Multivibrators and Its Types (With Waveforms) | Electronics | Engineering

Multivibrators constitute an important class of pulse and switching circuits.

Multivibrators perform a variety of useful functions like:

i. Generation of square waveforms,

ii. Counting,

iii. Frequency division,

iv. Generation of time-delays, and

v. Storage of binary bit of information.

Types of Multivibrator:

Multivibrators are of the following three types:

1. Astable Multivibrator (also called Free Running Multivibrator):

It generates a square wave of known period. It does not have any permanent stable states. They have two quasi-stable states (i.e., temporary states).

The circuit changes state continuously from one quasi-stable state to another without any external stimulus or trigger after a predetermined length of time. This predetermined length of time is decided by circuit time constants and parameters. Thus, an astable multivibrator generates continuous square waveform without any external circuit. The output waveform of an astable multivibration.

2. Monostable Multivibrator (Also Known as One-Shot Multivibrator or Univibrator):

It has one stable state and another quasi-stable state or unstable state. When an external trigger or stimulus is applied, the circuit state is changed abruptly to unstable, i.e., quasi-stable state for a predetermined length of time and returns to stable state automatically. The time duration of unstable state is by the circuit time constants and parameters and is independent of triggering pulse duration.

It is used to generate precise time delays. The waveforms of a monostable multivibrator.

3. Bistable Multivibrator (Also Known as Flip-Flop, Binary and Scaler of Two Circuits):

The word ‘bistable’ means two stable states. Bistable circuit can stay in a particular state indefinitely. For this reason it can be used to store the binary bit of information.

To change the state of the binary, it is required to apply the external trigger. At the occurrence of each triggering pulse, the circuit state changes abruptly from one stable state to another. An important application of bistable circuit is in counting circuits.

BJT Saturating Astable Multivibrator:

Fig. 3.17 indicates the circuit diagram of an astable multivibrator using BJTs as active devices while Fig. 3.18 indicates the waveforms obtained. R_C1 and R_C2 are the collector resistances for T₁ and T₂ respectively. The capacitors C₁ and C₂ are the coupling capacitors. The capacitor C₁ connects the output of the transistor T₁ to the input base terminal of the transistor T₂. Resistors R_B1 and R_B2 provide ON-state base current to the transistors T₁ and T₂ during saturation region.

For a symmetrical astable multivibrator, we should have-

a. Operation:

To explain the operation of a saturating astable multivibrator, we proceed as follows:

i. At t = 0, when the power supply voltage V_CC gets applied abruptly due to slight mismatch, the current I_C1 flowing in the transistor T₁ is little more than the collector current I_C2 of the transistor T₂. Hence, the rate of fall of V_C1 is more than that of V_C2.

ii. For the transients the capacitors act as short circuits and voltage across them cannot change instantaneously. Thus the changes in collector voltages of T₁ from the initial voltage V_CC to V_C1 (V_C1 < V_CC) will make the base of the transistor T₂ negative which will reduce the conduction and increase the collector voltage V_C1 towards the supply voltage V_CC.

iii. This increase in V_C2 will be transferred through C₂ to the base of T₁ making its voltage more positive and thus, increasing the conduction in T₁. It will further reduce the collector voltage V_C1 which in turn will make the base of T₂ more negative, thus, reducing its conduction more.

iv. In this way due to positive feedback action with the loop-gain greater than unity, whole of the above sequence of operation occurs instantaneously and the transistor T₁ is in saturation region and T₂ is in the OFF region, which is a limiting condition.

Thus, when an astable multivibrator is switched, we have the following two conditions:

i. Transistor T₁ is in saturation region.

ii. Transistor T₂ is in OFF region.

b. Circuit Conditions for the Transistor T₁ in Saturation Region:

For the circuit indicated in Fig. 3.17 we have-

In the saturation region, the following relationship should apply:

The base voltage of transistor T₁ should be about 0.7V, for the silicon transistor. With collector- to-emitter saturation voltage V_{CE sat} ≈ 0.2V, this base voltage will be forward bias for both the junctions.

If we make the simplifying assumption that the transistor T₁ is an ideal one then equations (3.33) to (3.36) will reduce to the following forms:

Circuit behaviour in Quasi-Stable State:

For the time interval 0 < t < t₁ the capacitor voltage will rise from (V_B2)_{0ƒƒ/t = 0} towards V_BB. The charging path for capacitor C₁ is indicated in Fig. 3.19 assuming that (VCE) sat of the transistor T₁ is negligible.

At any instant of time, the voltage on C₁ or (V_B2)_0ff/t can be written as-

Fig. 3.20 shows the charging of capacitor as function of time, due to the presence of emitter-base junction of the transistor T₂ which enters into conduction, when the instantaneous base voltage equals (VB₂)_on. This occurs at the time instant t = t₁.

We have-

a. Behaviour at the Time Instant t = t₁:

At the time instant t = t₁, transistor T₂ goes into conduction. The collector voltage of T₂ begins to fall which is communicated to the base of transistor T₁ through capacitor C₂. As a matter of fact the conduction of T₁ reduces, with the increase of collector voltage of T₁. This increase is communicated to the base of T₂ through capacitor C₁ and thus, increases its conduction. This regenerative process continues and as a result transistor T₂ is in saturation region and T₁ is in the off region instantaneously.

b. Circuit behaviour during Quasi-State (t₁ < t < t₂):

During this time period, capacitor C₂ charges from (V_B2)_off/t=0, towards V_BB. At the time t = t₂, the instantaneous base voltage is (V_B) on which brings the transistor T₁ into conduction. The time interval (t₂– t₁) may then be expressed as-

Then equation (3.50) reduces to the form-

Equation (3.51) shows that to the first approximation, the time period of astable multivibrator is independent of supply voltage, temperature and junction voltages.

Voltage-Controlled Astable Multivibrator:

When the supply voltage V_BB is not same as V_CC, then with equations (3.45) and (3.48), the period T for symmetrical multivibrator can be expressed as-

From equation (3.52) it can be inferred that time period T can be varied by varying V_BB. However, this variation is not linear.

Dual Power Supply Monostable Multivibrator:

In a monostable multivibrator, one of the state is permanent, i.e., stable and the other is temporary, i.e., quasi-stable. When an external trigger is applied to the monostable at appropriate point the monostable changes its state from stable to quasi-stable. It stays in the quasi-stable state for a predetermined length of time and returns to stable state automatically. Monostable multivibrator can also be realized by BJT, JFET and a negative resistance device.

A dual PS monostable multivibrator is indicated in Fig. 3.21(a). Here the negative supply V_BB through resistors R₁ and R₂ keeps the transistor T₁ in reverse- biased condition in the stable state. Fig. 3.21 (b) shows the equivalent circuit in the stable state, i.e., the transistor T₁ in the OFF and the transistor T₂ in the ON is states.

Dual Power Supply BJT Bistable Multivibrator:

Fig 3.22 shows the circuit diagram of dual power supply bistable multivibrator. The dual power supply bistable has its output swing from V_CC volts to zero neglecting the collector-emitter saturation voltage (V_CE) of the transistor. This output swing is more compared to that in self-biased bistable multivibrator, where the output swing is from V_CC to V_EN only.

The stable states of the circuits are:

i. T₁ OFF and T₂ ON

ii. T₂ OFF and T₁ ON