Understanding MOSFET clipping

Ok, you got your soldering iron and nothing is going to hold you back, but you have no clue where to start or what to build. There were others before you with the same questions... read them first.

Understanding MOSFET clipping

Postby commathe » 29 Dec 2013, 10:39

I'm having a little trouble understanding mosfet clipping. My understanding is that there are two forms: the diode-connected mosfet (tying gate and drain together) and the body diode. However, when breadboarding around I found that it worked opposite from how I expected. Examples follow.

Here are the three layouts I used. I picked a mosfet symbol that showed the body diode as on the datasheet for the 2N7000 (the n-channel enhancement mode MOSFET I used):

Image

As you can see Mosfet A uses the diode to block the body diode. This is the set-up I expected to sound the most different. I expected Mosfet B to act simply as a diode with a higher forward voltage. I used the leds as a control for comparison (the forward voltage was fairly close). My results surprised me!

First, here is a comparison of each of them used to rails after a simple Pro-co RAT clone op-amp stage:

NOTE: the board style seems to be cutting off the far side of the spectrum analysis (where the most intersting stuff is). If you want to see it just right-click then "open in new tab"
LED:
Image

Mosfet A
Image

Mosfet B
Image

As you can see, the Mosfet A (blocking the body diode) and the LED to rail are almost the same. Almost! The spectrum analysis shows a little difference, and the difference was barely audible. I expected Mosfet B to act like that though, whereas it created a much more different sound. A noticeably different sound actually.

This stands out much more when I put then in the negative-feedback loop of my op-amp:

LED (op-amp feedback)
Image

Mosfet A (op-amp feedback)
Image

Mosfet B (op-amp feedback)
Image

Here you can see again that the LED and Mosfet A are close. The difference is more pronounced this time however. Mosfet A is definitely a bit more aggressive and brighter. Not in a bad way though! Probably not enough to be actually worth it though. Mosfet B on the other hand produces something noticeably different. It's got a lot less high end, and it's generally smoother.

So, my question is: why? What is actually going on that creates this behaviour?
commathe
Breadboard Brother
 
Posts: 97
Joined: 01 Jul 2013, 23:31
Has thanked: 6 times
Have thanks: 8 times

Re: Understanding MOSFET clipping

Postby bajaman » 29 Dec 2013, 11:12

sorry but i cannot see ANY images :!: :scratch:
User avatar
bajaman
Old Solderhand
 
Posts: 4000
Joined: 26 Jun 2007, 22:18
Location: New Brighton, Christchurch, NZ
Has thanked: 288 times
Have thanks: 674 times

Re: Understanding MOSFET clipping

Postby commathe » 29 Dec 2013, 11:14

bajaman wrote:sorry but i cannot see ANY images :!: :scratch:

Weird! They are visible for me. Try quoting the post, the links should be visible in the text
commathe
Breadboard Brother
 
Posts: 97
Joined: 01 Jul 2013, 23:31
Has thanked: 6 times
Have thanks: 8 times

Re: Understanding MOSFET clipping

Postby snofla1900 » 29 Dec 2013, 11:38

sorry but i cannot see ANY images


Can't see them either !
snofla1900
Breadboard Brother
 
Posts: 115
Joined: 23 Sep 2012, 14:33
Has thanked: 66 times
Have thanks: 13 times

Re: Understanding MOSFET clipping

Postby commathe » 29 Dec 2013, 11:58


For this message the author commathe has received thanks: 3
Intripped (30 Sep 2014, 22:43), Manfred (30 Dec 2013, 00:12), snofla1900 (30 Dec 2013, 10:18)
commathe
Breadboard Brother
 
Posts: 97
Joined: 01 Jul 2013, 23:31
Has thanked: 6 times
Have thanks: 8 times

Re: Understanding MOSFET clipping

Postby deafbutpicky » 29 Dec 2013, 15:01

Hi,
I'm not sure if I understand your question right. From your pics I would go for the obvious concerning the differences:

in the Mosfet B setup the clipping threshold (Vf) is lower (smoother wave shape, esp. in OPA-loop)
and more clipping induces more harmonics.

You could try some shottkys additional to the bodydiode setup to raise Vf and compare them again.

This might be interesting if you don't know already (i.e. higher C in body diodes):

http://www.muzique.com/lab/zenmos.htm
I'm using capacitors to replace my guitar effects and give more clarity to the sound.
deafbutpicky
 
Posts: 49
Joined: 03 Jun 2011, 11:07
Location: germany
Has thanked: 24 times
Have thanks: 20 times

Re: Understanding MOSFET clipping

Postby commathe » 30 Dec 2013, 02:33

deafbutpicky wrote:This might be interesting if you don't know already (i.e. higher C in body diodes):

http://www.muzique.com/lab/zenmos.htm
Fantastic. I had missed this detail. This must be it! The shape of the wave is radically different from the other clipping set ups. This slower-diode/higher-capacitance must be causing that.

