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Definition of a square wave | Astable Multivibrator with Electronic Pipes "la Triode" | ABRAHAM BLOCH Multivibrator |

Astable Multivibrator with Cathodic Coupling | CHARBONNIER Multivibrator |

**Electronic Tube Multivibrators : **

After having seen the functioning of the electronic tube triodes exposed in the previous lessons, we will study an extremely important category of circuits in electronics **: Multivibrators.**

**Multivibrators are square wave** generators.

They are divided into three categories **: astable, monostable, bistable multivibrators.**

The former produce a **square waveform with no control signal.**

The second ones **deliver a square pulse, after the application of a control pulse.**

The latter produce a sudden change in voltage, each time they are controlled **by an impulsive input signal.**

Before beginning the study of these circuits, it is necessary to give some precisions concerning the square or rectangular waves.

**
1. - DEFINITION OF SQUARE WAVE**

If a phenomenon reproduces exactly in regular time intervals, it is called **PERIODIC.**

The time interval **T** separating two identical phenomena is called **PERIOD.**

Frequency is the speed with which the phenomenon recurs (**F = 1 / T**).

Figure 1 shows a square wave.

On this graph, we have reported times **T, T1** and **T2.**

**T** represents the period of the signal (time interval separating two consecutive slots).

**T1** corresponds to the working time, that is to say to the positive part of the signal.

**T2** represents the rest time, that is to say the time during which the voltage remains zero.

The square wave shown in Figure 1 is characterized by its **shape factor**, its **duty cycle** and **its amplitude.**

The form factor applies to signals with high asymmetry.

For the signal of Figure 1, the form factor is equal to **T1 / T** and it represents the duration of the short signal with respect to the total period.

If **T2** was shorter than **T1**, it would be equal to **T2 / T.**

The duty cycle is obtained by dividing the working time by the rest period (**T1 / T2**).

**
2. - ASTABLE MULTIVIBRATOR**

It was invented in 1918 by **ABRAHAM BLOCH.**

It is a square wave generator formed by a two-stage amplifier with **RC** links, but whose output is connected directly to the input.

Figure 2 shows the diagram of an **astable multivibrator**, often called the **ABRAHAM BLOCH multivibrator**, equipped with two triodes.

**STARTING THE ASSEMBLY :**

As soon as the high voltage is applied, the two tubes **V1** and **V2** begin to drive.

Although the mounting is symmetrical, currents **Ia1** and **Ia2** are not equal.

Indeed, it is necessary to take into account the tolerances of the elements which make that their values are not rigorously identical.

Suppose that **Ia1** increases faster than **Ia2**, so **Va1** decreases faster than **Va2.**

The capacitor **C1** instantly transmits the voltage variation **DVa1** across the resistor **Rg2** and **C2** transmits the voltage variation **DVa2** across **Rg1.**

These voltage variations are negative and we can say that **Vg2** is more negative than the voltage **Vg1.**

As a result, the tube **V1** leads much more than the tube **V2.**

The phenomenon amplifies rapidly (cumulative effect) and **V2** is blocked, while **V1** leads to saturation.

At the same time, as soon as the circuit is energized, the capacitor **C1** quickly charges through **Ra1** and through the resistance equivalent to **Rg2** and to the internal grid-cathode resistance of the tube **V2.**

Similarly, the capacitor **C2** charges through **Ra2** and through the resistance equivalent to **Rg1** and internal resistance gate-cathode tube **V1.**

The charges of **C1** and **C2** are very fast because they are carried out through resistors of low values (anode resistors and gate-cathode resistors of each tube).

Figure 3 shows the charging circuits of the capacitors **C1** and **C2**, as well as the polarities of the voltages appearing across **Rg1** and **Rg2.**

**DETAILED OPERATION OF THE ASSEMBLY**

To explain the operation of the multivibrator, we will use Figure 4 where are represented the voltages **Va1, Va2, Vg1, Vg2,** and decompose a period in time intervals corresponding to different phases of operation.

Consider editing at time **t0**, that is just after the start period.

