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-line bill of materials. Compare supplier offers when available. A single-stage common-emitter amplifier — the textbook BJT amp that every analog student builds at least once.
Set the DC bias
+9 V → 47 kΩ (R1) → base. Base → 10 kΩ (R2) → GND. The divider sets the base at 9 × 10/(47+10) = 1.58 V. Probe the base with a DMM — confirm ~1.6 V before proceeding. If it's wrong, your divider has a bad solder joint.
Set the emitter and collector
Emitter → 1 kΩ (Re) → GND. Collector → 4.7 kΩ (Rc) → +9 V. Probe the emitter — should be ~1.0 V (= 1.6 V base − 0.6 V Vbe). Probe the collector — should be ~4.3 V (= 9 − 4.7 V × 1 mA × 1 kΩ). If both DC voltages are right, your DC operating point is correct and the amp is ready for signal.
Add input coupling
From your signal source, through a 1 µF film cap, to the base. The cap blocks DC (so you don't disturb the carefully-set bias) but passes AC at audio frequencies. Use a film cap, not electrolytic — electrolytics distort at low signal levels.
Add output coupling
From the collector, through another 1 µF cap, to your scope or next stage. Same reason as input — block the 4.3 V DC bias, pass the AC signal. Without this you'd be feeding a 4 V offset into your speaker or scope, which is bad.
Inject signal and measure
Drive 100 mV peak-to-peak, 1 kHz sine into the input. On the output you should see ~470 mV peak-to-peak, INVERTED (180° phase-shifted). That's a gain of −4.7. Now (final phase) add a 10 µF cap from the emitter to GND. The gain jumps to ~150× — but the output starts to clip at large signals because gain × Vin > supply rail. That's the trade-off: high gain or large signal swing, never both.