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.