US2688078A - Multivibrator - Google Patents

Description

Aug. 31, 1954 2,688,078

L. BESS MULTIVIBRATOR Filed Feb. 21, 1946 '5 PLATE VOLTAGE SUPPLY n lz l d u NEG TRIGGER m L I a OUTPUT 32 i' 3 INVENTOR LEON BESS ATTORNEY nil,

Patented Aug. 31, 1954 MULTIVIBRATOR Leon Bess, Kansas City, Mo., assignor to the United States of America as represented by the Secretary of the Navy Application February 21, 1946, Serial No. 649,419

3 Claims.

This invention relates to multivibrator circuits and more particularly to multivibrator circuits having a low average current drain.

In multivibrators as previously known in the art, the output voltage is normally taken directly from the plate of one of the electron tubes. The peak-to-peak magnitude of the square Wave volt age output from a multivibrator so connected is equal to the product of the plate load resistance and the plate current of the tube from which the output is taken. This method of obtaining the output has been used with both free running multivibrators, and with biased multivibrators of the one-shot type. By the term one-shot as used in this specification is meant a multivibrator having one equilibrium state, and which will undergo a single complete cycle of operation each time that it is triggered from the equilibrium state. This type of multivibrator is often used to provide an output gate voltage synchronized in time with an input trigger voltage. When a biased multivibratcr is used to produce a positive voltage pulse, the output is generally taken from the plate of the normally conducting tube; and the magnitude of the voltage pulse is equal to the product of the plate load resistance and the current flowing in the normally conducting tube. In order to obtain a large voltage pulse for the purpose of gating, or enabling, following circuits, it is necessary to have a large current flowing in the normally conducting tube. In many circuits using voltage gates produced in this manner, the gate duty cycle is very low; that is, the duration of the voltage gate is short when compared with the length of time between successive voltage gates. Under such circumstances, the average current drawn by the multivibrator is very nearly equal to the current drawn by the normally conducting tube. In the case where a large voltage gate is required with a low gate duty cycle, the average current of the multivibrator is, therefore, large.

In one embodiment of the present invention, the positive voltage gate output is taken from the cathode circuit of the normally non-conducting tube. With this type of connection, the magni tude of the voltage output pulse is substantially the product of the current through the normally non-conducting tube and the resistance of the cathode resistor. As the current through the normally conducting tube does not have much effect upon the magnitude of the voltage pulse, the circuit may be designed so that the current drawn by the normally conducting tube is relatively small. This will result in a small average current being drawn by a multivibrator of this design.

A further improvement introduced by this embodiment of the invention is the lowering of the internal impedance of the voltage gate source. In the proposed circuit the output gate voltage is generated across a cathode resistance which introduces a high degree of negative voltage feedback into the circuit. The result of this negative feedback is to reduce the internal impedance of voltage gate source.

A primary object of the present invention is to generally improve multivibrator circuits.

Another object of the present invention is to provide a multivibrator circuit suitable for producing positive voltage gates but which has a low average current drain.

A further object is to provide a multivibrator gate generator having a low internal impedance.

These and other objects of the invention will be apparent from the following description when taken with the accompanying drawings, which is a schematic diagram of one embodiment thereof.

