Varactor (Varicap) Diodes

Quick Answer

A varactor is a diode used for the capacitance of its own reverse-biased junction rather than for its ability to conduct. Reverse voltage widens the depletion region, and that width is the plate spacing, so the capacitance falls as the bias rises. Tuning a circuit becomes a matter of setting a control voltage.

Variable and trimmer capacitors change their value by moving metal: a shaft swings one set of plates into another, or a screw closes the gap. A varactor changes its value with nothing moving at all. Its plate spacing is the depletion region of a PN junction, reverse bias sets how wide that region is, and the adjustment that used to want a screwdriver becomes a voltage on a control line.

Every reverse-biased junction has that capacitance, and the PN junction lesson computes how it falls as the reverse voltage rises. A varactor is the part that treats it as the product rather than the nuisance, with a doping profile shaped to steepen the curve and a geometry that keeps the series resistance low.

The reverse-only rule follows from the same physics. Forward bias puts current across the junction, and a junction carrying current is a resistance in parallel with the capacitance you fitted it for: the Q falls, the response broadens, and the tuning quietly stops being tuning. So the bias network has to hold the junction reverse-biased on the signal peaks and not only on average. A varactor that swings forward on the tips of a large signal has not merely become worse — it has become a diode again.

How far it moves is quoted as a tuning ratio: capacitance at the bottom of the bias range over capacitance at the top. A part running from 45 pF at one volt to 5 pF at twenty has a ratio of nine, and those figures are invented for this entry rather than read off a datasheet. The ratio is the number to reason with, because resonance goes as the square root of capacitance, so nine to one in capacitance is three to one in frequency.

That buys the arrangement the part exists for. Across an oscillator's tuned circuit it makes frequency a function of a control voltage, which is what a voltage-controlled oscillator is; put that oscillator inside a loop driving the same control line from its error against a reference, and the result is a phase-locked loop.

One consequence of the definition catches people. The signal on the tuned node is part of the voltage across the junction, so a large swing moves the capacitance within its own cycle, which appears as distortion and as a resonant frequency that depends on how hard the circuit is driven. Keep the signal small against the bias, or fit two varactors back to back so that each undoes on one half of the cycle what the other does.