Bob tipped us about a simple oscillator that uses negative resistance:
Normally -according to the Ohm’s law– when the applied voltage is increasing, the current is increasing too, however some components can break this law. When the voltage increases, current decreases. This is called negative resistance.
One of the most know element that exhibits this behavior is a tunnel diode. Once very promising, today it isn’t widely used in popular designs and occupies a niche in microwave applications. It’s a bit challenging to get one, fortunately simple circuits that have negative resistance feature can be build from popular discrete elements. One of them I will present today.
Simple negative resistance oscillators – [Link]