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DIY Stompboxes => Building your own stompbox => Topic started by: Nico666 on October 28, 2005, 07:27:34 AM

Title: FETs as diodes
Post by: Nico666 on October 28, 2005, 07:27:34 AM
Hi I am new here and rather new to the world of DIY.
I know this had been discussed plenty of times before but it seems I can't get it.
How do you wire MOSFETs as diodes to implement them in the feedback loop of a TS clone?
Do you have to put two of them back to back or can you just put one with a diode (1n34a) and if so, how do you orient the FET and the diode?


Thanks for any help!


Nicolas
Title: Re: FETs as diodes
Post by: amz-fx on October 28, 2005, 07:59:21 AM
The Shaka 5 distortion on my site shows one way to use mosfets as diodes:

http://www.muzique.com/schem/shaka5.htm

The Fat Gnat on my cd-rom uses jfets are clipping diodes:

http://www.muzique.com/amz-cd.htm

regards, Jack


Title: Re: FETs as diodes
Post by: Bernardduur on October 28, 2005, 08:00:39 AM
I done both the FET as the MOSFET in my TS loop and I love it. Try it!
Title: Re: FETs as diodes
Post by: Nico666 on October 28, 2005, 08:17:21 AM
OK so let me put it together!

1. shortcut G and D of the MOSFET
2. connect a 1N34a to the S - which way?
3. connect GD to signal or ground and 1N34a to the other side

Right?

What is the voltage theshold of the whole thing (FET+1N34a)
How does it sound, compared to 1Si+1Ge in the feedback of a TS clone?

Thanks for the answers?
All the best,

Nicolas
Title: Re: FETs as diodes
Post by: Nico666 on October 28, 2005, 09:41:27 AM
Anyone to correct me?
Title: Re: FETs as diodes
Post by: R.G. on October 28, 2005, 10:10:29 AM
I don't know that I'll correct you so much as enhance what you have already put down.

The point of diode connection of any device, bipolar or MOSFET is that the active input nodes (gate and source, base and emitter) are set up so they act like a diode and the output node (drain, collector) provides the current. What that does is to in effect provide current amplification to the turn on characteristic of the device.

You only need a series diode for MOSFETs because as a side effect of the semiconductor process that makes them, there is an intrinsic diode junction in the reverse direction to normal conduction, so if you try to put signal through them, one side conducts like you want - the amplified gate-source turn on voltage - and the other is just the silicon body diode. So you use an external diode in the desired direction to prevent conduction in the body diode direction. It does not matter whether the series diode is in series with the gate-drain terminal or the source terminal as long as it is pointed the right way.

Notice that the desired diode direction is a function of the channel polarity - N channel or P channel.  N channels need the diode pointing (anode ->|- cathode) in the direction from drain to source. P channels need the diode pointing from source to drain. The less the series diode adds to the conduction voltage, the better. That makes germanium a good choice. However, if germanium is not easily available, you can also use schottky diodes like the 1N5819. These have a forward drop of 0.4V typically and are quite sharp in turn on, so they affect the desired composite diode voltage less.

Title: Re: FETs as diodes
Post by: Nico666 on October 28, 2005, 11:48:49 AM
Thanks for the answer.

So, if I am right, for a p-MOSFET such as the IRF 520 or 2N7000, the wiring is:

---anode ->|-cathode --drain/gate--source---

or:

---drain/gate--source--anode ->|-cathode---

And what about the voltage threshold of a MOSFET wired as a diode?

Nicolas
Title: Re: FETs as diodes
Post by: gez on October 28, 2005, 02:29:55 PM
http://www.diystompboxes.com/smfforum/index.php?topic=34013.0

Title: Re: FETs as diodes
Post by: Nico666 on October 28, 2005, 06:09:02 PM
I am sorry but I still don't get it.

Same question as in this other thread:

http://www.diystompboxes.com/smfforum/index.php?topic=34013.0
 
Mosfet's as clippers
« Reply #5 on: June 10, 2005, 05:23:04 AM »
 
I'm getting a little confused - but if I've got it correct, there's 3 ways of using a mosfet as a diode:
G+S - D = normal silicon diode
D+S - G = low leakage, high res diode
G+D - S = diode conneted mosfet = very soft knee = as used i the Shaka B.

Questions:
1) What exactly was John Greene's setup?
(I have a note from '98 stating that he used a VN0300(?) and RG said about his findings: You may have the  next quantumstep extension to the TS style distortion here)
2) Whichs is the resulting diodes anode/cathode for each of the above mosfetconnections?
3) Vf? Measuring the diode forward voltage - how?


Erik


Especially the different wirings and the corresponding threshold and clipping response?

Can someone clarify this?

Thanks
Title: Re: FETs as diodes
Post by: R.G. on October 28, 2005, 07:14:22 PM
Quote
I'm getting a little confused - but if I've got it correct, there's 3 ways of using a mosfet as a diode:
There are only two. I'll outline them below. There would only be one if it were not for the intrinsic body diode. While you read, remember that diodes only conduct when their anode (arrow) is positive with respect to the cathode (bar) end.

Quote
G+S - D = normal silicon diode
Correct. This is the intrinsic body diode. Notice that in an N channel, the G+S must be positive with respect to the drain to get the diode to conduct. The anode is the source, the cathode is the drain, and the gate actually plays no part other than being tied to the source. It is a silicon diode with a forward voltage determined by the silicon doping of the semiconductor substrate. Typical Vf is 0.5 to 0.7V when the diode is clearly into conduction. All diodes begin conducting tiny currents well before they nominally turn on.

Notice that if you had a P channel MOSFET, the polarities are reversed and the anode of the diode is the drain and the cathode is the G+S.

