What Is a Varactor (Varicap) Diode?
1 min read
Quick answer
A varactor (varicap) diode is a special diode whose capacitance changes with the reverse voltage applied to it. In short, it behaves like a "voltage-tuned capacitor".
How Does It Work?
Under reverse bias, a charge-free (depletion) region forms at the diode's junction; this region acts like a capacitor's dielectric. As the reverse voltage increases, the region widens and the capacitance decreases.
Voltage-Controlled Capacitance
So by changing just a DC control voltage you can tune the circuit's capacitance — an electronic alternative to a mechanical trimmer capacitor.
| Reverse voltage | Depletion region | Capacitance |
|---|---|---|
| Low | Narrow | High |
| High | Wide | Low |
Where Is It Used?
Frequency tuning in resonant circuits, voltage-controlled oscillators (VCO), PLLs and the automatic frequency tuning of radio/TV receivers.
Common Mistakes
- Driving it into forward bias: a varactor works under reverse bias; in the forward direction it loses its property.
- Leaving the control voltage noisy: noise on the control line maps directly onto the frequency.
- Miscalculating the capacitance range: the tuning range depends on the diode's rating.
Frequently Asked Questions
What's the difference between a varactor and an ordinary diode?+
A varactor is specially designed for the capacitance change under reverse bias; in an ordinary diode this is an unwanted side effect.
Where is it commonly seen?+
In the electronic frequency-tuning circuits of radio/TV and wireless receivers.
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