A second order active filter with single supply - Zin ? - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

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
Thread Tools Search this Thread
Old 20th May 2012, 01:12 PM   #1
diyAudio Member
Join Date: Dec 2009
Default A second order active filter with single supply - Zin ?

Hello! I was wondering if a high-Z-in unity gain buffer and a second order low-pass filter could be combined, so that when just one op-amp is used less noise is introduced? I intend to use a single supply, so the ground point is virtual via a resistor divider. If I want to use the second order filter with a single supply how should I bias it ? Is it possible that the high input impedance is preserved? To which point in the attached schematic should I connect the bias resistor? Will it interfere with the filter and can this interference be accounted for?
Attached Images
File Type: png filter.png (51.5 KB, 59 views)
  Reply With Quote
Old 20th May 2012, 01:26 PM   #2
DF96 is online now DF96  England
diyAudio Member
Join Date: May 2007
The filter will not have a high input impedance, and it will vary with frequency, although it will depend on component values. If you use high values, then you will get more Johnson noise. It depends on your application.

The bias is best applied at the input, and in many cases may be able to be taken directly from the previous stage. Applying bias straight to the opamp input pin will affect the filter operation, although by how much depends on component values.

Modern opamps are not very noisy. Saving an opamp, but therefore having to use higher value resistors, may give you more noise.
  Reply With Quote
Old 20th May 2012, 01:49 PM   #3
diyAudio Member
Join Date: Dec 2009
Does this filter invert the phase of the signal?
  Reply With Quote
Old 20th May 2012, 02:04 PM   #4
DF96 is online now DF96  England
diyAudio Member
Join Date: May 2007
No, not in the passband. Think about it: at low frequencies the capacitors can be ignored so the filter becomes a unity-gain buffer with a few unnecessary resistors in front. At higher frequencies it will add some phase shift - up to 180 degrees in the stop-band.

There is plenty on the web about this. Just google Sallen-Key filter.
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Active Bandpass Filter Functional Block Order - Which is better? HiFiNutNut Multi-Way 0 29th March 2010 09:32 AM
Active Filter for 6th order Vented Box bwhitejr Multi-Way 1 20th April 2009 05:28 PM
Active Single Stage Butterworth 3rd order HP Calculator? HiFiNutNut Multi-Way 3 28th January 2007 09:37 PM
1st attempt of 2nd order active lowpass filter failed, please advise SimontY Software Tools 111 15th February 2005 10:32 PM
Single supply active filter Maryus17 Everything Else 5 24th February 2004 09:59 PM

New To Site? Need Help?

All times are GMT. The time now is 10:32 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2