Sean Breheny wrote:

> Don't you want as much gain in this first amp stage as possible? Each
> amp in the chain creates noise, so you want to amplify it as little as
> possible, so concentrating your gain in the first stage would be best,
> wouldn't it?

  Oh dear, Sean, you embarrass me!  KA3YXM, what did you study?  The
noise figure is set by the first stage, assuming it has REASONABLE
(20dB or so) gain and the following stages aren't dreadful.  If for
example the NF of the first stage was 0.25µV with a voltage gain of 10
(20dB) only, then its internal noise appears as 2.5µV to the next stage.

  Even if the next stage has 1µV of internal noise, this is about 8dB
down on the amplified version of the first stage noise and will be
barely discernible.  2.5µV noise + 1µV noise = approximately 2.7µV or
so, NOT 3.5µV.

  (Figures typical for VHF radios rather than ultrasonics, by way of
reference.)

  Cheers,
        Paul B.