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John Greene's diode-connected MOSFET distortion

9/30/1998 8:13 AM
R.G.John Greene's diode-connected MOSFET distortion
"It still needs some EQ work but it's definitely showing a lot of promise. "
I took a look at the schematic and the I/V characteristic for a hypothetical enhancement mode MOSFET, and it occurs to me that what is really happening here is that the diode connection for the MOSFET causes it to look like a diode that traces out the VGS=Vth line on the characteristic. This makes for a -very- soft kneed diode characteristic, much more so than LED's.  
You may have the next quantum-step extension to the tube screamer style distortion here - at least for us soft-distortion guys. The EQ work may be just backing out some of the bass and treble cut in the stock ts circuit. With so soft a distortion, the high end doesn't need nearly as much taming.  
It struck me that this is a missing piece I was looking for for the theoretical extension of of the iterated-stage distortion that's been kicking around in the back of my head for a while. I've been using CMOS inverter stages, but they run at a much higher signal level and have too much gain. This offers essentially the same transfer function at a lower signal level and a simpler circuit.  
"The silicon diodes have to remain in place to prevent conduction in the MOSFETs intrinsic drainbody diodes. This results in a much higher level signal which means it has more output level and there is a increase in volume when the boost is activated. "
You could try putting both the "ordinary" diodes and the MOSFET clippers in parallel with a 4.7M resistor in series to keep the DC levels right, and having a single SPDT shunt the 4.7M on the side that is not to be clipping at the moment. In fact, now that I think about it, if you do this on the (-) input side, you can use JFET or MOSFET switches as the (-) input would give you a stable voltage to tie the sources to. To get to more of the MOSFET only sound, you could use germanium diodes for their low Vf in series with the MOSFETs. If you did it with soft-switching, you could use a couple of different types of MOSFET for the variation in sharpness between types, and even select two different types.

Jack Orman I had a couple of BS170 mosfets in ... -- 9/30/1998 9:02 PM