# Multivibrator

Posted on Feb 6, 2014

At one of the previous chapters we have learned how to use a transistor as an electronic switch. At this chapter we will see how a transistor can be used to switch another one. At the drawing you can see a pair of cross coupled transistors. S (set) and R (reset) are the input push buttons of the circuit, while Q and Q are the output clamps. While

using a transistor as a switch, the state of the device can be either "on" (the voltage drop at the emitter-collector line is nearly zero) or "off" (the voltage drop at the emitter- collector line equals nearly the supply voltage of the circuit). So what happens, if the left transistor (T1) is turned "on" The base pin of T2 is connected via a series resistor to the collector pin of T1, so if the voltage drop at the emitter-collector line of T1 is (nearly) zero, the voltage drop at the base pin of T2 and so the current running through those pin is also zero, hence T2 is turned "off". While the voltage drop at the emitter-collector line of T2 equals (nearly) the supply voltage, the voltage feedback passed to the base pin of T1 and so the current running through the base of T1 is at it`s maximum, causing T1 to be turned "on". So the state of the circuit is stable: While T1 is turned "on", T2 gets turned "off", providing a feedback voltage which sustains the switching state of T1 ("on"). The current running through the base pin of T1 prior to this is now cut off, because the emitter base line of the transistor is shortened. Consequently T1 is turned "off", which is why the voltage drop at the emitter collector line increases to nearly the supply voltage. The voltage feedback passed to the base pin of T2 causes the right transistor to alter it`s state from "off" to "on". Now that T2 is turned "on", the feedback passed back to...

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