Copyright: 1996 - 2024
BSEE - KB3UON
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Noise & Gain circle - RF Cafe Forums
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Below are the old forum threads, including responses to the original posts.
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Post subject: Noise & Gain circle
Unread post Posted:
Sun Feb 20, 2005 3:17 pm
I want to design an
LNA based on the noise parameters of the device (could be any device
for this matter). My question is:
The optimal reflection coefficient
for a given frequency and bias point is different from the S11 parameter
for the same frequency and bias point.
I need to design a matching
network for the input to obtain the optiaml reflection coeffieicnt in
the device's input port and and 50 ohm on the source. How should I design
such matching network?
Same solution could be applied to obtain
a specific gain..
Am I doing the right thing?
Mon Feb 21, 2005 9:27 am
some devices will have the optimum
load impedance for noise figure close to the optimum load impedance
for a match. However, these devices a rare and most of the time the
optimum noise load will give you a less than optimum noise figure. It
is not possible to achieve the "best" noise figure and optimum impedance
match for these devices unless you isolate the input from the device
by using a lange coupler ofr similar circuit.
Unread postPosted: Mon Feb 21, 2005
Sorry, maybe I wasn't clear in my post.
to design a matching network that will make an impedance transformation
to obtain the best noise figure. My question is: Should I design the
matching network that on the source side it will 'see' 50 ohm and on
the device input it will 'see' the optimal reflection coefficient for
the minimal noise figure?
Unread postPosted: Mon Feb 21, 2005 2:38
The answer to the question is yes. You do want to build a
matching network that sees 50 ohms on the source side and the optimal
load impedance for noise figure on the load side. But, once you connect
it to the device it will no longer see 50 ohms on the source side because
the optimal noise impedance for the transistor is not neccessarily the
conjugate load required for max gain and optimum input impedance.
Tue Feb 22, 2005 9:30 am
Thanks for the information.
I have another question regarding matching: In order to obtain the maximal
transducer gain, the matching for both the source and load has to be
for the conjugate impedance. Does it mean that the source and load impedances
have to be the conjugate of S11 and S22 (S11*, S22*) and the matching
networks will transfrom the impedances from S11 to S11* and S22 to S22*
instead of to 50 ohms as it is usually being done?
Tue Feb 22, 2005 3:35 pm
Do not quite understand what you are
asking, but for maximum gain you need to present S11* to input and S22*
to output. You design a matching network to transform 50 ohms to the