Tunnel diodes used for oscillators without loses
The tunnel diode has an N-shaped IV curve. This means that when the voltage drop across it increases, its resistance increases as well. So, it can compensate &quot;positive&quot; resistance when connected in parallel to it.
The LC tank shown (C1 in parallel to L1) has some internal &quot;positive&quot; resistance. It should be neutralized by equivalent N-shaped negative resistance (tunnel diode) in parallel or by S-shaped negative resistance in series.