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Design >> RF Design >> ft and fmax
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Message started by raja.cedt on Apr 27^th, 2011, 8:47am

Title: ft and fmax
Post by raja.cedt on Apr 27^th, 2011, 8:47am

hi,
i am new to rf design. can any one please explain which one we should consider for what. Because ft definition doesn't consider gate resistance and fmax does it. I know that looking at ft we can say that max operating frequency of transistors in that technology node. is this correct? For example 65nm has ft nearly 250G i guess but recently i have seen 300G VCO from razaavi group.

Please explain me what is ft
Thanks

Title: Re: ft and fmax
Post by unevb on May 4^th, 2011, 2:54pm

Fmax : Is the frequency at which you can get power gain of 0dB from the transistor

Ft : Is the frequency at which the current gain of the transistor is 0dB.

The standard expression for the Ft = gm/Cgs assumes the input impedance of the transistor is purely capacitive. This would mean that the power input of the transistor is 0 and therefore that the power gain of the transistor is infinite. When you account for the gate resistance you will start thinking about power gain, and the notion of Ft is replaced by Fmax.

Thinking about Ft is ok when you are thinking of lower frequency design where the capacitance of the MOS device does dominate the looking in impedance. It is used as a Figure of merit when comparing the performance of technology nodes for digital applications.

Title: Re: ft and fmax
Post by rfcooltools.com on May 6^th, 2011, 1:28pm

raja,

unevb answered this but in searching the internet for something completely different I dug up a presentation explaining this in more detail.
starting on page 24
http://class.ece.iastate.edu/ee501/2006/2006DevModel.ppt

http://rfcooltools.com