phase shift in uncompensated ldo explanation needed

[kannu]

pahse shift in ldo

hi ,i am beginer in ldo design could you plz tell the total phase sift in uncompensated ldo . why we use positive feedback when use pmos as pass transistor.what is the reason behind this

 

[goh_jiun_heng]

Re: pahse shift in ldo

I think the schematic you are referring is:
One Error Amplifier => PMOS.

And the feedback path is connected back to non-inverting input of error amplifier. But you couldnt look at it and conclude it is positive feedback.

If you look at whole system (Error Amp + PMOS), it is equivalent to Negative Feedback since there is one inversion through the PMOS (Input=Gate of PMOS=Output of Error Amp, and Output of LDO = Drain of PMOS). IF you look at PMOS, it is in Common Source structure which has one more gain stage to drive the output.

Hope it helps.