Dynatron oscillator

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A dynatron oscillator is a unique electronic circuit that uses negative resistance to keep an LC tank oscillating.

In theory, if an ideal capacitor is connected in parallel with an ideal inductor, they form an LC tank circuit that, once it begins oscillating, will oscillate forever as the energy is transferred back and forth between the capacitor and the inductor.

In practice, however, the two components are not ideal. Because "real" components have some associated series resistance (primarily contributed by the inductor), such a circuit is often represented either as an inductor, capacitor and resistor all in series or all in parallel.

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 forever as described earlier.

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.

Though the oscillation may appear to be a form of electrical perpetual motion, this is not the case as energy is constantly being provided from the power supply of the active device to sustain the oscillation. In fact, sufficient noise energy is available to allow energy to be coupled out of the oscillating circuit.

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