variablecapacitance

Variable capacitance refers to capacitors whose capacitance value can be adjusted during operation. The adjustment is typically achieved by altering the physical geometry of the plates, changing the dielectric between them, or by using a semiconductor device whose capacitance depends on an applied electrical signal. In an LC circuit, varying C shifts the resonant frequency according to f = 1/(2π√(LC)).

Mechanical variable capacitors use movable plates or dielectrics. Air-gap variants, including rotary and trimmer types, adjust plate distance or overlap to vary C. They offer high quality factors and are common in RF tuning, calibration, and laboratory equipment.

Varactor diodes, or varicaps, are semiconductors whose capacitance varies with reverse bias voltage, enabling electronic tuning without mechanical movement. They are widely used in voltage-controlled oscillators, phase-locked loops, and tunable filters. Their C-V response is nonlinear and temperature-dependent, and device characteristics must be managed in circuit design.

MEMS and other microfabricated technologies realize variable capacitance by tiny mechanical movement or by materials with voltage-dependent permittivity. These devices provide small, precise, and integrable tunable capacitors for RF front ends, tunable filters, and adaptive systems.

Typical ranges and performance vary by technology. Mechanical trimmer capacitors span fractions of a picofarad to tens or hundreds of picofarads; varactors and MEMS devices generally cover from a few picofarads to several tens of picofarads or more, depending on design. Key considerations include tuning range, linearity, quality factor, temperature stability, drift, switching speed, and reliability.