**-** **V1** strongly drives and **Va1** is equal to **Vo - Ra1 x Ia1** that is to say a value lower than **Vo** (**Va1 mini**).

**-** **V2** is blocked, so **Va2** is equal to **Vo.**

**-** **Vg1** is very close to the potential of mass, since **Rg1** does not receive any variation coming from the anode of **V2.**

**-** **Vg2 **is strongly negative (well below the blocking voltage).

**-** Capacitors **C1** and **C2** are charged to a value close to the high voltage.

Between **t0** and t1 :

The capacitor **C1** charged to the high voltage sees the tension of its positive armature decrease sharply, as soon as the tube **V1** begins to drive.

It can then be discharged through the internal resistance of **V1** and the resistance **Rg2.**

**This discharge is slow** because the gate resistance **Rg2** has a high value.

The discharge current **Id1** causes a voltage drop across **Rg2**, according to the polarities indicated in Figure 5.

This negative voltage on the gate of **V2** keeps the tube blocked, but it decreases as the load is extended.

The voltages **Va1** and **Va2** respectively keep their value Va1 mini and **Vo.**

At the instant **t1 :**

The gate voltage **Vg2**, which rises slowly towards the ground potential, reaches the cut-off voltage of the tube **V2.**

The tube **V2** starts to drive and therefore the voltage **Va2** goes from the value **Vo** to the value **Va2 mini.**

This sudden negative variation of the anode voltage of **V2** is transmitted instantaneously to the gate of **V1** by the capacitor **C2.**

The tube **V1** is blocked and the anode voltage **Va1** rises from **Va1 mini to Vo.**

In reality, the voltage change on the anode of **V1** is in an exponential form.

Indeed, as soon as the voltage **Va1** increases, the capacitor **C1** is charged through the circuit described in **Figure 3**.

The charging current **Id1** brakes the rising of the anode voltage and causes the appearance of the small positive peak across the resistor **Rg2.**

At the instant **t1** to **t2 :**

The tube **V1** is blocked and the tube **V2** leads.

The capacitor
**C2** can be discharged through the tube **V2** and the gate resistor **Rg1** of the tube **V1** (**see Figure 5**).

The discharge current **Id2** determines a negative voltage on the gate of **V1** and maintains the latter blocked.

This negative voltage (**Vg1**) rises slowly towards the ground potential until time **t2.**

Between instants **t1** and **t2**, we find that **Va1** remains equal to **Vo** and **Va2 to Va2 mini.**

At the moment **t2 :**

the capacitor **C2** is almost discharged and the negative gate voltage **Vg1** reaches **the cut-off voltage of the tube V1.**

The latter begins to drive and sees its anode voltage Va1 decrease sharply.

The negative variation is transmitted instantaneously to **Rg2** by the capacitor **C1.**

The tube **V2** is blocked and the anode voltage **Va2** goes from the value **Va2 min to the value Vo.**

This rise is exponentially because during this time, the capacitor **C2** is charging again to the high voltage through its circuit shown in Figure 3.

Just as at time **t1**, the charge of capacitor **C2** causes the rounding of voltage **Va2** and the small positive peak of voltage **Vg1.**

This positive voltage of the gate voltage of the tube **V1** has an effect on the anode voltage **Va1.**

Indeed, at this precise moment, the gate becoming positive, the tube leads even more strongly and the anode voltage decreases slightly.

At the instant **t2** to **t3** **:**

The tube **V1** leads and **Va1 is equal to Va1 mini.**

The tube **V2** is blocked and **Va2 is equal to Vo.**

The gate voltage **Vg1** remains in the vicinity of the ground.

The gate voltage **Vg2** rises slowly according to the time constant of the discharge circuit of the capacitor **C1.**

At the moment when this voltage is equal to the **cut-off voltage** of the tube **V2** (time **t3**), it is released and the cycle can start again.

On the anode of the tube **V2**, a square-shaped voltage is collected.

