This is the last circuit proposed as working for an astable multi-vibrator.
It's now time to understand why this can or not correctly operate and if some issues are still present.
Due to the presence of R and C for each branch we're sure that the timing feature is somehow guaranteed; but how exactly is a matter we're going to see.
As already said at the switch-on one of bjts surely goes in saturation before the other which is so forced to go to interdiction.
We added the capacitors not only to make a timer as proposed but to alternates the states for the outputs too.
Let's suppose bjt2 wins the lottery and goes to saturation: its collector drops form Vcc to nearly 0.2V.
Due to the transitory nature of this falling signal, the capacitor won't block it at first instance so that it (the drop of ~Vcc) will be replicated at its other top which connects directly to base1, the new value of which becomes:
What essentially is an astable circuit useful for?
Just by thinking at its behaviour it can be considered a rudimentary timer with its two outputs alternating between on-off states and one each other in opposition.
First of all let's take again a first look to the circuit we saw in the previous post, here beside.
The links between the collectors and the positive supply Vcc up, and between emitters and ground down are dotted to indicate missing parts.
Suppose bjt1 on the right is interdicted while bjt2 is in saturation: if we connect both the collectors and emitters directly to the supply lines then the bjt1 would be charged quite entirely by the power supplier with the risk (depending by the bjt) to disrupt.
This suggests us to put a resistor in each branch: we choose between collectors and Vcc.
Let's start from the definition.
An astable (multi-vibrator) device is one unable to keep one state firmly but continuously oscillates between two states.
Up-down, one-zero, forth-back, all-nothing: whatever the way is in which you consider them the goal is the alternation between only two conditions.
By concentrating on the hardware we want to realize it: so where to start from?
High and low signals as referred to a ground one are furnished by every transistor and this means it's our first brick in the wall: let's say a very common one is enough for the purpose (for example the cheap BCxxx family bjts).
What kind of alternation is possible in output?
Not as the value of signals in itself but as the activation and control of the on-off.