The symbols most commonly used for the varicap diode and a first approximation for
its equivalent circuit in the reverse-bias region are shown in Fig. 16.8 . Since we are in the
reverse-bias region, the resistance in the equivalent circuit is very large in magnitude—
typically 1 M or larger—whereas R S , the geometric resistance of the diode, is, as indicated
in Fig. 16.8 , very small. The magnitude of C T will vary from about 2 pF to 100 pF
depending on the varicap considered. To ensure that R R is as large (for minimum leakage
current) as possible, silicon is normally used in varicap diodes. The fact that the device
will be employed at very high frequencies requires that we include the inductance LS even
though it is measured in nanohenries. Recall that XL = 2pfL, and a frequency of 10 GHz
with LS = 1 nH results in XLS = 2pf L = (6.28)(1010 Hz)(10-9 F) = 62.8 . There is
obviously, therefore, a frequency limit associated with the use of each varicap diode. Assuming
the proper frequency range and a low value of R S and XLS compared to the other
series elements, then we can replace the equivalent circuit for the varicap of Fig. 16.8a by
the variable capacitor alone.