On Dec 15, 7:57 am, (Joseph H Allen) wrote:
What do you do if the input impedance of an LNA does not equal the source
impedance which gives the lowest noise?

The source might be an antenna, transmission line or filter which will not
work properly if it does not see the correct impedance.

What exactly do you mean by "will not work properly"? There are a lot
of effects possible; reflections on a transmission line may cause
"ghosts" in an analog video image, or an improperly terminated filter
may not give the needed frequency response. But there are ways around
those problems: put the LNA at the antenna, or design the filter to
work into the load impedance it will actually "see." Or maybe the
lowest possible noise is not a requirement, and you can optimize system
performance with a tradeoff between noise and some other parameter.

Lucky indeed is the designer who is able to optimize all parameters at
the same time. A big part of design is often selecting the tradeoffs
so the overall system goals can be met with enough margin to let the
system operate reliably.

Also, you may be surprised to find out just how little gain is
sacrificed by using an optimal noise match instead of an optimal power
match, or how little a filter's performance is altered by tuning for
minimum noise figure.

Finally, can you think of a way to alter the amplifier's input
impedance in a way that does not degrade noise, so that you come closer
to an impedance match while maintaining a match for optimal noise?

Cheers,
Tom