The astable multivibrator is also called as a free running multivibrator. It has two quasi-stable states i.e. no stable state such. No external signal is required to produce the changes in state. The component values used to decide the time for which circuit remains in each state. Usually, as the astable multivibrator oscillates between two states, is used to produce a square wave.
The circuit looks like a Schmitt trigger except that the input voltage is replaced by a capacitor. As shown in fig. 6.15 the comparator and positive feedback resistors and form an inverting Schmitt trigger.
When is at , the feedback voltage is called the upper threshold voltage and is given as
When is at , the feedback voltage is called the lower threshold voltage and is given as
(i). When power is turned ON, automatically swings either or to since these are the only stable states allowed by Schmitt trigger. Assume it swings to .
(ii). Now capacitor starts charging towards through the feedback path provided by the resistor to the inverting input. As long as the capacitor voltage is less than , the output voltage remains at
(iii). As soon as charges to a value slightly greater than , the input goes positive with respect to the input. This switches the output voltage from to
(iv). As switches to , capacitor starts discharging via . The current discharges capacitor to 0 V and recharges capacitor to When becomes slightly more negative than the feedback voltage , output voltage switches back to .
- Frequency of Oscillation: The frequency of oscillation is determined by the time it takes the capacitor to charge from to and vice versa.
Initial voltage ( at t=0 ) across Capacitor
Here Role of supply Voltage will play, the output voltage
Voltage across Capacitor at time is given by the eq
Total time period ( )