Maker Pro
Maker Pro

fet for automatic gain control?


George Herold

Jan 1, 1970
Le Wed, 11 Dec 2013 10:08:52 -0500, Phil Hobbs a écrit:
In fact, for the usual circuits conditions a JFET is not a MOSFET
(indeed :)

The main difference is that, at ordinary low level currents, you use the
JFET in its quadratic region, while you use the MOSFET in its
ubthreshold region. And that makes for all the difference...

See, for a JFET, in the triode region:

ID = k ((vgs-vt) vds - 1/2 vds^2)
when substituting vgs for vc+1/2*vds (vc for control voltage)
you get
ID = k vds (vc - 2 vt)
which indeed is the mark of a resistor.

Nice! Thanks Fred. I tried a Jfet for agc following J.Williams.
(Not as good as a light bulb... and a much more involved circuit.)