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Subject: Stumped by astable multivibrators!

Date: 02/13/03 at 2:44 AM
Posted by: Franklin DeMatto
E-mail: not@available.com
Message Posted:

Hi. I'm trying to understand the basic two transistor astable multivibrator (for instance, http://physics.usask.ca/~angie/ep326/lab1/intro.html ), but I am STUMPED.

I'm really stuck on a few points (the component numbers should be obvious, but can be seen from the above URL)

1) I understand that when Q2 is off, Q1's base is fed by current through R3 and R4. C2 is charged by the current going through R4 and "through" C2 to Q1's base. When Q2 goes on, however, C2 discharges through it. This somehow takes away the current from Q1. How? Why can't current still go through R3? How does C2's discharge prevent this? (Also, could someone elaborate on what happens to the current coming through R4?)

2) Let's assume Q1 is on and Q2 off. Why can't the current through R2 turn Q2 on? Is it because it is so small, since R2 has a high value? Moreover, I understand that when C1 charges to the threshold voltage of Q2's base, it turns Q2 on. How can this be? As soon as a bit of current flows out from C1, shouldn't it's voltage drop below the threshold again? And so shouldn't Q2 go on only for a moment.

I'm sure that my difficulty must be rooted in not completely understanding a basic concept here (regarding current, voltage, and capacitance). Could someone help me with this? Also, could someone recommmend a good source (book or URL) on multivibrators, oscillators, and amplifiers?

Thank you very much.

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