In Reply to: Astable Multivibrators - again! posted by Franklin DeMatto on 02/20/03 at 12:02 PM:
If you don't understand capacitors, I suggest you tackle them before taking on transistors. That's the order it was done in when I was in school. :)
The voltage across a capacitor cannot change instantaneously. In the circuit in question, if the voltage at the collector of Q2 drops abruptly, the voltage at the base of Q1 has to change by the same amount. See http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/experiment/diff/diff.html
Do you understand that the base-emitter junction of a biploar transistor is a diode? Current through that junction can only flow in one direction. When the voltage at the base of Q1 suddenly drops below 0 volts, no base current flows, so Q1 turns off. That voltage then rises towards +V through R3. When it reaches 0.7v, Q1 turns on, its collector voltage falls, that voltage change couples across C1 to the base of Q2, turning it off, and the same thing now happens at the base of Q2 as just happened at the base of Q1.