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How and why can a transistor be used as a switch?

Why: A bipolar transistor is an electronic device that has three terminals called emitter, base and collector. Usually the emitter is employed for signal input; in the collector we get the amplified input signal and the base serves to control the amplitude of the signal between the emitter and the collector.


In these circumstances, by changing the polarization of the base, it is possible to control the passage of current from the emitter to the collector so that we can get two limits states for the current between the input and the output. These states are the “conduction,” which is equivalent to a closed switch (the amplitude of the signal is maximum), and the “cut,” which is equivalent to an open switch (the amplitude of the signal is equal to zero).


How: For an NPN transistor with direct polarization, that is, with emitter connected to the negative terminal and the collector to the positive terminal, if the base is connected to positive terminal the transistor is saturated and enters in the conduction state (closed circuit). On the other hand, when the base is connected to the negative terminal the transistor enters in the cut state (open circuit).

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