Astable Multivibrator-What is Astable Multivibrator Full Explanation And Applications

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

Astable Multivibrator
Astable Mul­ti­vi­bra­tor

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 

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