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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). --- End 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. --- 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? --- End quote --- Pick a different opamp :) --- End quote --- 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. --- End quote --- 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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