How is this circuit a negative resistance oscillator
Each oscillator with a frequency-selective feedback loop can also be seen as a negative-resistance oscillator.

In some cases, the oscilator function can best be explained using the &quot;negative-resistance&quot; view, but

In most cases, it is easier to explain its working principle with the &quot;loop gain&quot; method.


Example: The well-known WIEN-Oscillator is mostly (if not always) described using the bandpass like feedback (damping 1/3 at fo) and a fixed opamp gain of &quot;3&quot; (unity loop gain). However, looking into the inverting input (all resistors remain connected), the input resistance at this point is the parallel combination of the grounded resistor (Ro) and a negative (frequency dependent) resistance (-Rin) of the same value. At the oscillating frequency fo we have Ro||(-Rin) &gt;&gt;&gt; infinite.