[sdiy] jfet driver
David G Dixon
dixon at mail.ubc.ca
Wed Feb 27 09:07:25 CET 2019
For my part, I can say that I have always found MOSFETs to be confusing, with those built-in diode thingies, etc. Horowitz and Hill talk about MOSFETs, but many of the ones they advocated are no longer available. And I do everything through-hole, because I have clumsy hands and less than optimal eyesight, and my few attempts to do SMD by hand have been nearly suicide-inducing.
> -----Original Message-----
> From: Declare Update [mailto:declareupdate at gmail.com]
> Sent: Tuesday, February 26, 2019 6:39 PM
> To: David G Dixon
> Cc: Mike Beauchamp; synth-diy at synth-diy.org
> Subject: Re: [sdiy] jfet driver
> So! this has me wondering, why jfefs instead of mosfets?
> habit? old synth schematics? Since I started with synths, I
> leaned toward jfets at first at my day job, but quickly found
> a wide world of tighter spec’d and better behaved mosfets.
> for instance, PMV55ENEAR, N-CH with a Vgs-th (gate to source
> threshold voltage) of 2.7 max, with an on-resistance of ~50
> milliohms (!!) with Vgs of 5 and up. And crazy affordable. To
> be specific and fair, I do everything surface-mount, so maybe
> through-hole availability is part of it ¯\_(ツ)_/¯
> Sent from my iPhone
> On Feb 26, 2019, at 7:29 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
> >> Is the -10V shut off voltage for the 2N4391 the same as
> the "Voltage
> >> - Cutoff (VGS off) @ Id" characteristic?
> >> If so I'm seeing N-Channel FETs on Digikey with a VGS(off)
> >> between 180mV and 10V.
> >> Something like J106FS-ND has a VGS(off) of -2V typical and is
> >> through-hole.
> >> Mike
> > Yes, I believe so. For 4391, V_GS (cutoff) is minimum -4V,
> maximum -10V.
> > One of the problems with JFETs is that you really cannot
> rely on any
> > of the numbers to be at some sort of average value, so you
> just have
> > to assume that it might take -10V to cut the damn thing off. 5485
> > specifies V_GS (cutoff) of minimum -0.5V, maximum -4V.
> I've also used
> > 3819, which is maximum -8.0V (it doesn't specify a minimum).
> > Bottom line, taking the Gate to the negative rail (or
> nearly) is the
> > most reliable way to shut off JFETs. However, the 5485 is a nice
> > option for -5V square waves, which is why I use it for all my saw
> > shaping and similar circuits.
> > In Multisim, the 4391 has a Vto (which I assume means "turn-off
> > voltage") of 5.803. It is completely shut off at -5.803V, with an
> > infinite source-drain resistance. It doesn't quite get down to the
> > specified on resistance of 30 ohms. At a V_GS of 0V
> (grounded gate)
> > the on resistance is about 35 ohms according to the simulation.
> > However, it is only 100 ohms at a V_GS of about -3.9V, so
> it has about a 2V active range.
> > In Multisim, the 5485 shuts off at -2.2776V, has a grounded-gate
> > resistance of 200 ohms. Interesting. I guess it pays to really
> > understand what a simulation is telling you, because you
> could use one
> > of these based on a simulation result and then find that they don't
> > all work because they were closer to their maximum cutoff
> values than the simulation suggested.
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