1. . Varactor
. Construction P N

2. . Theory
. For testing purposes, a front to back ratio of 10:1 is considered normal.
. The size of the depletion region in a varactor diode is directly proportional to the amount of bias applied.
. As forward bias increases, capacitance (Depletion region) decreases.
. As reverse bias increases, capacitance (Depletion region) increases.
. In the capacitance equation below, it is shown that only the distance between plates can be changed.
Where: A = Plate Area
C = Ak
k = Constant d
d = Distance between plates

3. . An increase in reverse bias increases the width of the gap (d) which reduces the capacitance of the PN junction and vice versa.
. Advantage: Allows DC voltage to be used to tune a circuit for simple remote control or automatic tuning function.
. Operation
. used to replace old style variable capacitor tuning circuits.
. They are used in tuning circuits of more sophisticated communications equipment and in other circuits where variable capacitance is required.

4. 3V 6V
20F
5F P N
Depletion Region