Input impedance of a non-inverting op amp? - 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 17th May 2004, 03:39 AM   #1
glennb is offline glennb  Australia
diyAudio Member
 
Join Date: May 2004
Location: Melbourne
Default Input impedance of a non-inverting op amp?

I have some doubts in my mind about the input impedance of a common non-inverting op amp circuit which appears in the data sheet for the LM4780 chip amp, and my research has not revealed a definitive answer.

Here's a couple of simple questions for which I am seeking the correct answer, and why:

1. What is the AC input impedance of the circuit below, for frequencies above the f3 point determined in part by Cin ?

2. What value should RB actually be to balance the DC input offset currents, so that the output DC offset is minimised?

Please reply if you know the correct answer. I would prefer not to waste bandwidth with any off-the-top-of-my-head guesses.
Attached Images
File Type: jpg inpimpedopamp1.jpg (8.1 KB, 473 views)
__________________
Glenn.
  Reply With Quote
Old 17th May 2004, 11:28 AM   #2
moamps is offline moamps  Croatia
diyAudio Member
 
Join Date: Oct 2002
Location: Croatia
Default Re: Input impedance of a non-inverting op amp?

Hi,
your question is about non-inv. opamp but your picture shows inv.amp.........wasting bandwidth?

Answers are here http://en.wikipedia.org/wiki/Operational_amplifier

Regards
  Reply With Quote
Old 17th May 2004, 12:12 PM   #3
diyAudio Member
 
Ouroboros's Avatar
 
Join Date: Dec 2003
Location: Nottingham UK
DC offset is caused by other things as well as by the PD due to input currents!

But to answer your question, 14K for Rb is about right (20k in parallel with 48k). But frankly, unless your input bias currents are high, I wouldn't have though it will make much difference.

But why is this an inverting amp when you call it non-inverting?
  Reply With Quote
Old 17th May 2004, 12:15 PM   #4
diyAudio Member
 
Ouroboros's Avatar
 
Join Date: Dec 2003
Location: Nottingham UK
Oh, and your input impedance will be about 980 Ohms in the audio range.
  Reply With Quote
Old 17th May 2004, 12:17 PM   #5
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Glenn, first I think you should take a close look at the datasheet of LM4780 and also AN-1192 and also application notes for the LM4780.

The 47 k is totally unnecessary. If you want to do it really right the 953 ohms resistors should be 20 kohms if you want to minimize the offset voltage. This resistor had also a purpose of protection during power down and the recommend value was 1 kohms.

The input impedance = Rin => 1 kohms in your case => the input cap is way too small. 10 uF at least should it be. f = 1 /(2*pi*R*C)

EDIT: I have checked the datasheet and I see that you have taken the exact circuit as suggested but something is wrong, according to me. Am I wrong?
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy
  Reply With Quote
Old 17th May 2004, 12:47 PM   #6
glennb is offline glennb  Australia
diyAudio Member
 
Join Date: May 2004
Location: Melbourne
Whoops, I actually meant an inverting amp circuit. My apologies.

The wikipedia says Zin = Rin for the inverting amp, as does every other reference I could find. In which case, Cin = 1 uF is indeed way too small for this circuit as audio amplifier:

f3 = 1 / ( 2 . pi . C . Rin || Ri ) = 163 Hz (no bass!)

(where Rin || Ri = 979 ohms)

Also, RB should really be Rf || ( Ri + Rin) = 14118 ohms, rather than the Rf || Ri = 953 ohms given in the circuit.

Does anyone think that NS stuffed up these two aspects of this particular app circuit for the LM4780?
__________________
Glenn.
  Reply With Quote
Old 17th May 2004, 01:06 PM   #7
diyAudio Member
 
Ouroboros's Avatar
 
Join Date: Dec 2003
Location: Nottingham UK
953 Ohms would be correct if you didn't have the input capacitor or course.
  Reply With Quote
Old 17th May 2004, 01:09 PM   #8
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Quote:
Originally posted by glennb
Does anyone think that NS stuffed up these two aspects of this particular app circuit for the LM4780?
I think NS has done a little too much copy and pasting I'm afraid.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy
  Reply With Quote
Old 18th May 2004, 04:22 AM   #9
glennb is offline glennb  Australia
diyAudio Member
 
Join Date: May 2004
Location: Melbourne
Quote:
Originally posted by Ouroboros
953 Ohms would be correct if you didn't have the input capacitor or course.
Yes, because then Ri would be DC coupled to a low source resistance to ground, which is the general assumption made for calcuating RB = Ri || Rf in a classic inverting opamp circuit. I think NS failed to realise that adding Cin actually changes the DC considerations around the opamp inverting input.
__________________
Glenn.
  Reply With Quote
Old 18th May 2004, 04:32 AM   #10
glennb is offline glennb  Australia
diyAudio Member
 
Join Date: May 2004
Location: Melbourne
Quote:
Originally posted by peranders
...The 47 k is totally unnecessary....
Yes, without the 47K there is still a DC charge path for Cin, which is through Ri and Rf.

The resistive load on the input which determines the f3 then becomes Ri alone, ie.

f3 = 1 / ( 2 . pi . Ri . C )

It then follows that RB = Rf, as there is now no DC path through Ri.

My beef with the inverting circuit is that to have a reasonable input impedance (say >20K) and a resonable gain (say 20) requires Ri = 20K and Rf = 400K. In my mind, 400K is too large for a feedback resistor. The overall noise would be higher and the DC offset input currents would make the DC output offset voltage much higher.

Quote:
Originally posted by peranders
..I have checked the datasheet and I see that you have taken the exact circuit as suggested but something is wrong, according to me. Am I wrong?
I cut/paste the application circuit from the LM4780 PDF, wiped out the power supply connections, and moved the Rf label from below the resistor to above.
__________________
Glenn.
  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
opamp inverting input sounds better? dorkus Solid State 89 24th November 2011 11:53 AM
Class D input LM311 Inverting input zox2003 Class D 1 1st February 2008 07:48 AM
Inverting opamp input resistors calculation (pic) redrabbit Parts 3 18th July 2007 09:12 AM
Aleph-phase lag with non-inverting input jupiterjune Pass Labs 38 21st February 2007 04:05 PM
Input impedence of inverting gainclone PHilgeman Chip Amps 1 7th January 2004 03:33 PM


New To Site? Need Help?

All times are GMT. The time now is 08:11 AM.


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