US2764677A

US2764677A - Monostable multivibrator

Info

Publication number: US2764677A
Application number: US327611A
Authority: US
Inventors: Robert E Graham
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1952-12-23
Filing date: 1952-12-23
Publication date: 1956-09-25
Anticipated expiration: 1973-09-25

Description

p 25, 1956 R. E. GRAHAM MONOSTABLE MULTIVIBRATOR 2 Sheets-Sheet 1 Filed Dec. 23, 1952 POTENTIAL SIGNAL souecs SYNCl-IRON/Z/NG INVENTOR R. 1:. GRAHAM BY A1 4 & 4/-

A TTORNEV United States Patent 'Ofiice 2,764,677 Patented Sept. 25, 1956 MONOSTABLE MULTIVIBRATOR Robert E. Graham, Chatham Township, Morris County, N. 1., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 23, 1952, Serial No. 327,611

4 Claims. (Cl. 250-27) This invention relates to signal generator systems and more specifically to such systems of the multivibrator type which operate only in response to a triggering pulse.

In some types of electronic circuits, a signal generating system is frequently required that will, in response to an external timing or triggering pulse, provide a non-sinusoidal output pulse for a brief period. One such system, known as a monostable or one-shot multivibrator, includes a pair of coupled electron discharge tubes, one of which conducts only during the stable or off period of multivibrator and the other of which conducts only during its unstable or on period. The system provides an output pulse during the on period. The beginning of the on period is determined by the external triggering pulse and the duration of this period (and hence, the duration of the output pulse) is determined by the time constant of the coupling network connecting the two tubes.

It is an object of this invention to simplify and make more compact monostable multivibrators capable of pro viding output pulses whose duration-may be of the order of microseconds or less and which can be varied over a wide range.

This invention contemplates the use of a pair of crosscoupled electron discharge devices or vacuum tubes. The coupling is (1) between the anode of one tube (called the first tube) and the grid of the other (second) tube and (2) between the cathodes of the two tubes. The cathode coupling network includes a capacitor connected between the cathodes which acts as a low impedance connection during the initial stage of the multivibrator on period to produce regeneration. During the remainder of this period, the cathode network serves as a timing circuit which controls the multivibrator on period. Unlike other monostable multivibrators of this type, the multivibrator circuit requires but two points of fixed potential throughout.

Additionally, and also unlike previously known monostable multivibrators of this general type, the grid of the second tube is biased by an internal impedance arrangement in the circuit whereby the grid is maintained at a constant potential with respect to its corresponding cathode during the off period irrespective of any fluctuations in the potential levels of the coupled cathodes. The biasing means provides an important advantage by insuring that the multivibrator will not be triggered into an on period by inopportune fluctuations of the voltage level in the cathode circuit, and further insures that the grid-to-cathode bias will always be maintained within such limits as to permit the small triggering pulse reaching the grid of the second tube to trigger it into its conductive state.

Other advantages will be apparent and the invention will be more clearly understood from the following descrip tion taken in connection with the accompanying drawings forming a part thereof, in which:

Fig. 1 is a circuit diagram of a multivibrator in accordance with the invention; and

Figs. 2A to 2F, inclusive, show voltage wave forms appearing on various elements of the multivibrator circuit.

With specific reference to the drawing, there is shown in Fig. 1, asan exemplary embodiment of the invention, a multivibrator system which includes a twin triode vacuum tube VI comprising a first triode section including an anode or plate element 11, a grid element 12 and a cathode element 13, a second triode section comprising an anode or plate element 14, a grid element 15 and a cathode element 16, a heater element common to both cathode elements, and a shield. The anode 11 is connected to the positive terminal of a potential source 17 through series-connected load resistor 19 and resistor 18. A condenser 20 connected between the junction of

resistors

18 and 19 and ground potential provides for decoupling the plate from the potential source. Anode 14 is connected to the positive terminal of source 17 through resistor 21. The anode 14 is connected to ground through a condenser 29.

