[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)_/¯ 
> 
> Chris 
> 
> 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) 
> anywhere 
> >> 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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