Why use a combination of capacitors in decoupling circuits?

[lennybogzy]

i assume on my input bias line the capacitors to ground are there to decouple the ac contribution/nosie that might arise on the DC bias.

Why do i consistantly see two caps in parallel, something like a 10uF and a.1uF forming a 10.1 total.

One is 100 times bigger than the other, why not just put the 10uF in and call it a day? why do people consistantly go with 10.1

 

[davidrit]

The reason you use two capacitors rather than one is that the capacitors aren't ideal.

That is they have parasitic resistance and inductance so if you plot the impedance versus frequency you will see the impedance decrease for a period and then increase as the parasitics begin to dominate.

Typically the larger the capacitance the larger the parasitics. So the large capacitor provides lower impedance at low frequencies and the smaller capacitor has lower impedance at high frequencies. The combination has a low impedance over a larger range than either by itself.

 

[lennybogzy]

i love this forum