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Are these used in our trade, and if so, where? Why (what is a JFET's advantage over other transistors)?

The main advantage of all FETS in general is-
There very low on resistance !
Which means that they can control large currents
with very little heat dissipation !

But they can be kantankerous beasts sometimes !

Also they are current controlled devices (like thermonic valves)
As compared to transistors which are voltage controlled.
 

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JFETS are mostly obsolete replaced by mosfets. I don't know why anyone would still use them. Still didn't stop my teacher from doing a course on them though.
For all power related applications, MOSFETs win over JFETs. MOSFETs have lower on resistance, and can handle way higher voltages and currents.

JFETs are very good in low noise instrumentation applications and can do much better than bipolar transistors (BJTs) when their driving signals have high source impedances.

JFETs are depletion mode devices which are on when no voltage is applied to their gates (Vgs=0). This is handy when you want something shorted out with no power applied.

MOSFETs are typically enhancement mode which turn on when the gate is biased and are off when Vgs=0. MOSFETs are also available in depletion mode for sold-state relay applications.

I don't think JFETs were ever used for power applications.

For power applications there is a whole zoo of critters like SCRs, TRIACs, and IGBJTs.

EJPHI
 

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JFETs are very good in low noise instrumentation applications and can do much better than bipolar transistors (BJTs) when their driving signals have high source impedances.
This might be the only purpose to ever use a JFET, but you'll probably only ever come across them in an OP-AMP package for specialized applications. Linear makes the LT1115 which still uses JFETS, but it's not suited for any low impedance driving sources. You'd probably use it to amplify signals from say a pH or a piezoelectric sensor. You wouldn't ever see them used in an audio application.

Even if you did have a device with a JFET, it's most likely just buffering the input stage with the proceeding gain stages being handled by MOSFETS.
 
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