nrk1 wrote on Jun 20th, 2011, 8:38pm:* main amp BW < aux amp BW
Slight correction to the above(maybe this is what was meant):
Closed loop BW of the amp < aux amp unity gain frequency
The aux amp is in unity feedback. The main opamp may not be. So it is possible for main opamp unity gain frequency to be more than the aux amp unity gain frequency and still things to work out fine. The closed loop bandwidth of the circuit using the main opamp is (feedback fraction) * (main opamp unity gain frequency). In Fig. 3 of the paper, the unity gain frequency of the aux amp is less than the unity gain frequency of the main amp. But Fig. 5 shows that the former is more than the _closed loop bandwidth_ of the main amp. This is also clearly stated in the paragraph starting with "Our approach here ..." in Section IV.
* aux amp BW < main amp second pole
This inequality if not required for stability. For instance, things would be just fine if the aux amp was ideal(infinite bandwidth). There will be a pole followed by a zero due to the aux amp's unity gain frequency and because of this, the phase dips down and back up. If the maximum dip coincides with the unity loop gain frequency[ = (feedback fraction) * (main opamp unity gain frequency) ], stability will be affected, but it won't matter if it occurs at a much higher frequency.
I believe Sackinger and Guggenbuhl published this before Bult.
Cheers
sorry to dig up an old thread, but I was reading up on gain boosted opamps and the settling response issues which can arise from the pole zero doublets.
Just a clarification here, when people are referring to the main amp. in this thread, are you guys talking about the amplifier itself without gain boosting or the amplifier with the gain boosted included? If I understand it right, it is the former.