**The frequency of this wave depends on the time constants of the discharge circuits of the capacitors C1 and C2. **

We have seen that the successive discharges of these capacitors cause the unblocking of the tubes and consequently the tilting of the assembly.

If the discharge circuits of the capacitors **C1** and **C2** are not identical, the duty cycle of the square wave obtained at the output will be different from **1.**

For example, if **C1** is greater than **C2**, the tube **V2** will remain locked longer than the tube **V1.**

If we take the output signal on the anode of **V2**, we will have a longer working time than the rest time and, consequently, a **T1 / T2 duty cycle greater than 1.**

**
2. 1. - IMPROVEMENTSOF THE MULTIVIBRATOR ABRAHAM BLOCH**

We have seen that the square tension delivered by this assembly is quite deformed (front fronts rounded and rear fronts having a negative peak with respect to the voltage **Va mini**).

This peak is due to the fact that the gate receives a positive voltage during the charging of the capacitor which is connected thereto.

To prevent the gate voltage from becoming positive, **stop resistors** are mounted as in Figure 6.

These resistors must have a value much greater than that of the grid-cathode space. In this way, almost all of the positive impulse is dropped across **Rs.**

This system nevertheless has a serious disadvantage.

The stop resistors are put in series with the charging circuits of the capacitors **C1** and **C2.**

The time constant of the circuit is therefore longer and it follows that the fronts leading to the square wave are more rounded.

To overcome this disadvantage, we can use penthodes tubes.

The charge of the capacitors **C1** and **C2** is no longer made through the anode resistances but through the screen resistors **RE1** and **RE2.**

Since there is no more load by **Ra1** and **Ra2**, the rising edges are practically rectilinear and no longer exponential.

Figure 7 shows a multivibrator equipped with two penthodes

Despite all the improvements made to the **ABRAHAM BLOCH multivibrator**, this one still presents a major defect **:** **the instability in frequency.**

The instability is due to the angle of the exponential rise of the gate voltage, with the line representing **the cut-off voltage of the tube** (Figure 8).

When the gate resistors are connected to ground, the exponential rise is towards **the potential 0.**

A small external variation causes the cut-off line to cut off earlier or later and causes large differences in frequency.

By connecting the gate resistors directly to the high voltage, the discharge circuit passes through the high voltage source and the exponential rise is in the direction of the high voltage **Vo.**

In this way, small external variations cause only small displacements of the cutoff point between the gate voltage and the line representing the cut-off voltage.

With this means, the stability of the whole is significantly improved.

Figure 9 shows an astable multivibrator, whose gate leakage resistors **Rg1** and **Rg2** are connected to the high voltage.

With this assembly, it is necessary to rethink the value of the elements, because the gate resistances **Rg1** and **Rg2** must have very high values, so that the gate potentials close **to 0 volt.**

**
3. - ASTABLE MULTIVIBRATOR WITH CATHODE COUPLING**

This multivibrator is an astable and asymmetrical assembly.

The cathodes are united and connected to the ground by a polarization resistor **Rk.**

The schematic diagram of such a circuit is shown in Figure 10.

As soon as the high voltage is applied, the two triodes start driving, but the voltage drop of **Va1** is transmitted to **Rg2** by the capacitor **C.**

Since this variation is negative, we have a negative voltage on the **V2** grid.

Very quickly, we get **V2 blocked** and **V1 driving normally.**

Simultaneously with this, capacitor **C** charges at high voltage across **Ra1** and **Rg2.**

The gate of **V1** is connected to the ground by a low value resistor and is practically grounded.

**V1** leading, the capacitor **C** can exponentially discharge through the internal resistance of **V1** and the resistance **Rg2.**

The current in **V1** creates a voltage drop **Vk** in the resistor **Rk** and the cathodes are at a positive potential with respect to the ground.

As soon as **Vg2** reaches the critical point of cut-off, a current is established in **V2.** The voltage drop in **Rk** becomes more positive and that is to say that the grid of **V1** becomes more negative.

The current **Ia1** in **V1** decreases and the voltage **Va1** increases.

