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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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    \$\begingroup\$ For one, the top one is inverting and the bottom is non-inverting. \$\endgroup\$ – Null Nov 12 '14 at 2:11
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    \$\begingroup\$ Also, the top one can have passband gain, while the lower one has gain of 1. \$\endgroup\$ – The Photon Nov 12 '14 at 2:19
  • \$\begingroup\$ @confused, you might be interested in this book from TI: PLL Performance, Simulation, and Design Handbook \$\endgroup\$ – The Photon Nov 12 '14 at 2:22
  • \$\begingroup\$ @ThePhoton I take it that the second comment was meant for confused's other question about PLL's? \$\endgroup\$ – helloworld922 Nov 12 '14 at 2:59
  • \$\begingroup\$ @helloworld922, I'm assuming this question is also about PLLs (specifically, about the loop filter). \$\endgroup\$ – The Photon Nov 12 '14 at 3:36
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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.

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