I've been trying to understand the astable multivibrator shown above. Wikipedia has a pretty good description. But I've tried to write it up on my blog in my own words here: http://41j.com/blog/2014/12/astable-multivibrator-oscillator/
I'm uncertain about the following things:
- R1 and R4 are low value resistors, am I right in thinking they just limit the current going into the transistors? Do they have any other purpose?
The way I understand the process, at startup one of Q1 or Q2 will be conducting (or will randomly start conducting first?). That will allow it's capacitor to charge up (because current will be flowing past it). This drives the capacitors other terminal negative, forcing the other transistor open.
The negative terminal of the capacitor slowly discharges (is my terminology correct?) though the larger resistor (R3 or R2). As the negative terminal becomes positive it no longer holds the other transistor open, allowing the other capacitor to charge up and the process to start again with the other transistor.
- Is there anything fundamental missing in my understanding?
Best Answer
"R1 and R4 are low value resistors, am I right in thinking they just limit the current going into the transistors? Do they have any other purpose?"
It is a common misunderstanding that these collector resistors would have the task to "limit" the collector current. However, the BJT always acts as a controlled current source - and the collector current is determined by the base-emitter voltage only. It is the only purpose of these resistors to create/allow a voltage drop at the collector node. Otherwise the collector would have a fixed dc voltage +V.