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Subject: Re: Stumped by astable multivibrators!

Date: 02/14/03 at 4:28 PM
Posted by: Ron Harrison
E-mail: rmaxh@yahoo.com
Message Posted:

In Reply to: Stumped by astable multivibrators! posted by Franklin DeMatto on 02/13/03 at 2:44 AM:

When Q2 is off, it is off because the base is below ground. Take this on faith for a moment. Remember that Q1 is saturated (collector near gnd, low on resistance). C1 charges through R2 until Q2 starts to turn on (base at 0.7v). When this happens, the Q2 collector voltage starts to drop. This voltage change couples through C2, pulling down the base of Q1 and stealing its base current. Q1 starts to turn off, so its collector voltage starts to rise. This couples through C1, turning Q@ on even harder. As you should be able to see, a regenerative process occurs, with the transistors changing state very rapidly. The base of Q1 at (-VCC+0.7v), so Q1 is now cut off. The base of Q1 charges with the R3*C2 time constant until the voltage reaches 0.7 volts, and the circuit changes state as before.

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