US2784309A - Monostable multivibrator circuit - Google Patents

Description

March 5, 1957 J. D. SABLE 2,784,309

MoNosTABLE MULTIVIBRATOR CIRCUIT Filed Feb. 4, 1953 I l I l l I l I cz/Mf grief arr/fai Z/ f/f" \,.300 v INVENTOR. TERUMED- Salar-r By /wwn/ ATTORNEY MONOSTBLE MULTIVIBRATOR CIRCUIT Jerome D. Sable, Stratford, N. J.,assignor to Radio Corporation of America, a corporation of Delaware Application February 4, 1953, Serial No. 335,098

6 Claims. (Cl. Z50-27) This invention relates to monostable multivibrator circuits, and more particularly to such circuits including a cathode follower in a cross-coupling circuit to provide a very short recovery time.

Multivibrators normally comprise two vacuum tube electrode structures with their input and output electrodes cross coupled so that when one tube conducts the other is cut 01T, and vice versa. In a monostable multivibrator, there is a stable oit condition with the rst tube conducting and the second tube cut oi (i. e. with negligible or no output current flowing in the tube). When the multivibrator receives a trigger pulse of desired sense and magnitude, the multivibrator circuit shifts to an unstable on condition with the first tube cut olf and the second tube conducting. The on condition obtains for a period of time determined by the time constant of the cross-coupling between the tubes and then the circuit automatically shifts back (returns) to the stable off condition.

Multivibrators are commonly used in radar systems to provide a gating or shaped waveform starting atthe time of the transmitted radio-frequency pulse and ending prior to the time of the following transmitted pulse. It is often desirable to generate a gating waveform wherein the on time is almost equal to the time between transmitted pulses. In this case, the multivibrator must have a very short recovery time, that is, after switching itself oi it must immediately become ready to be switched on again by a trigger pulse. The desirability of having a multivibrator with avery short recovery time is particularly great in systems of the type having a modulated pulse repetition rate. This is so because the shorter the recovery time, the greater the distance range of the radar system with a given minimum period between transmitted pulses, and, unless the recovery time of a multivibrator is short, the on time may not remain constant when triggered by a pulse train wherein the time between successive pulses varies. It is therefore an object of this invention to provide a multivibrator having a low possible ratio of olf time to on time, in the order of live percent or less.

lt is another object to provide an improved multivibrator having a low impedance output point so that the utilization circuits do not alter the operation of the multivibrator. A

It is a further object to provide an improved multivibrator having an output wave which swings both positive and negative and which can be directly connected to the grid of a utilization tube without using a coupling capacitor or attenuation network,

It is a still further object to provide a multivibrator wherein the output electrode of one tube is directly connected to the input electrode of the other tube, without the use of a coupling capacitor, so that faster response may be obtained.

,In accordance with the teachings ofthis invention, there is provided a monostable multivibrator circuit having a rst normally-conducting tube electrode structure ited States Patent y 2,784,309 Eiatented Mar. 5, 1957 the grid side of the timing capacitor discharges sutii# ciently so that the voltage on the grid of the rst multi-` vibrator tube exceeds the grid cut-off voltage, at which time the lirst multivibrator tube becomes conductive and the second multivibrator tube is cut olf.

A feature of the invention is the arrangement whereby the anode of the second multivibrator'tube is coupled through a cathode follower and a timing capacitor to the grid of the lirst multivibrator tube. The capacitor is part of an RC circuit which determines the on time of the multivibrator. After the multivibrator switches itself off (i. e. restores itself to the stable condition), the cathode follower rapidly recharges the timing lcapacitor to provide a very short recovery time for the multivibrator. The cathode follower also provides a low impedance output point in the multivibrator.

Another feature of the invention is the use of values of direct current supply voltages on the multivibrator tubes that permit the plate of the lirst multivibrator tube to be directly connected to the grid of the second multivibraor tube. The absence of coupling elements results in faster response by the second multivibrator tube.

