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How does an n-p-n transistor amplify a small input signal?

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NMOS transistorNegative-channel metal-oxide semiconductors (NMOS) employ a positive secondary voltage to switch a shallow layer of p-type semiconductor material below the gate into n-type. For positive-channel metal-oxide semiconductors (PMOS), all these polarities are reversed. NMOS transistors are more expensive, but faster, than PMOS transistors. — transistor NMOS transistor
Negative-channel metal-oxide semiconductors (NMOS) employ a positive secondary voltage to switch a shallow layer of p-type semiconductor material below the gate into n-type. For positive-channel metal-oxide semiconductors (PMOS), all these polarities are reversed. NMOS transistors are more expensive, but faster, than PMOS transistors.

An n-p-n transistor amplifies a small input signal by utilizing its structure of two closely coupled p-n junctions, typically configured with a forward-biased emitter-base junction and a reverse-biased base-collector junction.12

In this setup, the forward bias on the emitter-base junction injects a stream of electrons into the base region. Because the base is very narrow, the strong reverse bias on the collector attracts the vast majority of these electrons across the base and into the collector region.12

When an AC signal is applied to the base, it causes the base current to fluctuate. Because the transistor is designed so that a large number of electrons reach the collector for every one that reaches the base, these small fluctuations in base current result in much larger, proportional variations in the collector current. This varying collector current flows through a series resistor, creating a significantly larger voltage drop that serves as the amplified output signal.12