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19th August 2008, 06:21 PM  #1 
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Join Date: May 2006
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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
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19th August 2008, 07:01 PM  #2  
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Re: How much NFB around an opamp?
Quote:
120dB  40dB =80dB feedback this is also frequency dependant
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19th August 2008, 07:36 PM  #3 
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19th August 2008, 09:05 PM  #4 
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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...
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19th August 2008, 09:55 PM  #5  
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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
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