an oscillator

An oscillator is a circuit that makes its own repeating signal out of nothing but a steady DC supply. Plug it into a battery and, with no input wiggling it, the output starts swinging up and down all by itself, over and over, at a steady rhythm. It is the heartbeat of electronics: the clock that paces a computer, the tone in a doorbell, the carrier that lifts your voice onto a radio wave. Where an amplifier needs a signal to amplify, an oscillator is the source of the signal itself.

How can a circuit make a signal from nowhere? The trick is feedback that reinforces instead of corrects. Take an amplifier and feed a slice of its output back to its input in a way that adds to whatever is already there (positive feedback). The ever-present tiny noise inside any circuit gets amplified, fed back, amplified again, and grows until it settles into a steady swing. A frequency-selective network in the feedback path picks out one frequency to favour, so the result is a clean repeating waveform rather than random noise. Two broad families exist: sine-wave oscillators (smooth, like RC and LC types), and relaxation oscillators (square or triangle, made by charging and discharging a capacitor through a threshold).

Oscillators are everywhere a circuit needs to keep time or make a tone. The key honest point is that a real oscillator is a careful balancing act: too little feedback and the wiggle dies out; too much and the output slams into the supply rails and distorts. The art is to start up reliably from noise, then settle to exactly the gain that sustains a steady amplitude. The rest of this field is the toolbox for doing that well.