- It has already been discussed that a forward biased diode conducts easily whereas a reverse biased diode practically conducts no current. It means that forward resistance of a diode is quite small as compared with its reverse resistance.
1. Forward resistance: The resistance offered by the diode to forward bias is known as forward resistance. This resistance is not the same for the flow of direct current as for the changing current.
- Accordingly; this resistance is of two types, namely; d.c. forward resistance and a.c. forward resistance.
(i) d.c. forward resistance. It is the opposition offered by the diode to the direct current. It is measured by the ratio of d.c. voltage across the diode to the resulting d.c. current through it. Thus, referring to the forward characteristic in Fig. 3.5, it is clear that when forward voltage is OA, the forward current is OB.
Therefore, DC forward resistance, R
f = OA/OB
(ii) a.c. forward resistance. It is the opposition offered by the diode to the changing forward current. It is measured by the ratio of change in voltage across diode to the resulting change in current through it i.e.
- The a.c. forward resistance is more significant as the diodes are generally used with alternating voltages. The a.c. forward resistance can be determined from the forward characteristic as shown in Fig. 3.3. If P is the operating point at any instant, then forward voltage is ob and forward current is oe. To find the a.c. forward resistance, vary the forward voltage on both sides of the operating point equally as shown in Fig.
3.6 where ab = bc. It is clear from this figure that :
- It may be mentioned here that forward resistance of a crystal diode is very small, ranging from 1 to 25 O.
2. Reverse resistance: The resistance offered by the diode to the reverse bias is known as reverse resistance. It can be d.c. reverse resistance or a.c. reverse resistance depending upon whether the reverse bias is direct or changing voltage. Ideally, the reverse resistance of a diode is infinite.
- However, in practice, the reverse resistance is not infinite because for any value of reverse bias, there does exist a small leakage current. It may be emphasized here that reverse resistance is very large compared to the forward resistance. In germanium diodes, the ratio of reverse to forward resistance is 40000: 1 while for silicon this ratio is 1000000 : 1.
Wilfykil answered the question on
August 14, 2019 at 06:48