Op-amp input impedances and a pot/attenuator - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

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
Reply
 
Thread Tools Search this Thread
Old 31st January 2005, 05:28 PM   #1
breez is offline breez  Finland
diyAudio Member
 
Join Date: Jul 2004
Location: Finland
Question Op-amp input impedances and a pot/attenuator

Hello

I have been wondering. If I have a potentiometer/attenuator in the usual configuration as a voltage divider in front of the op-amp input and there is also an input cap (series) and a shunt resistor from the input to ground:

What is the impedance seen by the source? I presume it will be the pot value in parallel with the shunt resistor. Correct?

What happens to the DC offset of the amp when turning the pot the resistance from wiper/input pin to ground changes? At the other end of the pot the resistance reaches very low values. Wouldn't this unbalance the input impedances of the amp and therefore cause large DC offset at output?

Thanks,
Lauri


edit: If the second point is correct, is there any easy way out of it?
Attached Images
File Type: gif circ.gif (2.9 KB, 231 views)
  Reply With Quote
Old 31st January 2005, 05:48 PM   #2
macboy is offline macboy  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Ottawa, Canada
Here's the trick, the AC Impedance is the parallel combination of the input resistor and both halves of the pot, just as you suspected. But the DC impedance as seen from the input of the chipamp/opamp is only the input resistor. Why? Because of the input cap, which blocks DC. So what is the effect to the DC offset of changing the position of the pot? None.
  Reply With Quote
Old 31st January 2005, 11:03 PM   #3
diyAudio Member
 
TwoSpoons's Avatar
 
Join Date: Oct 2004
With the pot set for zero output the source will see just the pot impedance. At max setting it will see pot in parallel with shunt R. In between it will see [input->wiper] in series with ( [wiper->ground] in parallel with shunt R). Make sense? Its easier to see if you draw it out as three resistors.
  Reply With Quote
Old 31st January 2005, 11:33 PM   #4
azira is offline azira  United States
diyAudio Member
 
Join Date: Dec 2003
Location: Near Seattle
DC impedance it'll just see the full value of the POT, however, the AC impedance it'll see is:

lets call R1 resistance above the wiper, and R2 below the wiper, so R1 + R2 = Rpot, Xcap is the AC impedance of the cap which is frequency dependant. Xcap = 1/jwC;

R1 + ( R2 || (Rshunt + Xcap) )

If C is sufficiently large so that it's impedance is perhaps 1/10th or so of Rshunt, the it can be safely calculated as R1 + (R2 || Rshunt)

Which by the way, varies as you adjust R1, at 0-volume, you'll have Rpot as your input resistance, at full volume, you'll have Rpot || (Rshunt+Xcap)
--
Danny
  Reply With Quote
Old 1st February 2005, 12:11 AM   #5
homer09 is offline homer09  Canada
diyAudio Member
 
homer09's Avatar
 
Join Date: Jan 2004
Location: Montreal
What if one does not have any input cap or input resistor, just a pot and a shunt resistor? any problems there?
  Reply With Quote
Old 1st February 2005, 02:17 PM   #6
macboy is offline macboy  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Ottawa, Canada
Quote:
What if one does not have any input cap or input resistor, just a pot and a shunt resistor? any problems there?
Yes, there is a potential for problems. The DC impedance, looking back from the opamp input (which is a source of leakage current) to the audio source, will be all over the place. That will affect the DC offset, because the leakage current will induce a different voltage depending on the position (resistance) of the pot. Even though this voltage is small, it is amplified by the gain of the opamp.
  Reply With Quote

Reply


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
Simple ReAmp Mod? Help with Input Impedances, Please! slor Tubes / Valves 7 17th March 2008 08:56 PM
Input transformers and IC input impedances duffy Chip Amps 4 2nd December 2004 07:08 AM
line input impedances? Dominique Parts 5 5th August 2004 12:19 PM
Aleph P input attenuator ElectroNick Pass Labs 1 16th September 2003 09:28 PM
How does a summing circuit work in terms of input/output impedances? Saurav Everything Else 3 16th May 2003 05:10 PM


New To Site? Need Help?

All times are GMT. The time now is 11:34 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2