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MSG and S21 - 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.

Post subject: MSG and S21
Unread postPosted: Wed May 11, 2005 1:49 am
User avatar

Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
Hi All :

I have one problem regarding MSG and S21. Currently I simulate small signal performance of power amplifier.
I observed that MSG is around 14.6dB, however |S21| is only 4.1dB.

I understand that MSG= |S21| / |S12|
the |S21| is 1.6018
while |S12| is 0.1097.

My doubt is, why in this case the |S21| is much smaller than MSG?
besides |S12|, are there any other factors that affect?

Thank you.
Your advice is greatly appreciated.

Best Regards,


Post subject:
Unread postPosted: Wed May 11, 2005 2:48 am

Hello Jean,

Your calculation is not correct.
The MSG ratio you obtained is not in dB units. You should multiply this ratio - 14.6 by 10*log >>>>>10*log 14.6=~11.65dB

You should check why your S21 is so small? I assume that this can be caused by poor matching or wrong bias operation point. If you can tell more details about your circuit, I might be able to help you.

Good luck!!!


Post subject:
Unread postPosted: Wed May 11, 2005 5:28 am
User avatar

Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
Thanks for ur advice.

Yes, I think the matching network is not good.
I'm currently designing for 5GHz , 2 -stages power amplifier.

This problem of low gain (S21) is on my second stage.
This is my first-time designing on 5GHz range.
Any design tips? :idea:
Because I observe that it's very hard to have "gain-matching" network at this frequency range.

any advice is greatly appreciated. :wink:

Thanks and Regards,


Post subject:
Unread postPosted: Wed May 11, 2005 5:41 am

Hello Jean,

You are welcome.

What is your operation bandwidth?

The matching netwroks done in this freqency range are consisted of transmission lines. They can be broadband or narrowband.

Which substrate are you using? What is the device that you use in your final stage?

S21=4dB for a device, which is capable for a gain of at least 5dB more shows about very poor match. If you can provide more details about the questions above it will help.

Posted  11/12/2012
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