MOSFET source follower anti-static zener protection?

[Scruffie]

I'm building a lowish voltage (40V) tube overdrive and using a BS170 as a source follower in it to drive the tone stack and my question is, do I still need to do the zener anti-static protection trick with it?

Would I want to use 2 back to back from gate to source or a single diode like in the super hard on and am I better going for a higher or lower voltage rating?

[R.G.]

MOSFET gates are separated from the channel by a layer of high-purity glass 20V thick. If the voltage on the gate exceeds 20V plus some safety factor from the source, the glass punctures and the device no longer works the way it should.

In a 40V supply situation, as a source follower, the device will be conducting at most a couple of milliamperes, so the gate will be at Vthreshold plus a few millivolts all the time. Any zener that keeps it from going more than perhaps 10V above the source will protect it from positive voltage excursions. If the gate is below Vthreshold, the device is cut off, so it's not amplifying. In this direction, it doesn't matter if it's clamped to -0.7V or -[any voltage less than 20]. So you may as well use a single zener and let that zener protect against positive excursions with its zener voltage and negative excursions with its forward diode voltage. I can't think of any reason to use nose-to-nose zeners here.

Whether you really NEED to put a zener in is open to debate. But zeners are cheap, so why not?  I'd use a single zener of 10V to 15V. The actual voltage rating is not very important as long as it's well under 20V.

[Scruffie]

Excellent explanation as always, thanks R.G  (Oh and thanks for the MOSFET follies article too while i'm here!).