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biasing transistor for LNA

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dili Neo

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Hi,
I am newbie in RF IC design and presently working to design an LNA at 2.4GHz.I have a doubt .I used a cascode arrangement with current mirror for biasing.the problem is once I did the dc analysis and printed the dc operating point in cadence virtuso(UMC 180nm tech) the value obtained for Cgs=-124.3f and gm=34.74m.the transition frequency ,wt=gm/Cgs=279G .is this value very large since Ls=Rin/wt and if Rin=50ohm,Ls will be 0.179nH !!!!
where shall be the mistake?????

Thanks in advance
 

I am also attaching my work,kindly take a look and help out
 

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  • biasing.tar.gz
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Hi,

First, ft=gm/Cgs, and therefore Ls = 50/(6.28*279G) = 28.5 pH :)
Second, seems like you got wrong value for Cgs, i measured Cgs for transistor in 65 nm technology with gm = 34m and it equal 270 fF.

If value of Ls very small, one of possibilities is decreasing ft of transistor with additional capacitor or increasing channel length.
 

@sarge

I am attaching the dc operating point details.kindly take a look and tell me where I went wrong.

parameter1.png

I used only minimum length for channel,L=180nm. How it is affecting the capacitance ? what is the optimal choice for channel length

Thanks and Regards

- - - Updated - - -

sarge said:
Hi,

First, ft=gm/Cgs, and therefore Ls = 50/(6.28*279G) = 28.5 pH :)
Second, seems like you got wrong value for Cgs, i measured Cgs for transistor in 65 nm technology with gm = 34m and it equal 270 fF.

If value of Ls very small, one of possibilities is decreasing ft of transistor with additional capacitor or increasing channel length.
Click to expand...

hi kindly take a look in the parameters list attached above
 

I don't use for measuring the dc operation point window, it's look strange :)
For gm you can use dc sweep for input voltage, and then plot gm from direct plot form. For Cgs i run AC analysis and mesure capacitance in it.
 
sarge said:
I don't use for measuring the dc operation point window, it's look strange :)
For gm you can use dc sweep for input voltage, and then plot gm from direct plot form. For Cgs i run AC analysis and mesure capacitance in it.
Click to expand...

kindly tel me hw to measure capacitance using ac analysis? I am newbie to cadence virtuso!! In ac analysis I found only gain measurement options.

- - - Updated - - -
 

dili Neo said:
kindly tel me hw to measure capacitance using ac analysis? I am newbie to cadence virtuso!! In ac analysis I found only gain measurement options.
Click to expand...
I place vsin source with ac amplitude on input and measure ac current throw it. After that i place in calc:
Code: 
abs((imag(value(IF("/V0/PLUS") VAR("fr"))) / (twoPi * VAR("fr") * VAR("vam"))))
where:
fr - input frequency
vam - ac amplitude
 

why we need to do ac analysis to measure Cgs.DC operating point is also giving Cgs value.How to two shall be different???

- - - Updated - - -

dili Neo said:
kindly tel me hw to measure capacitance using ac analysis? I am newbie to cadence virtuso!! In ac analysis I found only gain measurement options.

- - - Updated - - -
Click to expand...

why we need to do ac analysis why dc operating point data not sufficient?
 

DC operation point also giving gm, but is it correct? When i measure gm with sweep input voltage it gives another results.
I can't explain why this values is different, but i think that there is more experienced people who can give answer for you.

- - - Updated - - -

DC operation point also giving gm, but is it correct? When i measure gm with sweep input voltage it gives another results.
I can't explain why this values is different, but i think that there is more experienced people who can give answer for you.
 

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