mosfets as clippers, help please... aron?

[m_charles]

Hey RG, Aron, everyone,

I've been learning more about mosfets, and mosfets as clippers and I wanted to double check something.

was scanning old posts and saw this thread:

http://www.diystompboxes.com/smfforum/index.php?topic=57926.0

Going by RG and Aron's comments, is it correct to assume that if you are not running 2 mosfets as clippers, say you're running a mosfet and a silicone diode for asymmetrical clipping, than the series ge diode is not really nessicary? I've read that post a few times now, and I still don't understand the purpose of the ge diode with a mosfet. I believe it's not just for the tone of the distortion.

Just to double check, I'm using a mosfet, drain and gate tied, source alone, as a diode, NO GE diode, and a silicone diode opposite. To me it sounded much better without the ge, but I wanted to make sure  I wasn't totally missing something, or using the mosfet as a regular old silicon diode.

thank you!

chuck

[R.G.]

Every discrete MOSFET you can buy has a built-in reverse-connected silicon diode. In the N-channel stuff, there is an implicit Si diode with its anode on the source and its cathode on the drain. If you connect the gate to the drain, then in the normal "drain positive" direction, the thing does not conduct at all until the drain gets up to the threshold voltage Vth for the particular MOSFET, then the gate lets the channel conduct. The forward voltage/current curve that it traces is the Vgs/Id curve.

If you then reverse the voltage, the channel would not conduct at all for quite a few volts, but the body diode starts conducting the reverse way at its silicon forward voltage. So you don't really have to put another silicon diode in parallel at all. It's an asymmetrical clipper all by itself. An external diode doesn't hurt, but may be useless depending on the diode voltages.

If you want to restrict the body diode from conducting, THEN you need the series diode to keep the MOSFET body diode from conducting in the reverse direction.

[aron]

I seriously can't remember how this works anymore. I'm going to read R.G.'s post! :-)

For the record, I don't use MOSFET clippers anymore. It worked in the Shaka 3 pretty well though.

[amz-fx]

Just to double check, I'm using a mosfet, drain and gate tied, source alone, as a diode, NO GE diode, and a silicone diode opposite. To me it sounded much better without the ge, but I wanted to make sure  I wasn't totally missing something, or using the mosfet as a regular old silicon diode.

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

regards, Jack

[m_charles]

thanks guys...

[WGTP]

http://www.diystompboxes.com/smfforum/index.php?topic=38581.0

This is a big a$$ post that I link to whenever this question comes up.   Great stuff. 

[nokaster]

i have some bs250 with no specific use for them.
can i try these as clippers in distortion boxes?

i know they are positive ground, so direction might be important?

help would be appreciated!

[R.G.]

They aren't positive ground, they are P-channel. That means the substrate/body diode points the other direction. Otherwise, all the comments count.

In electronics, if all the terminals of the component are not identical, direction ALWAYS counts.

[PaulC]

Here's a simple switching setup I did a while back that can let you play around with mosfets in different setups on the fly.  It uses a DPDT on/off/on switch.  It gives the series mosfet effect,the parallel body diode effect, and a single mosfet for the asym effect R.G. stated above.  You can use it in feedback loops or shunts.   http://i58.photobucket.com/albums/g259/ocluap/moswitch.gif

Later, PaulC
Tim & timmy pedals