Astable Multivibrator

Definition: An astable multivibrator is a type of multivibrator that does not require a triggering pulse thus, sometimes known as a free-running multivibrator. It necessarily provides square wave as its output.

Astable multivibrator is called so because both the states present here are unstable i.e., quasi-stable. This means output swings between 0 and 1 and neither 0 nor 1 is the stable state.

Here, the successive transition is made from one quasi-state to other after some pre-decided time interval.

Circuit and Operation of Astable multivibrator

In this section, we will discuss the operation of an astable multivibrator by collector coupling and by using a complementary pair of transistors. Let us first understand collector coupled astable multivibrator.

1. Collector coupled Astable Multivibrator

The figure below shows circuit of an astable multivibrator:

The two transistors Q₁ and Q₂ are employed here which are back to back connected. It is to be noted here that the two transistors cannot be exactly identical. This means, on applying power V_cc both transistors starts conduction.

But, due to variation in operating characteristics, one conducts more than the other.

Let us assume that Q₁ conducts more than Q₂, so we consider the conduction of Q₂ as almost negligible. Hence, considering only Q₁ is conducting.

When Q₁ starts conducting it starts to move towards saturation region. This causes the collector potential V_c1 at Q₁ to become minimum. As we can see in the figure above that collector of Q₁ is connected to the base of Q₂. So, the low voltage at the collector of Q₁ is fed to the base of Q₂.

Due to applied low potential, transistor Q₂ reaches the cut-off region, thereby causing an open circuit. This causes, a high potential at the collector of Q₂ represented by V_c2.

This increased potential as shown in the figure above is supplied at the base of transistor Q₁. Thus, causing it to get short circuit by reaching a saturation region.

Hence, the assumption taken at the beginning is satisfied.

After some duration, Q₁ completely reaches the saturation region and Q₂ at the cut-off region. As we are taking output at the collector of Q₁, thus at saturation region V_c is minimum. So, the first unstable state achieved will be 0.

Now, moving further, V_BB comes into action, due to which a current start to flow from V_BB to Q₂. As Q₂ is in the cut-off region and behaving as an open circuit. So, the current charges the capacitor C₁ connected in the circuit. Thus, producing a voltage across C₁.

This high voltage is fed to the base of Q₂. Hence causing it to move saturation from the cut-off region by starting conduction. This ultimately reduces the collector voltage at Q₂, thereby providing minimum voltage.

This low voltage is then fed to the base of Q₁, hence now it moves to the cut-off region. Ultimately, the collector potential at Q₁ is increased rapidly.

Hence, the next unstable state achieved is a high voltage i.e., 1.

In this way, the full operating cycle repeats in order to provide a complete square wave at the output. The duration of each of the unstable state is dependent on the time constant. Let us now understand how a complementary transistor astable multivibrator works.

2. Complementary transistor astable multivibrator

An astable multivibrator can also be formed by making use of a complementary pair of transistors. In this case, unlike two transistors astable multivibrator both the transistor either gets on or off at the same time when supply is provided.

As here the power is taken for some short period of time from the supply at the time when the transistor is on. Thus, it consumes very less power as compared to collector coupled astable multivibrator.

The figure below shows the circuit of a complementary transistor astable multivibrator

As we can see, Q₁ is the PNP transistor and Q₂ is the NPN transistor.

When power is applied at the circuit, the capacitor in the circuit starts charging up to the applied potential. When the voltage across the capacitor reaches the summation of zener voltage and the cut-in voltage of the transistor, the emitter-base junction of NPN transistor Q₂ gets forward biased.

As Q₂ starts conduction, now the collector current starts to flow through the emitter-base junction of PNP transistor Q₁.

This means that the collector current of transistor Q₂ acts as the base current of transistor Q₁ denoted by I₂. Also, the collector current of Q₁ denoted by I₁ is the base current of Q₂ which can be clearly seen from the figure shown above.

Hence, when the base of Q₂ is increased, both the transistor comes to saturation region, causing it to come to on state. Ultimately discharging the capacitor.

When the capacitor discharges, the base current of both the transistors now gets decreased, thus causing the transistors to get off.

The process repeats in order to provide continuous output in case of the astable multivibrator.

Advantages of Astable Multivibrator

• Due to continuous oscillation between 2 unstable states, it is power efficient.
• Its design is simple.
• It is inexpensive.

Disadvantages of Astable Multivibrator

• The feedback provided in case of astable multivibrator does not completely provide entire output to the input.

Applications of Astable multivibrator

These are widely used in radio equipment, in timing circuits and the systems that require square wave as output.