Dynatron oscillator

In theory, if a perfect charged capacitor is connected in parallel with a perfect inductor, the circuit will oscillate forever as the energy is transfered back and forth between the capacitor and the inductor.

However, in practice, the two components will not be perfect. Since the main source of imperfection comes from the series resistance of the inductance, such a circuit is often represented as an inductor, capacitor and resistor in series. The circuit can be re-represented as an inductor, capacitor and resistor all in parallel - it actually doesn't matter whether the resistor is in series or parallel with the inductor/capacitor pair, the circuit can be represented either way.

Now if a negative resistance equal in magnitude to this positive resistance can be connected in parallel with the above circuit, then the two resistances will cancel and the circuit will oscillate for ever as outlined in the first paragraph.

Although no passive negative resistors exist, some active devices exhibit a negative resistance characteristic. Examples of such devices are the tetrode thermionic valve (or tube) and the tunnel diode. Using these devices to provide the negative resistance causes the tuned circuit to oscillate as long as power is provided to the active device.

The oscillation may appear to be a form of electrical perpetual motion, but in fact the oscillation energy comes from the noise energy in the active device. In fact sufficient noise energy is available to allow energy to be coupled out of the oscillating circuit.

This type of oscillator is known as a 'dynatron' oscillator, though technically, only the version using the tetrode valve is a true dynatron oscillator.