Student Question

# 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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The basic idea is that a transistor can effectively "amplify" the output of a logic gate (or more complex low-voltage signal such as a CPU output). Usually the direct output from a logic gate is at too weak a current to power anything of significance (LEDs maybe, but not motors or solenoids).

It can be enough voltage, however, to bias a transistor either forward or backward---and then the transistor can carry the necessary current to power the high-load devices. The current necessary to bias the transistor can be made far lower than the current necessary to run the output.

So when the logic gate says to turn the output OFF, the transistor can be biased backward, cutting off the current; but when the logic gate says to turn the output ON, the transistor can be biased forward, allowing the current through. In practice there is always a little bit of current even in the OFF position, and not fully maximal current even in the ON position; but transistors can be made to approximate the ideal case fairly well.

It's also possible to chain multiple transistors and thereby increase the gain even further (in principle, arbitrarily large); this is called a Darlington Transistor.

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