These and other objects, advantages and features of the invention will be apparent to those skilled in the art from the following description taken together with the appended drawings, wherein:

Fig. l is a circuit diagram of a presently preferred form ot' the invention; and

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

In Fig. 1, various leads are shown going to a direct current voltage source (not shown) which in the present example provides potentials of 300, 150, +150 and +300 volts relative to ground. Voltage sources are well known in the art and in describing Fig. l, an element will be merely saidto be connected to a certain voltage. A trigger input terminal 9 is connected through coupling capacitor 10 to the grid 13 of buffer vacuum tube or electron discharge device 15. A grid bias resistor 11 is connected from grid 13 to ground, and another grid bias resistor 12 is connected from grid 13 to -150 volts. Resistors 11. and 12 constitute a voltage divider which serves to bias buffer tube 15 below cut-off, that is, to the condition of no current flow through the tube. Bulfer tube 15 includes a cathode 16 connected to ground and an anode 17 connected to grid 19 of cathode follower vacuum tube o1' electron discharge device 20. Tube 20 includes an anode 21 connected to +300 volts anda cathode 22 connected through cathode resistor 23 to -150 volts. Cathode 22 is also connected through an RC circuit consisting of capacitor 24 and resistor '24 to the grid 26 of a lirst vacuum tube or elect-ion discharge device 25 of a multivibrator including tubes 25 and 30. One end of resistor 24 is connected to 150 volts. Multivibrator tube 25 includes a cathode 27 connected to 300 volts and an anode 28 connected through anode resistor 29 to -150 volts. Anode 28 is also connected across grid resistor 35 to the grid 31 of the second vacuum tube or electron discharge device 30 of the multivibrator. Tube 30'includes a cat-bode' 32 connected-'to 150 volts and an anode 33 connected through' a plate 3 resistor 34 to +150 volts. Anode 33 is also connected to grid 19 of cathode follower tube 20 and plate 17 of buffer tube 15.

In the absence of a trigger input pulse of desired sense and magnitude, buffer tube is cut off by the grid bias provided by voltage divider grid resistors 11 and 12 connected between -150 volts and ground. First multivibrator tube 25 is-conducting heavily because, while the grid 26 is connected through resistor 24 to -150 volts, the cathode 27 is connected directly to a more negative voltage of 300 volts, as a result of which the grid is positive relative to its cathode. Second multivibrator tube 30 is cut olf because its cathode 32 is connected to --150 volts and its grid is connected to a more negative voltage on the anode 28 of first multivibrator tube 25. The anode 28 ofiirst multivibrator tube 25 isxmore negative than -150 volts because of the voltage drop ,in anode resistor 29 due to current drawn'therethrough by the conducting tube 2S. Cathode vfollower tube is conducting because its grid 19 is connected through resistor 34 to +150 volts and its cathode 22 is connected through cathode resistor 23 to -150 volts.

The operation of the circuit when a positive pulse is applied to input terminal 9 will now be described. Reference will be made to the voltage waveforms shown in Fig. 2 wherein the designations (a) through (e) are used to identify the waveforms at correspondingly designated points in the circuit of Fig. 1. A positive pulse applied from input terminal 9 through coupling capacitor 10 to grid 13 of buffer tube 15 renders the tube conductive. The current drawn through anode resistor 34 causes the voltage at (b) to fall as shown in curve (b) of Fig. 2. This voltage applied to the grid 19 of cathode follower 20 causes the voltage on cathode 22 to fall also because of the reduction in current through cathode resistor 23. The voltage drop on cathode 22 is coupled through `capacitor 24 to the grid 26 of first multivibrator tube 25, as shown in wave (d) of Fig. 2, as a result of which the flow of current through tube 2S ceases. When current through anode resistor 29 ceases, the voltage on anode 2% rises to about -150 volts shown in wave (e) of Fig. 2. This increased Voltage is applied to grid 31 of the second multivibrator tube 30 causing tube 30 to conduct heavily, and draw anode current through anode resistor 34, and grid current through resistor 3S. The potential at (b) remains at the low value due .to current flowing through resistor 34 and tube 30, while buffer tube 15 returns to the cut-off condition at the end of the trigger pulse applied to input terminal 9. As long as first multivibrator tube remains cut off, the -150 volts on its anode 23 which is applied to grid 31 of second multivibrator tube 30 maintains tube 30 in a heavily conducting condition.

