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Unity gain amplifier stability

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magic:

--- Quote from: exe on February 11, 2020, 11:16:59 am ---Like, few textbooks explain why openloop gain should have a rolloff of -20db/decade (I figured out myself what this means).

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I'm not familiar with the details but rolloff steepness appears to be tied with phase.
There are opamps where it's faster than 20dB/decade at certain frequencies, see NE5534 or LT1128. Phase margin suffers.


--- Quote from: exe on February 11, 2020, 11:43:30 am ---
--- Quote from: ricko_uk on February 10, 2020, 09:27:56 pm ---if it does not say "unity gain stable" in the datasheet?

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Pick a different opamp :)

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These days almost everything is unity gain stable. The datasheet may not say it explicitly.

Wimberleytech:

--- Quote from: Wimberleytech on February 11, 2020, 11:59:20 am ---
I have a great set of lecture notes from, uh, 40+ years ago.  I will scan them and post later.


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Here they are: http://www.wimberleytech.com/techblog/wp-content/uploads/2020/02/Second-order-systems-and-compensation_opt.pdf

Wimberleytech:


--- Quote ---I'm not familiar with the details but rolloff steepness appears to be tied with phase.
There are opamps where it's faster than 20dB/decade at certain frequencies, see NE5534 or LT1128. Phase margin suffers.


--- End quote ---

For a second order system, if Aß crosses 0dB at 20dB/decade then you will have a minimum of 45° of phase margin.  No such amplifier exists, but that is where the rolloff rule of thumb comes from.

741:
A very simple approach that can work "resonably" is simply to "pot down" (scale down) the inverting input, then allow for this by introducing a gain of the same. When computing the feedback resistance, note the input impedance will be equivalent to the 2 input resistors in parallel.

The above works by simply reducing the "closed-loop gain", and it compromises op-amp circuit performance accordingly.

rstofer:

--- Quote from: Wimberleytech on February 11, 2020, 01:38:22 pm ---
--- Quote from: Wimberleytech on February 11, 2020, 11:59:20 am ---
I have a great set of lecture notes from, uh, 40+ years ago.  I will scan them and post later.


--- End quote ---
Here they are: http://www.wimberleytech.com/techblog/wp-content/uploads/2020/02/Second-order-systems-and-compensation_opt.pdf

--- End quote ---

Thanks for the notes.  They clearly point out why I walked away from analog and focused on digital in college.

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