about bi-cap

[TheNaive]

I would like to know about Bi-cap. From what I know, we use small value C connect in parallel with the big one. My question is how do we chose the value of small C.
For example, if I connect small C to 10uf C, is there any difference between 0.1uf, 0.22uf, 0.01uf, 0.047uf or any other value.

Thank you.

[Miles Prower]

Originally posted by TheNaive
I would like to know about Bi-cap. From what I know, we use small value C connect in parallel with the big one. My question is how do we chose the value of small C.
For example, if I connect small C to 10uf C, is there any difference between 0.1uf, 0.22uf, 0.01uf, 0.047uf or any other value.

Thank you.

The whole idea behind doing this is that the large capacitors, whether they're electrolytic or PiO, or plastic film, have an inductance since they're all constructed by rolling up a long length of metal-dielectric-metal. There isn't enough stray inductance there to make any difference at low frequencies, but there might be at high frequencies, and certainly so at RF. The smaller capacitor bypasses high frequency currents if the stray inductance becomes effective in blocking those high frequency currents. The actual value doesn't much matter, and 0.1uF is almost universally used for this bypass purpose.

Whether it's necessary at audio frequencies or not is questionable, but it doesn't do any harm, and a few extra 0.1uF capacitors don't add very much cost.