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Design >> RF Design >> MOS sizes in RF Design
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Message started by eng on Jul 12^th, 2007, 12:16pm

Title: MOS sizes in RF Design
Post by eng on Jul 12^th, 2007, 12:16pm

Hi all,
Is it necessary to use minimum length for an RF circuit. i.e. LNA. I doubled both length and width of my LNA (Lmin=0.18um) there is a .6 dB NF improvement and -5 dBm P1dB improvement. Gain dropped 4.5 dB. The transient results look good. I don't know if I'm missing a important point.

What are the consequences of increaing L in RF design?

Title: Re: MOS sizes in RF Design
Post by James Bond on Jul 12^th, 2007, 12:40pm

do you have the same current when increase the size ?

Title: Re: MOS sizes in RF Design
Post by eng on Jul 12^th, 2007, 1:43pm

current increased only 23u. It was 795u became 818u.

Title: Re: MOS sizes in RF Design
Post by aaron_do on Jul 12^th, 2007, 6:45pm

It depends on your system. If your system has a high NF requirement, then 0.6 dB NF won't make a dent in the overall system NF. 4.5 dB gain however might be very important.

Title: Re: MOS sizes in RF Design
Post by ACWWong on Jul 13^th, 2007, 2:20am

eng wrote on Jul 12^th, 2007, 12:16pm:

My thoughts are as follows:

Increasing L will decrease ft of the device and increase capacitance, although cgs is usually matched out. This doesn't really matter if the input gm transistor is operating at 0.1*ft anyway and in cascode configuration. The cascode device however, which develops the voltage gain, may suffer from larger L as a consequence of lowered ft means less gain available.

Obviously if current and W are not changed, overdrive increases which means less gm, but more linearity.

But mostly your choice depends on your specifications as Aaron_do said. I must admit the CMOS LNAs i have designed (few hundred MHz to few GHz) have seldom need minimum geometry (expect at >~0.35um), also having larger L might help ESD robustnbess as well.

cheers
aw