Current is supplied to grid 12 from the positive terminal source 17 by the grid resistor 25 which is connected to the grid through the anti-sing resistor 24. Through the use of a potential dividing arrangement comprising the series connected

resistors

32 and 33, one end terminal of which is connected to the junction of

resistors

24 and 25, the common terminal of which is connected to grid 15 through anti-sing resistor 31 and the other end terminal of which is connected to the negative terminal of source 17, the potential between

grids

12 and 15 is held substantially constant during the OE period. It is obvious that a potentiometer may be used instead of

resistors

32 and 33. Cathode 13 is connected to ground and to the negative terminal of source 17 through load resistor 26, and cathode 16 is coupled to cathode 13 and resistor 26 through a parallel-connected capacitor 27 and resistor 28. Anode 11 is coupled to grid 15 through a series-connected capacitor 30 and anti-sing resistor 31. The external timing or triggering wavefrom the synchronous signal source 22 is applied to grid 12 through the series-connected coupling capacitor 23 and the resistor 24. One output terminal 34 is connected to the anode 11 and the other terminal 35 is connected to the negative terminal of source 17. This output terminal pair in turn supplies an output pulse to a useful output load here represented by 36. With reference to the mode of operation of the circuit arrangement just described, prior to the appearance of the triggering pulse in the circuit the multivibrator is o with the first section of tube VI conducting at approximately zero grid-to-cathode potential and the second section of the tube substantially cut-off with the cathode positive with respect to the grid. The first section of tube V1 is maintained at zero grid-to-cathode potential by an appropriate choice of values for the potential source 17 and for the

resistors

24, 25 and 26. Values for the

resistors

18 and 19 are chosen in light of the value already given to, resistor 26, so that the anode dissipation of the first section will not be exceeded. -Further, the size of resistor 19 will be determined by the magnitude of the output pulse desired. The second triode section of V1 is held substantially at cut-oii by biasing

means comprising resistors

32 and 33, and the cathode resistor 23.

When a negative timing or triggering pulse shown in Fig. 2A is applied from source 22 to the grid 12, the grid voltage drops as shown in Fig. 2B and starts cutting ofi the first section, thereby causing an amplified positive pulse shown in Fig. 2C to appear on the anode 11. A regenerative cycle is then initiated by two actions that take place in the circuit. The first takes place instantaneously with the initial negative swing of grid 12 in the first triode section which causes the voltage on cathode. 13 to fall by cathode follower action. This sudden drop in voltage is communicated to the cathode 16 of the second section which causes the voltage on this "cathode to also drop. At the same instant, the second (and most important) action takes place at grid of the second section. The positive pulse on the anode 11 is impressed on grid 15 "through coupling condenser 30 and resistor 31, thereby causing the voltage on grid 15 to rise as shown in Fig. 2D. The combined efiec't of these simultaneous actions on the .grid and cathode of the second section is to suddenly change the grid-to-cathode bias from a cut-off level to a conducting level. Current begins to flow through the second seeti'onof the tube and, because the plate'voltage of the tube'is held constant by the plate decoupling condenser 29 during the timing wave interval, a cathode follower action takes place and the voltage on cathode M'rises as shown in Fig. 2E to approximately'the voltage on grid 15.

The rise in cathode voltage in the second section is communicated to cathode 13, as shown in Fig. 2F, through condenser 27, thereby tending to further reducethe current in the first triode section, increasing the magnitude ofthe positivepulse on anode '11, and further raising the voltage on grid 15. The gain around this regenerative loop substantially exceeds unity so that the action proceeds with great rapidity until the first triode section is cut off by having its cathode driven highly positive with respect to its grid. At the same time the second triode section becomes conducting withits grid-to-cathode potential at substantially zero.

