Generic AC equivalent circuit of a shunt <a href="https://en.wikipedia.org/wiki/negative_resistance" class="extiw" title="en:negative resistance">negative resistance amplifier</a>.  Negative resistance amplifiers, using devices like tunnel diodes and Gunn diodes, are used at microwave frequencies.   The amplifier just consists of a negative differential resistance device,  represented here by a resistor with a negative value <i> −r</i>  in parallel with the source and load resistances.
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