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  Bistable Multivibrator to electronic tubes "la Triode"       Footer   

Electronic Tube Multivibrators - Monostable ... :


The monostable multivibrator, sometimes called FLIP-FLOP or UNIVIBRATEUR, presents a stable state and an unstable state.

A trip pulse switches the mount to the unstable state. Then, the assembly returns of itself to the stable state.


This assembly, shown in Figure 13, differs from the astable cathodic coupling multivibrator by two points :


1°) The gate leakage resistance Rg2 is connected between the control gate of the tube V2 and the cathode of the same tube (and not between the gate and the ground).

2°) The resistance Rg1 which determines the quiescent potential of the gate of V1, receives positive triggering pulses.


As soon as the high voltage is applied, both tubes V1 and V2 tend to drive.

But the capacitor C1 charges through Ra1, the resistance equivalent to Rg2 and the gate-cathode space of V2 and Rk (see the load circuit of C1 in Figure 14).


The charging current of this capacitor creates a positive voltage across Rg2.

The V2 gate being positive with respect to the cathode, the V2 tube leads to the maximum.

The anode current Ia2 creates in Rk a positive voltage such that it suffices to keep blocked the tube V1, even when the charge of C1 is complete.

In this type of assembly, we always have, at the beginning, V2 leading to the maximum and V1 blocked.


The different waveforms recorded on this assembly are given in Figure 15.


At time t1, a positive pulse arrives at the control gate of the tube V1.

The tube V1 starts to conduct and the anode current Ia1 causes a voltage drop in the resistor Ra1.

This negative variation is retransmitted by the capacitor C1 on the control gate of the tube V2.

This is blocked and the anode voltage Va2 rises from the value Va2 mini to the value Vo.

From t1 to t2, the current Ia1 causes a voltage drop in the resistor Rk, but this voltage is not large enough to cause the blocking of V1.

V1 leading, the capacitor C1 can discharge through its internal resistance r and the resistor Rg2 (Figure 16).


The discharge current Id causes across the resistor Rg2 a negative voltage sufficient to keep the tube V2 blocked.

During the discharge, the voltage Vg2 rises slowly towards the potential 0, according to the time constant of the discharge circuit.

At time t2, the voltage Vg2 reaches the cut-off of the tube V2 and the tube begins to drive.

The current Ia2, crossing Rk causes a voltage Vk very much higher than the cut-off voltage of the tube V1.

The latter is blocked and its anode voltage Va1 goes from Va1 mini to Vo.

This rise is not done instantly, because at the same time, the capacitor C1 is charged through Ra1, Rg2 and Rk.

The charging current maintains an exponential form voltage across the resistor Ra1, and causes a small positive peak on the gate voltage Vg2.

From t2 to t3, the assembly remains in its stable state, that is to say :

  • V2 leading to the maximum (Va2 = Va2 mini)

  • V1 blocked by voltage Vk (Va1 = Vo)

  • C1 charged at high voltage.

At time t3, a new positive pulse on the grid of V1, switches the assembly and a new cycle begins again.


The square voltage collected on the anode of the tube V1 has defects in shape (exponential rise and peak below the value Va1 min).

This voltage collected on the anode of V2 is already more acceptable, but also has negative peaks with respect to the minimum voltage due to the increase of the anode current, when the gate voltage Vg2 becomes positive.

As in the multivibrators, this defect can be mitigated by placing a stopper resistance between the resistor Rg2 and the control grid of the tube V2.

This resistance drops a part of the positive voltage appearing on the gate, and, consequently, the increase of the anode current is less important.

The stopper resistor decreases the peak of the voltage Va2, but it accentuates the rounding of the voltage Va1.

Indeed, the resistor is placed in series in the load circuit of the capacitor C1. The time constant of the RC circuit is lengthened and the exponential rise is slower.

Since the signal used is in most cases the one appearing on the anode of V2, we consider the defect brought by the stopper resistor on the anode signal of V1 to be negligible.

The frequency of the multivibrator is very stable because it is slaved to the input signal.

Only the cyclic ratio of the square wave can vary, because the exponential rise of Vg2 intersects the line representing the cut-off voltage at an angle too low, which causes a release of V2 before or after the normal time.

To remedy this defect, the gate resistor Rg2 is connected to the high voltage (this mode of connection has already been explained during the study of the ABRAHAM BLOCH multivibrator).

Figure 17 shows an improved multivibrator with a stop resistor and whose gate resistor Rg2 is connected to the high voltage.




This command mode is the most used ; it is the one we saw during the explanations concerning the operation of the monostable multivibrator.

The positive pulses cause the unblocking of the tube V1 and consequently the tilting of the assembly from the stable state to the unstable state.


When only negative pulses are available, these can be applied to the control gate of V2 (Figure 18).


The operation of the assembly remains identical to that which we have just explained.

The negative pulse causes a decrease in the current Ia2, therefore the voltage Vk.

This sudden decrease of Vk unblocks the tube V1 and allows the discharge of C which carries the tube V2 to the prohibition.


This injection mode requires very large amplitude pulses.

The negative signal applied in this way decreases the voltage of Va1. The capacitor C transmits the variation on the gate of V2 and the tube V2 is blocked. The voltage Vk decreases and V1 begins to drive.

The capacitor C can be discharged through V1 and Rg2 and the discharge current keeps the tube V2 blocked until the voltage Vg2 reaches the unlocking voltage.

