[quote author="louder"]
AND:these cog/npo,only in very small values(till 18pf,if my memory....).
[/quote]
There is a tradeoff in ceramic capacitors between dielectric constant and temperature stability. You can have good size/capacitance ratio or good temperature stablity, but not both. Here is a good rundown from the latest IMAPS "Advancing Microelectronics" journal:
"The dielectric material is primarily barium titanate mixed with materials called ?shifters,? which shift the Curie point higher or lower in temperature and ?depressors,? which lower the dielectric constant in a given temperature range with the effect of making the dielectric constant lower but more consistent with temperature. Thus, by shifting the Curie point higher in temperature, a region with a low dielectric constant but which is relatively insensitive to temperature changes is created. This type of material is called ?COG? or ?NPO.? These capacitors are used in applications, such as tuned circuits, that require high stability. By shifting the Curie point to room temperature, a material is created that has a high dielectric constant, but which varies widely above and below room temperature. Materials of this type, called ?Y5V? or ?Z5U,? are commonly used as decoupling capacitors in commercial equipment that does not see temperature extremes. Materials with both depressors and shifters added, called ?X7R,? have the Curie point at room temperature also, but the peak is wider and these materials may be used as decoupling capacitors over an extended temperature range."
-Chris
