Basterdised Fatpants2/Overdrive w/ Mosfet clipping. What you guys think?

[kothoma]

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According to Jack Orman in http://www.muzique.com/news/mosfet-body-diodes/ it's the other way round:

The top body diode can now conduct from drain to source [...]

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I think he may just be using conduction in the inverted sense there. His diagram shows it the right way around (the bright red diode between body/source connection and the drain). Also, this aligns with the diagram of the body diode on the Fairchild 2N7000 spec sheet.

From there, just looking at the flow form the body diode indicates to me that the blocking diode has to be oriented the opposite direction to the body diode.

Yeah, that makes perfect sense. So maybe Orman is talking of electron flow.
Need to think about it some more. But every time I do this nearly drives me crazy.

I'm fiddling with it now and I'll report back once I hear something.  

Hey, that would be great!

[kothoma]

I will probably put a light on to indicate the MOSFET clipping mode being on. When you say diodes in series do you mean like the 8th example on this link?

http://www.muzique.com/lab/zenmos.htm

Well, no. That's for Zener diodes and is much like example 2 of that article (antiserial).

OK, to make this clear. You can use any normal diode here for getting the high level soft clipping, but you also need a second anti-parallel branch, of course.

But referring to the Shaka5 was nonsense, I stand corrected. Sorry for the confusion.

[kothoma]

The following three configurations have similar effects (especially with low voltage Zeners):

[kothoma]

So you need to do one of these to get maximum swing:
(That's missing in the otherwise great article http://www.muzique.com/lab/zenmos.htm)

[midwayfair]

The following three configurations have similar effects

Similar, but remember that, according to RG's description, the GD-S arrangement of the MOSFET has some very strange properties. It begins conducting at ~1.2v but doesn't seem to ever reach hard clipping, at least not with the signal levels we're playing with in a 9v circuit. As far as I know, no other diode will have this effect when crowbarred to ground (not sure about in a feedback loop, though) -- they'll all conduct when you reach their Fv (or voltage rating in the case of a 3v zener). Well, except germanium diodes to a much smaller extent. They have a softer knee and different types of germanium diodes can be harder or softer. I don't know the mechanism there but I suspect it's leakage.

AMZ's second example should indeed be another way to avoid body diode conduction with absolutely minimal parts. The conduction would begin around 2v (similar to LEDs) but have nice roundy edges in anything coming out of a 9v op amp. Personally I find that very attractive as a solution, but then again I have a thing about reducing the parts count in any design most of the time.

[RobA]

So, I've built up a bit of a modified Distortion+ using a split rail (+/-9V) supply and using 2N7000's with Ge, 1N914, and BAT46 diodes as the blocking diode. My conclusion is that it if you want the MOSFET clipping, then it has to be done with the cathode of the blocking diode pointing in to the G+D connection on the MOSFET (or the equivalent of the anode attached to the source). This is consistent with the 2N7000 spec sheet showing the body diode with the cathode at the drain and the anode at the source (which is also what makes sense looking at the PN junctions between the body connection and the drain.

I then did the bit with the G+D - S<->S - G+D arrangement. Using this method, you get the MOSFET clipping, which is different and sounds nice too me, and it is a really simple arrangement. The only downside I see to this is not being able to use the Ge diode as the blocking diode (or a Schottky). But really, I've already gained the thing up to some pretty good levels to get the MOSFET clipping anyway, so it seems more sensible to use that extra gain to get something else out of it rather than just throw it away using a lower total forward voltage .

Given that I've already figured out what to do with that extra gain, I'm going to have to play with distortion circuits now, even though I promised myself I wouldn't do anymore distortion designs -- just gotta figure out a tone control setup to use...

[RobA]

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AMZ's second example should indeed be another way to avoid body diode conduction with absolutely minimal parts. The conduction would begin around 2v (similar to LEDs) but have nice roundy edges in anything coming out of a 9v op amp. Personally I find that very attractive as a solution, but then again I have a thing about reducing the parts count in any design most of the time.

I was writing my last response as you posted this and was too lazy to change what I'd already written. But, this is just what I found when testing just now and I really liked the results. The solution is really elegant to me.

[kothoma]

The following three configurations have similar effects

Similar, but...

Sure, quite right! I only wanted to get the "arrows" right...

AMZ's second example should indeed be another way to avoid body diode conduction with absolutely minimal parts. The conduction would begin around 2v (similar to LEDs) but have nice roundy edges in anything coming out of a 9v op amp.

I then did the bit with the G+D - S<->S - G+D arrangement. Using this method, you get the MOSFET clipping, which is different and sounds nice too me, and it is a really simple arrangement.

I need to try this too. It is analogous to example 5, which I have tried and liked, it is the Zener analogue to example 2. But my Zeners started somewhere over 3V. (2.7+0.7 and 3.0+0.7).

[kothoma]

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AMZ's second example should indeed be another way to avoid body diode conduction with absolutely minimal parts. The conduction would begin around 2v (similar to LEDs) but have nice roundy edges in anything coming out of a 9v op amp. Personally I find that very attractive as a solution, but then again I have a thing about reducing the parts count in any design most of the time.

I was writing my last response as you posted this and was too lazy to change what I'd already written. But, this is just what I found when testing just now and I really liked the results. The solution is really elegant to me.

True, but with "doing it right"-diodes you could save some headroom, as Ge/Schottky diodes would have less forward voltage than the body diodes.

[hammerheadmusicman]

You guys have gone on an absolute rampage! But, it is all relevant and super interesting information, so do continue i will change those mosfet clippers tonight, and make some amendments.


thanks again

George

[RobA]

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True, but with "doing it right"-diodes you could save some headroom, as Ge/Schottky diodes would have less forward voltage than the body diodes.

Absolutely and in some cases I can see that as being a fairly critical difference.

[RobA]

You guys have gone on an absolute rampage! But, it is all relevant and super interesting information, so do continue i will change those mosfet clippers tonight, and make some amendments.


thanks again

George

Yeah, we've gone off a bit . Hopefully it'll be useful information. I'll be interested to hear what you think when you get a chance to test the MOSFET clipping.

[hammerheadmusicman]

Well, I'm away on a gig until around oct 31st, so I won't get a chance, i'm just going eagle crazy while i'm away and have loads of free time, Shed load of layouts being fabbed, and trying to cobble a few more designs together, maybe do some etch layouts and get on the case when i get home!!

All this stuff is super useful though! Keep it coming

George

[kothoma]

i will change those mosfet clippers tonight, and make some amendments.

Any progress?