Transistors are a special type of semi-conductor. They come in NPN and PNP (named after their diode counterparts). Basically, The middle letter represents a "base", while the outer two act as "emitter" and "collector." The difference between "emitter" and "collector" varies per how you use it. The forward and reverse bias works the same way as a diode, except, depending on which transistor you use, you can have two different voltage sources or two different grounds.

For a simple example, we'll assume you have a microphone that can't handle more than 1.5 volts (AA battery) and a speaker that needs 9 volts to even be heard.



So, Q1 happens to be a NPN-transistor. In real life, you'd probably have at least a resistor between the battery and the mic for safety, but this isn't the real world. The electricity can only flow through the speaker if it also flows through the microphone. That is to say, electrons can only flow from the emitter (1) to the collector (3) if electrons can also flow from the emitter to the base (2). This, then, amounts to a basic amplification circuit. Boring, right? Well, at least it does something. Essentially, though, the transistor is making a decision: electricity may only flow on the other side if there's electricity on the one side.

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