With the triggering action completed, the circuit is in temporary equilibrium, the first triode section being cut oil, the second section being conducting and a positive output pulse being supplied at the

terminals

34 and 35. It becomes apparent that this temporary equilibrium condition exists only so long as the current through condenser 27 and resistor 26 maintains the cathode 13 positive with respect to the grid 12 in an amount exceeding the cut-off potential of thetube. Immediately after the triggering action is completed, the condenser 27 begins to charge and the potential of cathode 13 consequently begins to fall towards a voltage E1 established by the voltage divider comprising resistors -26 and 28 as shown in Fig. 2F. Inasmuchas the internal impedance of cathode 16 is very small compared to the values of

resistors

26 and 28, the rate of fall is determined by the parallel value of these two resistors and the cathode coupling capacitor 27.

When the voltage on cathode 13 of the first section has fallen to a value that permits the first triode section to conduct again, the triggering action is repeated in reverse. A negative pulse appears on the anode 11 of the first section which is communicated to the grid 15 of the second section thereby lowering its potential with respect to cathode 16 and tending to drive this section beyond cutoif. Cathode follower action lowers the voltage on cathode 16, which through coupling capacitor 27 further lowers the voltage on cathode 13 of the first section. This, of course, places the first section in a more stable operating condition and further tends to cut oh" the second triode section so that the circuit is returned to its stable state with great rapidity and the positive pulse at

terminals

34 and 35 is cut ofi.

It is desirable that before the multivibrator is again triggered into its unstable state, the capacitor be discharged to its stable state potential. The time required to do this is determined by the value of condenser 27 and the value of resistor 28, which resistor provides a parallel discharge path across condenser 27.

The choice of values for

resistors

26 and 28 in this circuit is seen to be restricted by several conditions. In the first instance, resistor 26, together with

resistors

19 and 18, must limit the plate dissipation of the first triode section. Secondly, the value of

resistors

26 and 28 in parallel, together with the value of condenser 27, must determine the on period of the multivibrator. Thirdly, the values of

resistors

26 and 28 which form a voltage divider must establish a voltage towards which the capacitor 27 charges that is well below the cathode voltage required to cause the first triode section to conduct. Finally, the time constant established by resistor 28; and condenser 27 must be short enough to permit the multivibrator to arrive again at its stable state so that it may be triggered when next called upon by the synchronous signal source to provide an on period.

The resistor 28 not only serves in the functions described above, but it also serves as one element in the biasing means for the second triode section, which biasing action We shall now consider.

One element of the biasing means is that portion of the potential divider connecting the grid circuit of the first section and the grid circuit of the second section, which potential divider comprises

resistors

32 and 33. The values of the resistors are chosen so that the current in the

resistors

32 and 33 is small as compared with the grid current in resistor 24 during the off condition, and resistor 33 is large compared with resistor 32. Another element in the biasing means is resistor 28 connecting the two cathodes by which any change in the potential of either cathode causes a similar change in the potential of the other cathode. Any change in the cathode voltage of the first section affects the grid current flow through resistor 25 which causes the grid voltage of the first section to change in magnitude and direction according to the change on the cathode voltage. This change in grid voltage in the first section is repro duced on the grid of the second section by the potential dividing arrangement thereby causing the grid-towathode bias of this section to be substantially constant.

To better understand this biasing action, consider the condition wherein the voltage levels of the cathodes have increased above their given values during the off period. There follows then a decrease in the grid-to-cathode potential in the first section which is at this time conducting, and a decrease in the grid current'in this first section. Inasmuch as the resistor 25 is large compared to anti-sing resistor 24 and the internal grid-to-cathode impedance of the first section, the decrease in grid current causes the voltage drop across resistor 25 to decrease and the voltage level of the grid 12 to increase both by an amount substantially equal to the increase in the voltage level of the cathode 13. Thus, the gridto-cathode potential of the first *section remains substantially constant. With resistor 32 small as compared to resistor 33, the voltage level of the grid 15 of the second section is caused to be increased by an amount similar to that of the grid 12 of the first section and by an amount similar to that of cathode 1650 that the grid-to-cathode potential of the second section remains substantially constant. It is clear that inthe case where the voltage levels of the cathodes have decreased, appropriate changes in the voltage levels of the grids will occur to maintain the grid-to-cathode potentials constant. The advantage of this arrangement is that the second triode section, which is non-conductingduring this off period, is prevented from being tripped into a conducting state by fluctuations in the voltage levels of the cathodes and similarly there is assurance that the grid to-cathode bias of the second section will stay relatively constant during this period and. permit thezpulse appearing on grid 15 of the second section to trip this section into its conductive state.