This variation of the potential of **Va1** is plotted on the gate of **V2** through capacitor **C**, which further increases the current in **V2 ;** hence, further increase of **Vk** and faster decrease of the current in **V1.**

The effect being cumulative, **V1** is quickly blocked while **V2** leads to the maximum.

As soon as **V1 is blocked** (**Va1 = Vo**), the capacitor **C** is charged through **Ra1** and **Rg2.** When this charge is complete, the gate of **V2** is brought back to the potential of the mass **;** the current in **V2** then decreases slightly, which causes a decrease in the cathode voltage.

This reduction is sufficient to unlock the **V1** tube that starts driving again.

Current **Ia1** creates a voltage drop in **Ra1**, which is plotted on the grid of **V2.** The cumulative action quickly brings **V2** to the prohibition and **V1** to the maximum conduction.

Capacitor **C** can discharge through **V1** and **Rg2** and a new cycle begins again.

In Figure 11, you will find the different waveforms present in the circuit that we have just described.

**
4. - MULTIVIBRATOR CHARBONNIER**

This cathodic coupling multivibrator, represented in Figure 12, is a variant of the previous assembly.

The gate voltage of the tube **V1** appears across the capacitor **C**, which forms with **R2** a long **RC.**

As soon as the high voltage is applied, the two tubes begin to drive. But the gate of **V2** is brought to a positive potential by the resistor bridge **Ra1, R1** and **Rg2** placed between the high voltage and the ground.

The tube **V2** thus leads much more than the tube **V1.**

The gate of **V1** is at a potential close to ground (the capacitor **C** has received no charge) and the current **Ia2** creates a positive voltage across **Rk.**

Very quickly, we get **V1 blocked** and **V2 driving heavily.**

The positive voltage **Vg2** appearing across the terminals of **Rg2** is also applied across the **RC** circuit formed by the resistor **R2** and the capacitor **C.**

Capacitor **C** will thus slowly charge through **R2** at voltage **Vg2.** As soon as its armature connected to the gate of **V1** is at a positive potential corresponding to the unblocking voltage of the tube **V1** (**Vg1 = Vk - V cut-off**), the latter begins to conduct and its anode voltage **Va1 decreases.**

The decrease of **Va1** causes a decrease of **Vg2** (thanks to the resistors **R1** and **Rg2**) sufficient to block the tube **V2.**

The voltage drop of **Vg2** is also present across **R2.** Capacitor **C** can therefore be discharged through **R2** and **Rg2.** (It should be noted that the discharge can not be made through the gate-cathode space of **V1** and **Rk**, because the voltage **Vk** is higher than the voltage **Vg1**).

The voltage **Vg1** decreasing exponentially, the tube **V1** leads less and less and the voltage **Vk** is less and less important.

After a certain time, the gate voltage **Vg1** is no longer sufficient to maintain the tube in conduction and the latter is blocked. At the same time, the voltage **Va1** goes back to the potential **Vo.**

This positive variation is transmitted by the resistance bridge **R1** and **Rg2** to the gate of **V2.**

The increase of the gate potential and the low cathode polarization voltage make the **V2** tube very easy to unlock.

The current **Ia2**, very important, causes a high voltage drop in Rk which keeps the tube **V1** to prohibition.

The edit has returned to the starting conditions (**V2 leads, V1 blocked**) and a new cycle can start again.

The signal collected on the anode of **V1** has deformations, due to the fact that the current **Ia1** follows the variations of the gate voltage **Vg1.**

On the other hand, the signal collected on the anode of **V2** is perfectly rectangular.

The frequency of the square wave delivered by a **CHARBONNIER multivibrator** is relatively stable and largely determined by the time constant of the circuit **R2 C.**

By design, this assembly can only provide perfectly symmetrical slots (cyclic ratio equal to 1).

To transmit more easily the variations of the anode voltage **Va1** on the control gate of **V2**, the assembly can be improved by connecting a small capacitor across **R1.** This capacitor is marked in dotted line in Figure 12.

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