Common-emitter amplifier

The common-emitter amplifier is the BJT's workhorse voltage-gain stage — the bipolar cousin of the MOSFET common-source amplifier. The signal goes into the base, comes out (much bigger and flipped upside-down) at the collector, while the emitter sits as the shared 'common' reference. It is the first amplifier most students build, and the building block of countless audio and instrumentation circuits.

Its voltage gain is roughly minus the collector resistance times the transconductance (A_v ≈ −g_m·R_C), which can easily reach a few hundred. The minus sign means it inverts: a rising input gives a falling output. Adding an emitter resistor trades some of that raw gain for stability, predictable gain, and wider bandwidth — a bargain almost always worth taking.