Quote
D+S - G = low leakage, high res diode
This does not exist for MOSFETs. MOSFET gates are insulated by 20 volts thickness of high purity glass. This connection exists for JFETs, where you are using the gate-channel junction as a diode forward biased, instead of reverse biased as it is in normal JFET operation. The junction is typically lightly doped, low leakage, and will show typical silicon diode drops (0.5-0.7V) as above. The only reason to do this or the connection above is to get a slightly different silicon diode junction, hoping that a diode not normally intended for use as a diode is different a little bit.

Quote
G+D - S = diode conneted mosfet = very soft knee = as used i the Shaka B.
This is the different one. The gate shorted to drain means that the "diode" doesn't start to conduct until the gate/drain is more than the threshold voltage more positive than the source (positive for N-channels, negative for P channels that is). So the forward voltage of these diodes is usually 1.5 to 3V. The knee of conduction is quite soft, and so diodes using this don't go from full off to full on, they gradually turn on. This happens to be quite good for soft distortion. This is the only FET diode connection worthy of any special study or pursuit IMHO. The others are just too similar to ordinary diodes.

It's this connection that needs the series ordinary diode to keep the body diode (connection 1) from conducting when the signal reverses.

Quote
1) What exactly was John Greene's setup?
The third one I just talked aobut.
Quote
(I have a note from '98 stating that he used a VN0300(?) and RG said about his findings: You may have the  next quantumstep extension to the TS style distortion here)
I said that because the MOSFET diode connection preserves signal wiggles even in the "clipped" region. I like it - a lot. It's dramatically underused, and I can only think that's because most players only like gain, megadistortion and more distortion.

Quote
2) Whichs is the resulting diodes anode/cathode for each of the above mosfetconnections?
As noted.
3) Vf? Measuring the diode forward voltage - how?
With a voltmeter. Put a resistor in series with the diode across a 9V battery. Measure the voltage. That's Vf. The resistor can be between 100K and 1K. If you use a pot, you can acctually see the Vf change as the current changes.
Title: Re: FETs as diodes
Post by: Nico666 on October 28, 2005, 07:22:40 PM
 :D

That is what I thought I understood and wanted to see written in a clear and concise way!
Nice job clarifying that!

Nicolas
Title: Re: FETs as diodes
Post by: johngreene on October 28, 2005, 09:03:08 PM

Questions:
1) What exactly was John Greene's setup?

Maybe if you tried asking him, he'd tell you....  :icon_wink:


I tie the Drain to the Gate and connect the anode of the diode to the source. Then treat the whole thing, Drain of MOSFET to cathod of diode, as a single diode. MOSFETs with low gain will give the most 'color'.

I've also used FETs as diodes by using the Gate as the Anode and the Source as the cathode. When connected back-to-back the resulting clipped waveform has the most perfectly rounded corners you ever seen. However, it didn't sound all that remarkable to me in a circuit. I didn't really spend much time with it though.

--john greene
Title: Re: FETs as diodes
Post by: Nico666 on October 29, 2005, 04:27:40 AM
Thanks Mr. John Greene!

Bernardduur, what did you use in the loop of your TS?


Nicolas
Title: Re: FETs as diodes
Post by: Elektrojänis on October 29, 2005, 02:34:07 PM
It's this connection that needs the series ordinary diode to keep the body diode (connection 1) from conducting when the signal reverses.

How about using just one mosfet without the series diode as the clipping device. (and I mean just one mosfet replacing the whole "standard" two diode back to back thing) Would it give asymmetrical clipping where the tops of the waveform would be clipped very softly and the bottoms would be clipped much harder?

If it works like this the harder clipping would happen a earlier than the softer, but it could probably be biased towards the softer side.

I might have understood it all wrong though...

Edit: Based on the other MOSFET-clipper thread it seems that it should work as I thought.
Title: Re: FETs as diodes
Post by: johngreene on October 29, 2005, 07:21:36 PM

How about using just one mosfet without the series diode as the clipping device. (and I mean just one mosfet replacing the whole "standard" two diode back to back thing) Would it give asymmetrical clipping where the tops of the waveform would be clipped very softly and the bottoms would be clipped much harder?

If it works like this the harder clipping would happen a earlier than the softer, but it could probably be biased towards the softer side.

I might have understood it all wrong though...

Edit: Based on the other MOSFET-clipper thread it seems that it should work as I thought.

yes, it would work just as you described.

 Try it!

--john
Title: Re: FETs as diodes
Post by: Elektrojänis on October 30, 2005, 12:49:00 PM
yes, it would work just as you described.

 Try it!

Got to get some mosfets first... I haven't tinkered with those before so I don't have any.

Other side would be softly clipped and the other hard clipped... Fuzzface? :D
Title: Re: FETs as diodes
Post by: WGTP on November 04, 2005, 04:46:13 PM
http://www.diystompboxes.com/cgi-bin/webbbs_scripts/webbbs_config.pl?read=343

Check out the Mosferatu the use of the Mosfet diodes

Also not the interesting positioning of the diodes in the loop and to ground.
Title: Re: FETs as diodes
Post by: johngreene on November 04, 2005, 06:14:46 PM
They are configured such that it will use the body diode of the MOSFET and not the MOSFET as a diode. The 1N34s would have to be connected the opposite way for that.

--john
Title: Re: FETs as diodes
Post by: aron on November 05, 2005, 02:56:01 AM
I still stand by the way I used them in the Shaka 3.  ;)

(http://www.diystompboxes.com/pedals/shaka3.jpg)
Title: Re: FETs as diodes
Post by: R.G. on November 05, 2005, 07:59:16 AM
Quote
I still stand by the way I used them in the Shaka 3.
That's the body diode mode. You do get more apparent distortion that way.

The diode-connected MOSFET is not enough distortion for everyone, especially at low overdrives.