I am still confused as to why I am not seeing much in the way of the gate-tied-to-drain effect though. In theory, it shouldn't behave like a normal diode at all as it will "slowly" switch itself on until the gate is eating up all the current... yet it is acting almost identically to my led clippers :? I'm wondering if putting a small resistance between the gate and drain will have an effect or not (slowing down the turn on of the gate). Definitely need to go back to the breadboard and do some more tests.

I guess my real issue is: Why is the the Mosfet A set-up behaving nearly identically to the LEDs? This set up *should* be the most different, and yet it is not.
commathe
Breadboard Brother
 
Posts: 97
Joined: 01 Jul 2013, 23:31
Has thanked: 6 times
Have thanks: 8 times

Re: Understanding MOSFET clipping

Postby ggedamed » 30 Dec 2013, 03:26

Maybe LEDs are behaving differently than the standard diode clippers. Try the usual silicon diodes so you have the whole spectrum.
User avatar
ggedamed
Resistor Ronker
 
Posts: 319
Joined: 21 Jun 2009, 12:43
Location: Bucharest, Romania
Has thanked: 138 times
Have thanks: 141 times

Re: Understanding MOSFET clipping

Postby commathe » 30 Dec 2013, 03:41

ggedamed wrote:Maybe LEDs are behaving differently than the standard diode clippers. Try the usual silicon diodes so you have the whole spectrum.
Even if this is the case, the Mosfets are still not behaving like they should. In theory they will switch on slowly, which should give a very rounded and soft clipped waveform. Any circuit simulation I run of mosfets does this, I'm confused as to why my real life ones are not.

I guess I should add that pretty much every commercial pedal I see that uses mosfets is also using the higher capacitance body diode (OCD, Fulldrive, Zendrive etc.). Even the AMZ Shakka Bradda does. Although I read about the theoretical soft clipping mosfets could provide when you block the body diode and use the gate tied to drain configuration (a number of R.G. posts on here about it), I see no pedals using it. So something is up there too. :?
commathe
Breadboard Brother
 
Posts: 97
Joined: 01 Jul 2013, 23:31
Has thanked: 6 times
Have thanks: 8 times

Re: Understanding MOSFET clipping

Postby deafbutpicky » 30 Dec 2013, 11:57

The problem with the Mosfet A type clippers is their high Vf (1.2V-2n7000 - 2.5V-BS170).
They need a lot of gain and headroom to be used as heavy clippers.
I'm using capacitors to replace my guitar effects and give more clarity to the sound.

For this message the author deafbutpicky has received thanks:
commathe (31 Dec 2013, 02:26)
deafbutpicky
 
Posts: 49
Joined: 03 Jun 2011, 11:07
Location: germany
Has thanked: 24 times
Have thanks: 20 times

Re: Understanding MOSFET clipping

Postby commathe » 31 Dec 2013, 02:30

Thanks. I didn't consider this. I wasn't using the maximum possible gain in that set-up so it definitely warrants a going back to try it as such. I also found that the diode loop use of Mosfet A seems to behaving like it should. I might try putting some resistors at the source or something similar and see what effect this has too.

Lots of fun experimentation to do!
commathe
Breadboard Brother
 
Posts: 97
Joined: 01 Jul 2013, 23:31
Has thanked: 6 times
Have thanks: 8 times

Re: Understanding MOSFET clipping

Postby Intripped » 30 Sep 2014, 23:10

hey! thanks for this, very interesting.
have you done some further experiments?
what is the best sounding configuration on your opinion?

here's a video from proguitarshop where Andy makes a demo of the new highly modded YATS from Earth Quacker Devices:
http://www.youtube.com/watch?v=O-71XWcLtmk
from 3:44 you can listen to different clipping options, mosfets included.
(there's a nice shot on the clipping section on the PCB too (...what do you guys think of D8? what is it for?))

@commathe: can you confirm these sonic differences, expecially between LEDs and mosfets? (what's the corresponding configuration?)

thanks!
Intripped
Resistor Ronker
 
Posts: 439
Joined: 25 Aug 2009, 02:03
Has thanked: 473 times
Have thanks: 204 times


Return to Absolute Beginners

Who is online

Users browsing this forum: No registered users and 2 guests

cron