Joseph H Allen wrote:
In article .com,
K7ITM wrote:

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?

Negative feedback increased input impedance. It looks like lossless
negative feedback will do this without also increasing the noise. Perhaps
use this to make the input impedance equal the noise-opt impedance- then you
can match the two together to the source (using a transformer or a tuned
circuit).
....

Right. Well, actually, you can use negative feedback to either
increase or decrease the input impedance, and to either increase or
decrease the output impedance. Negative feedback applied as a current
into a summing junction (as in an inverting op amp circuit) decreases
the input impedance. Negative feedback applied as a voltage in series
with the input signal (as in a non-inverting op amp circuit, or by
using a source/emitter impedance) increases the input impedance.
Feedback derived from the output voltage decreases the output impedance
(tries to hold the output voltage constant, for a constant input
signal). Feedback derived from the output current increases the output
impedance (tries to hold the output current constant, for a constant
input signal).

Just as you say, though, feedback can be difficult to apply at RF, and
especially so at the very high frequencies where noise figure is
typically the most critical.

Cheers,
Tom