Just goes to show, there's no one way that works for everyone's sound preferences.
Title: Re: FETs as diodes
Post by: amz-fx on November 05, 2005, 08:28:06 AM
Check out the Mosferatu the use of the Mosfet diodes

The Mosferatu arrangement is the same as that used on the Shaka 3/5 projects (http://www.muzique.com/schem/shaka5.htm)...  the diode steers the current flow through the integral body didoe of the mosfet formed by the substrate to drain junctions. The body diode is fairly slow switching and that may account for its softer sound.

regards, Jack
Title: Re: FETs as diodes
Post by: R.G. on November 05, 2005, 08:55:51 AM
I'm always trying to see how soft I can make distortion, which probably accounts for why the body diode on MOSFETs just sounds like any other silicon diode to me. After hearing substantially every silicon diode clipper ever made, I just keep thinking that there must be something more.

That's one reason I'm always interested in clipping knees - the fun stuff for distortion is all there. A new silicon diode - or for that matter germanium or LED - clipper is just one more angel on the head of the pin.

Been there, done that, what's next?
Title: Re: FETs as diodes
Post by: ahermida on November 05, 2005, 08:29:29 PM
Has anyone looked at what Fender is doing in a few of their amps?  I think this is what they call "Dyna-Touch" or something along these likes.  Here's a pic of a section of the Champion 30 solid state amp.

(http://www.hermidaaudio.com/champion30dist.jpg)


I think their creating a "piece-wise" approach with this one.

Alf
Title: Re: FETs as diodes
Post by: amz-fx on November 05, 2005, 08:42:31 PM
Quote
After hearing substantially every silicon diode clipper ever made, I just keep thinking that there must be something more.

I'm quite in agreement with you on that...

regards, Jack

Title: Re: FETs as diodes
Post by: R.G. on November 06, 2005, 11:42:59 AM
Quote
Has anyone looked at what Fender is doing in a few of their amps?
That's funny. I wondered when any commercial outfit would finally tumble to the idea of putting (ideally) good sounding limiting before a solid state amp to emulate a tube amp. I've been telling anyone that would listen about that ever since I figured out what Thomas Organ was doing with their solid state amps in the 60's. I stumbled onto it in the 80's, only 20 years late. That's better than I usually do.

Now all they have to do is figure out the rest of it. Given their track record, that may be a long time.
Title: Re: FETs as diodes
Post by: ahermida on November 06, 2005, 03:04:08 PM
I've been studying the Fender designs and if you look at the pic I posted in a previous message they come up with something interesting but  they go overboard.  The have a pair of clipping diodes right before this more complex clipping stage (CCS).  What do they expect is going to happen?  They already chopped off a lot of "information" from the signal before it reaches the CCS  I've seen designs where they have multiple clipping sections that really are more about distortion than trying to emulate anything even remotely close to a tube sound.

I think one of your comments also hits it in the head:  most people focus on distortion, sustain, gain etc rather than dynamics, response, clarity etc.  It all depends on who their "market" is wrt these solid-state amp models.  I've been working on designs aiming for dynamics, transparency etc for the longest time and when people check my stuff out some ask me right off the bat if I can add more gain to it!

Alf
Title: Re: FETs as diodes - or alternatives
Post by: MR COFFEE on November 06, 2005, 09:36:55 PM
If you want really soft limiting, try using cadmium sulfide photocells biased partially on (with light, natch) to around 4K or greater instead of diodes. You can't get much clipping with a 9volt supply, but if you get the signal swing up to around 10 volts or better the nonlinearity rounds things off very nicely and smoothly. Think Bassman pushed gently.
 
You can make the distortion dynamic as well if you modulate the light intensity with the guitar signal just a little. Think "give". Vactec photomods like the VTL5C2 do well in this application. FWIW
Title: Re: FETs as diodes
Post by: R.G. on November 06, 2005, 10:31:22 PM
Quote
If you want really soft limiting, try using cadmium sulfide photocells biased partially on
Or even softer, the old voltage variable resistors (varistors) like they used in the Magnatone amps for vibrato. You need upwards of 70V across them for a big change in conductance.

Really, really soft.
Title: Re: FETs as diodes
Post by: A.S.P. on November 07, 2005, 02:31:02 AM
information lost...
Title: Re: FETs as diodes
Post by: Nasse on November 07, 2005, 11:20:13 AM
Been thinkin this note on Headwize limiter page...

"One way to vary clipping levels is to apply a bias voltage to the normally grounded side of the diode array"

But what kind of bias should it be and how much does it alter sound...
Title: Re: FETs as diodes
Post by: WGTP on November 07, 2005, 12:58:29 PM
See if I got this right.  Modified Aron's drawing and barrowed parts from Joe Davidson's Obsidian schematic.  This should be the high threshold soft knee version that we have been discussing.

Let me know if I messed it up.

http://aronnelson.com/gallery/WGTP/MosfetDiode
Title: Re: FETs as diodes
Post by: johngreene on November 07, 2005, 02:08:07 PM
See if I got this right.  Modified Aron's drawing and barrowed parts from Joe Davidson's Obsidian schematic.  This should be the high threshold soft knee version that we have been discussing.

Let me know if I messed it up.

You have it correct. Here's a simulation plot showing what this does for you. Half way through the high frequency tone is inserted and you can see how it is visable even when the lower frequency is clipped.

