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Re: Motsenbacher and Fitchen "Low Noise Electronic Design"

They were comparing a circuit having no feedback with the exact same circuit with feedback. The equivalent input noise (squared per Hertz) parameter basically takes the mean-square Volts per Hertz (voltage spectral density) value at the output and divides it by the magnitude squared of the voltage gain. This reflects the output noise back to the input, giving a mean-square Volts per Hertz equivalent input noise.

So with no feedback, you have a high output noise, but also a high gain. With feedback applied, the output noise is lower because the gain is correspondingly lower. But when you divide the output noise by the gain, it all comes out in the wash to the same equivalent input noise, assuming the contribution of the feedback resistors to the total noise is negligible.

So if the non-feedback circuit had high noise, it was probably a topology-related issue rather than the absence of feedback per se that was causing the problem.


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