multiple feedback low pass filter question

graygem · 5th July 2011, 02:29 PM

All the multiple feedback lowpass filters I see are inverting type. Is it possible to do a non-inverting multiple feedback low pass filter?
Just something that I keep asking myself.

Michael Bean · 5th July 2011, 03:13 PM

No, MFB filters are inherently inverting. If you're thinking of trying to input signal to non-inverting input of MFB filter, it won't work correctly. You must correct signal inversion elsewhere in the design. One way to accomplish that is to use another inverting circuit to "un-invert" final output, could be an inverting buffer or amp, or in the case of a 4th order lowpass for example, use two MFB filters in series, I've done that in some of my projects.

Mike

graygem · 5th July 2011, 03:35 PM

Thanks, that was my thoughts as well.

graygem · 5th July 2011, 04:31 PM

So, how would I make an inverting unity gain amp with a 28Hz roll off in front? Below is the non inverting buffer they use. What should I do to make into inverting amp?
unity.jpg

DF96 · 5th July 2011, 04:40 PM

I may have misunderstood the question, because the answer is obvious? Use an inverting op-amp configuration, with a capacitor in series with the input resistor.

graygem · 5th July 2011, 04:52 PM

I need an inverting amp from the non inverting pictured. I know it is very simple but I'm still learning.

DF96 · 5th July 2011, 05:09 PM

OK. Like I said, use the inverting configuration with a capacitor.

graygem · 5th July 2011, 05:22 PM

Like this?
unity2.jpg

Michael Bean · 5th July 2011, 05:26 PM

Here ya go.

INVERTING OPAMP.JPG

Mike

Davey · 5th July 2011, 05:34 PM

I thought you were interested in a non-inverting.....multiple feedback.....and low-pass filter?

Now, you've got an inverting.....non-multiple-feedback....high-pass filter.

Cheers,

Dave.

5th July 2011, 02:29 PM	#1
graygem diyAudio Member Join Date: Jul 2010	multiple feedback low pass filter question All the multiple feedback lowpass filters I see are inverting type. Is it possible to do a non-inverting multiple feedback low pass filter? Just something that I keep asking myself. __________________ Professional Newbie.

5th July 2011, 03:35 PM	#3
graygem diyAudio Member Join Date: Jul 2010	Thanks, that was my thoughts as well. __________________ Professional Newbie.

5th July 2011, 04:31 PM	#4
graygem diyAudio Member Join Date: Jul 2010	So, how would I make an inverting unity gain amp with a 28Hz roll off in front? Below is the non inverting buffer they use. What should I do to make into inverting amp? unity.jpg __________________ Professional Newbie.

5th July 2011, 04:52 PM	#6
graygem diyAudio Member Join Date: Jul 2010	I need an inverting amp from the non inverting pictured. I know it is very simple but I'm still learning. __________________ Professional Newbie.

5th July 2011, 05:22 PM	#8
graygem diyAudio Member Join Date: Jul 2010	Like this? unity2.jpg __________________ Professional Newbie.


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

5th July 2011, 03:13 PM	#2
Michael Bean diyAudio Member Join Date: Sep 2004 Location: Minneapolis, Minnesota, USA	No, MFB filters are inherently inverting. If you're thinking of trying to input signal to non-inverting input of MFB filter, it won't work correctly. You must correct signal inversion elsewhere in the design. One way to accomplish that is to use another inverting circuit to "un-invert" final output, could be an inverting buffer or amp, or in the case of a 4th order lowpass for example, use two MFB filters in series, I've done that in some of my projects. Mike

5th July 2011, 04:40 PM	#5
DF96 diyAudio Member Join Date: May 2007	I may have misunderstood the question, because the answer is obvious? Use an inverting op-amp configuration, with a capacitor in series with the input resistor.

5th July 2011, 05:09 PM	#7
DF96 diyAudio Member Join Date: May 2007	OK. Like I said, use the inverting configuration with a capacitor.

5th July 2011, 05:26 PM	#9
Michael Bean diyAudio Member Join Date: Sep 2004 Location: Minneapolis, Minnesota, USA	Here ya go. INVERTING OPAMP.JPG Mike

5th July 2011, 05:34 PM	#10
Davey diyAudio Member Join Date: Dec 2001 Location: Bremerton, WA.	I thought you were interested in a non-inverting.....multiple feedback.....and low-pass filter? Now, you've got an inverting.....non-multiple-feedback....high-pass filter. Cheers, Dave.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
DIY One fader mixer with Filter Low Pass, Hi Pass, Notch Pass Filter PGM	stevep314	Analog Line Level	0	10th August 2010 09:15 PM
Low Pass Filter question	synthius	Chip Amps	18	28th February 2008 12:13 PM
Low pass filter?	happyb	Car Audio	5	11th December 2006 09:47 PM
simple low pass RC filter noob question	mr.duck	Parts	13	22nd July 2006 02:16 AM
know of a web-based calculator for a multiple feedback band pass filter?	travis	Everything Else	2	13th August 2003 05:08 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read