[sdiy] jfet driver

[Declare Update]

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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