What Is Astable Multivibrator
Astable Multivibrator contains no stable states, both states are quasi stable states. It produces continuous square wave signal without applying any external trigger pulses. So it is also called as free running multivibrator
Astable Multivibrator Explain
The terminals of Q₁ and Q₂ are connected to the supply voltage through Rc₁ and Rc₂ respectively. The collector of Q₁ is connected to the base of Q₂ through C₁. Similarly the collector of Q₂ is connected to the base of Q₁ through C₂
The resistors RB₁ and RB₂ provide proper bias to both the transistors.
In a symmetrical astable multivibrator, Q₁= Q₂, Rc₁= Rc₂, RB₁ = RB₂ and C₁ = C₂ When supply is switch ON, one transistor conducts more than other, because of the slight mismatch between the two transistors. (No other transistors are 100% identical)
We assume that the conduction of Q₁ is faster than Q₂. The current Ic₁ is slightly greater than lc₂. The regenerative feedback action, makes to increase the conduction of Q₁ and to decrease the conduction of Q₁ By this effect Vc₁ decreases and Vc₂ increases. Finally the transistor Q₁ goes to saturation and Q₂ goes to cut-off. So the output of Q₁ (Vc) is low and the output of Q₂ (Vc) is high. When Q₁ is at saturation, the full supply voltage also dropped across RC₁
Since Q₂ is at cut off, the voltage across Rc₂ is zero. During this condition the voltages at the base of Q₁ (VB₁) is less positive and the base of Q₂. (VB) is ‑Vcc, because the initial voltage occurs at the collector of Q₁ is dropped across C₁
Now the capacitor C₁ is going to charge from — Vcc to positive supply voltage through RB₂ When the capacitor C₁ attains a sufficient voltage (more than VB.) to drive Q₂ the conduction of Q₂ increases. Now the Ic₂ flows through the collector of Q₂
By the regenerative feedback action, the conduction of Q₂ again increases, and the conduction of Q₁ decreases. Finally the transistor Q₁ goes to cut-off and Q₂ goes to saturation.
Now Vc₁ goes to high level and Vc₂ goes to low level. The voltage already placed at the collector of Q₂ is fully dropped across the capacitor C₂. The base of Q₁ goes to ‑Vcc, and base of Q₁ goes to less positive.
Now the capacitor C₂ is going to charge from ‑Vcc to supply voltage through RB₁ is applied to the base of Q₁ When the capacitor C₂ attains a sufficient voltage to drive Q₁ again Q₁ goes to conduction and attain saturation.
Immediately Q₂ goes to cut-off. By this cumulative actions, continuous square wave signals are produced at its collectors. The output of Q₁ (Vc₁) is always 180° out of phase with respect to the output of Q₂ (Vc₂). Both outputs are complement to each other.
The ON period of Q₁ is equal to the OFF period of Q₂. This period (T₁) depends upon the charɡinɡ effect of the capacitor C₁ throuɡh RB₂
T₁=0.69 RB₂ C₁ sec
The ON period of Q₂ is equal to the OFF period of Q₁. This period (T2) depends upon the charɡinɡ effect of the capacitor C₂ throuɡh RB₁
T₂=0.69 RB₁ C₂ sec
If R=RB₁ =RB₂ and C=C₁ =C₂
Total period, T=T₁+ T₂ = 1.38 sec
What is the Applications Of Astable Multivibrator
- These are widely used in radio equipment in timing circuit and the systems that require square wave as output
- Due to continuous oscillation between 2 unstable states it is power efficient
- it is design is simple
- it is inexpensive