Double stage of first order low-pass filter

jenson

Joined Jun 2, 2011
1
Hi,

What is the purpose for using double stages of the low-pass filter as the firgure shown below and is there any improvement in filtering when using double stages of the low-pass filter? Thanks.

Jenson

crutschow

Joined Mar 14, 2008
25,445
Two first-order stages have double the rolloff outside the passband (12dB/octave instead of 6dB/octave).

But the corner rolloff is very soft for two such stages in series so it's not often used in practice. It's much better to use an active two-pole filter (such as a Butterworth) where the feedback gives a sharper rolloff at the corner.