Multivibrator And Basics of Multivibrator

Multivibrator are sequential logic circuits that operate continuously between two distinct states of high and low

Multivibrator are can also classified depending on the coupling also.

                 

·       Collector coupled multivibrator

1.  Astable multivibrator

2.  Monostable multivibrator

3.  Bistable multivibrator

·       Emitter coupled multivibrator

1.  Schmitt trigger

·       Emitter coupled multivibrator are more stable than collector coupled ,but have less variation than collector coupled multivibrators

Astable multivibrator

·       It is free running multivibrator

·       It does not have any stable states

·       It has two quasistable states

·       It doesn’t require triggering pulses.

·       The duration of two quasistable state is dependent upon the two rc time constants.

·       Total time period = t₁ + t₂

                                 = 0.69 R₁ C₁ + 0.69 R₁ C₁

                                                   =1.38 R₁ C₁

·       Frequency = 1/(time period)

                    = 1/(1.38 R₁ C₁)

·       Duty cycle = t₁ / ( t ₁+t₂ )  =  50%

ASTABLE MULTIVIBRATOR USING OP-AMP:

                                   

                                    

          
   ·       Vi = - β V_sat

·       Charging time T1 = RC ln( (1+ β) / (1- β) )

·       Total time period of output wave form T = 2T1   

  = 2RC ln( (1+ β) / (1- β) )

  =  2RC ln( (1+(2R₂) / R1 )

·       Frequency = 1/T

·       R1 = 1.16 R₂ 

·       frequency = 1/(2RC)

DUTY CYCLE: 
    Duty Cycle = (R_A + R_B) / (R_A + 2R_B)
if the duty cycle is less than the 50% then the diode is place across R_B

Frequency is independent of supply voltage

ASTABLE MULTIVIRATOR USING TRANSISTOR

·        It is free running multivibrator

·        It does not have any stable states

·        It has two quasistable states

·        It doesn’t require triggering pulses.

Total time period = T₁ + T₂

=0.69 R₁C₁  + 0.69 R₁C₁

= 1.38  R₁C₁

Frequency of astable multivibrator= 1/time period

                                                             = 1/ 1.38 R₁C₁

Monostable multivibrator

·       It has one stable state & one quasi-stable state

·       When triggering pulse pulse is applied it enters quasi-satble state from stable state.

·       It is also called as a single shot multivibrator

·       The pulse width of monostable multivibrator is

        t = 0.69 RC

Bistable multivibrator

·       In bistable multivibrator resistor coupling is used between the two transistor stages

·       It has two stable states. By triggering the output it can be change from one state to another i.e. from low to high or high to low

·       Triggering pulses are given to the base of transistor

·       Transition time is the time taken for the off transition to on state immediately whenever we applied triggering signal to it.

·       Commutating capacitors or speed up capacitors are used to reduce the transition time.

Triggering:

 We have two types of triggering are there

·       Symmetrical triggering: in symmetrical triggering method triggering pulses are taken from single source

·       Asymmetrical triggering: triggering signals are taken from different signal sources