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?
Unread postPosted: 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.
Mon Feb 21, 2005 1:07 pm
Sorry, maybe I wasn't clear in
I need 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 pm
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?
Unread postPosted: Tue Feb 22, 2005
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 conjugate.