[sdiy] JFETs (arrggh!)

[Oren Leavitt]

Aaron Lanterman wrote:
> 
> On Oct 8, 2009, at 2:08 AM, David G. Dixon wrote:
> 
>> I Can anybody here point me to a good source of information on  
>> selecting JFETs
>> for this application?
> 
> 
> You want a very low "on" resistance.
> 
> You can never go wrong if you see what parts Ian Fritz chooses: 2N4391  
> or PN4391. Hmmm, 2N4391 doesn't seem to show up on findchips but  PN4391 
> does.
> 
> I believe Ray Wilson lists PN4391 as the FET of choice in his latest  
> iteration of his VCO (which I think rolls up a lot of advice from Ian  
> and perhaps other folks):
> 
> http://www.musicfromouterspace.com/analogsynth/VCO20090724REV0/VCO20090724REV0.html 
> 
> 
> - Aaron

Basically devices that are optimized for "high speed switching" or 
"chopper" applications.

* low "On" resistance
* very high "Off" resistance
* minimal "turn on" and "turn off" delay times (aka rise and fall times)

JFETs typically take a little time to "turn on" after the reset pulse 
turns on, and "turn off" after the reset pulse turns off. Good switching 
FETs are well less than 50ns. A comparator with a fast rise/fall time 
and integrator op amp with a fast slew rate are also important.
The 2N/PN4391 is spec'd at 15ns on and 20ns off, with an "On" resistance 
of 30 ohms. 2N4856 also has similar specs.
For the classic integrator-with-reset saw core often discussed here, a 
reset pulse duration of one micro-second or less is best for minimal HF 
tracking error.
A JFET/capacitor combination that can do a complete reset in that time 
frame is what you're after.

- Oren (Bedtime...time to "reset" for next day. ZZZzzzz.....)