A demonstration circuit showing how a tunnel diode can amplify a signal using negative differential resistance.  The DC voltage Vb  biases the tunnel diode TD into its negative resistance region and also supplies the power used in amplifying the signal.  For stability the resistor R must be less than the diodes negative resistance  r.  Therefore the total resistance of both devices in series, r - R is a small negative value.  Therefore a small increase in the input voltage Vi causes a large decrease in the current, causing a large increase in the voltage across the diode .   Using the equation for a voltage divider, the AC output voltage is
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