(http://www.greene-pedals.com/MOSclip.gif)

--john
Title: Re: FETs as diodes
Post by: WGTP on November 07, 2005, 03:13:21 PM
Very Cool. :icon_cool:
Title: Re: FETs as diodes
Post by: lovekraft0 on November 08, 2005, 02:08:06 AM
 :icon_redface: OK, now I'm confused - it looks to me like WGTP's drawings has the diodes oriented opposite to the direction shown in John's sim schemo. Somebody check this out and tell me if this drawing is the correct setup for soft clipping, please:
(http://home.mindspring.com/~williamlove/MOSFET_clippers.png)

One more silly question - am I missing something, or does it not matter whether you connect the diode to the source or the drain, so long as the correct polarity is observed?
Title: Re: FETs as diodes
Post by: R.G. on November 08, 2005, 08:48:39 AM
Quote
OK, now I'm confused - it looks to me like WGTP's drawings has the diodes oriented opposite to the direction shown in John's sim schemo. Somebody check this out and tell me if this drawing is the correct setup for soft clipping, please:
Here's how to tell on your own: the diode must let the MOSFET conduct from drain to source as though it were in normal operation. It *is* in normal operation, it's just feeding itself an odd gate drive waveform.

Quote
One more silly question - am I missing something, or does it not matter whether you connect the diode to the source or the drain, so long as the correct polarity is observed?
And that's a fundamental principle that no one will believe until they do it for themselves: as long as you don't connect to or look at what happens in the middle of the series string, the order of components in a series string is completely arbitrary. All orderings of parts in a series string have equal effects at the ends of the string. You may swap series components around at will with no effect outside the string.
Title: Re: FETs as diodes
Post by: lovekraft0 on November 09, 2005, 01:00:35 AM
Quote
...the diode must let the MOSFET conduct from drain to source as though it were in normal operation...
OK, so the "polarizing" diode has to be wired opposite to the polarity of the body diode - thanks, that makes perfect sense, now that you've broken it down.
Title: Re: FETs as diodes
Post by: WGTP on November 10, 2005, 09:20:48 AM
In retrospect, I think I tried this before and didn't think it was working because it wasn't distorting much and the signal got louder. ::)

It appears that applications using this method with the goal of a soft knee would benefit from a rail to rail op amp to minimize the harder distortion produced by the op amp.  Of course, mixing the 2 varieties of clipping could be desireable.   Maybe an LED/Mosfet combo.  :icon_twisted:
Title: Re: FETs as diodes
Post by: johngreene on November 10, 2005, 11:36:49 AM
Not surprising. Using MOSFETs as diodes introduces a bunch of addtional variables that you need to consider.
1. The clipping threshold is higher, depending on the MOSFET, it can be much higher. (therefore louder).
2. The diode you use doesn't add much if anything to the clipping so Schottkys or Germanium will help lower the threshold.
3. Using a low gain MOSFET will give a softer clip than a high gain one.
4. You can get low threshold MOSFETs but they tend to be high gain and therefore clip harder sounding not much different than a Si diode.

Just to name a few.

--john
Title: Re: FETs as diodes
Post by: WGTP on November 11, 2005, 11:07:56 AM
John, I don't know much about Mosfets, where do the BS170/2N7000 and IRF510 fit into that?   :icon_cool:
Title: Re: FETs as diodes
Post by: R.G. on November 11, 2005, 12:20:25 PM
There are neato datasheets that you can find just by googling with the part number and "datasheet".

For the BS170, the gate threshold voltage is listed as 0.8 to 3.0V, with a typical value (whatever that is...) of 2.1V. The gain is the forward transconductance, which is listed as 320mSiemen, or 0.32 amps per volt.

For the 2N7000 the gate threshold is the same, but the forward transconductance (gain) is listed as 80 minimum, 320 mS typical.

For the IRF510, the gate threshold is 1.0 to 4.0, the gain is 1.2S minimum, or about four times the typical BS170 or 2N7000.
Title: Re: FETs as diodes
Post by: johngreene on November 11, 2005, 12:51:16 PM
There are neato datasheets that you can find just by googling with the part number and "datasheet".

For the BS170, the gate threshold voltage is listed as 0.8 to 3.0V, with a typical value (whatever that is...) of 2.1V. The gain is the forward transconductance, which is listed as 320mSiemen, or 0.32 amps per volt.

For the 2N7000 the gate threshold is the same, but the forward transconductance (gain) is listed as 80 minimum, 320 mS typical.

For the IRF510, the gate threshold is 1.0 to 4.0, the gain is 1.2S minimum, or about four times the typical BS170 or 2N7000.

Then you have something like the Si1012R/X from Vishay that has a MAX gate threshold of .9V and a MIN of .45. Transconductance is a little high at 1S but in pedal circuits you are playing mostly in the knee area anyway because of the low amount of current. Something to remember too is that the gate threshold is most commonly measured at a DS current of .25 mA. You will start to see conduction at lower voltage with lower currents.

From this you can get a taste of just how much there is to chose from. What sounds the best? It depends. ;)

--john
Title: Re: FETs as diodes
Post by: stm on November 11, 2005, 01:21:46 PM
My 2c.

John, your graphics are very informative.  This thread has been particularly interesting to me, since I had the gut feeling that when people talked about MOSFET diodes they were not talking about the same thing necessarily. Now that it has been properly clarified, I think it is in order to be specific when refering to the subject:

So, a MOSFET-AS-DIODE produces a softer knee with a higher threshold, as opposed to the MOSFET-BODY-DIODE, which is essentially another silicon diode, or not ?...

...I've always had the feeling that the IRF520 MOSFETS sounded better/different in the Shaka 3 because said MOSFETs have a very large Drain to Source intrinsic CAPACITOR in parallel with the integral body diode, acting as some sort of filtering.  In fact, this capacitance is in the order of 1000pF IIRC, thus, very near to what you may find in parallel with the clipping diodes  of a Ross distortion, liquid drive and others.  I think this explains why it sounds different/better than a standard pair of silicon diodes and was preferred by Aron.  Moreover, the addition of the Ge diodes in this case do add some additional softness to the sound.  Remember that some poeple has reported good results in the TS workalikes when using mixed silicon and germanium diodes in series.

