How much NFB around an opamp? - 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 19th August 2008, 06:21 PM   #1
cuibono is offline cuibono  United States
diyAudio Member
 
cuibono's Avatar
 
Join Date: May 2006
Location: City of Angles
Default How much NFB around an opamp?

Hi,

Here's a newbie question I'm trying to answer - how much feedback (in decibels) is around a typical opamp? Lets take for instance an opa2134 - here is the datasheet .

The chip has 120dB open loop gain. At the top of page 5 are a couple graphs showing open and closed loop behavior. Lets assume the chip is operating at a voltage gain of 100 (the top line in the closed loop graph). That is equivalent to 40dB of gain.

Would the equation for NFB in decibels = 10log(120/40) = 4.8dB? That doesn't seem right - I calculated it another way and got about 60dB.. Anyone have the answer?

Thanks
  Reply With Quote
Old 19th August 2008, 07:01 PM   #2
diyAudio Member
 
myhrrhleine's Avatar
 
Join Date: Jan 2006
Location: Avalon Island
Default Re: How much NFB around an opamp?

Quote:
Originally posted by cuibono
Hi,

Here's a newbie question I'm trying to answer - how much feedback (in decibels) is around a typical opamp? Lets take for instance an opa2134 - here is the datasheet .

The chip has 120dB open loop gain. At the top of page 5 are a couple graphs showing open and closed loop behavior. Lets assume the chip is operating at a voltage gain of 100 (the top line in the closed loop graph). That is equivalent to 40dB of gain.

Would the equation for NFB in decibels = 10log(120/40) = 4.8dB? That doesn't seem right - I calculated it another way and got about 60dB.. Anyone have the answer?

Thanks
subtraction.
120dB - 40dB =80dB feedback
this is also frequency dependant
__________________
Just because you can't hear it doesn't mean no one can.
  Reply With Quote
Old 19th August 2008, 07:36 PM   #3
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
These books may help you.

Opamp Applications

Opamps for everyone
__________________
/Per-Anders (my first name) or P-A as my friends call me
Tube Buffered Gainclone in work |Thread ||
  Reply With Quote
Old 19th August 2008, 09:05 PM   #4
cuibono is offline cuibono  United States
diyAudio Member
 
cuibono's Avatar
 
Join Date: May 2006
Location: City of Angles
Hey, thanks.

Here's one that might be a little harder - how much NFB is there around a current feedback opamp, such as the AD815 ? There is no stated open loop gain, and page 10 describes the chips gain structure some, saying the amps gain is bandwidth independent, more or less. Lets say I'm using a voltage gain of +5...
  Reply With Quote
Old 19th August 2008, 09:55 PM   #5
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
Quote:
Originally posted by cuibono
Hey, thanks.

Here's one that might be a little harder - how much NFB is there around a current feedback opamp, such as the AD815 ? There is no stated open loop gain, and page 10 describes the chips gain structure some, saying the amps gain is bandwidth independent, more or less. Lets say I'm using a voltage gain of +5...
AD815
gain of x5 = +14 dB

It looks as AD815, current fb opamp, has got
open loop transimpedance
.. which seems like being sorta open loop gain

In case of AS815, this is 120 dB .. in lower audio band.

Now a transimpedance is load dependent
so the gain depends on how much impedance the output will see
This load impedance of course, as in any feedback amplifier,
will include the feedback resistance.
Lower load resistance (impedance) = lower open loop Voltage gain.

The formula is probably quite simple
to find out the Open Loop Voltage gain, for any load impedance.

I will wait for the audio mathematical guys ( proud Electronics Engineers and higher = EE and EE+ )
to come here tell us these simple formulas.
Thanks Lineup


See my attached figure from AD815 datasheet
Attached Images
File Type: png ad815_open-loop.png (3.5 KB, 86 views)
__________________
lineup
  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
CF-Opamp Onvinyl Parts 6 3rd April 2006 03:09 PM
Single supply opamp - unequal rail voltage and opamp selection Hans L Parts 8 19th February 2006 02:15 PM
Opamp Vcc computeruser Solid State 5 13th October 2005 03:27 AM
Regulate the psu of a power opamp with a similar power opamp? Franz G Chip Amps 34 19th October 2004 10:49 AM
opamp-amp petera Solid State 5 4th December 2001 11:39 AM


New To Site? Need Help?

All times are GMT. The time now is 01:59 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