The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> Is S21 the actual gain of a LNA??
https://designers-guide.org/forum/YaBB.pl?num=1215827287

Message started by zhangjerome on Jul 11^th, 2008, 6:48pm

Title: Is S21 the actual gain of a LNA??
Post by zhangjerome on Jul 11^th, 2008, 6:48pm

There are three kinds of gain, power gain, transuducer gain, availble gain. Which one is the gain we concern??

In Spectre, with a 50 ohm port as the input, is S21 the gain of the LNA even the input is not well matched??

Title: Re: Is S21 the actual gain of a LNA??
Post by pancho_hideboo on Jul 11^th, 2008, 10:38pm

zhangjerome wrote on Jul 11^th, 2008, 6:48pm:

There are three kinds of gain, power gain, transuducer gain, availble gain. Which one is the gain we concern??
In Spectre, with a 50 ohm port as the input, is S21 the gain of the LNA even the input is not well matched??

S21 is a transducer gain.
You have to learn measurements using actual instruments. Not "EDA Tool Play".

Title: Re: Is S21 the actual gain of a LNA??
Post by zhangjerome on Jul 13^th, 2008, 4:23am

H, Pancho. As I know, the transducer gain is a function of S21 and other parameters, such as source and load reflection coefficients. Why do you think S21 is the transducer gain??

Title: Re: Is S21 the actual gain of a LNA??
Post by pancho_hideboo on Jul 13^th, 2008, 7:26am

zhangjerome wrote on Jul 13^th, 2008, 4:23am:

H, Pancho. As I know, the transducer gain is a function of S21 and other parameters, such as source and load reflection coefficients.

All incomplete knowledges are always useless.

You don't seem understand basic of RF. But this is not restricted to you.
1/S21 is a insertion loss.
Definition of insertion loss is same as inverse of sqrt(transducer gain).

See circuit theory text book, especially on filter design.

http://en.wikipedia.org/wiki/Power_gain

If you think the following, you must consider reference impedance of S-parameters definition. In the following, what values are used as reference impedances(Z01 and Z02) of S11, S12, S21, S22 ?

http://www.maurymw.com/support/faqs/faqs/faq9.html

Gamma_S=(ZS-conj(Z01))/(ZS+Z01), ZS is source impedance.
Gamma_L=(ZL-conj(Z02))/(ZL+Z02), ZL is load impedance.
If Z01=conj(ZS), Gamma_S=0.
If Z02=conj(ZL), Gamma_L=0.
Here these are not matching.
So S21=sqrt(Transduder Power Gain) is satisfied.
Usually ZS and ZL are real impedances such as 50ohm.

Again You have to learn measurements using actual instruments. Not "EDA Tool Play".

Title: Re: Is S21 the actual gain of a LNA??
Post by zhangjerome on Jul 13^th, 2008, 6:21pm

Hi, pancho. Thank you for you patience.

Seems I get it. Both gamma_source and gamma_load are zero. So S21 is actully equal to transducer gain. Am I right?