US2858427A - Stabilized cathode-coupled multivibrator - Google Patents

Description

Oct. 28, 1958 A. c. LUTHER, JR I 2,858,427

V STABILIZED CATHQDE-COUPLED MULTIVIBRATOR Filed April 29, 1955 2 Sheets-Sheet 1 v1 Girl/00E INVENTOR.

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. ATTORNEY STABILIZED CATHODE-COUPLED MULTIVIBRATOR Arch C. Luther, Jr., Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 29, 1953, Serial No. 351,951 2 Claims. (Cl. 250-27) This invention relates to cathode-coupled multivibrators, and more particularly, to monostable multivibrators of the cathode-coupled type which operate in response to a recurring trigger pulse wave to generate output pulses having a predetermined width.

A monostable multivibrator is one which generates an output pulse or square Wave having a predetermined width or duration immediately following the application to the multivibrator of a trigger pulse. A multivibrator usually comprises two cross-coupled vacuum tubes and the duration of the output pulse is usually determined by a timing circuit coupling the plate of one tube to the grid of the other tube.

The duration of the output square wave is normally influenced by the characteristics and conditions of the tubes employed and by the values of the power supply voltages applied to the circuit. In the electronic arts it is often desirable to employ a monostable multivibrator which continues to provide an output square wave of a given predetermined duration despite changes in the tubes and power supply voltages. A general object of this invention is to provide an improved monostable multivibrator which operates in response to a recurring trigger pulse to generate an output pulse wave the width or duration of which is substantially unaffected by changes in the tubes and in the power supply voltages.

Another object is to provide an improved monostable multivibrator which will generate output pulses of substantially the same predetermined width and can be manufactured in quantity using circuit components having relatively broad tolerances as to the exact values thereof. Uniformity and stability of operation is thus achieved while effecting economies in manufacture.

it is a further object to provide an improved multivibrator, for operation in response to a recurring trigger wave, which combines circuit simplicity with stability of operation.

In one aspect, the invention comprises a first and a second vacuum tube and a timing circuit coupled between the plate of the first tube and the grid of the second tube. The cathode of the second tube is coupled to the cathode of the first tube through a capacitor of large enough value so that, with a given minimum trigger pulse repetition rate, the charge thereon remains substantially constant. The cathode of the first tube is returned to a point of reference potential through a large degenerative cathode resistor and the grid of the first tube is biased an appropriate positive amount relative to the point of reference potential. The large degenerative cathode resistor in the circuit of the first tube stabilizes the amplitude of the voltage step generated on the plate of that tube and applied to the timing circuit. The large capacitor coupling the cathodes of the two tubes permits the use of the degenerative cathode resistor while providing means whereby moderate voltage changes on the cathode of the second tube are coupled undiminished to the cathode of the first tube to turn this last tube on and off.

Other objects, advantages and aspects of the invention willbe apparent to those skilled in the art from the following more detailed description taken together with the appended drawing, wherein:

Fig. 1 is a circuit diagram of a multivibrator constructed according to the teachings of this invention;

Fig. 2 is a chart of voltage waveforms appearing at various designated points in the circuit of Fig. 1;

Fig. 3 is a chart of voltage waveforms, appearing at one point in the circuit of Fig. 1, which will be used in explaining a feature of the operation of the circuit;

Fig. 4 is a circuit diagram of another embodiment of the invention; and

Fig. 5 is a chart of voltage waveforms appearing at various identified points in the circuit of Fig. 4.

Referring to the drawings in more detail, Fig. 1 shows a cathode-coupled monostable multivibrator having a first vacuum tube V1 and a second vacuum tube V2. The word tube as used herein is intended to designate an electrode structure whether in a separate envelope or in a common envelope with another electrode structure. Tube V1 includes a grid 10 coupled through a coupling capacitor 11 to a trigger input terminal 12. Grid 10 is also connected to the junction between grid-biasing voltage-divider resistors sistor 13 is connected ,to the B+ terminal of a source of unidirectional potential (not shown). The other end of resistor 14 is connected to the negative terminal of the source which is at ground or reference potential. Cathode 15 of tube V1 is connected through a large degenerative cathode resistor 16 to ground. Plate 17 is connected through plate resistor 18 to the B+ terminal.

