MOSFET follower/clipper

From circuitbending to homebrew stompboxes & synths, keep the DIY spirit alive!

Moderators: Joe., lisa, luketeaford, Kent

Post Reply
User avatar
devinw1
Super Deluxe Wiggler
Posts: 1190
Joined: Tue Aug 07, 2018 11:20 am
Location: Portland, OR
Contact:

MOSFET follower/clipper

Post by devinw1 » Thu Mar 26, 2020 10:56 am

I was wondering if anyone sees any pitfalls in using the circuit (falstad link below) as a follower for a comparator's 24Vpp square wave to clean it up and take it to 10Vpp by using a common-source Nchannel/Pchannel MOSFET follower with the drains set to 5V rails.

I built it up using VP2106/VN2106s and it works great, but figured I'd run it by ya'll anyway. It's basically similar to what's inside a CMOS inverter but with the MOSFETs flipped.

I added the 100 Ohm drain resistors because there was some ringing on transitions but I'm thinking this might have been more messy routing/long wires on the protoboard. They may not be necessary on a PCB.

The circuit:

http://tinyurl.com/vnntyh2

User avatar
guest
Super Deluxe Wiggler
Posts: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: MOSFET follower/clipper

Post by guest » Thu Mar 26, 2020 11:31 am

as long as the Vgs,max and Vgd,max requirements are met, it should be ok. this version doesnt have the shoot-through issues of the more common inverter configuration, so slow moving signals are ok with it. the ringing might be due to gate capacitance, so a resistor to the gate might be in order.
openmusiclabs.com

User avatar
devinw1
Super Deluxe Wiggler
Posts: 1190
Joined: Tue Aug 07, 2018 11:20 am
Location: Portland, OR
Contact:

Re: MOSFET follower/clipper

Post by devinw1 » Thu Mar 26, 2020 11:50 am

Cool, looks like I should be good as it's +/-20V for both those specs on the FETs I used.

I'll try a gate resistor instead of the 2 100Ohm resistors to see if that works well.

User avatar
devinw1
Super Deluxe Wiggler
Posts: 1190
Joined: Tue Aug 07, 2018 11:20 am
Location: Portland, OR
Contact:

Re: MOSFET follower/clipper

Post by devinw1 » Thu Mar 26, 2020 4:40 pm

No drain resistors and a 3.9k gate resistor works fantastic. Ringing is gone, and the transistion is even smoother than the other configuration. It's nice and snappy, about 15ns transisition. Just what I needed. Thanks for the suggestion!

User avatar
guest
Super Deluxe Wiggler
Posts: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: MOSFET follower/clipper

Post by guest » Thu Mar 26, 2020 6:54 pm

cool, glad it worked! ive seen mosfets in class D outputs basically destroy themselves because of ringing on the gate. its relatively large capacitance coupled with the inductance of the line feeding it, and it spends way more time in the middle than it should, and they overheat. you can also overvoltage the gate during the ringing.
openmusiclabs.com

User avatar
devinw1
Super Deluxe Wiggler
Posts: 1190
Joined: Tue Aug 07, 2018 11:20 am
Location: Portland, OR
Contact:

Re: MOSFET follower/clipper

Post by devinw1 » Fri Mar 27, 2020 10:14 am

Ah yes, that makes sense for a power amp for sure.

I haven't dealt at all with MOSFET power amps but just read a page about them and indeed the output stage of a class D amp is what I have built here.

Post Reply

Return to “Music Tech DIY”