In most cases, the signal applied to the monostable multivibrator is composed of positive and negative pulses (these pulses are obtained by applying a square voltage to a differentiating RC circuit).

The control being carried out either with positive peaks or with negative peaks, it is necessary to eliminate unnecessary pulses.

For this purpose, a diode connected is used so as to let only the pulses having the desired polarity pass.

Figure 19 shows a univibrator controlled by the negative pulses on the anode of V1.

Diode D blocks the positive pulses and passes the negative pulses.



The assemblies we are going to study now, have two stable states and they are essentially designed to move from one state to another, when sending a command pulse on a given input. These are the bistable multivibrators.


The diagram of a bistable multivibrator "ECCLES JORDAN" is given figure 20.


This assembly is deduced from the "ABRAHAM BLOCH" multivibrator by replacing the two connecting capacitors with resistors.

The new circuit now has two direct links. It is always a tilting mount (one of the tubes is conductive while the other is blocked), but because of the direct links, one or the other state is maintained indefinitely, as long as no external cause modify it.

The control is effected by positive pulses applied simultaneously, through C1 and C2, to the control gates of the two triodes.


As soon as the circuit is switched on, both tubes start driving.

Despite the symmetry of the circuits, currents Ia1 and Ia2 are not perfectly equal.

These two currents, crossing Rk, give a cathode polarization Vk common to both tubes.

The gate bias voltages of the triodes are given by the currents flowing in the resistor bridges R1, Rg2 and R2, Rg1.

These voltages are positive and depend directly on the values taken by the anode voltages of each tube.

The grid voltage of the tube V1 is given by the expression :


We immediately notice that for each tube, the gate and cathode voltages are positive.

The positive voltage on the cathode having the same effect as a negative voltage on the gate, we can write that the bias voltage Vgo is equal to the difference between Vk and Vg.

For the tube V1, we obtain Vgo1 = Vg1 - Vk and for the tube V2, Vgo2 = Vg2 - Vk.

For the tubes to operate under normal conditions (negative Vgo or at most 0 volt), the gate voltages Vg1 and Vg2 must be lower than the cathode voltage.

Suppose Ia1 is larger than Ia2. The voltage drops in the resistors Ra1 and Ra2 are unequal and therefore Va1 is lower than Va2.

Since R1 = R2 and Rg1 = Rg2, the values Vg1 and Vg2 are no longer equal and we obtain Vg1 larger than Vg2.

The polarization of the tube V1 (Vg1 - Vk) is lower than that of the tube V2 (Vg2 - Vk). As a result, the triode V1 leads even more than V2.

Very quickly and thanks to the cumulative effect, the tube V1 will lead to the maximum and V2 will be blocked, (polarization below the cut-off voltage).

Without external action, the system remains infinitely in this stable state.

Apply to the input E, a positive pulse of sufficient amplitude to unlock V2.

This impulse also arrives on the grid of V1, but as this tube already leads, it does not cause any important reaction.

On the other hand, the pulse unlocks the tube V2. This results in a sudden decrease in the anode voltage Va2 and consequently a decrease in Vg1.

At the same time, the anode current Ia2 is added in Rk to the anode current of V1, and the voltage Vk increases.

These two combined effects cause the bias voltage Vgo1 to fall below the cut-off voltage of the tube V1.

Ultimately, after the trigger pulse, we get V1 blocked and V2 driving strongly.

The assembly remains in this stable state until a new impulse comes to unblock the tube V1 and causes a new switch which returns the multivibrator to the starting position. Figure 21 shows the different forms of tension that we can see on this assembly.


We note the appearance of peaks on the anode voltages of the two tubes. They are due to the positive control pulses which cause a conduction of the tube that they release.

The bistable multivibrator we have just studied is controlled by positive pulses.

It is also easy to control the assembly by negative pulses applied to the control gates of tubes.

The first pulse, instead of unblocking the tube V2, blocking the tube V1. The rise of the anode potential Va1 has repercussions on the grid of V2 which starts to drive.

This control mode is widely used in practice, because these pulses do not make the grid positive with respect to the cathode and therefore the small peaks, due to the conduction of the tubes at the time of release, disappear.

Another method is to send the control pulses (positive or negative) directly to the cathodes. The operation is the same but this arrangement has the advantage of using only one input capacitor since the cathodes are interconnected.

The two capacitors C3 and C4, of low values (50 to 100 pF), connected in parallel on R1 and R2, do not introduce a time constant peculiar to the circuit.

Their role is to transmit instantaneously on the opposite grids, variations in anode voltage and thus promote changes in the mounting state.

The "ECCLES JORDAN" bistable multivibrator controlled by negative impulses, delivers a nearly perfect square tension.

The frequency of the signal is constant since it depends solely on the frequency of the control pulses and the duty cycle is equal to 1.

If we want to obtain an asymmetrical square wave (cyclic ratio other than 1), we must separate the inputs on the control gates and inject two pulse trains of the same frequency but having a constant offset over time (Figure 22).


We thus finish our lessons of explanations of the astable, monostable and bistable multivibrators concerning the electronic tubes, and, we will continue the multivibrators based on transistors.

The astable, monostable and bistable multivibrators based on transistors work in the same way as the astable, monostable and bistable multivibrators with electronic tubes and have the same role.


Nombre de pages vues, à partir de cette date : le 27 Décembre 2019

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