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Design >> RF Design >> transconductance measurement
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Message started by sivacha on Dec 14^th, 2006, 10:32pm

Title: transconductance measurement
Post by sivacha on Dec 14^th, 2006, 10:32pm

Hi,

For my Gilbert mixer, how can i measure its transconductance(Gm)??
Theoritically it is (2/pi)*gm, gm being the transconductance of the RF transistor.

Which analysis i can use to measure the Gm??

Thanks,
Siva.

Title: Re: transconductance measurement
Post by ACWWong on Dec 15^th, 2006, 2:37am

You can measure the gm of you input transconductance stage.
Typically for a Gilbert this is a source-coupled-pair or emitter-coupled pair. The gm reported by the DC operating point is usually a good start. For hand calculations you look at something like Gray and Meyer.
If your Gilbert transconducatnce stage uses local series feedback (i.e degeneration) then the gm becomes gm/(1+gm*Zdeg). Again this is covered in Gray & Meyer and other standard analog IC design texts.
Anyway irrespective of linearity technique/input toplogy, I often just DC sweep the output differential drain current versus input differential volatge. The derivative of the current with respect to the input voltage gives gm. Its a quick look see and is often sufficient if devices are operating upto a tenth of ft.