1
\$\begingroup\$

In a self-biased JFET circuit, the gate voltage must be approximately zero so that the reverse voltage at the gate-to-source will be equal(but negative) to the voltage across the source resistance. This can be achieved by putting very large resistance at the gate so that no current will flow.

BUT, what would happen if I left the gate open so that the infinite resistance will block all current? enter image description here

\$\endgroup\$
1
  • \$\begingroup\$ With an open gate node the FET is NOT self-biased. It needs a VOLTAGE between gate and source - hence, a fixed and well-defined potential difference! \$\endgroup\$
    – LvW
    May 15 '20 at 7:27
2
\$\begingroup\$

If you leave the gate open on a JFET, you end up with a circuit that will react to all variations in the electrical fields around it.

This is an electroscope that makes use of an open gate JFET to detect static electricity.

enter image description here

It is very sensitive to variations in the electrical field.

\$\endgroup\$
5
  • \$\begingroup\$ ...and in that case, the behavior depends on the JFET package. For TO-92, typically the leads go G, D, S, automatically supplying many gig-ohms of plastic surface-leakage connecting G to D. So, after half a minute of picoamps leakage current, the JFET turns partway on. (Or speed this up, by breathing your human breath upon the TO-92 package!) Once it's biased into the active region, the Gate is capacitively picking up all the voltage signals of nearby conductors (often dominated by 60Hz, if unshielded 120VAC line is within many cm of the floating Gate.) \$\endgroup\$
    – wbeaty
    May 15 '20 at 7:00
  • \$\begingroup\$ @wbeaty Nice to see you around. \$\endgroup\$
    – JRE
    May 15 '20 at 7:45
  • \$\begingroup\$ @wbeaty Curious circuit...Battery seems also to be an "antenna", since it is FET-channel-connected. Perhaps the battery hand-held might enhance antenna sensitivity? \$\endgroup\$
    – glen_geek
    May 15 '20 at 13:51
  • \$\begingroup\$ @glen_geek yep, holding the battery slightly increases sensitivity. Or just solder on a longer wire. Also: lift the whole device by its antenna, and the battery then is the "field detector plate," but it gives opposite LED behavior. Also, here's three hundred of those, in an array: amasci.com/electrom/sel-back.gif, amasci.com/electrom/electrom.html \$\endgroup\$
    – wbeaty
    May 16 '20 at 3:58
  • \$\begingroup\$ @JRE I'm lying low, hiding on stackexchange, also Quora.com science-answers \$\endgroup\$
    – wbeaty
    May 16 '20 at 4:00
1
\$\begingroup\$

For an N-channel JFET: I suppose it would not conduct if there was stored negative charge on the gate since it wouldn't be able to drain out (or drain out very slowly). If it had zero or positive charge it would conduct.

Positive charge on the gate would be able to escape into the channel and equalize resulting in equivalent to Vgs = 0 (or slightly V>0 due to the PN junction voltage drop so not all positive charge could drain from the gate)

While any negative charge would be trapped and thus be floating relative to the source causing Vgs < 0.

Very similar to a MOSFET gate capacitance trapping charge when you disconnect the gate.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.