Electronic – what’s the difference between these two low pass filter types


Is there any difference between these two types of lowpass filters? I see the equation is the same, I assume they're both first order then. Just curious if there's any practical difference. Maybe one allows me to set gain and the other doesn't?

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Best Answer

You can write their transfer function from input to the output.

The first is a active filter, assume a ideal Op amp:

$$ H(s)=\frac{-R_{2}}{R_{1}} \times \frac{1}{1+R_{2}C_{2}s} $$

The second it's basically passive RC filter followed by a voltage follower.:

$$ H(s)=\frac{1}{RCs+1} $$

Then, you can see some differences, such as the first one has negative gain and can have non-unity gain at low frequency.