-----------------------------
Now regarding different unexplored diode types:

a. Has anybody experimented with "lambda" diodes?  I've seen one schematic (Pearl Overdrive?) that used two back-to-back diodes in the feedback loop of an OpAmp. One os them said "lambda diode" and had a strange part number (like MA150 or something).

b. What about tunnel diodes (very nonlinear!)  I haven't been able to get some, but there would be something new in there.
Title: Re: FETs as diodes
Post by: johngreene on November 11, 2005, 01:49:30 PM
Thanks for the comments, I always understand things better where there's pictures involved. ;)

I did a lot of work with MOSFETs as clippers back when I first designed this pedal:

http://www.greene-pedals.com/glasspak.htm (http://www.greene-pedals.com/glasspak.htm)

I think I built the first one back in 1999 or 1998. I must have tried a hundred different MOSFET parts. Each one sounded different. The one I liked the most was discontinued but I managed to find something like 5000 of them from a broker and bought them all.

The body diodes are not anything to ignore either. I've liked several versions I had put together only to realize later than I put the other diode in backwards and was actually using the body diode. They tend to have more spank than a 1n4148 or similar diode. I'm not too familiar with the transfer characteristic of the body diode but whenever you put 2 or more semiconductor/diode devices in series, the transfer characteristics will follow that of the lowest transconductance device in the chain.  So if you put a germanium diode is series with the MOSFET body diode and the germanium has the dominant curve, it probably doesn't sound a whole lot different than if you used a regular silicon diode with the same forward voltage drop.

--john
Title: Re: FETs as diodes
Post by: WGTP on November 11, 2005, 04:38:15 PM
Thanks R.G., but what would be really neato is if I understood the stuff on the data sheets.  :icon_biggrin:

Thanks John and others for helping clarify this for the rest of us.  It ought to get some more play around here now.

About the time I think I've covered the cool stuff, something else comes along.   :icon_cool:
Title: Re: FETs as diodes
Post by: stm on November 11, 2005, 07:04:06 PM
I've been studying the Fender designs and if you look at the pic I posted in a previous message they come up with something interesting but  they go overboard.  The have a pair of clipping diodes right before this more complex clipping stage (CCS).  What do they expect is going to happen?  They already chopped off a lot of "information" from the signal before it reaches the CCS...

...

Alf

Alfonso, if you look carefully at the schematic you posted, you will notice the first pair of diodes is located at the very (-) input of U2B. Under normal operating conditions, the (-) input is a virtual ground (just a few millivolts apart from the voltage set at the (+) input).  Only when U2B is heavily overdriven so that its output starts clipping, then the first pair of diodes will start conducting.  As such for low to medium gain settings it is indeed a soft clipping design.

Best regards.
Title: Re: FETs as diodes
Post by: ahermida on November 11, 2005, 09:41:16 PM
I should breadboard it and listen to it.  Thanks!

Alf
Title: Re: FETs as diodes
Post by: amz-fx on November 12, 2005, 01:38:04 AM
Quote
For the IRF510, the gate threshold is 1.0 to 4.0, the gain is 1.2S minimum, or about four times the typical BS170 or 2N7000.

While that (gain) is true of the IRF510 under the conditions of the datasheet, which are pulses of high current, I would expect the '510 to behave differently under small signal audio conditions with much less current. It is not made to be an audio amplifier and behaves differently from the BS170.  Some people prefer its softer (?) sound...  I think Aron did some tests with it.

regards, Jack
Title: Re: FETs as diodes
Post by: NoFi on November 12, 2005, 07:56:13 AM
Thank you for all the useful info in that thread.

I was just wondering, is the Ibanez MT10 mostortion (mosfet overdrive) made during the eighties using that sort of diode connected mosfets ?
The schematic is here but i don't really have the skills to understand it :
http://www.freeinfosociety.com/electronics/schematics/audio/pictures/ibanezmt10.gif
Title: Re: FETs as diodes
Post by: ahermida on November 12, 2005, 08:22:17 AM
Thank you for all the useful info in that thread.

I was just wondering, is the Ibanez MT10 mostortion (mosfet overdrive) made during the eighties using that sort of diode connected mosfets ?
The schematic is here but i don't really have the skills to understand it :
http://www.freeinfosociety.com/electronics/schematics/audio/pictures/ibanezmt10.gif

Its really a diode clipper but the dual op-amp chip itself has the mosfets.  I think those chips have been discontinued for a while now.

Alf
Title: Re: FETs as diodes
Post by: luix on November 12, 2005, 11:10:14 AM
Hi guys, sorry for the intromission but I like this argument to much...
I made some experiment whith JFET in the feedback loop of opamp (circuit like the tube screamer) and this is the result: CLICK  (http://www.diyitalia.com/index/elettronica/effetti/Distorsione/The%20Valvolo.htm)

It seem like the distortion of a tube, the only problem is the gain of the circuit, is a bit low so for guitar use (i tested it with a CD and a sinusoidal track) so I made the 1.2k resistor in series with the drain of the jfet a 500k log pot...
The sound is not bad for my hear, if you try it let me know the result...
Title: Re: FETs as diodes
Post by: WGTP on November 12, 2005, 11:50:36 AM
The fender schematic looks like some of the dual op amp stuff around, but what is going on in the loop of the second op amp?  I guess that is where we would want to put the Mosfet clipper?  Sort of goes back to the thread about the "spacing" between clipping stages.   :icon_cool:
Title: Re: FETs as diodes
Post by: WGTP on November 20, 2005, 11:57:41 AM
OK, I'm set up on the breadboard with the 510,s as clippers but the op amp contributes more distortion than the clippers.  Guess I need to try a rail to rail op amp, or get mosfets that clip at a lower threshold.  Any other good ones to try?   :icon_cool:
Title: Re: FETs as diodes
Post by: lovric on January 06, 2006, 07:15:45 PM
Hi Ahermida,

On L2XH site was said that emulation of a tube amp goes in two(3) steps:

