LearnTronics
LC Tank Oscillator
Recipe #41 — lc tank oscillator
A freely ringing LC tank loses energy and fades. Add an amplifier that returns just enough energy in the correct phase and the ringing can become a sustained oscillator.
The tank chooses the frequency; the amplifier replaces the losses. A small part of the oscillation is fed back with the correct phase so the active device gives the LC circuit a little energy each cycle.
An oscillator is controlled positive feedback
Too little feedback and the ringing dies out. Enough properly phased feedback and a steady waveform develops. Too much gain can drive the amplifier into clipping and distort the waveform.
Many practical LC oscillators use this same idea
Hartley oscillators use a tapped inductor or two inductive sections. Colpitts oscillators use a capacitive divider. Armstrong oscillators use transformer coupling. Their details differ, but all provide gain plus frequency-selective positive feedback.
This drawing is intentionally a principle diagram, not a complete buildable oscillator. It keeps the new idea visible before adding the bias networks and device-specific details of a Hartley or Colpitts circuit.