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kalee20 · 17th Apr 2017, 11:10 pm

Sounds good! How about sharing the circuit, as I don't have that book?

Regarding your protection diode, it shouldn't be necessary! The 2N3819 is a junction FET, so taking the gate positive, it will just conduct - and the current is limited by the resistors in your attenuators anyway. Taking it negative - no current will flow till you reach the gate-channel breakdown voltage, and then you do need to limit the current, just like a Zener diode. But again, the input attenuator will do this. MOSFETs would die, JFETs not! (Though to be fair, some people have reported an increase in noise after Zenering the gate junction).

A 5.1V Zener diode has a rather soggy turn-on and with some samples of the 2N3819 and Zener, you might find that input sensitivity is compromised, so, if you do wish to add extra protection, choose a diode not far below the breakdown voltage (25V for the 2N3819).

17th Apr 2017, 11:10 pm	#4
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Another FET voltmeter Sounds good! How about sharing the circuit, as I don't have that book? Regarding your protection diode, it shouldn't be necessary! The 2N3819 is a junction FET, so taking the gate positive, it will just conduct - and the current is limited by the resistors in your attenuators anyway. Taking it negative - no current will flow till you reach the gate-channel breakdown voltage, and then you do need to limit the current, just like a Zener diode. But again, the input attenuator will do this. MOSFETs would die, JFETs not! (Though to be fair, some people have reported an increase in noise after Zenering the gate junction). A 5.1V Zener diode has a rather soggy turn-on and with some samples of the 2N3819 and Zener, you might find that input sensitivity is compromised, so, if you do wish to add extra protection, choose a diode not far below the breakdown voltage (25V for the 2N3819).