- the preamp stage soft clipping is emulated by two antiparallel :icon_biggrin: Si diodes to ground and
- the power stage clipping is emulated with simmetrical opamp clipping
- speaker/cab emulator is a must

I've listened to fender amp on a gig and it does sound sort of 'retro'. But it is great when you know the subject, only the liking of the sound is a different problem altogether. It's great when I fantasize about old marshalls and voxes and tweed amps than all of a sudden I would like sustain and heavy tone of a rectifier or something simmilar. Companies offer us three channels and what not, but we probably can't have it in a singlebox. Is there a device with such a sensitivity span that coud translate my pick attack from country all the way to the santana sustain. gary moore sustain. Whatever. EH says it's a reverb...
Title: Re: FETs as diodes
Post by: lovric on January 06, 2006, 07:19:49 PM
oh, I meant Fenderr solid state amp advertised with dyna-touch'ability'. From a far it looked like blackface deluxe. A far => three tables away.
Title: Re: FETs as diodes
Post by: WGTP on March 13, 2006, 11:29:32 AM
In the continuing saga of clippers, I experimented with the IRF520 and BS170 in the feedback loop of an inverting op amp lightly driven by a non-inverting op amp this weekend and found:

1.  They both sound nice, seemingly more organic or earthy than LED's.

2.  The GE 1N34/BS170 combo has a lower clipping theshold than the GE 1N34/IRF520.  When adding and removing them from the FBL the BS170 is quieter and more distorted.  The IRF520 isn't much quieter or distorted than the op-amp alone.  I was using a rail-to-rail dual op-amp to maximize the distortion of the mosfets and minimize the distortion from the op amp. 

3.  The CMOS op-amp doesn't sound bad by itself.

4.  They both seem to have less treble content than LEDs, which I'm assuming has to do with the softer knee generating fewer high order harmonics.

5.  The body diodes measured under .8v for the IRF520 and over .8v for the BS170.

6.  As stm pointed out, the IRF520 has almost .001uf of capacitance. The BS170 has significantly less (almost X10 if I understand the info).

7. Using a switch to rout the clipping thru the body diodes or the mosfets would result in a nice sounding clipping distortion with 2 different thresholds.  Around .8v for the body diodes and 2+v for the mosfets.   :icon_cool:
Title: Re: FETs as diodes
Post by: RedHouse on March 14, 2006, 10:09:23 AM
They are configured such that it will use the body diode of the MOSFET and not the MOSFET as a diode. The 1N34s would have to be connected the opposite way for that.

--john

Ok one more time, by "opposite" do you mean the 1N34 is flipped (in the Shaka picture above in Reply #18) so it's anode connects to the MOSFET ...or... do you mean the 1N34 is moved over to the G-D connection on the MOSFET?
Title: Re: FETs as diodes
Post by: WGTP on March 14, 2006, 11:07:04 AM
Look at my drawing.  You can hook it to either connection, as long as it is "facing" the right directionl.  IIRC in Arons drawing, the 1N34 is connected the same, just turned the opposite direction.

What I have found on the bread board is that the body diodes are usually lower threshold, so that connection results in a lower volume more distorted sound.  When the mosfet connection is used, the output is higher and the sound less distorted.  This is the one we are after.   When either connection is out of the circuit, it is the loudest and least distorted, just the op amp.

Remember, in a Dist+/Rat diode to ground set up, the op amp will start distorting at some point and you will have your smooth Mosfet distortion after the harder op amp distortion.  Could be an effect to be "balanced" for nice transition from hard to smooth distortion over the decay period.

If you just want the Mosfet distortion, you need to use them in the feedback loop like a TS or Muff Fuzz to minimize the op amp contribution.   :icon_twisted:

I have a nice Brown sound using dual feedback loop mosfet clipping and a BMP tone stack.  That is the way I have been dealing with it.   :icon_cool:
Title: Re: FETs as diodes
Post by: WGTP on August 15, 2006, 09:44:30 AM
Edit: sorry wrong post location, still interesing thread.

How about using just one mosfet without the series diode as the clipping device. (and I mean just one mosfet replacing the whole "standard" two diode back to back thing) Would it give asymmetrical clipping where the tops of the waveform would be clipped very softly and the bottoms would be clipped much harder?

If it works like this the harder clipping would happen a earlier than the softer, but it could probably be biased towards the softer side.

I might have understood it all wrong though...

Edit: Based on the other MOSFET-clipper thread it seems that it should work as I thought.


yes, it would work just as you described.

 Try it!

--john
Title: Re: FETs as diodes
Post by: Constantin Necrasov on September 12, 2006, 11:16:30 PM
Thanks for the comments, I always understand things better where there's pictures involved. ;)

I did a lot of work with MOSFETs as clippers back when I first designed this pedal:

http://www.greene-pedals.com/glasspak.htm (http://www.greene-pedals.com/glasspak.htm)

Wonderful thread gentlemen! Very informative!
Mr Greene, are those mosfets as clippers? http://www.greene-pedals.com/Overdrive%20pedal_files/inside.jpg
Title: Re: FETs as diodes
Post by: johngreene on September 13, 2006, 12:01:45 AM
2 of them are.
Title: Re: FETs as diodes
Post by: Constantin Necrasov on September 13, 2006, 12:15:32 AM
How would you describe the change in tone they produced i na tubescreamer circuit?
I am currently using two 1n914s reversed-parrallel (stock configuration), although have tried every popular mod, but involving only signal diodes and rectifier deiodes. None of them produce quite the dynamic responce I want. I will certainly give the mosfets a try.

