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.

--
/* AB1GO */ /* Joseph H. Allen */
int a[1817];main(z,p,q,r){for(p=80;q+p-80;p-=2*a[p])for(z=9;z--q=3&(r=time(0)
+r*57)/7,q=q?q-1?q-2?1-p%79?-1:0%79-77?1:01659?79:0158?-79:0,q?!a[p+q*2
]?a[p+=a[p+=q]=q]=q:0:0;for(;q++-1817printf(q%79?"%c":"%c\n"," #"[!a[q-1]]);}

Lowest noise does not equal highest gain.
Highest gain equals source impedance equals system impedance.
So, LNA equals source impedance gives not necessarely lowest noise.
Tune LNA for lowest noise, the gain will be somewhat lower.
Its the S/N ratio that counts especially important for an LNA.


"Joseph H Allen" schreef in bericht
...

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.

--
/* AB1GO */ /* Joseph H.
Allen */
int
a[1817];main(z,p,q,r){for(p=80;q+p-80;p-=2*a[p])for(z=9;z--q=3&(r=time(0)
+r*57)/7,q=q?q-1?q-2?1-p%79?-1:0%79-77?1:01659?79:0158?-79:0,q?!a[p+q*2
]?a[p+=a[p+=q]=q]=q:0:0;for(;q++-1817printf(q%79?"%c":"%c\n","
#"[!a[q-1]]);}

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

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).

Here is a nice reference:

http://rfdesign.com/mag/radio_unique...tive_feedback/

The lossless feedback can be as simple as an inductor in the emitter or
source lead.

I've found lots of other references (mostly for-pay IEEE papers), it looks
like stability can be a big issue- no big surprise in a feedback network.

--
/* AB1GO */ /* Joseph H. Allen */
int a[1817];main(z,p,q,r){for(p=80;q+p-80;p-=2*a[p])for(z=9;z--q=3&(r=time(0)
+r*57)/7,q=q?q-1?q-2?1-p%79?-1:0%79-77?1:01659?79:0158?-79:0,q?!a[p+q*2
]?a[p+=a[p+=q]=q]=q:0:0;for(;q++-1817printf(q%79?"%c":"%c\n"," #"[!a[q-1]]);}

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