What is 'fm' of a MOSFET?

[Christian Auner]

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

 

[Pooh Bear]

Christian Auner wrote:

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?

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

 

[Christian Auner]

On Wed, 01 Dec 2004 04:10:38 +0000, Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote:


[...]

So fm is a frequency that is always below ft, but what does it mean?

Any ideas?

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.

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