Praveen K
Junior Member
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Posts: 25
Bangalore
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hi Raj,
In my simulation I posted before, the values for the two stages were,
GM1 = 9.5mS , R1 = 11.36K , C1 = 11pF GM2 = 18.82mS, R2 = 1.15K (after loading of 1mA) , C2 = 10nF miller Cc = 0pF , 100pF
I tried to find the poles and zeros of the whole transfer function Vout/Vin with small signal models for the two stages with miller cap across it, and as you know its a long expression.
then I plugged in the above values to that expression in octave and tried to see how the pole and zero moves with variation of miller cap Cc
for Cc = 0pF the Vout/vin =
1.626e+15 --------------------------------- (s + 8.737e+04) (s + 8.004e+06)
for Cc = 10pF the Vout/Vin =
8.534e+14 (s - 1.883e+09) --------------------------------- (s + 7.172e+04) (s + 5.104e+06)
for Cc = 100pF the Vout/Vin =
1.631e+14 (s - 1.883e+08) -------------------------------- (s + 2.71e+04) (s + 2.554e+06)
for Cc = 1nF the Vout/Vin =
1.958e+13 (s - 1.883e+07) ---------------------------- (s + 3727) (s + 2.039e+06)
you can see from above, increasing the miller Cc results in both the poles moving to lower frequencies and there is no pole splitting. The zero is again away from the two poles at a higher frequency.
So with no approximation to the transfer function, clearly when C2*R2 > C1*R1 , miller cap Cc doesn't pole split nor helps in PM???
or am i missing something in the above?
thanks, Praveen
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