Varactor Diode
Understanding Varactor Diodes
Varactors are the enabling component for electronically tuned RF circuits. Their voltage-controlled capacitance allows frequency, phase, and impedance to be adjusted electronically without mechanical components.
Varactor Properties
- Capacitance range: 0.1 pF - 100 pF, depending on junction area.
- Tuning ratio: C_max/C_min = 3:1 (abrupt junction) to 10:1 (hyperabrupt).
- Q factor: 50-500 at 1 GHz. Decreases with frequency. Limits tuned circuit loaded Q.
- Tuning voltage: 0-30V typical. Higher voltage = more tuning range.
- Linearity: Varactors are inherently nonlinear. This limits use in high-power circuits.
Varactor Types
- Abrupt junction: C proportional to V^(-1/2). Moderate tuning range. Best Q.
- Hyperabrupt: C proportional to V^(-n), n > 0.5. Wider tuning range. Linear frequency vs voltage.
Frequently Asked Questions
What is a varactor?
A varactor is a voltage-variable capacitor using a reverse-biased diode junction. As voltage increases, capacitance decreases. Used for VCO tuning, tunable filters, and phase shifters. Tuning ratio 3:1 to 10:1.
What limits varactor performance?
Q factor decreases with frequency (series resistance). Nonlinearity limits use at high RF power (generates harmonics). Capacitance tolerance and TC affect tuning accuracy. Voltage breakdown limits the maximum tuning range.
What is a hyperabrupt varactor?
A hyperabrupt varactor has a specially designed doping profile that gives wider tuning range and more linear frequency-vs-voltage characteristic than standard abrupt varactors. Preferred for wideband VCO applications.