The resistance offered by the p-n junction diode in forward biased condition is called forward resistance. The forward resistance is defined in two ways
1) Static Forward Resistance
This is the forward resistance of p-n junction diode when p-n junction is used in d.c. circuit and the applied forward voltage is d.c. This resistance is denoted as RF and is calculated at a particular point on the forward characteristics.
Thus at a point E shown in the forward characteristics, the static resistance RF is defined as the ratio of the d.c. voltage applied across the p-n junction to the d.c. current flowing through the p-n junction.
RF = Forward d.c. voltage/Forward d.c. current RF =OA/OC at point E
2) Dynamic forward resistance
The resistance offered by the p-n junction under a.c. conditions is called dynamic resistance denoted as rf.
Consider the change in applied voltage from point A to B shown in the Fig. 1.18. This s denoted as AV. The corresponding change in the forward current is from point C to D. I is denoted as J. Thus the slope of the characteristics is t\l/AV. The reciprocal of the e is dynamic resistance rf.
rf = V/ I
=1/slope of forward characteristics
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