Hi,
reading some course notes I got from a collegue I came accross a definition which is not explained. I wonder what it means:
For a MOSFET transistor the maximum usable frequency is discussed. The transition frequency ft is defined as
ft = gm / (2 pi Cgs)
So far so good, but then a frequency fm is defined as
fm = ft / (1 + Cdb/Cgs + Cgs_overlap/Cgs)
So fm is a frequency that is always below ft, but what does it mean?
Any ideas?
Regards Chris
Nope. Never heard of it. In what context does it appear in those notes ?
College / University tutors often come up with stuff that is unknown to the real world.
Graham
Intriguing. Never come across mosfets used in currrent mirrors. I dare say they may be found in ICs but never seen a discrete one. Can't imagine such a topology to be honest.
Graham
Hi Graham,
thank you very much for replying!
I think you made me find the answer: the formula appears in the context of the maximum usable frequency of a current mirror.
Hence the frequeny fm could be that maximum frequency. At least now it seems to make sense. In a current mirror one transistor is diode-connected therefore Cdb (drain-bulk depletiong capacitance) comes into play. Yet I have no idea why Cgs_overlap is needed in the current mirror only, I'd expect that it should degrade the transition frequency of a single transistor as well.
Regards Chris
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