Hi Raj,
Please send it to kpraveenthomas@gmail.com

thank you,
k.praveen

hello rajkumar,
i am about to post ans for this query, but one question do you think miller fail in case of o/p pole dominate and first stage pole non-dominate?

Thanks,
raj.

hello rajkumar,
you are worng, let me explain. If outpole dominate then entire explanation is wrong. Miller works in any case for your reference plz check this fig (http://www.designers-guide.org/Forum/YaBB.pl?num=1314197543). So from that fig you can say miller works in any case. Because when you have o/p pole dominates, then you will see miller multiplied resistance rather miller cap, which is very difficult to explain and unfortunatly i forgot where i read this. Plz belive me that miller works in either case.

Thanks,
Raj.

Hi Raj, Thanks for sending me those papers. I'm going thru them and working on it. Let me see if i can convince myself on what you say, if not i'll post my arguments to you shortly.

hi Rajkumar, your perception of AV2 falling below 0db if output pole is dominant is new to me, i've never thought of it that way. let me verify this in simulation, then i think i can appreciate it well.

thanks,
Praveen

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

hello pravin,
have you got conclusion regarding this or still strugling? try the cap what i gave in the previous post check once, what i found is in the case of o/p pole dominate miller caps needs to be much higher... Hope some one who has good understanding will clarify further..

Thanks,
raj.

yes praveen,
you are correct, i am also in confusion. But here miller still works but not worthy i guess. Did you try in your sim?

Thanks,
Raj.

raja.cedt wrote on Oct 3^rd, 2011, 7:02am:


I partially agree with Raja. It is correct to say that Miller effect will always take place, regardless the value of CL, since such effect depends on the DC gain of the second stages. Thus CL does not play a role in such multiplication effect.
What it is also true that if the o/p pole is the dominant one then the expression for it is not 1/(gm2*R2*Cmiller*R01) anymore since now the contribution from CL has to be considered. In any case Miller capacitor will have to be very large in order to make the o/p pole non-dominant, since actually in that case the location of p1 and p2 will have to be actually inverted when considering the closeness to the imaginary axis.
But Miller effect will still be there anyways.

Best
Tosei