In Chapter 9, on page 304, you mention
"Note, as discussed in Ch. 8, that looking at the size of the output noise alone gives no
indication of the noise performance of the amplifier or circuit."
However, isn't output noise important to determine the noise figure of an amp? Could
you elaborate on that statement? I looked through Ch. 8 but couldn't find the passage
that clarifies this statement.
Yes, even for the NF looking at the size of the output noise alone gives no indication
of the noise performance of the amplifier or circuit. You would have to also know the
portion of the measured output noise due to the noise in the input signal (e.g., from
the thermal noise of a source resistance if it's an ideal signal).
For general noise performance you have to refer the output noise to the input since
doing so takes into account the gain of the amplifier. This "input-referred noise"
isn't really physically present on the input of the amplifier but it does allow the
designer to calculate the circuit's SNR. For example, if two amplifiers have the
same output noise but one has a gain of 10 and the other 100 then the noise performance
of the gain of 100 amplifier will be better, see top of page 223 and other
discussions in Ch. 8.