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pcardoso73

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Portugal

Urgent - Strong vs Weak inversion
May 27^th, 2011, 12:02am

Hi all,

Please try to help me out on this. I am simulating a VCO using 130 nm technology.
As BJT's have a lower flicker, I tried the simulation on weak inversion for the MOS, expecting a better behavior than in the strong inversion, in terms of 1/f and 1/f^3. But, by simulations, I cannot reach this conclusion.
Any ideas ?

Best regards,
Pedro

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solidstate

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Munich, Germany

Re: Urgent - Strong vs Weak inversion
Reply #1 - May 27^th, 2011, 10:53am

As far as I know, a mosfet does not become a BJT by biasing it in weak inversion, it just shows similar characteristics. The main reason BJTs have low 1/f noise is because they are not surface devices; 1/f noise in mosfets is due to defects at the Si/Si02 interface. The only solution to reduce that noise is by increasing device area..

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rfidea

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Re: Urgent - Strong vs Weak inversion
Reply #2 - May 27^th, 2011, 11:38am

solidstate wrote on May 27^th, 2011, 10:53am:

As far as I know, a mosfet does not become a BJT by biasing it in weak inversion, it just shows similar characteristics. The main reason BJTs have low 1/f noise is because they are not surface devices; 1/f noise in mosfets is due to defects at the Si/Si02 interface. The only solution to reduce that noise is by increasing device area..

I have the same opinon.

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aaron_do

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Re: Urgent - Strong vs Weak inversion
Reply #3 - May 28^th, 2011, 8:17am

Hi Pedro,

the signals in a VCO are large. So I'm not sure that it's fair to say its in the weak inversion or strong inversion region, since more likely it goes from off to weak inversion to strong inversion and finally to triode region, and then back again. There are a few papers on VCO phase noise around, so probably they will have better information.

cheers,
Aaron

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pcardoso73

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Portugal

Re: Urgent - Strong vs Weak inversion
Reply #4 - May 28^th, 2011, 10:43am

Hi all,

Thank you for your replies. Maybe I wasn't very clear. Well I know that both devices are different. Please notice that, in weak inversion, the MOS device operates according an exponential law rather then operating according to a square law. Thus here is some similarity with BJT's. With this assumption, I was expecting that the exponential law behavior would cause a better noise performance.
As solidstate said I just need to increase the size of the MOS to get better 1/f noise, but that is to go in the direction of weak inversion.
Moreover I will have more problems wih parasitic capacitances of the transistor if the gain of he mosfet is high.

Concluding, how is it possible to have a good phase noise behavior on weak inversion ?

Tanks to all.
Pedro

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Urgent - Strong vs Weak inversion Reply #5 - May 28^th, 2011, 6:11pm do some research on phase noise in VCO's there are many papers out there. IEEE JSSC is your friend here.
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