The negative square voltage wave applied from the cathode 22 of cathode follower 20 through capacitor 24 to grid 26 of first multivibrator tube 25, in the first instant appears entirely across resistor 24', and drives grid 26 far below cut-off. Ffhe voltage on grid 26 immediately starts increasing exponentially as negative charge on the grid side of capacitor 24 discharges through resistor 24. After a period determined by the time constant of capacitor 24 and resistor' 24', the voltage on grid 26 reaches and exceeds the cut-off value and tube 25 starts conducting. The voltage on anode 28 of tube 25 and grid 31 of tube 30 then starts to fall, causing the voltage of anode 33 of tube 30, grid 19 and cathode 22 of cathode follower 20 to rise. This rise in voltage is coupled by capacitor 24 to grid 26 of tube 25 causing a further increase in anode current drawn by the tube.

The action continues in a rapid regenerative manner until first multivibrator tube 25 is conducting heavily and second multivibrator tube is cut olf. The initial conditions then obtain and the cycle is repeated when another positive ,trigger pulse is applied to terminal 9.

A multivibrator constructed according to known systems of the prior art would have the anode 33 of second multivibrator tube 30 coupled through capacitor 24 to the grid 26 of rst multivibrator tube 25. In the circuit of the invention illustrated in Fig. l, the feedback coupling between the anode 33 and grid 26 includes a cathode follower 20 and the capacitor 24. This arrangement permits of a very short recovery time in the transition from the on condition to the off condition. The re covery time depends on the speed with which capacitor 24 can be recharged after iirst multivibrator tube 2S starts conducting. The Vcharging current reaches capacitor 24 through cathode follower tube 20 which has a very low impedance, and consequently the charging time constant is low. ln conventional multivibrators, the charging current reaches the capacitor through the anode resistor 34 of the second multivibrator tube 30. Since an anode resistor has a relatively high impedance, consequently the charging time constant is relatively long.

The multivibrator of Fig, l is also characterized by a very rapid transition from the olf condition to the on condition. This rapid transition results from the construction whereby the direct current supply voltages to the multivibrator tubes have such values as to allow the use of a direct connection feedback coupling between the anode 2S of first multivibrator tube 25 and the grid 31 of second multivibrator tube 30 and the absence of coupling elements in this direct connection.

The following values are given by way of example, for a .cir-cuit constructed according to Fig. 1.

Resistor 11 ohms 100,000 Resistor 12 do 470,000 Resistor 23 do 33,000 Resistor 24' do- 1,000,000 Resistor 29 do 10,000 Resistor 35 do 100,000 Resistor 34 do 15,000 Capacitor 10 c micromicrofarads 470 Capacitor 24 do 1,000

The circuit provided waveforms as shown in Fig. 2

when operated with a frequency modulated trigger input Wave having a repetition rate of about 1400 pulses per second.

What is claimed is:

1. A monostable multivibrator comprising, a source of uni-directional potential including first, second, third, fourth and fifth terminals, in order, from relatively most positive potential to relatively most negative potential; first, second, third, fourth and fifth resistors; a first normally conductive electron discharge device including a cathode connected to said fth terminal, a grid connected thru said first resistor to said fourth terminal, and a plate connected thru said second resistor to said fourth terminal, and also thru said third resistor to said third terminal; a second normally non-conducting electron discharge device having `a cathode connected to said fourth terminal, a grid directly connected to the plate of said first device, and a plate connectedthru said fourth resistor to said second terminal; a cathode follower device including a plate connected to said first terminal, a grid counected to the plate of said second device, and a cathode connected thru said fifth resistor to said fourth terminal; and a timing capacitor coupling the cathode of said cathode follower device .with the grid of said iirst device; whereby said capacitor and said tirst resistor constitute a timing circuit which determines the length of time said first device remains non-conductive.

