[sdiy] Does Source = Drain???

Ian Fritz ijfritz at comcast.net
Sun Jun 13 19:30:43 CEST 2010

At 03:54 AM 6/13/2010, John Richetta wrote:

>Anyone want to recommend a truly up-to-date and
>complete transistor reference, that is still useful in the context of
>hobbyists dealing with discrete (as well as IC) transistors (as
>opposed to a reference intended just for those designing integrated
>circuits)?  Such texts seem to be almost impossible to find (thus,
>lots of time spent on my part reading a variety of sources, piecing
>little bits together, in a fairly non-authoritative fashion).

Horowitz and Hill has a pretty good chapter (Ch 3) on FETs, which addresses 
many of your questions.  That's the place to start for an advanced hobbyist 
level of understanding.

As one example, they write: "... FETs ... are nearly symmetrical, but the 
gate-drain capacitance is usually designed to be less than the gate-source 
capacitance." (What Antti said.)

As far as operating an N-JFET with a positive gate voltage, remember it is 
the gate voltage wrt the more negative of the source or drain voltage that 
defines the gate current.  So if you have AC signals present you have to 
look very carefully at what these differential voltages are.

In forward bias at limited gate current you will not get much of a field in 
the channel, so you won't get much change in drain-source current.  But if 
you come up with some clever way of getting interesting operation under 
these conditions, please let us know!


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