LND150 MOSFET as first gain stage

[ElectronAvalanche]

And is the only possible source of this voltage whatever the user puts into the input?

Yes - including scuffling across a carpet and then touching the end of the guitar cable to plug it in. Several kV is pretty common for things like that. Then there's the static electrical gradient, about 120V/meter on a dry day. The LND150 has a specified maximum of +/- 20V on the gate to the channel, so raising the end of a cable by 20/120 = 1/6 of a meter could theoretically puncture the gate isolation. Then there's mistakenly plugging in the cord to a phantom-powered socket on a mixing board.

Putting the zeners in is a far superior way to do the circuit.

Interesting side-trip. Static electricity zaps happen on all input cords. It's common for old bipolar transistor gear to get noisy. It turns out that zapping a bipolar base is non-fatal, as the junction recovers, but every zap/breakover of the base-emitter junction of a bipolar permanently degrades the trasistor's noise figure. Fixing old noisy bipolar inputs can be as simple as replacing the input transistors with a new low-noise device. If you ever get into this, add a 1N4148 or similar in parallel with the base-emitter junction, normally reverse biased. This completely stops static zaps from breaking the base-emitter and degrading noise performance.

Very interesting! Thank you R.G.!

[Raja_Kentut]

the LND150 has a ESD (zzzap!) protection built inside. No need for zeners to do that job. ( but it is a second parachute )
To protect the input from higher voltages, e.g. phantom supply a resistor before the zeners can be used. ( input - resistor - zeners to ground - gate )

[R.G.]

Hmmm. I did go look at the datasheet before posting (here: https://ww1.microchip.com/downloads/aem ... 05454A.pdf ), but I missed the statement "ESD gate protection" right up front on the first page. The predecessor Supertex datasheets don't specify how ESD protection is done, but they do contain the one note that it's there.

I was looking for the internal schematic with zeners that is typically shown for protected-gate MOSFETs or the specs on the gate-to-channel voltage or specs on ESD resistance. I didn't find those, so I reverted to normal MOSFET procedure.

So yeah - the LND150 is ESD gate protected. Not sure how, as they don't say, but it is likely that added zeners would be a form of belt and suspenders, probably not needed. Good catch.

[Ten Over]

ESD protection devices are great for dissipating transient surge voltages, but are useless for mundane voltages such as input signals exceeding +/- 20V. Zener diodes or other strategies are needed to prevent device destruction from these mundane voltages. Also, it is not clear whether a zener diode will dissipate a transient surge voltage since they have no ratings for such.

[R.G.]

ESD protection devices are great for dissipating transient surge voltages, but are useless for mundane voltages such as input signals exceeding +/- 20V. Zener diodes or other strategies are needed to prevent device destruction from these mundane voltages. Also, it is not clear whether a zener diode will dissipate a transient surge voltage since they have no ratings for such.

This is a reasonable explanation of my unease with the single statement "ESD protected". Protection is all about the details. With no particular spec on either the protection mechanism or the limits of ESD, I get suspicious.

With external zeners, we have working experience. External zeners have been used extensively for a long time, nearly as long as MOSFETs have been available, and they do seem to work for scuff-across-the-carpet stuff. Transient dissipation depends mostly on the thermal capacity of the thing doing the absorbing, and its sheer mass. This is serious business in the specs of MOVs and TVS devices. External zeners will probably, maybe, have more mass than the chip inside the MOSFET and might withstand more energy/heating.

Whether internal or external zeners work OK very much depends on what ESD transient the device is hit with. Lightning is an ESD transient, and pretty much nothing survives a solid lightning strike. My former neighbor had every piece of electronics in his house killed by a strike. It hit the tree next to his house and BURNED A 3MM HOLE THROUGH A WINDOW PANE to get to the soft, chewy electronics inside. My wife's parents had a 20 foot tall 8ft by 3ft stone chimney that had the top six feet broken off by a strike; fell over into their back yard.

The variation in ESD event energy is why I was looking for specs on protection. Most things that spec ESD protection use an energy maximum, and often specify a "human body model" for the ESD transient source. This both fixes the total energy to be dissipated and the source impedance.

And yeah - put a DC voltage source of greater than +/- 20V on a protected input, and the continuous fault current can simply burn out the protection devices by constant heating. Even the external zeners will have some level of dissipation where they will overheat and die. That can be pretty high, as 500mW zeners are cheap and widely available, but eventually a 10V zener (or the presumably 20V ones inside the LND150) will overheat and die. A resistor of 100 ohms to 1K directly in series with the LMD150 gate might also help. This puts at least some kind of bound on how much current can flow in the first instants of an ESD event.

We won't even get into protection for EMP events.

[JD0x0]

Not only do you want the Zeners on the input, but, IMO, you also want a coupling cap on the input to block DC in either direction. This will protect the MOSFET as well as the player, in the event the LND150 shorts, it's possible to get DC from the B+ line on the tip of the input jack.

[bepone]

ofcourse..but there is no need to rely on weak internal ESD protection on any devices, put external it is cheap..