US2904756A - Astable multivibrators - Google Patents

Description

Sept. 15, 1959 M. J. G. HINTON ASTABLE MULTIVIBRATORS Filed May 13, 1957 ttorney United States PatentOfiice Patented Sept. 15 1959 ASTABLE MUETIVIBRATORS Maitland Joseph Gresley- Hinton, Belmont, England, assignor to NagardLimited, .Belmont,.England, acorns.

pany of Great Britain Application May 1 3, 1957, Serial No.-658,'677

2. Claims (CL. 331-144).

This invention relates to astable; multivibrators of the I kind in which two valves are connected in a circuit With capacitive coupling between-"the anodeof each valve and the control grid of the other; The invention has for its principal object to enable'greater accuracy-in the calibration of multivibrators-of this'kind:

The applicant'has' found that an important Aca-use of frequency instability in a multivibrator over a-period of=time is the variation-during the-life of the-valves of the maximum current. which-the valvescan pass; and that an importantcause of frequency variation-When the valves arereplaced is the variation of this current from one valve to: anot-lier;

According to the present invention the control grids of the two valves are each connectech through-'ah individual rectifier to a point of steady potential the rectifier beingso arranged that the grid potential isnotperrnitte'cl to=risepassediby each valve; to less than its,- fullemission, current.

In order that the inventionmaybebetter understood.

an embodiment thereof will now be described with reference to the aecompanyinggdrawingsin which:

Figure l is a. diagram. ofa conventional multivibrator circuit; 7

Figure 2 shows waveforms atfour points in the circuitof Figure I; and-- I Figure 3 shows a multivibrator circuit according to the invention.

In the known form of multivibrator shown in Figure 1 the two electron discharge valves V1 and V2 are connected through anode resistors R1 and R4 to a positive H.T. supply conductor 10. In addition the anode of V1 is connected through a capacitor C1 to the control grid of the triode V2 and the anode of triode V2 is connected through capacitor C2 to the control grid of valve V1. Leak resistors R2 and R3 are connected between control grids of V2 and V1 respectively and the positive conductor to enable charges on the two capacitors to leak away. The resistors R2 and R3 may be connected to earth instead of the positive conductor 14 At any given time one of the valves in the circuit of Figure 1 is conducting and the other non-conducting, and the states of conduction of the two valves are reversed when the charge on the capacitor connected to the control grid of the non-conducting valve has leaked away to an extent sufiicient to enable the latter to conduct. In Figure 2, prior to moment a, valve V1 has been nonconduc'ting and its anode potential Va; is therefore at a high potential, and valve V2'has been conducting, with its anode potential Va at a low value. At moment a, the control grid potential Vg of valve V1 reaches a level at which the valve begins to conduct. The anode potential of this valve immediately falls, taking With it the potential of the control grid of valve V2, as shown by the controlgrid potential Vgg in-Figur'e 2. The fall-Kin the" control gridpotential of valve V2 reduces anode current through this valve and increases i-tsranode potential, which".

results in the application of a positive potential through" the capacitor C2 to'thecontroi'grid of-valve VI. This" positive potential increases conductionin the valve vl" still further andthe processcontinuesin cumulative man neruntiltheconduction "in valve-Vl reachesitsr limitr The' change-ever of'cond'uct'ion from valvevftovalvef V2tak'es"place in an extremely short'p'eri'o'd of timearrd" can be considered to' commence and be' comp'leted at thesamemoment min Figure 2. As-soon as thecumula r tive action stops, the charge on capacitor C1""be'gin's -to leak away through resistor R2 and'this continues until atmoment I) thepotential on the-control gridofvalve V2 issufiicient to enable the latter-valve to conduct. Thecumulative process is then reversed 'until valve V2 is' fully conductingand valve fl cut' off." At moment'-c'the process is again reversed and is thereafter repeateddridefinitelyg' theosci'llationsproduced on theanodesof-thetwo valves being approxirnately" in the-form of square waves."

A- di-fiiculty" wit-h: multivibrators of the form" shownin" Figure l is-tha-t theirfrequency va-ries'during theli'fe" of the valves; and alsofrequently varies considerably when the' valves are replaced by othersof'the same type. A

further disadvantage of this circuit is that the 'vvaveforms produced at the=- anodes'ofthet-wo valves are not perfect square waves.

in attempting tofollow-- the anode potential of the other waveform-Vg at moment 11 andi n the' waveforrn'ivg at moments a and c, and leads to the-produetion'ofia large-negativepeak in the corresponding anodefpotential I at these points.