Also, there has been a design by Joe Davisson, called Amber Boost (?). I beleive it had 2N7000's as clippers in NFL, yet they were not followed by any other diode then themselves. Would this mean that they both could be clipping "like" regular diodes (because the setup seems similar to WGTP suggestion, but with two mosfets)?
Title: Re: FETs as diodes
Post by: bancika on September 13, 2006, 08:02:08 AM
can other FETs be used for this? like BS170, MPF102 or J201?
Tnx
Title: Re: FETs as diodes
Post by: WGTP on September 13, 2006, 09:55:02 AM
Yes, see above.  I thought this was a great thread, which is why I keep posting places.   :icon_cool:
Title: Re: FETs as diodes
Post by: johngreene on September 13, 2006, 11:37:12 AM
How would you describe the change in tone they produced i na tubescreamer circuit?
That particular picture is of a tubescreamer clone that I put MOSFETs in as diodes. The date on the PC board looks like 12/97 (dang, nine years....where did the time go?). They cause the bass reposne to be significantly increased giving a much darker sound. The distortion is softer and most people that tried it didn't like it as they were more used to the harder distortion of silicon diodes. It gives a pretty transparent overdrive sound that crunches nicely on 5ths but never seemed to have enough 'cut' to work for leads. Which is why I changed everything to this design:
http://www.geocities.com/greene_pedals/images/glasspak.jpg (http://www.geocities.com/greene_pedals/images/glasspak.jpg)

I am currently using two 1n914s reversed-parrallel (stock configuration), although have tried every popular mod, but involving only signal diodes and rectifier deiodes. None of them produce quite the dynamic responce I want. I will certainly give the mosfets a try.
I think there is a lot to be said for using higher-threshold devices in the tubescreamer type circuit. Try different LEDs. Every MOSFET sounds different, trying to find the one that sounds good is a long and lengthy process. The threshold voltage and transconductance variation from one model MOSFET to the other makes huge differences. The higher the gain of the MOSFET, the more like a silicon diode it will sound.

Also, there has been a design by Joe Davisson, called Amber Boost (?). I beleive it had 2N7000's as clippers in NFL, yet they were not followed by any other diode then themselves. Would this mean that they both could be clipping "like" regular diodes (because the setup seems similar to WGTP suggestion, but with two mosfets)?
A lot of people like the sound of the MOSFET body diode. It clips much harder than the 'MOSFET wired as a diode' but not as hard as a 1n914.

--john
Title: Re: FETs as diodes
Post by: Constantin Necrasov on September 13, 2006, 01:46:07 PM

A lot of people like the sound of the MOSFET body diode. It clips much harder than the 'MOSFET wired as a diode' but not as hard as a 1n914.

--john

Thanks for the info! Could you possibly explain about this MOSFET body diode, (for the slow group) ::)? How should I connect the MOSFET transistor to get this diode?
thanks again.
Title: Re: FETs as diodes
Post by: WGTP on September 13, 2006, 02:27:23 PM
See above.   :icon_cool:
Title: Re: FETs as diodes
Post by: grolschie on March 16, 2009, 04:51:27 PM

How about using just one mosfet without the series diode as the clipping device. (and I mean just one mosfet replacing the whole "standard" two diode back to back thing) Would it give asymmetrical clipping where the tops of the waveform would be clipped very softly and the bottoms would be clipped much harder?

If it works like this the harder clipping would happen a earlier than the softer, but it could probably be biased towards the softer side.

I might have understood it all wrong though...

Edit: Based on the other MOSFET-clipper thread it seems that it should work as I thought.


yes, it would work just as you described.

 Try it!

--john

Replying to an old thread. Did anyone give this a go? If so, what were the results please?

cheers,
grol
Title: Re: FETs as diodes
Post by: WGTP on March 17, 2009, 10:24:04 AM
I have tried it a number of times and it is another option in the large number of possible clipping combinations.

Using the typical BS170 or 2N7000 Mosfets, the "body diode" clips around .6v and the actual mosfet around 2v.  This creates very asymmetrical clipping and may be to your liking YMMV.  It is similar to using a std. 1N914 diode and an LED in clipping threshold.

As John Greene mentioned, and I have noticed on some "boutique" pedals, the body diode is the one being used, not the mosfet.  Also, as John has pointed out, there are a number of different mosfets that will work, each with a slightly different sound.  The lower the Transconductance, the better.  Supertex has a few that work and some will have a mosfet clipping threshold lower than 2v. making the combination of the body diode and mosfet more symmetrical.     :icon_twisted:

Here is a switch that allows either method or neither to be used.   :icon_cool:

http://www.aronnelson.com/gallery/main.php/v/WGTP/Mosfet_Switching.jpg.html?g2_imageViewsIndex=1
Title: Re: FETs as diodes
Post by: twangquack on March 17, 2009, 06:06:12 PM
Using the typical BS170 or 2N7000 Mosfets, the "body diode" clips around .6v and the actual mosfet around 2v.  This creates very asymmetrical clipping and may be to your liking YMMV.  It is similar to using a std. 1N914 diode and an LED in clipping threshold. As John Greene mentioned, and I have noticed on some "boutique" pedals, the body diode is the one being used, not the mosfet.

I really like Mosfets as clippers and always use the body diode. I first used IRF510 andIRF520 Mosfets with 1N34A diodes as "traffic cop," then started using BS170 Mosfets ... but found (as previously mentioned) that due to the approx. .001uF of capacitance of the IRF5** and the ultra-low capacitance of the BS170, I was soon adding various values of caps (often between .0005uF and .001uF, sometimes a bit higher) parallel to the clippers to give that somewhat warmer clipping tone. It usually has to be adjusted for the particular circuit. I think I added higher capacitance values to the Mosfet clippers (body diode) in an OCD clone than the BS170s in several TS clones.

