I want to use a well known AGC circuit (see below) with 3.3V instead of 5V. The description of the circuit can be found here, and state the following about the JFET:

Other JFETs with VGS(OFF) of 5V or under, such as the 2N5019 or 2N5116, should work equally well in this circuit, although they haven’t been tried. To use JFETs with higher VGS(OFF), such as the 2N3993 (it was tried and worked equally well), increase the supply voltage to 12 V.

Can I still use the J176 when the supply voltage is 3.3V, or do I need to select a different JFET?

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  • \$\begingroup\$ Be aware that pinch-off gate voltage for almost any FET has a notoriously wide range. For J176, data sheet suggests 1 - 4 volt range. So some may not have a wide AGC range with a 3.3V supply. \$\endgroup\$
    – glen_geek
    Oct 15 '17 at 13:37
  • \$\begingroup\$ Sorry, but I am a novice when it comes to these things. So using the J176 with 3.3V will not give me an AGC for 40mV to 20V? Would it be better to supply LM358 with 12V and use one of the suggested JFETS instead? \$\endgroup\$
    – JohnDonut
    Oct 15 '17 at 16:21
  • 1
    \$\begingroup\$ LM358 with 3.3V supply is very close to its minimum. Certainly, a higher DC supply will help both LM358, and the FET AGC range. If you must use 3.3V supply, a J177 would be suitable. Some, perhaps most J176 would be OK while a few (whose pinch-off is near 4V) would have poorer AGC range. \$\endgroup\$
    – glen_geek
    Oct 15 '17 at 18:16
  • \$\begingroup\$ @glen_geek Thanks! That's all I wanted to know. If you add an answer with the same information, I will mark it as accepted. \$\endgroup\$
    – JohnDonut
    Oct 16 '17 at 9:26

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