How does an n-p-n transistor amplify an electrical signal?

An n-p-n transistor amplifies an electrical signal by using a small input current to control a much larger output current.1

The device consists of three regions—the emitter, base, and collector—configured with two parallel p-n junctions. In a typical amplifier circuit, the emitter-base junction is forward-biased, while the base-collector junction is reverse-biased.12

When an AC signal is applied to the base, it causes the base current to fluctuate. Because the base is thin, most electrons injected from the emitter into the base are pulled across the reverse-biased base-collector junction rather than exiting through the base connection. Consequently, small variations in the base current trigger proportionally larger variations in the collector current—often by a factor of 100 or more. This varying collector current flows through a resistor, creating a voltage drop that represents an amplified version of the original input signal.12