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A Bipolar Junction Transistor or BJT consists of a three-layer sandwich of semiconductor material, arranged N-P-N or P-N-P. The central section is usually much narrower than the other two and is connected to a terminal called the base. The outer areas are connected to terminals called the collector and the emitter.
At first glance a BJT looks a bit like two diodes placed back-to-back, and indeed if one tests a BJT using a multimeter it may be treated as one when testing between the base and either of the other two terminals. However, its physical behaviour is not the same as two diodes back-to-back, because of the very close physical proximity of the two junctions. This is an important point to note to understand the working of a BJT.
In a circuit, a current flows from the collector to the emitter through the device. If the base terminal is not connected, in fact no current will flow because at least one of the internal junctions is reverse biased, so just like the diode, carriers are repelled away from the junction join to form an insulating depletion layer. Unlike the diode, this depletion layer is not altered by reversing the voltage across the device, but by injecting extra carriers via the base terminal.
If a small current is arranged to flow into the base terminal, it adds carriers to the depletion layer, making it narrower. This reduces its resistance and allows more current to flow across the junction from collector to emitter. This current can be very much greater than the current flowing the in the base, yet is proportional to it, thus the base current acts as a control for the collector-emitter current.
The ratio between the base current and the collector current is called the gain of the device (also called its beta), and could be 100 times or even more. Thus in one sense a transistor amplifies the signal - the much larger current changes in sympathy with the smaller base current. However this relationship is not linear - overall it follows a square law - the collector current varies in proportion to the square of the base current. However over limited ranges the output is more or less linear, and usually for amplification purposes a transistor is operated on this part of the curve. The fact that it is a curve is where distortion arises when amplifying music or other small signals.
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