Depletion Mode MOSFETS for phasers / compressors?

[polaris26]

Has anyone tried depletion-mode mosfets for phasers?  I was looking at the sheet for IXTP08N50D2 and the like and it seems like the so-called "ohmic" region covers a larger ground, in terms of drain to source voltage, which is a limiting factor with JFET-based phasers.  I don't have any but was considering ordering some.  Anyone else looked at these?  They might be good in an orange-squeezer type compressor also?

Dave

[R.G.]

It's worth looking at, with the notable worry about the intrinsic (that is, you can't get rid of it) body diode which limits voltages in quadrant 3 to under about 300-400mV before bad distortion happens.

[polaris26]

Ah ok - so some sort of dc bias to keep the signal more in the positive realm would be needed - and that might offset whatever 'extra headroom' there had been in the ohmic region Vs. a JFET... interesting - I guess I should just buy some and mess around with them.   

On a tangent - can triodes be used as variable resistances (again assuming staying in the positive realm) ?


Dave

It's worth looking at, with the notable worry about the intrinsic (that is, you can't get rid of it) body diode which limits voltages in quadrant 3 to under about 300-400mV before bad distortion happens.

[PRR]

> can triodes be used as variable resistances

MOSFETs are triodes.

Do you mean vacuum triodes?

Yes, kinda. The zero-voltage resistance is very dubious and depends too much on "insignificant" construction details. Normally you want to run with DC current, though this complicates your circuit. The resistance changes much slower than current, typically square-root. This means clipping at the low-I hi-R end or cooking at the hi-I end. Resistance is rarely lower than 10K which puts your other end very much in high-impedance turf. (Instead of plate resistance, you can use cathode resistance relative to grid, the 1/Gm curve. This mostly does the same thing at same current but lower resistance, lower undistorted signal power.)

FWIW, very useful audio compressors have been built NOT using concepts from other devices (which had not yet been born) but using tubes the way they want to be worked. UA-175 is typical:
http://www.waltzingbear.com/Schematics/Urei/UA_175.htm

Not sure how Depletion MOSFETs have real advantage over Enhancement MOSes. You do have to re-think the control voltage relative to "JFET" values.