Referring to the drawing, an input trigger voltage obtained at terminal I I is connected through condenser l2 to the plate of triode electron tube !3. The plate of triode l3 is also connected throughplate load resistor 14 to the plate voltage supply at terminal l5, and is coupled through condenser 20 to the grid of triode electron tube M. The cathode of triode l 3 is connected directly to the cathode of triode 2! and also to ground throughresistor 22 andresistor 23. In parallel withresistor 22 iscondenser 24. The grid of triode I3 is connected to the junction point ofresistor 22 andresistor 23 throughresistor 25, and is also coupled to the plate of triode 2! through condenser 38. In the absence of an external signal, triode 2! will be conducting since its grid is returned to the plate voltage supply at terminal I5 through resistor 3!. The voltage drop established acrosscommon cathode resistor 22 by the current through triode 2| will keep triode l3 cut-off. This state of equilibrium is interrupted when a negative trigger is introduced at terminal II. The negative trigger is capacitively coupled to the grid of triode 2! through condenser I2 and condenser 20 and results in multivibrator action cutting off triode 2| and turning on triode l3.Cathode resistor 22 is substantially by-passed bycondenser 24 with respect to the heavy current pulses through triode [3, in order that the grid of triode l3 may be maintained at a high voltage during the gate. With the multivibrator in this stage of the cycle, the charge on condenser 20 increases exponentially until the voltage of the grid of triode 2|, which is connected to one side of condenser 20, is raised above the eut-ofi voltage for triode 2|; and the multivibrator action then causes triode iii to be cut-01f and triode 2| to conduct. The multivibrator then remains in this steady state condition until another negative trigger pulse is introduced at input terminal Under steady state conditions with triode 2| conducting and triode i3 non-conducting, the voltage available atoutput terminals 32 is equal to the product of the current through triode 2| and the resistance ofresistor 23. When a negative trigger is applied to the circuit at terminal H, triode |3 starts conducting and triode 2| is cut-01f, the output voltage is increased from its steady state value to a voltage equal to the product of the resistance ofresistor 23 and the combined current through triode i3 and the charging current of condenser 3t, which is connected between the cathode of triode 2| and the junction ofplate load resistors 35 and 35. The chargin current ofcondenser 34 is approximately equal to the current that flows in triode 2|, and, therefore, the magnitude of the voltage pulse is substantially equal to the product of the resistance ofresistor 23 and the current through triode Hi. In order to obtain a good output Waveform it is necessary that the time constant of the charging circuit for condenser 3 t be long compared with the time constant of the charging circuit for condenser 20 which determines the length of the voltage gate. For the same reason it is also required that the time constant ofresistor 22 and condenser M be long in comparison to the length of the voltage gate.

The invention need not be limited to the details shown in the foregoing specification which are considered to be illustrative of one embodiment thereof. The scope of the invention is defined by the appended claims.

What is claimed is:

1. An unsymmetrical multivibrator having a low current drain comprising first and second electron tubes each having a cathode, an anode and a control grid, anode-cathode circuits for said electron tubes including a common source of space current, a resistance-capacity coupling between the anode of at least one of said electron tubes and the control grid of the other tube, said first and second electron tubes conducting alternately, the period of oscillation of said multivibrator being substantially determined by the time constant of said anode to control grid couplingcircuit, the conducting period of said first electron tube being a very small part of the total period of said inultivibrator oscillations, means for deriving an output voltage from said space current, and a condenser effectively in parallel with said second electron tube to draw during the ofi time of said second electron tube a charging current from said space current source substantially equal to the space current drawn by said second electron tube.

2. An unsymmetrical multivibrator having low current drain comprising first and second electron tubes each having a cathode, an anode and a control grid, the cathode circuits for said tubes including a common cathode resistor, anodecathode circuits for said electron tubes including a common source of space current, a resistance capacitance coupling between the anode of each of said electron tubes and the control grid of the other tube, said first electron tube being normally non-conducting and said second electron tube being normally conducting, means to apply a control signal voltage to initiate a single cycle oscillation of said multivibrator, the period of said oscillation bein substantially determined by the time constants of the capacities of said anode to control grid coupling circuits and the resistances effectively in series therewith including tube resistances, the conducting period of said first electron tube being a very small part of the total period of oscillation, an output circuit to obtain an output voltage established by the current flowing in said common cathode resistor, said output voltage being obtained at a low impedance level, and a condenser efiectively in parallel with said second electron tube to draw from said source during the oii time of said second electron tube a charging current substantially equal to the space current drawn by said second electron tube.

3. A low current unsymmetrical multivibrator comprising, first and second electron tubes each having a cathode, an anode and a control grid, the cathode circuits for said tubes including a common resistor, a source of space current for said tubes, a resistance-capacity coupling between the anode of one of said tubes and the grid of the other of said tubes, said first tube being normally nonconducting and said second tube being normally conducting, means to apply a control signal voltage to the control grid of said second tube to initiate a single cycle oscillation of said multivibrator, the period of said oscillation being substantially determined by the time constant of said coupling, an output circuit to obtain an output voltage at low impedance from the current flowing in said common cathode resistor, and a condenser effectively in parallel with said second tube to draw during the oil time of said second tube a charging current substantially equal to the space current drawn by said second tube when normally conducting.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,495,237 Ruhlig Aug. 6, 1946 2,416,201 Nagel et al Feb. 18, 1947 FOREIGN PATENTS Number Country Date 456,450 Great Britain Nov. 3, 1936

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