US2540539A

US2540539A - Multivibrator type oscillator

Info

Publication number: US2540539A
Application number: US779283A
Authority: US
Inventors: James R Moore
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1947-10-11
Filing date: 1947-10-11
Publication date: 1951-02-06
Anticipated expiration: 1968-02-06

Description

Feb. 6, 1951 J, MOORE 2,540,539

MULTIVIBRATOR TYPE OSCILLATOR Filed Oct. 11, 1947 @501 wag Emma Patented Feb. 6, I951 MULTIVIBRATOR TYPE OSCILLATOR James R..Moore, Washington, D. 0., assignor'to Radio Corporation of America, ,a corporation voi Delaware .Application october :11, 1947, Serial No. 779,283

invention :relates to oscillators of the multivibrator type, and more particularly to the stabilization of such oscillators.

The :principalobject of the invention is to proyide multivibrators in which the frequency and the amplitude-of the output voltage are .sub- .stantially independent of ordinary variations in characteristics-of the component vacuum tubes.

Another. object :is to provide oscillators of the described type wherein the frequency of oper- I ation can be controlled by :means of .a bias volt- .age.

Itively, and grid leaks Hand I3. The plate of the tube 1 is coupled to the grid of the tube 3 through a capacitor I5. The plate .of the tube .3 may be likewise coupled .to the grid of the tube I through a capacitor I'.'I., .although this is not strictly essential to the operation of the circuit. Both tubes I and 3 conduct .initially when the plate supply voltage is turned on. The potentials at both, plates decreases, owing to drop in the fi separate plate load resistors .I and 9, respecload resistors I and .9, and .at the same time the cathodes become increasingly positive with respect to ground because the. plate currents of both tubes flow through the common cathode resistor 5. The decrease in potential at .each plate is communicated through the respective coupling condenser I5 or I! to the grid of the other tube. This, together with the positive cathode potential, tends to drive both tubes toward plate current cutoff.

I4Any slight difierence between the tubes 1 and {or their associated circuit elements causes one oI the tubes, for example the tube I, to .cut .off first. The plate current of the other tube '3 will then increase suddenly, causing a further sud .den decrease in the potential at its plate. The

resulting negative pulse appearing at the grid of the tube I by way of the condenser I"! drives theltiibe 1 still lurtherbeyond cutofii.

tube 3 is now fully-conducting and the 3 Claims. (01. 25036) tube .I is well below cutoff. This condition sists while the condenser I1 discharges, through the resistor II to a potential approximating that between the plate of the tube 3 and ground. This.

:allows the grid of the tube 1, whichwas-driven negative by the pulse from the plateof 'the tnhe to approach ground potential at arate much depends upon the capacitance of the condenser 41 and the resistance of theeresistor I .I.

Finally the grid of the tube I vbecomes isufficiently less negative to allow conduction-to in the tube I. This increases the drop-fin iih cathode resistor 5 and. also applies-accegatire pulse to the grid of the tube 3.. These :gether operate to reduce the plate current imthe tube 3, causing an increase imthe potential-attire plate of the tube 3. The increa i y l flfiiliive voltage at the plate of the tube 3 drives the grid of the tube I in a positive directiom'accelerating the increase-of plate current inthe tube .1 and quickly driving the tube -3 beyond cutoff.

Conditions are now reversed, with .thetube I conducting and the tube 3 cutofi. .The condenser i5 discharges through the resistor 1.3,,andthe cycle is repeated. The tubes I .and 3 .conduct alternately, each for a period determined by the 3-6 product or time .constant .of its respective grid leak and condenser. The time required f or the actual transfer of conduction .from one tube to the other is extremely small. Output may be taken from the plate of the tube 3asshown, or from the plate of the tube i, or from both tubes.

Oscillators of the above described type are used to produce sharply defined non-sinusoidal waves for various applications. Often the iregu'ency is set approximately by the 3-0 networks and controlled exactly byinjectinga timing wave, for example at the control grid or one of the tubes. The present invention contemplates oscillators of the multivibrator type which are 'suflicientl'y stable so that they will operate at a definite selected frequency without being controlled by. a externally produced timing wave.

The frequency of operation of an oscillator like that of :1, assuming no timing W611i) be applied, depends not only upon the time nonetants orthe coupling networks but also upon the voltage of the supply source and the individual tube characteristics. The supply voltage determines how iarpositive the plate of either tube will go when the plate current is out off; the tube itself, as well as the supply voltage, determines how low the potential at the plate will go when theitub si o ductins Moreover. the potential at 'the plate or the tube vanes-11mins i A further tube 25 also has its cathode connected 3 the time that tube is conducting. All of these factors affect the length of the conducting periods for each tube, and thus the frequency. The amplitude of the output wave and its shape are also functions of the supply voltage and of the tube characteristics.

According to the present invention, the negative grid swing, i. e. the most negative potential which can appear at the grid of either of the multivibrator tubes, is set at a definite level which does not depend on the minimum plate potential at either tube and is preferably a predetermined fraction of the plate supply voltage.

