the common-emitter amplifier

The common-emitter amplifier is the workhorse voltage amplifier — the configuration you reach for when you want to make a small voltage bigger. The signal goes IN at the base, comes OUT at the collector, and the emitter is the terminal shared by (common to) both input and output, which is where the name comes from. It is the only one of the three BJT configurations that gives you both voltage gain and current gain at the same time, so it delivers the most raw power gain.

Here is how it amplifies. A small wiggle on the base changes the base-emitter voltage, which changes the collector current a lot (that is the transistor's transconductance at work). That changing collector current flows through the collector resistor Rc, and by Ohm's law it makes a large changing voltage across Rc — your amplified output. With the emitter resistor bypassed for AC, the voltage gain is approximately -gm times Rc, or equivalently -Rc/re where re is the tiny internal emitter resistance (about 25 ohm per milliamp of current). Concrete numbers: at 1 mA the internal re is about 25 ohm, so with Rc = 4.7 kohm the gain is about -4700/25 = -188. The input resistance looking into the base is about beta times re (a few kilohms), and the output resistance is approximately Rc itself.

Two features define its character. First, it INVERTS: when the input goes up, the output goes down, hence the minus sign on the gain — push more base current and the collector is pulled lower toward ground. Second, the high gain that makes it useful also makes it sensitive, so in practice you almost always tame it with emitter degeneration (gain = -Rc/Re) to make it predictable, or accept the high gain with a bypass capacitor and live with its variability. The common-emitter stage is the heart of countless preamplifiers, the gain stage inside discrete op-amps, and the first thing every electronics student builds to hear a signal get louder.