Kevin wrote:
> What's the difference between Vgs and Vgs(th) I thought the
> Vgs(th) was the voltage to turn it on fully ?

Vgs refers to the voltage on the gate with respect to the source.  This is
basically the input control signal into the FET.  It determines whether the
FET will be fully on, fully off, or somewhere in between.

Vgs(th) is the specif gate voltage called the "threshold".  I don't think
there is a universal definition for it, but it is roughly the voltage at
which the device transitions between off and on.  Of course it doesn't do
this at a single voltage, so that voltage is actually a in between point.
Sometimes Vgs(th) is define as the voltage at which there is the highest
change in output characteristic as a function of the gate voltage.  In other
cases I've seen it be where the output just starts to come on by some
specific definition of "starts".

In general, Vgs(th) is the voltage you want to be sure to be solidly on
either side of when using the FET as a switch.  In most switching
applications, Vgs(th) is of little use.  The full on and off voltages are
what you need to make sure your circuit drives the gate to.


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