How to model stability analysis of a linear regulator such as this? - Page 2 - diyAudio
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Old 3rd January 2013, 07:12 PM   #11
jcx is offline jcx  United States
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-30 degrees phase margin at the 0 dB gain intercept means its an oscillator, not a regulator

base pullup R is always used in real circuits

the pass Q adds gain depending on load current to the loop so compensation is required somewhere - heavier overcompensation is required for larger load range, C on the reg output has a big impact on stability
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Old 4th January 2013, 02:35 AM   #12
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Quote:
Originally Posted by Elvee View Post
Here it is; the circuit had to be adjusted to display something usable as the original values were too crazy, but the principle remains the same whatever the exact circuit.
Hmm, Elvee for some reason the circuit that you posted does not work for stability analysis. I had to put a large series L and large C to the AC source to get the analysis to work.

With that said, I'm finding this setup stable without any compensation. Why is this? I modeled my opamps to have a similar open loop frequency response as in the datasheet of the LM358. Is this because the low gain bandwidth product of the opamp?
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Old 4th January 2013, 12:32 PM   #13
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Fusion916 View Post
Hmm, Elvee for some reason the circuit that you posted does not work for stability analysis. I had to put a large series L and large C to the AC source to get the analysis to work.
Probably some restriction of the simulator you use.

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With that said, I'm finding this setup stable without any compensation. Why is this?
Could be many reasons, transistor not conforming to model, parasitic inductance in the input loop, etc
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Old 4th January 2013, 02:19 PM   #14
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It simulates stable with Simetrix.
If you add uF level load capacitance, it becomes borderline unstable. This is cured by small series resistance.
This is typical behaviour of this sort of regulator, never use low ESR decoupling
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