This type of compound amplifier uses an OTA instead of OPA input stage. It works only with bipolar input amplifiers. The OTA is a transconductance amplifier transforming the input error voltage into output current. The AD844, has the output of it's OTA available labeled Tz which can be used to drive the main amplifier. The input of the main amp and the output of the OTA being both of high impedance, the capacitance at this point determines the dominant pole. Here it happened to be about 15khz, but with an extra gain of 60db. The OTA then provides the main amp of gain 10 60db more NFB to smash the distortion.
The adjusted values for LM3886 gives for 60db extra NFB, phase margin of 62°
Hayk.
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