In order to decouple the pulses appearingon each anode of each tube section from the directcurrent source 17,. resistance-capacitance networks comprising resistor 18 and condenser 20, and resistor 21 and condenser 29 are provided. The time constants of these networks are long compared to the 'on period of the multivibrator.

Multivibrators have been built in-accordance with the invention having on periods of three microseconds and one microsecond, respectively, and it is'apparent that the circuit elements may be chosen so :asto provide a multivibrator having an on period that may be as brief as 0.1 microsecond. While there are a number of possible circuit constants for the various circuit elements, values used in an operable embodiment of the multivibrator system of Fig. 1 (for a multivibrator in which the applied trigger pulse is minus 2 volts and of 4 microseconds duration, the output pulse is 70 volts and the on period is of three microseconds duration) and which are reproduced below merely by way of example are:

V1 2C51 17 volts 300 18 ohins 6200 19 do 5600 20 micrornicrofarads 9100 21 "ohms" 4700 23 micromicrofarads 9100 24 ohms 100 25 megohms 0.56 26 0hms 3300 27 micromicrofarads 1000 28 oh-ms 18000 29 micromicrofarads 9100 30 do 9100 31 ohms 100 32 megohms 0.51 33 d-o.. 7.5

It is understood the above described arrangement is merely illustrative of the application of the principles of the invention. Numerous other arrangements might be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A multivibrator adapted to be connected to an input wave source comprising a first electron discharge device and a second electron discharge device, each of said devices including an anode, a cathode and a grid, means for coupling the anode of one of said devices to the grid of the other of said devices, means including a resistor and a capacitor connected in parallel to one another for coupling the cathodes of said devices, a source of potential having a negative and a positive pole, means for connecting both of said anodes to said positive pole, means including a resistor for connecting the cathode of one of said devices to said negative pole, means including a further resistor for connecting the grid of one of said devices to said positive pole, potential dividing means having two external terminals and an inner terminal, means for connecting one of said external terminals to the grid of one of said devices and to the said positive potential pole, means for connecting the other of said external terminals to said negative pole, and means for connecting said inner terminal to the grid of the other of said devices to maintain said grid at a constant potential with respect to its cathode.

2. A multivibrator according to claim 1 in further combination with means for applying an input wave to the grid of one only of said devices.

3. A multivibrator according to claim 1 wherein said two poles of said potential source are the only points of fixed potential in the multivibrator circuit.

4. A monostable multivibrator adapted to be connected to an input wave source comprising a first electron discharge device and a second elsctron discharge device, each of said devices including an anode, a cathode and a grid, means for coupling the anode of said first device to the grid of second said device, means including a resistor and a capacitor connected in parallel to one another for coupling the cathode of said second device to the cathode of said first device, a source of potential having a positive and a negative pole, said poles being the only points of fixed potential in the multivibrator, means for connecting both of said anodes to said positive pole, means including a resistor for connecting the cathode of said first device to said negative pole, means including a further resistor for connecting the grid of said first device to said positive pole, means for applying an input wave to the grid of said first device, potential dividing means having two external terminals and an inner terminal, means for connecting one of said external terminals to the grid of said first device, means for connecting the other of said external terminals to said negative pole, and means for connecting said inner terminal to the grid of said second device.

References Cited in the file of this patent UNITED STATES PATENTS 2,403,984 K-oenig et a1 July 16, 1946 2,459,852 Summerhayes Jan. 25, 1949 2,524,134 Palmer Oct. 3, 1950 2,526,000 Bliss Oct. 17, 1950