We have seen that accurate capacitance measurements can be made at audio frequencies
(generally at 1 kHz) using a variety of bridge circuits, described in Section 5.5.1. It is often
important to characterize capacitors at radio frequencies and at ultra-low frequencies, the
latter range being important in the characterization of capacitor dielectrics and dielectrics
used as electrical insulation. There are also voltage-variable capacitances associated with
semiconductor pn junctions, which are important to measure. These include the capacitance
of varactor diodes (reverse-biased pn junction diodes); the gate–source capacitance
(Cgs); gate–drain capacitance (Cgd); drain–source capacitance (Cds), Ciss, and Crss of FETs; and
the Cπ and Cμ of BJTs. We will describe in the following some of the nonbridge means to
measure capacitors and to characterize their equivalent circuits.