Glass capacitors

- an overview or tutorial about the basics of the glass capacitor, its construction, properties and the uses of glass dielectric capacitors particularly in RF circuits.

Glass capacitors are used where the ultimate performance is required for RF circuits. Glass dielectric capacitors offer very high levels of performance, although their cost is high when compared to many other forms of capacitor. Typically a glass capacitor will have a relatively low capacitance value. The values of glass capacitors may range between a fraction of a picofarad up to two to here thousand picofarads. As such these capacitors are used mainly in radio frequency circuit design.

While the performance of glass capacitors is exceedingly high, this is also usually reflected in the cost - it can run into many pounds or dollars for each component. As such glass dielectric capacitors are reserved only for the most exacting RF requirements, often on low volume products where cost is not such an issues as it is in high volume products. The supply of glass capacitors is also limited to a small number of manufacturers and suppliers, and the capacitors may not be available ex-stock.

Glass capacitor advantages and characteristics

Glass capacitors offer several advantages over types of capacitor. In particular glass capacitors are applicable for very high performance RF applications:

  • Low temperature coefficient:   Glass capacitors have a low temperature coefficient. Figures of just over 100 ppm / C are often obtained for these capacitors.
  • No hysteresis:   Some forms of capacitor exhibit hysteresis in their temperature characteristic. This is not the case for glass capacitors which follow the same temperature / capacitance when the temperature is rising and falling.
  • Zero ageing rate:   Many electronics components change their value with age as chemical reactions take place within the component. Glass capacitors do not exhibit this effect and retain their original value over long periods of time.
  • No piezo-electric noise :   Some capacitors exhibit the piezo-electric effect to a small degree. This can result in effects such as microphony on oscillators. Where this could be a problem, the use of glass capacitors could help solve the problem.
  • Extremely low loss / High Q:   Glass capacitors are very low loss as there is virtually no dielectric loss. This enables very high Q circuits to be built using them. provided the other components (e.g. inductors) are not lossy.
  • Large RF current capability:   Some capacitors are not able to withstand large values of current. This is not the case for glass capacitors which are suitable for use in RF high power amplifiers, etc.
  • High operating temperature capability :   Glass dielectric capacitors are able to operate at very high temperatures. Many are able to operate at temperatures up to about 200C without fear of damage or performance shortfall.

Glass capacitor construction

The construction of glass dielectric capacitors is relatively straightforward to understand. The capacitor consists of three basic elements: the glass dielectric, aluminium electrodes and the encapsulation. However the assembly of the glass capacitors is undertaken in a manner that ensures the required performance is obtained.

As the capacitance between two plates is not always sufficient to provide the required level of performance, the majority of capacitors use a multiplayer construction to provide several layers of plates with interspersed dielectric to give the required capacitance.

Although the glass plates are always flat, and tubular forms of construction are not applicable, the glass capacitors are usually available with leads emanating in either a radial or axial form. Essentially the leads either exit the encapsulation at the side or the end.

Glass capacitor applications

Glass capacitors can find applications in many areas as a result of their performance characteristics. They do tend to be specialist components and are normally fairly costly.

  • Circuits exposed to temperature extremes:   With the tolerance to a wide range of temperatures, both high and low, some circuits that may be exposed to very harsh environmental conditions may choose to use glass capacitors. Not only can they withstand high and low temperatures, but they do not change value at these extremes by a great amount. Accordingly remote sensors may choose to use glass capacitors.
  • Applications requiring a high Q circuit:   Many circuits including oscillators and filters may require high Q components to give the required performance. Filters will be able to attain their required bandwidth, and for oscillators there are a number advantages including improvement of phase noise performance, reduction in drift and reduction of spurious oscillations.
  • Low microphony requirements:   It may be expedient to use glass capacitors in circuits where microphony may be a problem. RF oscillators including those found in phase locked loops and PLL synthesizers may benefit from their use.
  • High power amplifiers:   The high current capability of glass capacitors may enable their use in RF power amplifiers where other forms of capacitor would not be suitable.
  • High tolerance areas:   In many areas such as filters or free running oscillators the high tolerance and precision accompanied by the low temperature coefficient may be required to maintain the tolerances within a precision circuit.

By Ian Poole

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