Negative resistance LC oscillator

Introduction

Negative resistance?
What on earth is it, and why is it useful ?
A negative resistor, also called negistor, is a resistor where current doesn't flow “into the resistor” when a voltage is applied over it as we are used to, but where current flows “out of it”, in the opposite direction. Really? Yes!
What is it good for ? Well, a lot of oscillator variations use negative resistance to sustain oscillation and cancel out the damping of a resonating LC circuit. A Colpitts, Hartley, Clapp, Pierce oscillator can be seen as using negative resistance to sustain oscillation of the resonating LC circuit.

In the image below, I try to show you the principle of a negative resistance and how it can be used to build an oscillator. An OP77 OPAMP is used to build the negative resistor, that virtually is located between Vin and ground. This is then used to babysit an LC circuit so it keeps on resonating. There are other ways to implement negative resistance, but with an OPAMP, I guess it is easier to understand how it works.