The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> NF min of a device
https://designers-guide.org/forum/YaBB.pl?num=1247138366

Message started by analog_cha on Jul 9^th, 2009, 4:19am

Title: NF min of a device
Post by analog_cha on Jul 9^th, 2009, 4:19am

My question may sound silly.

For a device, is NFmin constant irrespective of its size?
The expression for NFmin does have all technology constants. NFmin should be fixed by the process.
But when i simulated the device for its NFmin, it varies with size.

I am unable to understand this discrepency. Can anybody explain??
Thanks.

Title: Re: NF min of a device
Post by RFICDUDE on Jul 12^th, 2009, 5:57pm

NFmin is the minimum noise figure achievable for a particular device.

As a device, from any technology, is scaled up the transconductance tends to increase (and input referred noise decreases) and the input thermal noise due to resistance goes down, so it makes some sense that NFmin will reduce as the width increases from the minimum. But, of course, at some point the increased input capacitance is so high that the effective transconductance starts to decrease with increasing area, so NFmin will start to increase again at high frequencies.

Hope this helps a little.

Title: Re: NF min of a device
Post by rajesh14 on Oct 24^th, 2009, 11:45pm

@above
could u plz explain this point clearly "at some point the increased input capacitance is so high that the effective transconductance starts to decrease with increasing area"

Title: Re: NF min of a device
Post by RFICDUDE on Nov 2^nd, 2009, 7:09pm

I simply meant that as the device area gets bigger, so does Cgs which reduces ft (i.e. high frequency gain is limited by the capacitance). As the gain reduces, then the input referred noise increases, so NFmin increases with increasing frequency.