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H21 and S21 phenomenon - RF Cafe Forums

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Post subject: H21 and S21 phenomenon
Unread postPosted: Thu Apr 28, 2005 3:56 am
User avatar

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

I have one big doubt about H21 and S21.
There's a case that for MOSFET device, measured S21 is too low (<-15dB) range, for 1GHz to 10GHz range. However, the H21 shows larger than 0dB for the same frequency range.

From H21, the fT is around 3GHz.
I'm not sure whether due to this low fT, the S21 is not there?

I'm not sure whether it's related in anyway.
or maybe there's other reason? and why H21 and S21 shows different phenomenon?

I understand that H21 is current gain, while S21 is power gain, but how they are related to one another?

Thank you.

Hope somebody out there could help me clear my doubt :-)

Best Regards,


Post subject:
Unread postPosted: Thu Apr 28, 2005 4:25 am

Hello Jean,

There are transformations between H-Parameters and S-Parameters, they can be found in text books.
While H21 is measured in DC and equals to the transistor's hFE or betta, S21 has its own value, because it is measured under different conditions (50-ohm environment), while H-Parameters are measured in open/short circuit mixture.

If you can tell in more detail about how your transistor is connected, then I might be able to help you solve the problem of low gain in the operating frequency.

Good luck!

Posted  11/12/2012
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