BUF634 output resistor in feedback loop

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#2
If you want what the BUF634 is driving to see a certain amount of source resistance, then place the resistor with the desired value outside the feedback loop. If you want what you are driving to see a very low source resistance (a hard voltage source), then place the R inside the loop.
 
#3
I think it's purpose is usually not current limiting, but capacitive load isolation. If the output was to be shorted, the current would likely still be limited by the BUF634s internal current limiting, but I could be wrong.

And obviously what egellings said about the resistance seen by the load.
 
#5
The isolation resistor can be in the loop, but it makes everything slightly more complicated when it comes to ensuring stability. The BUF634A datasheet even shows an example somewhere. There are also few App Notes on this, I remembered one was from ST because and I found it easily, linked below. You can search for the "Analog Engineers Pocket Reference" from TI, it also touches on and it's free for download (I have the physical version - comes in handy now and then). But these only discuss OPAs by themselves driving capacitive loads, not with a buffer. Every configuration has benefits and drawbacks, but as an introduction I these app notes are okay to get one started.

https://www.st.com/resource/en/appl...ading-applied-to-ts507-stmicroelectronics.pdf

I know Jan has a headamp with AD797 and BUF634. Sadly my favourite version of the BUF634 is pretty much EOL. I really liked the BUF634T (TO-220-5) version, but I refuse to pay around 10 Euros each for the remaining ones on sale. Together with AD797 one composite opamp would cost me around 20 Euros purely in ICs and it would have to be driven from a really low source impedance as well. Hard to justify when you can buy 4 each of OPA1656 and BUF634A for the same price 🙁
 
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#7
I actually wanted to make a PCB that supports the SOIC-Powerpad versions of both BUF634A and LMH6321 in a composite loop with a dual OpAmp, very similar to the BUF634ADEVM evaluation board:

https://www.ti.com/tool/BUF634ADEVM

It can be done quite easily, you just need a few extra optional components then you can flexibly use either buffer on the same PCB. The evaluation board is based on the BUF634A version without a thermal pad though, that's why they were able to route the power traces very nicely. With thermal pads (and vias) the layout would have to be slightly different.
 
#8
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