In a Tubescreamer circuit, Mosfets sound great as clippers but sometimes, at least to my ear, the more "in your face" tone of the symmetrical 1N4148/1N914 can sometimes sound better. It may just gel with that design. But then again, a BS170/1N34A opposite a red LED is a pretty cool sound, and sometimes the Mosfets (and a more asym arrangement) seems to sound better. These days, there seems to be no less than three clipping options in pedals I build, and Mosfets (body diode) always figure into at least one of those. I also like 1N34A diodes in series ... but because they can be a bit darker, I never add capacitance to those, to soften that arrangement. They sure don't need "softening" but I really like 'em, too.

In the OCD clone I made, the Mosfets were a must (obviously) ... although I ended up switching in various clipping thresholds, using various combinations (in series) of both 1N34A and 1N270, with the BS170 Mosfets. I really like using a BS170+1N34A x BS170+1N270 as clippers ... then add a small amount of capacitance (tuned for each circuit) as needed. Even with the Ge diodes added to the BS170 Mosfets (to use the Mosfet's body diode), at least a little bit of added capacitance is usually in order. But that's a matter of taste and will vary.

I just worked up a Big Muff variant that sounds wonderful with the BS170+1N34A x BS170+1N270 combination as one of the clipping options (but, in this case, with no capacitance added parallel to the clippers).
Title: Re: FETs as diodes
Post by: grolschie on March 17, 2009, 06:44:59 PM
Thanks for the info guys.  :D

I was wondering if the order of clippers makes a difference? e.g. with a 2 diodes per side arrangement, would there be any difference between:
  ge->si  (on both sides),   versus,   si->ge   (on both sides).

I was wondering about that specifically in the MOSFET body diode clipping arrangement. If the ge diode clips at a lower threshold than the MOSFET and is also before the MOSFET in the circuit, whether the majority of the clipping heard was just the ge diode.

Also, would one hear any difference between a 1N60 and a 1N34/a as clippers? Thanks in advance.
Title: Re: FETs as diodes
Post by: grolschie on March 17, 2009, 07:01:37 PM
I suppose that instead of: 

D─┬──────               D─┬──────
G─┘                  or       G─┘
S────|◄──               S────►|──

Could you do this??:

D─┬──►|──              D─┬──|◄──
G─┘                  or      G─┘
S────────              S────────

Title: Re: FETs as diodes
Post by: WGTP on March 18, 2009, 10:41:28 AM
The important part is the direction of the "traffic cop" or "routing" diode.  It can be before or after the Mosfet.  You don't need a routing diode for the mosfet body diode because of the lower threshold the signal will naturally go in that direction.  It is only needed to force the signal thru the higher clipping threshold of the actual Mosfet.  Using a GE and body diode in series raises the threshold a little and insures that the clipping is soft.

If I understand correctly, diodes in series will produce the clipping of the "softest" diode at the threshold of the series value.  In other words, if you put a SI 1N914 (.7v) and a GE 1N34A (.3v) in series the result will be the softer clipping of the GE at 1.0v.  What I don't know, is if the clipping of the mosfet body diode is softer than a GE.  It's use in several circuits implies than it is.   :icon_cool:
Title: Re: FETs as diodes
Post by: grolschie on March 18, 2009, 05:42:38 PM
Thanks for that. I would've presumed that the majority of the clipping heard, would be from the Ge which has a much lower threshold - thus thinking all this MOSFET clipping could perhaps be only slightly different to plain Ge diode clipping. I guess the test would be to use an LED or silicon diode with the MOSFET? :D
Title: Re: FETs as diodes
Post by: WGTP on March 19, 2009, 10:33:59 AM
On the first page R.G. suggests using "schottky diodes like the 1N5819" in place of a GE diode.  It has a low threshold and a Sharpe turn on, so it impacts the other "diodes" less.  I think that means you will hear the clipping coming from the Mosfet better.  GE's are also used for their low threshold and "just to be safe" soft clipping.  SI's can also be used, but they raise the already high clipping threshold of the Mosfet.  You can also use the body diodes of each Mosfet as the "routing" diode for the other Mosfet as seen at AMZ and here.   :icon_cool:

http://www.aronnelson.com/gallery/main.php/v/WGTP/Moster.jpg.html?g2_imageViewsIndex=1
Title: Re: FETs as diodes
Post by: grolschie on March 19, 2009, 04:41:23 PM
Thanks for that.  :D
Title: Re: FETs as diodes
Post by: grolschie on March 24, 2009, 04:41:32 PM
Just a quick question:

When using MOSFET clipping (BS170s + 1N60's) in a TS-type pedal feedback loop (using them as MOSFETs, not the body diode method), which would be louder: said arrangement, or 3mm RED LEDs? Should one be much louder than the other? Thanks in advance.
Title: Re: FETs as diodes
Post by: WGTP on March 26, 2009, 11:06:21 AM
In my experience, Red LED's measure around 1.8v.  2N7000's measure slightly higher and BS170's are slightly higher yet, over 2.0v.  Then if you add in a GE diode for proper "routing" your at around 2.3v.  So, the Mosfet TS will be slightly louder.  With the softer Mosfet clipping, it will also sound less distorted.  Should work well as a boost with the higher/cleaner output.   :icon_cool:

Having said that, a Supertex TN0702 has a lower threshold more like a standard SI diode around .7v.  :icon_twisted:

http://www.supertex.com/pdf/misc/n_e_mode_mosfets_SG_device.pdf
Title: Re: FETs as diodes
Post by: grolschie on March 26, 2009, 04:13:33 PM
Thanks for that. I wired up both options, but one is considerably louder than the other. I haven't debugged yet, but am hoping I don't have a short somewhere. :-)
Title: Re: FETs as diodes
Post by: KHStudio on May 24, 2013, 02:39:41 AM
I have some info & questions concerning the OCD & the various clipping setups I've found in my own pedals.. I thought it fit this thread & wanted to share the link:

http://www.diystompboxes.com/smfforum/index.php?topic=102882.0 (http://www.diystompboxes.com/smfforum/index.php?topic=102882.0)