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[SOLVED] Distinguish feedforward path and feedback path

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minhchau

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Hello,
I am learning nested Miller Compensation to design LDO.
This image show the block diagram for a three-stage op amp with nested Miller compensation.

1600359800062.png
I have a few question:
Are Cm1 and Cm2 in feedforward path or feedback path? why?
And can you help me differentiate between feedforward path and feedback path
Thanks,
Chau
 

Dominik Przyborowski

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It is not good example to learn how to distinguish both of them.

Each capacitor is connected in negative feedback, but provides feed forward as well.
The reason for this is value of impedance seen from both cap ends, which is comparable.

If last OTA would be an opamp (low output impedance) than you would see feedback path only.
 

minhchau

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Thanks you, Dominik!
It is not good example to learn how to distinguish both of them.
Do you have any specific documents or examples about this distinction?
Each capacitor is connected in negative feedback, but provides feed forward as well.
If I use nested Miller Compensation to design LDO the system will have about 3 negative feedback loops,
when I evaluate the system's stability (by stb simulation in cadence) do i need to consider phase margin
and gain margin in 2 lnegative feedback loop, Cm1 and Cm2.
or I just evaluate the outer loop from Vout back to Vin.
Thanks you,
Chau
 

Dominik Przyborowski

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Stability is evaluated for the most outer loop (error signal flowing into OPAMP is interesting).
There is a lot of threads discussing stability on this forum.
 

    minhchau

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