Plate 17 of tube V1 is also coupled through a timing capacitor 19 to the grid 20 of second vacuum tube V2. A timing resistor 21 is connected from grid 20 to the B+ terminal. V Capacitor 19 and resistor 21 comprise an RC timing circuit which determines the width or duration of the output pulse of the multivibrator. The cathode 23 of tube V2 is connected through a resistor 24 to ground and through a large coupling capacitor 25 to the cathode 15 of the tube V1. The plate 26 of tube V2 is connected through a plate resistor 27 to the B+ terminal, and also directly to an output terminal 30.

Fig. 2 shows voltage waveforms at various identified points (the electrodes of tubes V1 and V2) in the circuit of Fig. 1.

In the operation of the circuit of Fig. 1, initially tube V1 is cut ofi and tube V2 is conductive. When a positive trigger pulse is applied through capacitor 11 to the grid 10 of tube V1, the tube is rendered conductive. The resulting drop in potential on the plate 17 is coupled through capacitor 19 to the grid 20 of tube V2 causing tube V2 to be cut oif. The drop in potential of the cathode 23 due to the cutting off of current flow through cathode resistor 24 is coupled through large capacitor 25 to the cathode 15 of tube V1. Tube V1 is thus maintained in a conductive condition until timing capacitor 19 is charged by current flow in the path including timing resistor 21. When the potential on grid 20 of tube V2 exceeds the cut-off potential, tube V2 starts conducting, the cathode 23 of tube V2 rises in potential and the rise is coupled through capacitor 25 to cathode 15 of tube V1. Tube V1 is thus cut 01f and it remains cut ofi until the following positive pulse of the periodic trigger wave is applied to grid 10. It will be understood that the multivibrator may, if desired, be triggered by the application of a negative pulse to terminal 12'.

In the construction of the multivibrator of this invention, the value of capacitor 25 is chosen to be large 13 and 14. The other end of re-,

pulses. The time constant of the circuit including capacitor 25 should be at least twenty times the period of the trigger pulse wave. When so constructed and operated, the multivibrator is extremely stable in operation.

The circuit of Fig. 1 will operate as an astable multivibrator when no trigger pulses are applied to input terminal 12. In the absence of trigger pulses, tube V2 is cut off for a period of time determined by the time constant of capacitor 19 and resistor 21, and tube V1 is cut off for a period of time determined by the time constant of the circuit including capacitor 25. Tube V1 is cut off for a much longer time than it is when the circuit is operated according to the teachings of this invention with the application of a periodic trigger wave. The circuit has the desired high degree of stability only when operated by a recurring trigger wave.

For an explanation of why the circuit of Fig. 1 is highly stable in the generation of a positive pulse of predetermined width on the plate of tube V2, reference will now be made to Fig. 2 showing the voltage waveforms on the electrodes of tubes V1 and V2. When a positive trigger pulse is applied at time t to the grid of non-conducting tube V1, tube V1 is rendered partially conductive, and the potential on the plate of tube V1 decreases. This decrease in potential is coupled to the grid of tube V2 causing a reduction in the current through tube V2 and a reduction in the potential on the cathode of tube V2. The reduction in potential on the cathode of tube V2 is coupled through large capacitor 25 to the cathode of tube V1 tending to make tube V1 conduct to a greater extent. This action continues in rapid regeneratvie manner until tube V1 is conducting fully and tube V2 is cut off. In the transition of tube V1 from a non-conducting state to a fully conducting state, a voltage drop E commonly called a voltage step, is generated on the plate of tube V1 and is applied to the RC timing circuit 19, 21. The amplitude of the voltage step E must remain constant if the timing circuit is to be operative to render tube V2 conductive at a constant predetermined time t following the step.

The amplitude of the voltage step E is made constant despite changes in the tubes, changes in the power supply voltages and variations in the values of circuit elements from the specified values, in part by reason of the use of a large cathode resistor 16in the circuit of tube V1. Cathode resistor 16 has a value which is such that the voltage developed thereacross when tube V1 is conducting is much larger than that required to self-bias the tube. An appropriate positive fixed bias is applied to the grid of tube V1 by means of a voltage divider 13, 14. The large resister 16 has a degenerative effect on the amplitude of the potential on the plate of tube V1 when the tube is conducting which stabilizes the amplitude of the step wave applied to the timing circuit.

