Hi Aaron,

At high frequencies (around the unity gain and higher), the Miller compensation cap is practically a short. Since this is connected between the gate and drain of the second stage driving transistor, you have a gate-drain short at high frequencies, and this "diode" offers resistance of gm2 to the output load cap CL for a second pole at gm2/CL.

Of course, the real situation is slightly different since the Cgs of the second stage will change the action a little bit, but that's the basic idea.

No black magic here, or anywhere else...

Vivek

Hi,

Pole splitting will occur only when the dominant pole is set by the compensation capacitance Cc. If you increase the value of this cap from zero to a certain optimum value, the dominant pole will rapidly move lower and lower, while the nondominant pole will move outwards. Beyond a certain point, increasing Cc will not cause the nondominant pole to move further or not much further. The second pole is set at Gm2/Cl. After this point, you need to increase Gm2 to move the second pole out.

Note in particular that it is highly undesirable practice in general to go on increasing Cc, and Cc >= Cl will give diminishing returns. In a good design, Cc < Cl and preferably by a factor of 2-3. Otherwise, you are wasting power. In some exceptional cases (very low-bandwidth circuits), you may want to increase Cc a lot to reduce total inband noise. That's it.

Vivek

aaron_do wrote on Oct 16^th, 2008, 6:02am:


I don't understand your question? Why should the gain be 3 dB lower than the DC value? Are you saying that you expect the gain of the 2nd stage to be 3 dB lower than its DC value at the frequency where the 2nd pole is to be found? In that case, your analysis is slightly off track. The gain would have been 3 dB lower for the 2nd stage at the frequency corresponding the its pole were there no feedback (in form of the Miller cap) across this stage. That changes the dynamics. So, gm2/Cl is the 2nd pole of a Miller-compensated opamp, not the open-loop pole of the 2nd stage. That is something different, and in certain compensation schemes like the Ahuja scheme where there is no direct feedback across the 2nd stage, that situation is seen more clearly.

I hope this answers your question.

Regards,
Vivek