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The VARACTOR is a diode that exhibits the characteristics of a variable capacitor. The depletion
region at the PN junction acts as the dielectric of a capacitor and is caused to expand and contract by the
voltage applied to the diode. This action increases and decreases the capacitance. The schematic symbol
for the varactor is shown below. Varactors are used in tuning circuits and can be used as high-frequency
amplifiers.
The SILICON CONTROLLED RECTIFIER (SCR) is a four-element, solid-state device that
combines characteristics of both diodes and transistors. The symbol for the SCR is shown below. A signal
must be applied to the gate to cause the SCR to conduct. When the proper gate signal is applied, the SCR
conducts or "fires" until the bias potential across the device drops below the minimum required to sustain
current flow. Removal of the gate signal does not shut off the SCR. In fact, the gate signal is often a very
narrow voltage pulse or trigger. The SCR is ideal for use in situations where a small, low-power gate can
be used to turn on larger currents, such as those found in rectifier and switching circuits. SCRs are used
extensively in power supply circuits as rectifiers.