The degenerative stabilizing cathode resistor 16 does not interfere with the process of switching tube V1 on and oif by reason of the use of a large capacitor 25 through which the switching signal is applied from the cathode of tube V2 to the cathode of tube V1. The capacitor 25 has a value such that the charge thereon cannot change appreciably in the period between trigger pulses. When tube V2 is cut off, the drop in potential on the cathode of tube V2 is coupled undiminished to the cathode of tube V1 to render and maintain tube V1 conductive. Also, when tube V2 is rendered conductive, the rise in potential on the cathode of tube V2 is coupled undiminished to the cathode of tube V1 to render and maintain tube V1 cut off. It will be noted from Fig. 2, that the waveforms on the two cathodes are the same except for a constant difference equal to the charge on capacitor 25.

The circuit of tube V1 is designed so that when tube V1 is conducting, the current flow through cathode resistor 16 makes the potential of the cathode of tube V1 slightly above the potential on the grid of tube V1 as determined by voltage divider 13, 14. When tube V1 is non-conductive, the cathode of tube V1 is maintained at a higher value due to the increased potential on the cathode of tube V2 being coupled through capacitor 25 to the cathode of tube V1.

The average potential E on the cathode 15 of tube V1 is represented by a dashed line in Fig. 2. Since tube V1 conducts only during the fractional part t of the repetition period T, and since the average potential E is developed across resistor 16, the current flowing through resistor 16 when tube V1 is conducting is given by the formula.

As can be seen by reference to Fig. 2, the voltage E equals the voltage on the grid of tube V1 plus the voltage difference between the grid and cathode of tube V1 plus the average value of the voltage coupled from the cathode of tube V2. Since the voltage difference between the grid 10 and the cathode 15 when tube V1 is conducting may be in the order of 0.8 volt, and since this voltage difference (which varies as between different tubes and with aging of a given tube) is such a small percentage of H it is apparent that variations in tube V1 have very little effect on the current flow through plate resistor 18, tube V1 and cathode resistor 16, when the tube is conductive. Therefore, the amplitude E of the negative pulse developed on the plate 17 of tube V1 and applied to the grid 20 of tube V2 is a stable value relatively unaffected by changes in tube V1.

The manner in which the circuit resists changes in the width of the output pulse due to changes in the tube V2 will be explained by reference to the waveforms on the grid of tube V2 shown in Fig. 3. If the waveform is as shown at a in Fig. 3, and then the electron emission of the cathode of tube V2 decreases, as with age, the voltage (Fig. 2) on the cathode 23 of tube V2 when V2 is conducting will become less than it was, as will the potential on the grid of tube V2 (wave b of Fig. 3). The potential on the cathode 15 of tube V1 when V1 is nonconducting will also become less than it was due to the coupling through capacitor 25. The potential on the cathode of tube V1 when V1 is conductive, however, remains the same, and therefore the average voltage E on the cathode of tube V1 is reduced. This results in a reduction in the amount of current flowing through tube V1 because, according to the formula given above, the current flowing through the tube when it is conductive is proportional to E Since the current through tube V1 is reduced, the voltage drop E developed across plate resistor 18 and applied to grid 20 of tube V2 (wave b of Fig. 3) is reduced. The drop E is reduced compared with the drop E (wave a of Fig. 3) by an amount which compensates for the reduction r in the voltage on the grid of tube V2 so that the timing waveform starts from the same voltage as previously and rises exponentially to the cut-off potential in the same time period t as existed prior to the reduction in electron emission from cathode 23. By the proper choice of a fixed bias on the grid 10 of tube V1 as determined by the values of voltage divider resistors 13 and 14, the above-described compensation can be made to be percent effective over a very broad range of variation in the emission of tube V2.

The circuit of Fig. 1 is further characterized in that the amplitude of the voltage pulse E on the plate 17 of tube V1 automatically adjusts itself to partially compensate for any cause of change in the width of the positive output pulse on the plate 26 of tube V2. As a consequence, resistors and capacitors used in the circuit can have values which depart considerably from the specified values without having much effect on the width of the output pulse. Therefore, inexpensive components having relatively large tolerances can be employed and yet the width of the output pulse will be maintained within relatively close tolerances.

