I have been trying to understand why the following astable multivibrator works, but I've hit a wall. Before I explain what's confusing me, I should mention that I don't have a background in electronics (though I'm trying to learn). My knowledge of electronics does include the following, though: (a) I know KCL/KVL, (b) I can set up and solve the differential equation that describes the voltage across a capacitor in a series RC circuit, and (c) I know that npn transistors act as closed switches if \$V_{BE} >= 0.6V\$.
The most useful explanation of the circuit that I've run across so far is this website. Ray's website helped me understand that we're basically dealing with two RC circuits, and the oscillation is driven each time by a capacitor turning on a transistor (when that transistor's base-emitter voltage crosses the 0.6V threshold), which induces a negative voltage across the opposite transistor's base-emitter junction and turns off that transistor. (That's my best rough description of the mechanism at play, anyway.)
A few things are still confusing me, though:
(1) I still don't fully understand the bit about a transistor turning on inducing a negative voltage. Is there a simple way to set this up with KVL/KCL or something along those lines to see this mathematically?
(2) One reason I've struggled with this circuit is that I feel like I should be able to write down some equations to fully describe this circuit, which I can then try to solve. Is there a simple way to do that here? Ray's page talks about the differential equation governing a series RC circuit, which I understand, but I don't get how that applies straightforwardly here. Don't we need to take into account all of the voltages and currents in the circuit here? I guess I just don't get why we can zoom in on one sub-RC circuit and analyze that.
Any help would be greatly appreciated!
Best Answer
If you abruptly turn on the power, imbalances in charges and currents and voltages will turn one transistor fully on and turn the other transistor full off ---- with that state continuing as the base of the Off transistor slowly rises from -VDD or similar levels (unless the emitter-base breakdown serves as a clamp on the negative_going excursion). And that bas continues rising, moving above ground to turn ON what was previously OFF. And the cycle continues.
On the other hand, if you bring up the VDD very very slowly.......
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Chua developed a chaotic circuit for electronics, decades ago.
But I recall it requires a non-linear element.
https://en.wikipedia.org/wiki/Chua%27s_circuit