imar said:hi freinds;
we have spoken later in (Problem with LDO design) about the positive and the negative feedback for the error amplifier and hopefully as LvW said, the PM is not always calculated by taking the origin in -180° and LVW has well explained it.
so at this point, a question could be put: what is the difference between having a positive feedback or a negative one? how could we choose between them?
in what point does this feedback affect ?
imar said:so, as you said, a negative feedback can turn into positive one after a critical frequency,and we can manage to know the nature of this feedback by simply draw the sensitivity function and pointing at the UGF to see in which side does it go.
--> then, we can know that our system is reaching a critical frequency if we designed to have a negative feedback and it appears, by virtu to the S function, to be a positive one!
imar said:you are right,
it was my mistake. S=1 means that the ampli has a unity gain frequency and it has not relation with UGF.
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