As an example of how the circuit partially compensates for any cause of change in the width 1 of the output pulse, assume that timing resistor 21 is increased slightly in value. This would increase the time constant of the timing circuit and tend to increase the width t. Tube V1 would then conduct for a longer period t and E would drop slightly. According to the formula;

the current in tube V1 when the tube is conducting would be reduced and this would reduce the voltage step E on the plate of tube V1. The decrease in the step E tends to shorten the timing waveform and reduce the pulse width t to partially compensate for the effect of increasing timing resistor 21.

Further stability results from the fact that the slope of the waveform on the grid of tube V2 as capacitor 19 discharges through resistor 21 is very steep at the point where it intersects the cut-oif voltage of the tube (time 1 in Fig. 2). This is accomplished by providing for a large voltage drop E on the plate of the tube V1 when tube V1 is rendered conductive at time t The large voltage drop E is coupled through capacitor 19 to the grid of tube V2 to cut off tube V2. The potential on the grid of tube V2 rises exponentially from a highly negative value toward B+ by reason of timing resistor 21 being returned to the B+ terminal. Since the voltage on the grid of tube V2 approaches the cut-E voltage at a steep angle, variations in tube characteristics and in the values of circuit components have little effect on the period of time t that tube V2 remains cut 01f.

The circuit of Fig. 1 provides an output pulse which is unaffected by changes in the B+ power supply voltage. As is set forth on page 190 of volume 19, Waveforms, of the M. I. T. Radiation Laboratory Series, the duration of the output square wave of a monostable multivibrator is in accordance with the formula;

where E; is the initial voltage from which the timing waveform begins, E is the ultimate voltage which the timing waveform would reach if it were permitted to do so, and E is the critical voltage or cut-off voltage of tube V2 at which the transition occurs to terminate the output square wave. It can be shown that in the circuit of Fig. 1, the voltages E E and B are all proportional to the 13+ voltage. Therefore, changes in the B+ voltage have little efiect on output pulse width. It was found that the B+ voltage of the circuit of Fig. 1 could be varied between 190 aud 490 volts without changing the output pulse width more than one percent.

Solely by way of example, it was found that a circuit constructed with the component values shown in the drawing and with a periodic trigger having a pulse repetition rate of 15,750 pulses per second provided an output pulse 10 microseconds wide which did not vary more than 2 percent when any one of a large number of type 12AT7 double triode tubes was substituted in the circuit. It was found that if each one of the resistors and capacitors in the circuit is off from the specified value by percent in one direction, the output pulse width is changed only 13 percent. In conventional multivibrator circuits, a given percentage variation in the value of a single component usually results in the same percentage variation in the width of the output pulse.

The circuit of Fig. 1 was found to be especially useful as a horizontal blanking pulse generator in the electronic view-finder of an image orthicon camera chain.

Fig. 4 shows another embodiment of the invention wherein the elements corresponding to those in Fig. 1 are given the same reference numerals with prime designations added. The voltage waveforms at designated points in the circuit of Fig. 4 are shown in Fig. 5. The

circuit of Fig. 4 difit'ers from the circuit of Fig. l in that the grid 10' of tube V1 and the timing resistor 21' are both returned to a junction point 32. The potential at point 32 is determined by voltage divider action result ing from the flow of current from B+ through voltage divider resistor 13 and timing resistor 21'. Thus current flows into the grid of tube V2 when the tube is conductive and into the timing capacitor 19 when tube V2 is non-conductive. By-pass or filter capicitor 33 maintains the potential at point 32 constant during the cycle of operation of the multivibrator.

By the construction shown in Fig. 4, the voltage divider resistor 14 of Fig. 1 is eliminated with the advantages that the power loss in the voltage divider 13, 14 of Fig. 1 is avoided, and the elimination of resistor 14 eliminates one component the value of which affects the width of the output pulse. It can be shown that the circuit is nearly insensitive to changes in the value of resistor 13' because the voltages E and E vary proportionally with the value of resistor 13'. The circuit of Fig. 4 has all the advantages as to stability that are attributed to the circuit of Fig. 1 except that the efiiect of changes in the emission of tube V2 cannot be 100 percent compensated for by the selection the values of voltage divider resistors 13, 14.

