# What's the best way to implement a MFB low-pass filter?

I'd like to design a second-order Multiple feedback low-pass filter. I am using the design in TI’s Op Amps for Everyone pdf.

There are 3 resistors and 2 capacitors that need to be selected. I'm looking for a voltage gain of 2.

What's the best method for calculating the other component values?

• The cutoff frequency is 100KHz. Oct 15, 2015 at 13:36
• Samee, for a cut-off at 100kHz you need an opamp with a GBW of at least several MHz. And dont forget the slew rate specification!.
– LvW
Oct 15, 2015 at 14:43

I'd use this tool: -

Found here. OK, I just realized that this one is for a band pass filter but the process is the same. LPF here

It models with perfect op-amps so there will be some small errors depending on your final choice of op-amp but you can experiment with values.

Note that you wanted a gain of 2 but an MFB filter has negative gain so I plugged in -2 to the tool. You also need to consider the damping ratio of your filter (note damping ratio = 1/2Q)

• That's way too easy ;-) Oct 15, 2015 at 14:09

That would be quite a good ;-) exercise to find all the component values. The DC gain is A0 so that means R2/R1 = 2. You can just fill in a value and see what you get as there is no single solution, only the ratios and products of the component values matter.

You probably don't know that this type of filter looks like a Sallen-Key filter but it's not ;-) So after a bit of Googling I found this. Which makes things a lot easier.

Nontheless if you want to become a real EE, you should at least once design a filter (even if it's a simple one) completely from scratch using some tables from an old book like the one written by Mr Anatol I. Zverev. After doing that, just use an online calculator ;-)

• No it's not (oh yes it is LOL) - it's not a sallen key filter at all - it's a multiple feed back filter and has an inverting gain. See my answer and go stand in the corner LOL. Oct 15, 2015 at 14:10
• Oh, it looks like a SK though, only 1 resistor difference ;-) And who needs inversion ;-) Oct 15, 2015 at 14:12
• And it inverts too. Oct 15, 2015 at 14:13
• Thanks, Fake. I do want to become a real EE so I'll take you up on designing one from scratch. Is this the book? wiley.com/WileyCDA/WileyTitle/productCd-0471749427.html Oct 15, 2015 at 14:13
• And can be used for higher Q filters. Oct 15, 2015 at 14:14