Test Your EQ (Engineering Quotient)

EQ #25

The classic two-transistor a stable multivibrator is shown below. Typically, R2 and R3 have at least 10 times the value of R1 and R4. This circuit oscillates, with Q1 and Q2 turning on alternately. From the point in time in a cycle where Q1 first switches on, describe what happens until Q2 switches on.

Right before the moment Q1 switches on, C1 is charged to VCC – VBE, with its left end positive, and the left end of C2 has just reached +VBE. The right end of C2 is being held at VCE(SAT) by Q2.

So, as Q1 begins to switch on, it pulls the left end of C1 low, and this also pulls the right end of C1 low, cutting off Q2. This in turn allows the right end of C2 to rise, emphasizing the turn-on of Q1 by increasing the voltage (and current) at the base of Q1. Once Q1 is fully on, the right end of C1 is now at VCE(SAT) – (VCC – VBE) (a fairly substantial negative voltage), and C1 now begins to charge in the other direction, through R2. Once the right end of C1 reaches +VBE, Q2 begins to turn on, starting the second half of the cycle.


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