Why does op-amp output impedance / parasitic input capacitance pole cause a noise peak?

Question

I was simulating the above circuit in LTspice... I realize the main problem here is the 2MHz peak (which should be fixable with proper compensation), but I'm curious about the smaller 398kHz peak that's seen in the output noise density plot. This matches up perfectly with the pole that is created by the LT1028 parasitic input capacitance and output impedance (5pF, 80 ohms). What is the mechanism of this creating an output noise peak though?

I understand that input capacitance increases the noise gain of an amplifier. However, I'd like to understand why this creates a peak in the noise rather than just an increase in the noise level.

Answer 1

When circuits have peaking, the noise is naturally also amplified.

Capacitance on Vin- will interact with the R_feedback, causing phase shifts. If the opamp is already of low phase_margin, you'll get peaking.

And given the Inductive output impedance of OpAmps, any Cload (unless well dampeded) makes that opamp provide output peaking.