2. A monostable multivibrator circuit having a stable state and an unstable state, comprising first and second electron discharge electrode structures each having input and output electrodes, and means to regeneratively cross-couple the input and output electrodes of said structures so that when one `is .conductive the `other is cut-off, and vice versa, said .cross-coupling means including a connection capable of vpassing direct current extending from the output electrode of said first structure to the input electrode of said second structure, a cathode follower in said regenerative cross-coupling means including an electron discharge device having grid and cathode electrodes and a cathode resistor connected to said cathode, a connection from said grid to the output electrode of said second structure, and a timing capacitorresistor combination connected between said cathode and the input electrode of said first structure, said capacitor-resistor combination having such values to determine the time the multivibrator remains in its unstable state.

3. A multivibrator circuit comprising a rst and a second discharge device, each having a grid and a plate, means to cross-couple the grids and anodes of said devices so that when said first device is conducting said second device is cut-off and when said first device is cut-off said second device is conducting, said cross-coupling means including: a direct connection devoid of concentrated impedance from the anode of said first device to the grid of said second device, a cathode follower including a discharge device having a grid, a cathode and a cathode resistor connected to said cathode, a connection from the anode of said second device to the grid of said cathode follower, a timing capacitor connected between the cathode of said cathode follower and the grid of said rst device, and a resistor connected from the grid of said first device to a source of bias potential, said timing capacitor having such value that it aids in determining the time during which said first tube is made to be non-conductive.

4. A multivibrator circuit comprising first and second electron discharge devices having cathode, grid and anode electrodes; regenerative cross-coupling means between the anodes and grids of said first and second devices making one device conductive when the other is cut-off, and vice versa, said cross-coupling means including a direct connection devoid of concentrated irnpedlance between the anode of said first device and the grid of said second device; a cathode follower including an electron discharge device having cathode, grid and anode electrodes and a cathode resistor connected to said cathode; a connection from the grid of said cathode follower to the anode of said second device; a resistance-capacitance timing circuit connected between the cathode of said cathode follower and the grid of said first device; and a buffer circuit having an output terminal connected to the grid of said cathode follower and having an input terminal receptive to an external triggering pulse.

5. A monostable multivibrator having alternate conductive states, comprising a Afirst normally conducting tube having grid and anode electrodes, a second normally non-conducting tube having grid and anode elec trodes, regenerative cross-coupling means between the sanodes and grids of said first and second tubes making one tube conductive when the other is cut-off, and vice versa, said cross-coupling means including a direct-connection feedback from the anode of said first tube to the grid of said second tube, a cathode follower tube in said regenerative cross-coupling means having grid and cathode electrodes, a connection from said grid of the cathode follower tube to the anode of said second tube, and a timing capacitor-resistor combination connected between said cathode of the cathode follower and the grid of said first tube, said capacitor-resistor combination having such values as to determine the duration of each of the alternate conductive states.

6. A monostable multivibnator comprising, first and second electron discharge devices which are alternately conductive, said devices each having input and output electrodes; first regenerative cross-coupling means couF pling an output electrode of said first device to an input electrode of said second device, said first coupling means constituting a direct connection devoid of concentrated impedance; second regenerative cross-coupling means coupling an output electrode of said second device to an input electrode of said first device, said second coupling means including a cathode follower electron discharge device having grid and cathode electrodes, a direct metallic connection from said grid to an output electrode of said second device, and a resistance-capacitance timing circuit coupled between said cathode and an input electrode of said first device, said timing circuit having values determining the duration of alternate states of conduction of said first and second devices; and means to bias said three devices so that said first and second coupling means make the first device conductive when the second is cut-off, and vice versa.

References Cited in the file of this patent UNITED STATES PATENTS 2,155,210 Young Apr. 18, 1939 2,442,769 Kenyon June 8, 1948 2,469,031 Canfora May 3, 1949 2,515,271 Smith July 18, 1950 2,642,532 Mofenson June 16, 1953 FOREIGN PATENTS 587,940 Great Britain Apr. 18, 1939

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