Astable multivibrator clock pulse generator

Astable multivibrator clock pulse generator

Astable multivibrator:- It consist of two transistor two base biasing resistance and two feed back capacitor connected as shown in fig.The frequency of square wave generated depends on the values of resistance and capacitance. The product of R and C is known as time constant of RC circuit. Time period of one cycle is equal to 2x0.693xRC. So frquency of square wave is equal to 1/2x0.693xRC hertz. For example resistace of 10k and capacitance of 0.1MFD. The clock frequency will be 1/2x0.693x10k×0.1×10^-6 =1/2x,0.693x0.1×10^-6=10×10^3 = 7.2 khertz. 

Working
When the circuit is switched on one transistor will driven to saturation  and other will driven to cutoff.  Consider T1 is ON and T2 is OFF. During this time Capacitor C2 is charging to Vcc through resistor R1. T2 is OFF due to the - ve voltage from the discharging capacitor C1 which is charged during the previous cycle.As C2 charged fully base current of T1 decay so T1 gets cut off and T2 gets switched on.base current of T2 is maintained by charging current of  C1. As C1 gets fully charged its charging current decay and T2 gets cut off.at this time T1 is swiched on which causes  C1 discharge which gives negative voltage to base of T2 for instant cut off of T2.this cycles repeat continously hence square wave output voltage is generated at the collector of both transistors. This type of oscillater are also designed with integrated circuits. For highly stable frequency clock pulses Cristal is used. crystal comes with various frequencies.In all digital devices which uses micro processer must have clock generator circuit. It controls all activities.