Varactor Diode

Varactor

/var-ak-ter/

A varactor (variable-capacitance diode) is a semiconductor diode whose junction capacitance varies with applied reverse bias voltage. As reverse voltage increases, the depletion region widens, reducing capacitance. Varactors are used for electronic tuning of oscillators (VCOs), voltage-controlled phase shifters, and parametric amplifiers. Tuning ranges of 5:1 or greater are achievable.

Category: Active Devices

Related to: VCO, Tuning, Capacitor, Diode

Units: pF, V

Understanding Varactors

Varactors provide the simplest way to electronically tune RF circuits. By varying a DC voltage, the varactor capacitance changes, shifting the resonant frequency of any circuit it is connected to. This is the tuning mechanism in virtually all VCOs and many electronically tunable filters.

Varactor Characteristics

Capacitance ratio: C_max/C_min, typically 3:1 to 10:1 depending on diode type.
Q factor: Ratio of reactance to series resistance. Higher Q = less loss. GaAs varactors achieve Q > 1000 at 1 GHz.
Tuning law: C(V) = C0 / (1 + V/phi)^n, where n = 0.5 (abrupt) or higher (hyperabrupt).
Breakdown voltage: Maximum reverse voltage before avalanche breakdown.

Varactor Types

Silicon abrupt junction: n=0.5 tuning exponent. Linear frequency vs voltage for VCOs.
Silicon hyperabrupt: n=1 to 2. Linear frequency vs voltage for oscillators.
GaAs: Higher Q, lower loss. Used for microwave applications above 5 GHz.

Varactor capacitance:
C(V) = C0 / (1 + V/phi)^n
where phi = contact potential (~0.7V Si, ~1.3V GaAs)

Abrupt junction: n = 0.5
Hyperabrupt: n = 1 to 2

Resonant frequency tuning:
f = 1/(2 pi sqrt(L x C(V)))
Tuning range = sqrt(C_max/C_min)

Common Questions

Frequently Asked Questions

What is a varactor diode?

A varactor is a diode whose capacitance varies with reverse bias voltage. It is used to electronically tune oscillators, filters, and phase shifters. Increasing the reverse voltage decreases the capacitance, shifting the resonant frequency higher.

How does a varactor tune a VCO?

The varactor is part of the oscillator's resonant circuit. Changing the varactor control voltage changes its capacitance, which shifts the resonant frequency of the circuit, thereby changing the oscillator output frequency. This is the tuning mechanism in all VCOs.

What limits varactor tuning range?

The tuning range is limited by the capacitance ratio (C_max/C_min), which depends on the diode junction profile and maximum voltage rating. Typical ratios are 3:1 to 10:1, giving frequency tuning ratios of 1.7:1 to 3.2:1 (since f scales as sqrt(1/C)).

Related Terms

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