I'm not realising this thing.
Hello, in my opinion it's also called dynamic resistance. It's derivative at given operational point on VA curve, dV/dI.
If you plot a V vs I graph for a resistor you get a straight line. The slope is the resistance.
Same graph for a diode is a curve so the slope (resistance) isn't constant. The small signal resistance is the slope at a particular point on the graph. That point is usually called bias or DC operating point. The resistance is only valid for small deviations from the bias point hence why its called the small signal resistance.
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