The resonator (LC tank for example) has parasitic resistances (RP) which prevents the resonator from oscillating because the stored energy will leak...

The resonator (LC tank for example) has parasitic resistances (RP) which prevents the resonator from oscillating because the stored energy will leak through the resistance. To compensate for this loss a positive feedback negative parallel resistances (-Rp) will be added to the resonator so that the energy loss in Rp is replenished by the negative resistance. The negative resistances implementation is typically an active network. What sets the Frequency and Amplitude of an Oscillator?