Mark wrote:
Note: For many LNA designs, the best signal/noise ratio occurs at an
impedance that is close to, but not really, a perfect conjugate match.
The signal is coupled to the amplifier best at the conjugate match
impedance, but sometimes the noise is enhanced even more.


That brings up an intersting question I never did get a good answer
to...

It is my assertion that an LNA that is physically at room temperature
(290K) can have a noise figure no better than 3 dB (i.e. its effective
noise temperature is 290K) IF it is also conjugatly matched i.e.
looks like 50 Ohms.

Yes you can make the noise figure better than 3 dB, but then you must
either cool the device or MISMATCH it to the line.

In other words if it looks like 50 Ohms and it is physically at 290 K,
it's effective noise temperature must also be 290K.

How could it be otherwise?

Comments plese.

Mark


That beer in your hand was also cooled in a 300K ambient. How is that
possible? (Hint: the fluctuation-dissipation theorem only applies to
systems in thermodynamic equilibrium. The moment you turn on the power,
that assumption is violated, just as it is in your domestic refrigerator.)

An ordinary room-temperature diode has a noise temperature of 150K
(Tambient/2) as you can show in about 3 lines of algebra, starting from
the diode equation and the shot noise and Johnson noise formulas.

Cheers,

Phil Hobbs

Cheers,

Phil Hobbs