Definitive explanation of FET clippers?

[stratcat]

I copied this question from the BYOC forum and I'm going to paste it here because I would also like to know the answer to this question:

OK, I must admit to being completely confounded by the different and apparently conflicting ways I see different FET's being used as clippers in various pedal circuit modifications. Hoping somebody here can explain to me what's going on. Here's why I'm confused:

    * The first example of this I came across was Dano's recommendation to use a pair of IRF510 or 520 MOSFET's as clippers in the Rat circuit. His layout shows the transistors' gate and drain legs tied together, and 1N34A germanium diodes in series with the source legs. I used this setup in a couple of modified Screamers I've built, and it gives a nice smooth & bluesy overdrive tone.
    * In the Mouse v.2.0, Keith has used a pair of BS170 MOSFET's as clippers on the overdrive side of the circuit. As with Dano's configuration, the drain and gate are tied together, but no series diodes are employed. Keith mentioned in another thread that he tried them with the BS170's (the spots for them are present on the Mouse v.2.0 PCB, but are then jumpered per the build instructions), but they sounded "horrible".
    * CallMeRog's Large Beaver Tweaking sticky in the LB forum suggests using a pair of J201 JFET's as clippers, but his directions explicitly state that the drain and source should be tied together, with no mention of a series diode at all. I used this configuration in the Triangle v.1 Beaver I recently built, and it works very well, giving the pedal a noticeably smoother, warmer tone in that clipping mode. I used this setup in place of one of the stock distortion modes on a v.2.0 Mouse I'm building for a customer, and it sounds quite good there, too.
    * Then, I turn to the "Simplemods" WIKI at DIYstompboxes, and about 2/3 down the page, it describes making diodes from FET's by connecting the gate and source together/to ground! That's it--I give up!


So what the heck is going on here? Is there a "right" way to connect the legs on these FET's for best clipping results? What is the function of the series diodes in these setups, and why do they sound good in one clipping application and lousy in the next? If I sound confused, it's because I am....

Any FET guru's care to explain this confusing topic.

[George Giblet]

There is no right answer.  You can connect them how you want most connections will give you some form of clipping.  Whether a particular connection sounds good to you is another matter.  There are misunderstandings about what is going on with MOSFET that's why you get "bad" (or more precisely unintended) behaviour.

The JFET is easy the G to D or S forms a silicon diode.  You can connect to D, S or both D and S.

The MOSFET clipper has two modes of operation:

The first is  the body diode.  The internal structure of a MOSFET ends up with a silicon diode pointing from the source to the drain.  Ideally you should connect the gate to the source so that when that diode is reversed biased the MOSFET doesn't conduct ie. it behaves as a diode.   

The second is the active MOSFET mode connected as a clipper, here the gate is connected to the drain.  The clip direction is when current flow from D to S.  In this mode the MOSFET doesn't follow the diode exponential law and the turn-on voltage is determined by the MOSFETs threshold voltage Vt - the the gate threshold is reach current flows from D to S.  In other words the MOSFET is behaving as a voltage limiter.    Now, here's the problem some people miss: if you reverse the voltage on this connection the body diode which is pointing from S to D starts to conduct.  So there is different limiting in both directions for a single MOSFET.   If you were to connect two MOSFET in parallel with this connection the overall behaviours ends-up being like two body diodes in parallel.  The MOSFET clipping behaviour is lost because MOSFET limiter has a turn-on voltage above the body diode turn-on voltage and so the other MOSFETs body diode conducts before the MOSFET limiting mode can occur.  If you want the MOSFET limiter behaviour you have to connect another diode in series with each MOSFET to stop the body diode of the other MOSFET interferring (cathode to drain).    You can of course connect diodes in series with the the body diode connection of a MOSFET but that is done for a different reason.

[valdiorn]

Hi there. Here's just a little input on how the JFETs work, at least. They are different from MosFETs in that the gate is not completely insulated from the channel, but is rather a reverse biased diode. As such, it works pretty much like a diode clipper, except if you also connect the drains you have some resistance that carries an (almost) undistorted signal to ground as well, making a clip a little softer

Take a look at these two pictures, one is 4 circuits and the second picture is the simulation of those circuits.
http://www3.hi.is/~voe1/Posted/jFET_clip_cir.jpg
http://www3.hi.is/~voe1/Posted/jFET_clip_plot.jpg

If you take a look at the ciruits on the left, and check the simulation (out1 and out2), you can see the output signals are very close.
There is a small difference, the out2 signal (with the diodes) is a little smoother at the top. This is because the channel resistance of a JFET is not linear, it grows for increasing voltages and currents through it. That's why the diodes + resistor in parallel is a little softer (a little!), the resistance of an ordinary resistor does not grow with current throughput.
Different JFETs have different channel resistances, the one I used has a rather low one, I used it so that the effect would be very visible.

Look at the out3 and out4 signal now, they are in fact very close, this way the JFETs are just acting as normal diode clippers

Edit: Oh, and I didn't model it, but if you tie the drain and source together, then the JFET also works just like a diode, except you got one P-substrate and TWO N-substrates... Imagine putting a pair of diode clippers in parallel, two diodes conducting each way at any given time... the effect you get with the JFET is kinda half-way between this and a single clipper pair (it's harder clipping than a normal pair, but less than two pairs) ... it's kinda like having 1.5 diodes in parallel