Silver mica capacitors are worth their high price for RF circuits. Find out how they work and when you can use a cheaper alternative.
This article is part of The engineer’s complete guide to capacitors. If you’re unsure of what type of capacitor is best for your circuit, read How to choose the right capacitor for any application.
What is a mica capacitor?
As a dielectric, mica provides capacitors with stable, highly accurate capacitance values. Mica capacitors exhibit low losses, which means they have a high quality factor (Q) and low dissipation factor (DF).
For an explanation of these terms, read: The engineer’s capacitor glossary: All terms and acronyms defined.
Mica capacitors can withstand high voltages, operate at high temperatures and have low leakage current. Because mica capacitors have a very small inductive characteristic and low losses, they are often used in radio frequency (RF) circuits. Silver is used to form mica capacitor plates. Other metals, like copper and aluminum, have been used, but do not perform as well.
Capacitance and voltage rating of silver mica capacitors
Silver mica capacitors offer tight tolerances from ±0.05% to ±5%. It is difficult to manufacture silver mica capacitors with large capacitance values, and they run from 0.5 pF to a few nanofarads. Typical capacitance values range from 1 pF to 91,000 pF, while voltage ratings range from 50 V to 2500 V.
The design of a silver mica capacitor does not permit internal air gaps. The entire assembly is sealed hermetically from the environment. Consequently, it is protected from the effects of air and humidity. This promotes long term stability. The average temperature coefficient for silver mica capacitors is ±50 ppm/°C.
Applications of mica capacitors
Silver mica capacitors are used in high-frequency RF tuned circuits such as those found in filters, oscillators and power amplifiers. In filters, the tolerances and low losses (high Qs) of silver mica capacitors result in precise and predictable tuned-circuit performance. These same benefits promote excellent RF oscillator operation in terms of frequency stability and very small drift with temperature. Their high voltage rating makes their application in RF power amplifier coupling and decoupling attractive despite their relatively high cost. Silver mica capacitors can have very large dv/dt ratings (e.g., 100, 000 V/µs) which promotes their use as snubbers in pulse applications.
Alternatives to silver mica capacitors
As passive electrical components go, silver mica capacitors tend to be expensive. In low power RF applications, a good replacement for silver mica capacitors is ceramic capacitors. If small capacitance tolerances, low losses and a low temperature coefficient are needed, Class I ceramic capacitors can be used. These ceramic capacitors have characteristics like silver mica capacitors, but at a fraction of the price.
