In a diode circuit, a DC bias voltage drives the diode current through a resistor, exhibiting exponential behavior in the current-voltage characteristic curve.

When a small, time-varying signal is introduced, it combines with the DC bias voltage centered around the Q point.

The exponential term in the total instantaneous diode current contributes to both the bias current and the signal current.

If the amplitude of the signal voltage is significantly smaller than the thermal voltage, the diode operates within a short, nearly linear segment of the characteristic curve.

Under this small-signal approximation, the exponential term simplifies.

As a result, the diode current becomes the sum of the DC bias current and the signal current.

The diode small-signal conductance is defined as the ratio of the small signal current to the small signal voltage, which is determined by the slope of the tangent to the I-V curve at the Q point.

The inverse of the diode small-signal conductance, called the diode small-signal resistance or incremental resistance, equals the thermal voltage divided by the bias current.