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Re: Motsenbacher and Fitchen "Low Noise Electronic Design"

This does get a bit confusing :-). But basically, the noisy circuit is modeled as a noiseless one with a noise voltage source in series with the open-circuit input signal voltage all the way back at the input. That noise source is eni, and is the figure of merit for the noise performance. It's computed by taking the output noise voltage and dividing by the gain from the open-circuit input signal voltage to the output voltage.

If I understand your question correctly, you're referring to the output noise contribution from a noise source at a certain point in the circuit. But the way the equivalent input noise figure of merit is formulated, the effect when that contribution is reflected back to the input (by dividing by the amp's gain) is independent of whether there is feedback or not. Feedback reduces the output noise contribution, but the gain is correspondingly lower too. So it comes out the same when reflected back to the input.

Anyhoo, I hope I've answered the question you asked and not a different one :-).

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