A capacitor that uses mica as the dielectric material. Mica capacitors have characteristics of low loss, low temperature coefficient of capacitance, and good frequency stability. There are several types.

• • Clamp-type mica capacitors (Fig. a) have the mica clamped between tin-foil electrodes. Alternate layers of foil are brought out on opposite sides. Metal lugs are soldered to the electrodes. This type of construction is used in the manufacture of standard capacitors but has been largely superseded for other applications.

• • Eyelet-construction mica capacitors consist of silvered mica plates fixed together with metal eyelets to form stacks. The possibility of relative movement and bowing of the plates in this construction leads to poor stability of capacitance and of temperature coefficient.

• • Bonded silvered mica capacitors (Fig. b) have mica plates that, except for the outer plates, are silvered on both sides, the silvering forming the appropriate electrode areas. The stacked plates are bonded together by firing. This arrangement gives a dimensionally stable stack with good stability of capacitance.

• • Button mica capacitors are circular capacitors with terminals in the form of a metal band round the perimeter and a metal eyelet in the centre. This type of capacitor also has good dimensional stability and is particularly suitable for high-frequency operation when mounted in a true coaxial arrangement.

(a) Simplified clamp-type mica capacitor (b) Simplified bonded silvered mica capacitor

The extremely good stability of most forms of mica capacitor both with temperature and frequency makes them highly suitable for use in filters.