Solely by way of example, a multivibrator constructed according to Fig. 4 with circuit elements having the values shown in the drawing provided an output pulse width of 8.5 microseconds when the multivibrator was triggered by a negative trigger pulse wave having a repetition rate of 15,750 pulses per second. The stability of the circuit was such that a 5 percent change in all of the elements 16', 18, 19' and 21 in a cumulative direction caused only an 8.1 percent change in pulse width. An additional cumulative change in the values of elements 13', 24' and 27' caused only an additional 1.0 percent change in the pulse width. In substituting a large number of type 12AT7 tubes, there was no more than a 2.0 percent change in pulse. width. If the emission of tube V1 is one-half the normal value, there is only a 1.1 percent change in pulse width. If the emission of tube V2 is one-half the normal value, there is only an 0.2 percent change in pulse width. The B+ voltage was varied over the range of l-140 volts without causing more than a 1.0 percent change in pulse width compared with the width when the B+ voltage was 280 volts.

The circuit of Fig. 4 is especially useful in the horizontal deflection circuit of a vidicon camera chain.

The circuits of Fig. 1 and 4 have the advantage common to all cathode coupled multivibrators that the output electrode (plate 26 of tube V2) is free, i. e., not used in a timing circuit of the multivibrator.

A multivibrator constructed according to the teachings of this invention will operate in the most stable manner set forth above when actuated by a periodic trigger pulse wave having a constant period of not more than one-twentieth of the time constant of the circuit including capacitor 25 (or 25). It should be understood that the circuit is also stable, to a lesser degree, when actuated by a trigger pulse wave which is not periodic but which is recurring and has a maximum time interval between pulses of less than one-twentieth of the above-mentioned time constant. Under such conditions, the output pulse width is stabilized against tube changes and power supply voltage changes but may vary somewhat according to the pattern of successive intervals be tween pulses. It is also true that with a given circuit, the width of the output pulse will vary according to the period of the periodic trigger pulse wave applied thereto. This may be a desirable characteristic in some applications.

What is claimed is:

l. A monostable multivibrator receptive to recurring trigger pulses having a maximum time interval therebetween, comprising first and second vacuum tubes having a cathode, a grid and a plate, a source of uuidirec 2' tional potential having positive and negative terminals, a resistor and a capacitor connected in series between said terminals, a timing capacitor connected between the plate of said first tube and the grid of said second tube, a timing resistor connected between the grid of said second tube and the junction between said resistor and capacitor, a connection from the grid of said first tube 'to said junction, a degenerative resistor in the cathode circuit of said first tube, and a capacitor coupling the cathode of said second tube to the cathode of said first tube.

2. A monostable multivibrator receptive to recurring trigger pulses with a predetermined maximum time interval therebetween, comprising, a first triode vacuum tube, a second triode vacuum tube, positive and negative terminals for applying a source of unidirectional current to said multivibrator, plate and cathode resistors for connecting said tubes in circuit across said terminals, a voltage divider connected between said positive terminal and the grid electrode of said first tube -to bias said grid electrode with a positive potential, a resistor and a capacitor coupled together to form a timing circuit for cross-coupling the plate electrode of said first tube to the grid electrode of said second tube, one end of said resistor second tube to the cathode electrode of said first .tube,

one of said cathode resistors being a degenerative resistor, said degenerative cathode resistor and said cathode coupling capacitor together having a time constant at least twenty times said maximum time interval between said trigger pulses.

References Cited in the file of this patent UNITED STATES PATENTS 2,405,237 Rulig Aug. 6, 1 946 2,422,698 Miller June 24, 1947 2,459,852 Summerhayes, Jr. Jan. '25, 1 949 2,494,353 Newman Ian. 10, 1950 2,526,000 Bliss Oct. '17., 1950 2,683,806 Moody July '13, 1954 2,750,502 Gray June 12, 1956 FOREIGN PATENTS 535,778 Great Britain Apr. 22, 1941

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