2N2222 Common-Emitter Audio Amplifier

Bias a transistor in the middle of its linear region, drop signal in, get gain out — the foundational analog amplifier circuit.

Difficulty: Breadboard. Estimated build time: about 45 minutes. Estimated parts cost: about US$3.60. 7-part bill of materials with live supplier pricing. A single-stage common-emitter amplifier — the textbook BJT amp that every analog student builds at least once.