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Noise & Gain circle - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


Moses
Post subject: Noise & Gain circle
Unread post Posted: Sun Feb 20, 2005 3:17 pm

Hi All,

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?

Many thanks!


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Guest
Post subject:
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.


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Moses
Post subject:
Unread postPosted: Mon Feb 21, 2005 1:07 pm

Sorry, maybe I wasn't clear in my post.

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?

Thanks.


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Guest
Post subject:
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.


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Moses
Post subject:
Unread postPosted: 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?

Many thanks.


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Guest
Post subject:
Unread postPosted: 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 conjugate.



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