said emitting electrode of said rst device, means for applying a unidirectional potential that is negative with respect to said point of reference potential to the other end of said lirst resistor and to said emitting electrode of said second device, a second resistor connected between said control electrode of said `first device and said point of reference potential, and a storage capacitor coupled between said emitting electrode `of said first device and said point of reference potential,

electrode, a control electrode, and a collecting electrode,

a timing capacitor coupled between said4 collecting electrode of said first device and said control electrode of said second device, a direct connection between said `collecting electrode of said second device and said control electrode of said first device, a first resistor connected at one end to said collecting electrode of said iirst device, a point of reference potential, means for applying a unidirectional potential that is positive with respect to said point of reference potential solely to the other end of said rst resistor, means for applying a synchronizing pulse wave to said multivibrator to control the frequency of operation thereof, a second resistor connected at one end to said emitting electrode of said first device, means for applying a unidirectional potential that is negative with respect to said point of reference potential to the other end of said second resistor and to said emitting electrode of said second device, an impedance element coupled between said control electrode of said first device and said point of reference potential, and a storage capacitor coupled between said emitting electrode of said first device and said point of reference potential, the time constant of said storage capacitor and said second resistor being at least fifteen times the period of said synchronizing pulse wave.

3. A stabilized multivibrator comprising first and second electron current devices each having an emitting electrode, a control electrode, and a collecting electrode, a timing capacitor coupled between said collecting electrode of said rst device and said control electrode of said second device, a direct connection between said collecting electrode of said second device and said control electrode of said first device, a first resistor connectedat one end -to-said-'col'lecting electrode ofsaid rst device, a point of referencerpotential, meansfor -applying aunidirectional potential that Vis positive with respect to saidpoint of jreference' potential solely to the other end ofsaid first resistor, means `for applying arsynchronizing pulsev wave to' said -multivibrator lto control the frequencyeof operation thereof, a second resistorconnected at'oneend to saidernitting electrode of said first fdevice, meansA for applying a unidirectional"potential that is negative `with respect to; said point of `referencepotential to the other'` y'end of saidsecond resistorfandto vsaid emittingrelectrode/ of said; secondrdevice, an impedancefelementf coupled between saidfcontrol electrode ofV said rst device vrand said point of reference potential, a third resistorcoupled-be tween said control electrode of said'4 second device and 'said point `of reference potentiaL-and a storage capacitor coupled between said emitting electrode of said rst device `and said point lof reference potential, the time constant Aosaid storage 'capacitor 'and said second resistor being 1 at least fifteen times the period'of said synchronizing pulse lwave. Y

4.` A multivibrator Yas defined in claim 3, wherein said impedance element lis a resistor.

` 5. yA multivirbatoras=dened in claim 3, wherein said impedance element comprises a rectifier element, acapacitor, and an inductor connected inf-parallel with each l other.

6. A fstabilized multivibrator ycomprising:rst'andsecond electron :current Ldevices each`r having at yleast an emittingelectrode a' controlelectrode, and a collecting- 'electrode7 Aa timingcapacitor coupled between said collecting electrodeof said rst deviceandlsaid control velectrode -ofl said: second device, a `directconnection between said col-lectingelectrode of '-said :second Vdevice ,and` said conrtrol *electrode of `said'first device, a firstresistorsconnected l atA one end to 1 said collecting electrode of said: first device, 'aft pointof reference potential, means for applying a uni- -directional-kpotentialthat ispositive with' respect to said f point ofrreference potentialsolely totheother Vend of said first resistor, means for applying asynchronizing pulse wavev to said multivibrator to -control the frequency of operation fthereof, a lsecond. resistor. coupled "between said ernittingelectrode of Isaidftirstadevice and -said .point of reference-potential, anvimpedance element coupled between said` control electrodeof saidrstideviceandtsaid '.point oflreferencepotential, adirectwconnection between said emitting electrode" Iof.-said"secondl device e and said `.pointfof reference `potential, `and a `ston'tge capacitor V coupled betweensa'id emitting electrodefof said rstdev'ice v`and/said A point of f 4reference fpotential, they time constant of -said storagecapacitor and said-second resistorllneingv at -least fifteen ltimesE the periodoff-'said synchronizing -pulse wave.

7. A stabilized multivibrator` comprising iirstand .sec-

#ond electron current devices 1 each `'having -at -leastll'an --emittingfelectrode, acontrol electrode,'iand afcoll'ecting electrode, ay timingcapacitor -A coupled between said collecting electrode of said rst device and said controlelectrode ofsaid second device7 a--directconnection between 'saidcollecting electrode ofv saidsecond-deviceandisaid Lcontrolelectrode of said-first device, a 'rstresistorfc'on- `nected'atone` end to saidy collecting electrodeiof-saidffirst device, -a point of reference potential,.means'for applying a unidirectional potentialfthat is. positive with respectl to said" point "ofY 'reference potential-solelyto the iotherten'd of-said .rst resistor, means for-applying a-synchronizing pulse wave to Asaid multivibratorrftocontrolfthefrequency lof-ioperation thereof, a second"` resistor coupledv between said emitting electrodeof saidrst 'device and'isaidlpoint 4vofreferencev potential, al direct 'connection tbetweenzsid emitting electrode of saidsecond .device'andtsaidtpointof reference potential; a ithird. resistory coupled between :said

:"control electrodeof.said'firstfdeviceiand said pointofir'efer- `Tence potential, Vaffourth` resistor coupled'betweenasaidifcon- '-trol.. electrode fof saidlsecond devicerandisaidiemitting Aelec- :trodeotsaid rstidevice, a fifth,resistor coupledrbetween `.said lcollecting electrodeA ofasaid .second device fand-,said tother end of saidrst resistolgland astorage capacitorc'ou- :pled between said emitting electrode. of ,-,saidrst device and zsaidtpoint of'. reference potential, the time. constantfo'f -said `storage,capacitor and said -second resistorffbeingiat `,least fifteen `times Vthe period of said-y synchronizingzpuls

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