A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically.
Variable Capacitor Basics
A variable capacitor is a capacitor
whose capacitance may be intentionally and repeatedly changed
mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio (therefore they are sometimes called tuning capacitors), or as a variable reactance, e.g. for impedance matching in antenna tuners.
Rotary variable capacitor
In mechanically controlled variable capacitors, the distance between
the plates, or the amount of plate surface area which overlaps, can be
The most common form arranges a group of semicircular metal plates
on a rotary axis (“rotor”) that are positioned in the gaps between a
set of stationary plates (“stator”) so that the area of overlap can be
changed by rotating the axis. Air or plastic foils can be used as dielectric
material. By choosing the shape of the rotary plates, various functions
of capacitance vs. angle can be created, e.g. to obtain a linear
frequency scale. Various forms of reduction gear
mechanisms are often used to achieve finer tuning control, i.e. to
spread the variation of capacity over a larger angle, often several
A Vacuum variable uses a set of plates made from concentric cylinders that can be slid in or out of an opposing set of cylinders
(sleeve and plunger). These plates are then sealed inside of a
non-conductive envelope such as glass or ceramic and placed under a
The movable part (plunger) is mounted on a flexible metal membrane that
seals and maintains the vacuum. A screw shaft is attached to the
plunger, when the shaft is turned the plunger moves in or out of the
sleeve and the value of the capacitor changes. The vacuum not only increases the working voltage and current handling of the capacitor it also greatly reduces the chance of arcing across the plates. The most common usage for vacuum variables are in high powered transmitters such as those used for broadcasting, Military and Ham Radio as well as high powered RF tuning networks.
Vacuum variables can also be more convenient since the elements are
under a vacuum the working voltage can higher than an air variable the
same size. By using a vacuum variable you can greatly reduce the size
of the capacitor. Under some high voltage conditions you can't use
anything else. When precise tuning and stability is required vacuum
variables work very well.
Very cheap variable capacitors are constructed from layered
aluminium and plastic foils that are variably pressed together using a
screw. These so-called squeezers can’t provide a stable and
reproducible capacitance, however. A variant of this structure that
allows for linear movement of one set of plates to change the plate
overlap area is also used and might be called a slider. This has practical advantages for makeshift or home construction and may be found in resonant loop antennas or crystal radios.
Small variable capacitors operated by screwdriver (for instance, to
precisely set a resonant frequency at the factory and then never be
adjusted again) are called trimmer capacitors. In addition to air and plastic, trimmers can also be made using a ceramic dielectric.
The thickness of the depletion layer of a reverse-biased semiconductor diode
varies with the DC voltage applied across the diode. Any diode exhibits
this effect (including p/n junctions in transistors), but devices
specifically sold as variable capacitance diodes (also called varactors
or varicaps) are designed with a large junction area and a doping profile specifically designed to maximize capacitance.
Their use is limited to low signal amplitudes to avoid obvious
distortions as the capacitance would be affected by the change of
signal voltage, precluding their use in the input stages of
high-quality RF communications receivers, where they would add
unacceptable levels of intermodulation. At VHF/UHF
frequencies, e.g. in FM Radio or TV tuners, dynamic range is limited by
noise rather than large signal handling requirements, and varicaps are
commonly used in the signal path.
Varicaps are used for frequency modulation of oscillators, and to make high-frequency voltage controlled oscillators (VCOs), the core component in phase-locked loop (PLL) frequency synthesizers that are ubiquitous in modern communications equipment.
Variable capacitance is sometimes used to convert physical phenomena into electrical signals.
- In a capacitor microphone
(commonly known as a condenser microphone), the diaphragm acts as one
plate of a capacitor, and vibrations produce changes in the distance
between the diaphragm and a fixed plate, changing the voltage
maintained across the capacitor plates.
- Some types of industrial sensors use a capacitor element to convert physical quantities such as pressure, displacement or relative humidity to an electrical signal for measurement purposes.
- Capacitive sensors can also be used in the place of switches, e.g. in computer keyboards or “touch buttons” for elevators that have no movable parts.
Special forms of mechanically variable capacitors
Very often, multiple stator/rotor sections are arranged behind one
another on the same axis, allowing for several tuned circuits to be
adjusted using the same control, e.g. a preselector, an input filter
and the corresponding oscillator in a receiver circuit. The sections
can have identical or different nominal capacitances, e.g. 2 × 330 pF
for AM filter and oscillator, plus 3 × 45 pF for two filters and an
oscillator in the FM section of the same receiver. Capacitors with
multiple sections often include trimmer capacitors in parallel to the
variable sections, used to adjust all tuned circuits to the same
A butterfly capacitor is a form of rotary variable capacitor
with two independent sets of stator plates opposing each other, and a
butterfly-shaped rotor arranged so that turning the rotor will vary the
capacitances between the rotor and either stator equally.
Butterfly capacitors are used in symmetrical tuned circuits, e.g. RF power amplifier stages in push-pull configuration or symmetrical antenna tuners where the rotor needs to be “cold”, i.e. connected to RF (but not necessarily DC) ground
potential. Since the peak RF current normally flows from one stator to
the other without going through wiper contacts, butterfly capacitors
can handle large resonance RF currents, e.g. in magnetic loop antennas.
In a butterfly capacitor, the stators and each half of the rotor can
only cover a maximum angle of 90° since there must be a position
without rotor/stator overlap corresponding to minimum capacity,
therefore a turn of only 90° covers the entire capacitance range.
The closely related split stator variable capacitor does not
have the limitation of 90° angle since it uses two separate packs of
rotor electrodes arranged axially behind one another. Unlike in a
capacitor with several sections, the rotor plates in a split stator
capacitor are mounted on opposite sides of the rotor axis. While the
split stator capacitor benefits from larger electrodes compared to the
butterfly capacitor, as well as a rotation angle of up to 180°, the
separation of rotor plates incurs some losses since RF current has to
pass the rotor axis instead of flowing straight through each rotor vane.
Differential variable capacitors also have two independent
stators, but unlike in the butterfly capacitor where capacities on both
sides increase equally as the rotor is turned, in a differential
variable capacitor one section's capacity will increase while the other
section's decreases, keeping the stator-to-stator capacitance constant.
Differential variable capacitors can therefore be used in capacitive potentiometric circuits.
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