FETs as diodes

[Nico666]

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

[amz-fx]

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

[Bernardduur]

I done both the FET as the MOSFET in my TS loop and I love it. Try it!

[Nico666]

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

[Nico666]

Anyone to correct me?

[R.G.]

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.

[Nico666]

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

[gez]

http://www.diystompboxes.com/smfforum/index.php?topic=34013.0

[Nico666]

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

[R.G.]

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.

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.

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.

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.

1) What exactly was John Greene's setup?

The third one I just talked aobut.

(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.

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.

[Nico666]

 

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

Nicolas

[johngreene]

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

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


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

[Nico666]

Thanks Mr. John Greene!

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


Nicolas

[Elektrojänis]

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.

[johngreene]

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

[Elektrojänis]

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?

[WGTP]

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.

[johngreene]

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

[aron]

I still stand by the way I used them in the Shaka 3. 

[R.G.]

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.