BiCMOS Amplifier with feedback for DC bias and AC
Is there a simpler approach?


My simple approach is that with the original circuit you have negative feedback at all frequencies therefore the input impedance at the gate is close to zero ohms. That's how negative feedback works. It's a virtual earth as per an op-amp virtual earth.

With the modified feedback (and C being a large capacitor as stated) there is only negative feedback at DC and very low frequencies hence, at significantly higher frequencies, there is no feedback hence the input impedance is 10 Mohm (the left resistor in your added/modified circuit).

That's about as simple an approach as you can get.