How a Transistor Oscillator Works

Written by j.t. barett
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How a Transistor Oscillator Works
Transistor oscillators can produce a variety of waveforms. (display of waveforms image by Albert Lozano from

Transistor oscillators generate regular, repeating signals for electronic equipment, including music synthesizers, radio transmitters, clocks and computers. They come in two basic kinds: resonant circuits with feedback, and relaxation oscillators.


A resonant network sets the transistor oscillator's frequency. It may consist of capacitors and resistors, capacitors and inductors, or an inductor and a quartz crystal. As with tuning forks and pendulums, a resonant network has a natural frequency.


A transistor amplifies current. If you connect a transistor amplifier to a resonant network and feed the output back into the amplifier's input, the transistor produces a pure frequency. You must tune such oscillators carefully, however, as too little signal will cause the oscillation to die out, and too much will distort the signal.

Relaxation Oscillator

A relaxation oscillator comprises two transistors in a configuration called an astable multivibrator. It uses a current source to charge a capacitor. When the capacitor is full, it triggers one transistor, which discharges the capacitor and turns off. The second transistor then turns on and recharges the capacitor, and the process repeats. This type of oscillator generates square and sawtooth waveforms.

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