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The miller cap makes A node as dominant pole, I expect that adding 10pF to A node will improve the dominant pole, but in fact the stablility is worse, why?
The miller cap makes A node as dominant pole, I expect that adding 10pF to A node will improve the dominant pole, but in fact the stablility is worse, why?
I think the Rc and Cc forms a LHP zero to cancel the 2nd pole (-gm5/C2) where C2 is the cap seeing from Vout node. If that is the case, original 3rd pole become the 2nd dominate pole (-1/Rc/C1) where C1 is the cap seeing from node A. Once you increase C1, the phase margain become small. Re-size Cc and Rc to make the time constant the same and increase Cc, decrease Rc.
Maybe because the 10pF is too large, and at a high frequency it act as "short" the gate of M5 to GND, and "shut off" M5. So the output resistance of the output stage became larger, and makes stability worse.
(BTW, the former message i post has misunderstood your meaning. You means add, but not replace)
Miller compensation produces pole-splitting, then increasing phase marging.
The addition of the other capacitor only makes the two parasitic poles to move left without producing pole splitting. The first non-dominant pole is:
p2=-gmout.Cm/(C1.CL+(C1+CL).Cm)
The second non-dominant pole is:
p3=-1/(Rm.C1)
with Cm= miller capacitor, Rm miller resistor, C1 capacitor in parallel with gate of output transistor, and CL the capacitor connected at the output.
there is another non-dominant pole @ w=1/(Rc*C1) where Rc is the miller resistor and C1 is the capacitance at node A (gate M5), so adding a cap. at node A will increase the non-dominant pole and hence PM goes Bad, so either increase the Miller cap. or just use the cap. at node A but not both simultaneously
refer to gray for more details
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