[tangent]

Can anyone tell me by looking at the AD797 datasheet what the gain bandwidth product is?

On the front page, they claim that GBWP is 8MHz at g=10 and 110MHz at g=1000. They say the same thing in the data table. Sooo....bandwidth goes up as gain goes up? I'm supposed to believe that? I also note that they don't have a GBW graph in the datasheet.

Elsewhere in the datasheet they talk about the bandwidth being 110kHz at g=1000. I hope this means there are just typos on the first two pages of the datasheet.

I went up to their web site, and I definitely have the current version of the datasheet.

 

[ppl]

The AD 797 data sheet open loop vs frequency Graph (See attached) shows that the Gain reaches 0 dB of gain at about 100 to 100+ MHz depending upon RS. When comparing op amp gain vs frequency I look at the open loop gain vs frequency to determine the real gain at any frequency and thus avoid any confusion about implications stated in the front cover of the data sheet. This cover page is often just an advertisement and in the game to play one up in specs vs the competition manufactures like to use whatever method gives the best numbers. However the graph like the one attached is the same for all op amps so this is what I use to get accurate comparisons,

 

[jcx]

i have studied the 797 data sheet for use in strain gage amplifiers so i can point out a few obscurities

under the heading "Distortion Reduction" the unusual option of decompensating the op amp with Cext to ground at the "compensation" terminal is discussed

i think understanding the gain bandwidths in table IV require looking at the feedback resistor to ground, at higher gains this is 10 Ohms, while in fig 36 they indicate 100 Ohms is desirable in series with a lead cap for unity gain stability, presumably the potential instability of low feedback resistance at unity gain is useful bandwidth extending peaking at high gains

 

[tangent]

That graph is the one I was looking for, ppl. I don't know why I didn't see it. I guess it's safest to call it a ~40MHz GBW chip.

Hmmm...looks like 10x gain is almost the worst case for stability. Good thing I'm not using the AD797 in my scope preamp any more. They were both running at 10x.

 

[ppl]

I have stated allot of times that High open loop gain op amps like the AD-797 and LT-1028 are intended for High Loop gain applications like Phono and Mic PreAmps However Designers of Commercial as well as the DIY crowd continue to use these devices at low gain line level stages. The High 160 or so dB of open loop gain of these devices are real attractive upon first glance in regulator stages because this high open loop gain results in lots of feedback and thus a ultra low output impedance for the regulator. I would look into a wideband Video op amp in this application and give up a an ultra low output impedance for the sake of stability and bandwidth.
http://www.allegromicro.com/sf/93018/
ic regulator simple Block diagram

The Series pass transistor used is an old design that used to be manufactured by G.E. while it is Rugged and has a high Ft its gain vs current is like a rollercoaster and not what i would consider suitable in modern high performance Designs I like these devices a lot better http://www.allegromicro.com/skncatlg/pwrxtr/pwrxtr.pdf

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