
Home  Forums  Rules  Articles  diyAudio Store  Gallery  Wiki  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
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 

Thread Tools  Search this Thread 
19th August 2008, 06:21 PM  #1 
diyAudio Member
Join Date: May 2006
Location: City of Angles

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
__________________
double complete rainbow all the way!! 
19th August 2008, 07:01 PM  #2  
diyAudio Member
Join Date: Jan 2006
Location: Avalon Island

Re: How much NFB around an opamp?
Quote:
120dB  40dB =80dB feedback this is also frequency dependant
__________________
[Grasshopper]:Old man, how is it that you hear these things? [master]:Young man, how is it that you do not? 

19th August 2008, 07:36 PM  #3 
Electrons are yellow and more is better!
diyAudio Member

__________________
/PerAnders (my first name) or PA as my friends call me PA03 LM4780 amplifier group buy, SIGN UP HERE for the group buy 0 boards left. 118 paid. 
19th August 2008, 09:05 PM  #4 
diyAudio Member
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...
__________________
double complete rainbow all the way!! 
19th August 2008, 09:55 PM  #5  
diyAudio Member
Join Date: Dec 2005
Location: the north

Quote:
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
__________________
lineup 

Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
CFOpamp  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 
opampamp  petera  Solid State  5  4th December 2001 11:39 AM 
New To Site?  Need Help? 