astable multivibrator transistors

Posted on Feb 7, 2014

An Astable Multivibrator or a Free Running Multivibrator is the multivibrator which has no stable states. Its output oscillates continuously between its two unstable states without the aid of external triggering. The time period of each states are determined by Resistor Capacitor ( RC ) time constant. In the above diagram we can find two transisto

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rs which is wired as a switch. Please do read the article Transistor as A Switch. When a transistor is ON, its collector and emitter act as a short circuit. But when it is OFF they acts as open circuit. So in the above circuit when a transistor is in OFF state its collector will have the voltage Vcc and when it is ON its collector will be grounded. When one transistor is ON the other will be OFF. The OFF time of transistor is determined by RC time constant. When the circuit is switched on, one of the transistor will be more conducting than the other due imbalance in the circuit or difference in the parameters of the transistor. Gradually the more conducting transistor will be driven to Saturation and the less conducting transistor will be driven to Cutoff. Q2 is OFF due to the -ive voltage from the discharging capacitor C1 which is charged during the previous cycle. So the OFF time of Q2 is determined by R1C1 time constant. When the Capacitor C1 charges to a voltage sufficient provide base emitter voltage of 0. 7V to the transistor Q2, it turns ON and capacitor C2 starts dischargeing. The negative voltage from the capacitor C2 turns off the transistor Q1 and the capacitor C1 starts charging from Vcc through resistor R and base emitter of transistor Q2. Thus the transistor Q2 remains in ON state.

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