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I am working on a design with cascode CS amplifier with resistors in the feedback. What are the advantages and disadvantages of such topology? How the resistors in the feedback affects noise figure? It will be great there's a book or paper that I can reference to.

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  • \$\begingroup\$ many SPICE simulators have good noise analysis where you can see the contributions of each resistor, transistor. Note that many transistor models have default parameters that yield optimistic noise magnitudes - but nevertheless, the SPICE noise model is good and useful. \$\endgroup\$ – glen_geek Mar 20 '17 at 19:34
  • \$\begingroup\$ Is the CS amplifier capacitively coupled? \$\endgroup\$ – Voltage Spike Mar 20 '17 at 20:17
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Consider this

schematic

simulate this circuit – 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. enter image description here

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.

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