Ok, ive got some BS170s enhancement type mosfets. They've just been sitting there, and finally last week i set a couple up on a bread board just to check them out. I set them up as buffers.
Ok, so that was fun.... learning how to bias them and all.
anyways, this got me thinking about depletion type mosfets.... they seem to act a lot like jfets.... do they have a lot of the same problems with the Vgs being way different on each unit, or are they more reliable.
ALSO..... anyone know of any pedals that they are used in. I'd like to see some examples.
Brian
I know of no stompboxes that use them. I'm only aware of Supertex making some, high voltage, small signal depletion mode MOSFETs. They seem maybe a bit more consistent than JFETs, but not much. After trying some out, I'm not sure they're worth the trouble, offering no rela benefits over JFETs, unless they're needed for HV application. Perhaps a SS "tube" amp? Anyhow, Mouser carries them.
Didn't Joe Davisson moved his mosfet designs from 2n7000 (nmos) to BS250(pmos)?
Check his site, or pm me an email address and i'll send some old versions of his pmos blackfire and obsidian.
The Eventide Instant Phaser used n-channel depletion-mode MOSFET's to tune the allpass stages. The mosfet (RCA 40468A, IIRC) is similar to the old RCA 3N128 single-gate mosfet (one of the first commercial mosfets). There are board pics at www.modezero.com.
Looking at the curves for the 3N128 suggests it won't have any better signal-handling than your typical JFET.
The mosfet's in the CD4049 chip are enhancement-mode.
Mike
Check this site for a nice Fuzz to try (the first distortion schematic).
http://www.angelfire.com/ak/voroslap/
I also used them with Gabe's Solid State Tube Sound Preamp
http://members.tripod.com/gillcar/id24.htm
Added... I just noticed you said depletion type mosfets... sorry.