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Design >> Analog Design >> CG LNA gate to source impedance
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Message started by myjasonlee on Feb 25^th, 2005, 7:00am

Title: CG LNA gate to source impedance
Post by myjasonlee on Feb 25^th, 2005, 7:00am

Hello All

Kindly see page
http://www.geocities.com/jason_class/circuit_Zs.jpg

I am reading a book on modern communication circuit, then I am stuck at equation 3.56
I do not know how to come up with this equation.
Any kind soul, please teach me step by step in getting this impedance Zs'.
I am not from electronic background but from pure physics back ground.
I am trying to learn but has been stuck here for two weeks. Kindly enlighthen. I must complete this reading as I am lag behind other student in this electronic class.
Kindly enlighthen
Thank you

rgds and thanks
jason

Title: Re: CG LNA gate to source impedance
Post by Paul Geraedts on Feb 26^th, 2005, 6:34am

Hi Jason,

Looks to me like a balun. As the two parts of the inductor are coupled, their mutual inductance comes into play. Although in the derivation of equation 3.56, two assumptions are made to make it simple again (perfect coupling and large inductances: grab a book on circuit theory to see what they're about). Just look at the two parts of the inductor as an ideal-transformer with ratio N_1/N_2. You'll figure out the rest.

Paul

Title: Re: CG LNA gate to source impedance
Post by myjasonlee on Feb 26^th, 2005, 7:21am

Hi Paul

I almost can figure it out by using the tutorial below recommended by a good guy
http://www.coilcraft.com/pdf/doc157.pdf

But then please see 3rd page on the 1st and 3rd and 4th equation after figure 5.
I can see that 1st and 3rd equations are contradicting.
Then for the initial Zs' question, I can use 4th equation to understand it.
However as for how to get 4th equation, I try to solve from the 1st equation(after Figure 5), then I find the other way round for the result.
It is V1/I1 = V2/I2. [(a+1)^2]
So which one is right?
Anyone please help

rgds and thanks
Jason

Title: Re: CG LNA gate to source impedance
Post by Paul Geraedts on Feb 26^th, 2005, 9:43am

Jason,

Put the tutorial aside for a moment and do the calculations yourself. You will have your answer in no time!

Paul

Title: Re: CG LNA gate to source impedance
Post by myjasonlee on Feb 26^th, 2005, 2:27pm

Hi Paul

I did my own calculation and that is the first equation after Figure 5 is wrong, then only it is meaningful .
Any comments?
Please enligthen

rgds and thanks
Jason