How much NFB around an opamp?

[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

[myhrrhleine]

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

[peranders]

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[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...

[lineup]

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