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Unity gain amplifier stability

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ricko_uk:
Hi,
how can I:
1) make sure an op-amp is unity gain stable (i.e. to use just as buffer) at a certain frequency and certain amplitude if it does not say "unity gain stable" in the datasheet?
2) how do I determine the minimum gain at a certain frequency and certain amplitude for it to be stable? What infos on the datasheet do I need to check?

Thank you :)

moffy:
If you get 180 degree phase shift (open loop) at a gain greater than 1 (open loop) then it is not unity gain stable. The open loop transfer curve showing open loop gain and phase shift vs frequency will show you this.

Prehistoricman:
I was always taught in uni that the unity gain operational amplifier configuration is the most unstable (or difficult to stabilise).


--- Quote from: ricko_uk on February 10, 2020, 09:27:56 pm ---1) make sure an op-amp is unity gain stable (i.e. to use just as buffer) at a certain frequency and certain amplitude if it does not say "unity gain stable" in the datasheet?

--- End quote ---
You may need to add a compensation capacitor (if the part supports that) or a resistor in the feedback path. In fact, a resistor in the feedback path is a good design, and you can see the reasons for that here (look at all the answers, they have good stuff):
https://electronics.stackexchange.com/questions/56727/what-is-the-purpose-of-a-resistor-in-the-feedback-path-of-a-unity-gain-buffer


--- Quote from: ricko_uk on February 10, 2020, 09:27:56 pm ---2) how do I determine the minimum gain at a certain frequency and certain amplitude for it to be stable? What infos on the datasheet do I need to check?

--- End quote ---
Datasheet won't tell you a lot because the opamp performance depends greatly on the external parts. Simulation is probably the best option.

exe:
If someone has a good textbook on how to design compensation networks -- please share. I went through gazillions of manuals, but still don't quite understand the topic. What often missing is a "logical" explanation why it is designed that way. Like, few textbooks explain why openloop gain should have a rolloff of -20db/decade (I figured out myself what this means). A lot of educational material starts with math with explaining much: "just put some zeros with your compensation network and you are good to go".  May be I'm searching wrong things, should I start with a generic textbook on feedback systems?

Wimberleytech:
What opamp are you considering?

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