Referring to Fig. 2, the tubes I and 3 are, asin Fig. 1, provided with a common cathode resistor 5. The plate of the tube I is not coupled directly to the grid of the tube 3, however, but goes to a cathode follower circuit IS. The cathode follower I9 includes a tube 2I with its entire load resistor 23 in its cathode circuit. The. plate of the tube 2| is connected directly to the plate supply source. The cathode of the tube. H is coupled through the condenser I5 to the'grid of the tube 3.

to t he resistor 23 and its anode connected to that of the tube 2|,- so that the space discharge paths ofthe tubes 2| and 25 are'in parallel. The cohtrol grid of the'tube 25 .is biassed positive with respect to ground by means of a voltage divider 3'0 21 connected across the plate supply source of the tubes I and 3. The'grid leaks and I3 of the-"tubes"I and 3 are not returned directly to ground'as in Fig. l, but are also biassed positive by a voltage divider 29, also connected across the platesupply source.

A s'eco'ndoathode follower circuit 3I is included in the connection between the plate of the tube 3 and the grid of the tube I. The circuit 3I is substantially identical with the circuit I9, and includes' -corresponding tubes 2I' and 25' with a common cathode load resistor 23. The grid of the tubal? is biassed like that of the tube 25 by the voltage divider 21.

1 The operation of the system of Fig. 2 is similar 'to that of Fig. 1, with certain significant exceptions The voltage divider 21 is adjusted to set the potential at the grids of the tubes 25 and 25' at a level E1 of, say, 50 volts. The positive bias supplied by the voltage divider 29 may be varied to adjust the frequency of oscillation to some extent, since this voltage will determine how far either ofthe coupling condensers I5 and I! must discharge before the respective tube 3 or I will be able to start conducting.

Assume that the tube I has just started to con duct. The potential at its plate assumes a relativelylow value, less than the supply voltage by the drop in the resistor I, and considerably less than'E1.' The actual potential at the plate of the tube I thus depends not only on the plate supply voltage but also upon how much current the tube Lean conduct.

Aslong as the potential at the plate of the tube Iis greater than E1, the cathode follower tube 2! conducts just enoughto maintain the drop in its lead resistor 23 equal to that potentiaL How cver, as soon as the tube I conducts and its plate potential drops below E1, the tube 25 starts conduotingiand provides a drop substantially equal to E1 across the resistor 23. This, being considerably greater than the potential at the grid of the tube2 I, cutsthelatter off, and any variations intheplatepotential at the tube I have no effect '5 an the voltage across the resistor 23.

Thus the voltage across the resistor 23 will have a substantially linear rectangular wave form, changing cyclically between 150 volts (the plate supply voltage) when the tube I is cut off and 50 volts (E1) when the tube I is conductive. These limits are determined solely by the supply and bias voltages, and any normal variations in "the characteristics of the tubes I, 3, 2I and 25 will not change them. If the voltage divider 21 is connected to the same source of supply as the plates of the tubes I and 3, the bias voltage E1 bear a constant ratio to the plate supply voltage regardless of variations in the latter. Thus I the maximum voltage across the cathode follower 18 load will always be three (or some other definite number) times the minimum voltage. This tends to make the frequency of operation of the oscillator independent of normal supply voltage variations.

It is apparent .without further discussion that the cathode follower circuit 3i acts exactly like the circuit I9, controlling accurately'the negative swings of the voltage reaching the grid of the tube I. Besidesstabilizing the frequency of the oscillator, this keeps the amplitude substantially constant. The output may be taken as in 1 from either of the tubes I and 3, or may be taken off either of the cathode follower load resistors. The latter is preferable because variations in the load resistance at this point have substantially noefiect on the system.

Summarizing briefly, the invention has been described as an improvement in oscillators of the multivibrator type. The frequency of oscillation is stabilized by limiting the negative swings at the grids of both tubes to a definite value, preferably some predetermined fraction of the plate supply voltage.

"I claim as my invention: 1.- An oscillator including two electron discharge tubes each including at least a cathode, a control grid, and a plate, a common cathode resistor connected to both of said cathodes and to ground; two cathode follower circuits, each in- 5 .'cluding an. electron discharge tube provided with a the cathode of each of said cathode follower tubes to the control grid of the other of said first mentioned tubes; two further tubes, each having its anode and its cathode connected to the anode and the cathode respectively of one of said cath- ,ode follower tubes, and means biassing the control grids of said further tubes to a potential in excess of the minimum voltage which appears at f' the plates of said first mentioned tubes during operation of the system.

' .2. The invention as set forth in claim 1, in-

cluding meansv applying an adjustable positive bias to the control grids of said first mentioned tubes to control the frequency of oscillation thereof.

{l ;3. A stabilized oscillator of the multivibrator type, including two electron discharge tubes with the plate circuit of each coupled to the grid circuit of the other to produce and sustain oscillations comprising alternate pulses of current in I. said tubes, and limiter means connected between the plate of each of said tubes and the grid of the other to prevent decrease below a predetermined value of the voltage applied to either of said grids, said limited means comprising a cathode follower circuit including a vacuum tube and 2,540,539 5 6 a cathode output resistor and also including v REFERENCES CITED another Vacuum tube havmg an anode to QZ The following references are of record in the ode space path that is connected in parallel confile of this patent.

ducting relation with the first mentioned vacuum tube of the cathode follower circuit, said limiter 5 I, UNITED S A S PATENTS means further comprising means for biassing the Number Name Dat control grid of said other tube to a p tential in 2,207,511 Geiger July 9, 1940 excessof the minimum voltage which appears 1 4 9 Dodington 25 1947 at the plates of the first mentioned l n 2,441,579 Kenyon May 18, 1948 charge tubes during operation of the system. 10

JAMES, R. MOORE.