Astable multivibrator circuits

Posted on Jul 29, 2012

What exactly is a multivibrator? I suppose one definition would be 'a circuit which has several states'. This will do for now, it's quite loose so leaves plenty to the imagination! Conventional multivibrators have only two stages and come in three flavours: astable (where neither state is stable and the circuit oscillates between the two states); monostable (where one state is stable, the other transient). And bistable, where the circuit can be flipped from state one to state two. The fact that it can be 'flopped' back again leads to another term 'flip-flop' - but you can call an astable an astable flip-flop! Then you can get tristables - and one circuit I shall introduce is a 'donkey simulator' where 3 transistors are combined to make 3 separate astable mulivibrators!

Astable multivibrator circuits
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Astable multivibrator circuits - image 1
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Assume that the Tr1 is turned hard on (if you've indexed this site by 'hard on' sorry - it's not for you!) and Tr2 is not conducting, so the right end of C2 is at Vcc: current is flowing into the base of Tr1 via R3 causing it to conduct, so the left end of C1 is at 0v. Assume C1 is not charged: its left end is clamped to 0v by Tr1 but its right end is being fed from Vcc via R1, so it will start to charge up through R1, its right end moving positive towards Vcc. At a certain point there will be enough voltage on the right end of C1 to cause Tr2 to start conducting. As Tr2 starts to turn on, Tr2's collector will start to fall and will pull down the right hand end of C2. Since C2 is a capacitor, the voltage across it cannot suddenly change so the left hand end of C2 will also start to fall. This will rob current from Tr1 which will start to turn off and its collector voltage will start to rise. As it does so, C1 will feed this rising voltage into Tr2's base, helping it to turn on - so the circuit will 'collapse' into a state where Tr2 is fully on and Tr1 is fully off. As Tr2 quickly switches from off to on, its collector falls from Vcc to 0v and (by capacitor action) the base of Tr1 will be reversed biased from about 0.6v to (Vcc-0.6v) below earth as the left end of C2 follows the right end downwards. Now R3 starts to charge C2's left end positively from near -Vcc (below the 0v line) towards 0v and past 0v towards...

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