Basic Electronics (3110016)

Varactor Diode

• The Varactor (also called the voltage-variable capacitance, varicap, epicap, and tuning diode) is widely used in television receivers, FM receivers, and other communications equipment because it can be used for electronic tuning.
• In Figure, the depletion layer is between the p region and the n region. The p and n regions are like the plates of a capacitor, and the depletion layer is like the dielectric.
• When a diode is reverse biased, the width of the depletion layer increases with the reverse voltage. Since the depletion layer gets wider with more reverse voltage, the capacitance becomes smaller.
• It’s as though you moved apart from the plates of a capacitor. The key idea is that capacitance is controlled by reverse voltage.
• The Varactor acts the same as a variable capacitance.

Varistor

• Lightning, power-line faults, and transients can pollute the ac line voltage by superimposing dips and spikes on the normal 120 V rms. Dips are severe voltage drops lasting microseconds or less. Spikes are very brief over voltages up to 2000 V or more.
• In some equipment, filters are used between the power line and the primary of the transformer to eliminate the problems caused by ac line transients. One of the devices used for line filtering is the Varistor (also called a transient suppressor).
• This semiconductor device is like two back-to-back Zener diodes with a high breakdown voltage in both directions. Varistors are commercially available with breakdown voltages from 10 to 1000 V. They can handle peak transient currents in the hundreds or thousands of amperes.