Building a Simple Transistor Audio Amplifier

A single transistor, a handful of resistors, and a capacitor turn a weak audio signal into something loud enough to drive a small speaker.

You'll need: an NPN transistor, a small 8Ω speaker, a coupling capacitor (try 10µF, mind polarity), a couple of biasing resistors, and a weak audio source (a phone's headphone output works well, through a resistor to keep the signal small and safe).

This is the simplest form of a common-emitter amplifier: the incoming audio signal, coupled in through the capacitor (which blocks any DC offset from the source while letting the audio's changing voltage through — the high-pass behavior from Lesson 26), varies the transistor's base current slightly. Because the transistor's collector current responds to base current at a multiplied ratio, small wiggles in the base current become much larger wiggles in the collector current — and that larger, amplified current swing is what drives the speaker.

The biasing resistors (typically a voltage divider from Lesson cb-06, feeding the base) matter as much as the signal itself: they hold the transistor partway "on" even with no audio present, so the incoming signal can swing the current both up and down from that midpoint, rather than clipping against a hard off state at the bottom of the swing.

This one-transistor design won't get loud or especially clean — it's a starting point for understanding amplification, not a hi-fi circuit — but it's a genuinely audible, working amplifier built from parts this section has already introduced.