vivkr
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Hi,
Indeed, there is a very simple way to predict that there is a doublet without doing any math at all. I assume that you have not read the classic paper by Bult and Geelen ("A fast settling CMOS Opamp for SC Applications with 90 dB DC Gain" which appeared in the JSSC, Dec. '90 issue).
Look at the gain profile of a gain-boosted amplifier for the case where the -3dB bandwidth of the main amp is f0, and for the boosting amp f1, where f1 << f0 (as a simple example). Say the main amp has gain G0, the boosting amp G1, both in dB.
Now the gain at DC is G0 + G1, and stays so until f1, where it starts to drop by 20 dB/dec, until it reaches f0 upon which it becomes constant at G0 dB until you reach f0.
So, the gain first drops and then it becomes flat, hinting that a pole is followed by a zero. That's all there is to it really.
Regards,
Vivek
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