How does resistive feedback affect noise figure?
Consider this







simulate this circuit &ndash; Schematic created using CircuitLab

Here is a 2-opamp circuit, Rnoise OA1=60 ohms, Rnoise OA2=6,000 ohms. Yet the RTI noise contributions are equal.


How is this done? We simply scale up the Rnoise and the 2 gain-set resistors of OA#2, by Gain^2 of the prior stage. Thus RG1=66ohm, RF1=594ohm; RG2=6Kohm, Rf2=56Kohm. (not exactly 100:1 scaleup, between stages, I know)

The final low-pass is needed to ensure the final opamp does not contribute more noise than the first opamp. That final opamp has nothing filtering its noise contributions, whereas the first opamp is "filtered" by the finite frequency response of 2nd stage.