What are good MOSFET's to use as diode clipping?

[Joe Hart]

And what about FET's? Or are all MOSFET's pretty much the same? And all FET's pretty much the same?
-Joe Hart

[RDV]

I think they may all react similarly,.....but, you should socket and plug them in till your eyes glow red and you've got it! :twisted:  :twisted:  :twisted:  :twisted:  :twisted:  :twisted:  :twisted:  :twisted:  :twisted:  :twisted:  :twisted:  :twisted:  :twisted:  :twisted:  :twisted:

RDV

[Peter Snowberg]

There are two basic flavors of FET. FET is just an acronym for Field Effect Transistor which means it switches via an electric field that is injected into the transistor via an insulated "gate" connection. The "regular" transistors are called BJTs or Bipolar Junction Transistors, and they switch via current flowing through the base connection.

Now we get into types of FETs. The two most common names you'll see are MOSFET and JFET. Each switches via a field applied to an insulated gate. Now the differences start. In a JFET, the insulation is provided by a reversed biased PN semiconductor junction and the switching field is injected via a silicon layer. In a MOSFET, the insulation is from a thin layer of silicon dioxide and the switching field is injected via a tiny metal (usually aluminum) plate.

JFETs are usually smaller and more rugged than MOSFETs. MOSFETs can get downright huge, but their fragile construction involving a super thin insulator between the gate and the switched current path makes them very easy to kill with static electricity.

Why am I rambling about all this? Now we get to the point when it comes to using MOSFETs for clippers.

To make MOSFETs more rugged and stop tiny amounts of random static from killing devices, semi fabricators started building reverse biased protection diodes into MOSFETs.  :!:    There's the clipping. The internal structure of a power MOSFET can have a fair amount of capacitance too. There's some filtering added to the picture.

What Aron did was to use the protection diodes and the "built-in" low value caps in parallel. The actual MOSFET device is passive and unused in this application.

PS: Not all MOSFETs have these diodes. :!:

[Joe Hart]

If they don't all have these diodes, how do I know which ones have them? In other words, what should I buy from Mouser to try?
-Joe Hart

[Peter Snowberg]

They're just another flavor of silicon diode so I'll bet the largest difference will come from the physical size of the die. Larger devices will probably cut more highs but I have absolutely no idea by how much. Try devices rated at 1A, 5A, 10A, 25A, 50A, etc. (that sort of thing)

As far as looking for different MOSFETs, I think that most of the power devices include the diode(s), but check the datasheets to find out.

[Steben]

Well... I wonder....
If its just the diode inside the MOSFET.... why using the MOSFET then?....
Why not a smaller diode with a cap?

Sorry for my European gründlichkeit, but yet...

[jplaudio]

Well... I wonder....
If its just the diode inside the MOSFET.... why using the MOSFET then?....
Why not a smaller diode with a cap?

Sorry for my European gründlichkeit, but yet...

You can use a MOSFET for it's clipping properties instead of using it as a Si equivalent. The trick is ( as RG has mentioned) to use a series diode with the MOSFET to prevent the intrinsic diode from conducting. The external diode can be Si or Ge. The cathode of the diode is connected to the gate/drain of the MOSFET. The MOSFET has a nice rounded conduction "knee", smoother and more tube like than other devices. The threshold is higher than other clippers so you end up with  much more dynamic range.

[Phorhas]

http://diystompboxes.com/sboxforum/viewtopic.php?t=23474&postdays=0&postorder=asc&highlight=mosfet&start=15

check this thread out

[WGTP]

Check out the Mosferatu in the Schematics II.  I think he got it right in an interesting arrangement of op-amps.  

[jplaudio]

If they don't all have these diodes, how do I know which ones have them? In other words, what should I buy from Mouser to try?
-Joe Hart

Try some 2n7000's or BS170's. The actual threshold can vary from .8 to several volts so you may want to get several of each type to try. The mismatched thresholds can add some asymetrical clipping. The last batch of 2n7000's I got were pretty consistant , all around 2 volts. You can always use them for gain stages or relay drivers if you don't like the way they clip.

[Joe Hart]

Thanks everyone! I checked out the schematic for the Mosferatu and it uses 2N7000's. Am I right that you do not hook them up back-to-back? I see that they are just one single transistor with a diode added on to ground. I am guessing that the transistor's internal diodes clip back-to-back and that's whay you don't need two?
-Joe Hart

[jplaudio]

Thanks everyone! I checked out the schematic for the Mosferatu and it uses 2N7000's. Am I right that you do not hook them up back-to-back? I see that they are just one single transistor with a diode added on to ground. I am guessing that the transistor's internal diodes clip back-to-back and that's whay you don't need two?
-Joe Hart

The design that Alf used was to have one side of the signal clip in the feedback loop and the other side clip at the output of the op amp. Everything being equal the fb loop device should clip before the one on the out put. You can hook the mosfets up back to back in the fb loop or on the output to ground. Note that in the schematic it appears that series diode is connected to the source pin of the  mosfet. This arrangment will not block the intrinsic diode from conducting. It is basically the equivalent of a Si diode and a 1N34 in series. To obtain the clipping qualities of the mosfet the cathode of the series diode should be connected to the drain and gate pins of the 2n7000. The source pin becomes the cathode and the open diode lead becomes the anode of the composite clipper diode.

[WGTP]

Sorry, I guess that is what I get for thinking again.

R.G. explained all this Mosfet glipping stuff in a thread not too long ago, but I don't remember the title.

I still think the Mosferatu is cool for the way it uses both FBL diodes and diodes to ground, one of each for assymetrical clipping of each flavor.  You could do the same thing with SI diodes Jfets, GE's, etc.