-Witha symmetrical multivibrator circui't'the period of one cycle is'equal to twice the time occupiedjby theris Ling grid waveform, and the frequency of the' oscillations" produced by the multivibrator" is therefore governed by the length' of time taken byeach grid-capacitor to discharge to such an extent that the grid potential of the valve to 'which it is connected is suflicient to cause this valve to conduct. This length of time depends primarily upon three factors. The first of these is the time constant of the capacitor and its leak resistance. The second is the potential to which the leak resistor is connected, this potential being usually that of the high tension conductor, as in Figure 1 or earth potential. The third factor is the value of the negative potential excursion of the control grid, due to the negative potential applied to the grid through the coupling capacitor as the other valve begins to conduct.

The first two factors can be controlled accurately since the first depends on the values of the capacitor and leak resistor, and the second is fixed at the potential of the HT. conductor or at earth potential. The third factor, however, is governed by the fall in potential at the anode of the conducting valve and this fall in potential is determined by the maximum current which the valve can conduct. The value of the maximum current depends upon the emission characteristic of the cathode of the valve and varies with the temperature and age. Moreover, the maximum current varies between valves of the same type. The result of this is that the frequency of the oscillations produced by a multivibrator having a particular pair of valves may be incorrect by as much as 10% of the calibrated value, and even larger variations The reason for this is that as the" multivibrator-switches frorn'on'e conducting state to the other, the grid of the valve-which is becoming conductive;

may occur when the valves are replaced by others of the same type.

The apparatus shown in Figure 3 greatly reduces this variation by determining the amplitude of the negative potential excursion at the anode of a valve which begins to conduct by means other than the emission characteristic of the valve. This circuit is generally similar to that shown in Figure l, but the control grids of valves V1 and V2 are connected through rectifiers D1 and D2 respectively to earth, the control grids being connected to the anodes of the rectifiers so that they are prevented from rising significantly above earth potential. Furthermore, the cathodes of valves V1 and V2 are connected through a common resistor R5 to a negative supply line 12, which may have a potential of, for example, -150 volts.

The cathodes of these two valves follow closely the grid potential of the conducting valve and are therefore effectively at earth potential. The current in the common cathode resistor R5 will thus be given by the value of the negative potential of the conductor 12 divided by the resistance of resistor R5. This current can, therefore, be precisely controlled. As only one valve is con ducting at any moment the current flowing through resistor R5 is the current flowing through the conducting valve and this current is switched from one valve to the other as the multivibrator changes its state of conduction. Since this current is constant and of known value,

the amplitude of the negative potential excursion of the valve anodes can be selected by choosing a suitable anode resistor, the amplitude of the negative excursion being given by the product of the valve current and the resistance of the anode resistor.

The resistor R5 has a value such that the current passing through either valve is always less than the full emission current of its cathode. Thus the negative po tential excursions of the anodes of the valves are almost independent of the characteristics of the valves themselves, and the square waves produced will be constant in amplitude.

The calibrated frequency of a multivibrator according to the present invention will therefore be substantially correct in spite of variation of the emissivity of the cathodes and in spite of the replacement of the original valves by others of the same type.

A further advantage obtained with a multivibrator according to the present invention is that the waveform of the oscillations produced at the anodes of the valves is more square, the peaks in the anode waveform being reduced in size. This is due to the reduction in amplitude of the peaks in the grid waveforms, which is in turn due to the fact that the capacitors are now charged through two diodes instead of by grid current flowing in the grid-cathode paths as in the circuit of Figure 1. The forward resistance of the diodes is much less than that of the grid-cathode path with the result that a much smaller peak is produced in the grid waveform before the charging current takes control and holds the grid substantially at earth potential.

The rectifiers employed may be metal rectifiers, but in the preferred embodiment thermionic diodes are used since they enable oscillations of a more stable frequency to be produced.

The circuit according to the present invention also improves the frequency stability of the output waveform during changes of heater voltage.

It is to be understood that the use of triode valves in the circuit of Figure 3 is described only by way of example. The invention is also applicable to circuits using other types of valve, such as tetrodes or pentodes, although it is most effective when triodes are employed.

I claim:

1. An astable multivibrator of the kind employing thermionic valves and having capacitive coupling between the anode of each valve and the control grid of the other valve, in which the control grids of the two valves are each connected through an individual rectifier to a point of steady potential, each rectifier being so arranged that the grid potential is not permitted to rise significantly above the potential of the said point, and the cathodes of the two valves are connected through a common resistor to a second point of steady potential which is negative with respect to the first point, the resistance of the said resistor being sufiicient to limit the current passed by each valve to less than its full emission current.

2. A multivibrator according to claim 1 in which the two rectifiers are thermionic diodes.

Geiger May 23, 1939 Geiger July 9, 1940

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