matched fet for expo?

mrmellon · Joined: Jul 25, 2010 Posts: 7 Location: up north

First of all im no expert and i mainly go by trial and error. when i was a child i liked stripping pcbs and i was lucky my dad kept the components. a few years back i got from my job ALOT of components and i found the old in my parents house. and .. now i im building a synth. did some practice with lunettas and is abut to finish a small machine. well to the question.

i have found an old u430 matched n channel pfet.

could this be used as the differentiel pair in a exponential converter?

i have done some research and cant seem to find any schem using fet. so the answer i think is no but why not then. hope someone will take the time to explain me.

gdavis · Posted: Mon Apr 17, 2017 12:19 pm Post subject:

n-channel PFET makes no sense, did you mean JFET (guess I could just look it up... yep, it's a JFET Laughing

)?

Your question is one I've wondered about as well. I believe it's theoretically possible to create an exponential converter from FETs. I think the reason you don't see them is simply history.

The basic principal of the FET was patented in 1925, but the first working MOSFET wasn't demonstrated until 1959. In the meantime, the BJT was developed as it was technically easier to fabricate than the MOSFET.

BJT's were the primary transistors in use at the time that analog synthesizers where being developed. They were cheap and well understood. By the time the MOSFET started becoming commonplace, I believe analog synths were starting to fall out of favor, being replaced by digital synths. So they were just never updated to use MOSFETs.

The circuits you see today are mostly still based on those early circuits from the 60's and 70's. They just haven't evolved much since the BJT's they use are still easy to obtain.

The operation of MOSFET's is fundamentally different from BJT's so the circuits would require significant redesign.

mrmellon · Joined: Jul 25, 2010 Posts: 7 Location: up north

thanks gdavis.

usefull info. and yes i dont know what i was thinking jfet ofc. Embarassed

but.
i found something.
a jfet differentiel pair.
https://paginas.fe.up.pt/~fff/eBook/MDA/MDA.html

and the outline of an expo.
https://www.allaboutcircuits.com/projects/diy-synth-series-vco/

for opamps i have alot of ltc1152 so im planning of putting those to use.

mrmellon · Joined: Jul 25, 2010 Posts: 7 Location: up north

i doubt it is this easy Very Happy

. and that the value of the resistors will be the same.

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blue hell · Posted: Mon Apr 17, 2017 2:08 pm Post subject:

The expo is based on the Base Emitter diode of the BJT doing exponential voltage to current conversion .. the BJT being a current controlled device converts the resulting current into an amplified Collector Emitter current.

A FET is voltage driven and so would need an extra diode to do the expo conversion .. does it even have a usable diode I wonder ... I'm not too good at FETs TBH.

mrmellon · Joined: Jul 25, 2010 Posts: 7 Location: up north

blue hell.

well thanks for the reply. i dont get much from the datasheet.

http://pdf.datasheetcatalog.com/datasheet/vishay/70249.pdf.

but then again im no expert in anyway. but so far i have gotten alot of research on the net and i find that most things i get from doing this but expo's is not something i get.

the reason i wanted to use the u430 is i have 4 of them so that would give me alot more posibilities. i have other matched transistors from back in the days ready for ysynth vco's.

gdavis · Posted: Mon Apr 17, 2017 3:51 pm Post subject:

mrmellon · Joined: Jul 25, 2010 Posts: 7 Location: up north

thanks gdavis and blue hell

i get it now. so i save the u430 for something else (any ideas) and then i buy some lm394 from ericasynth. so im sure to have enough and if the old components dont work. i test them as i go along.

i just like to challenge myself with this kind of stuff.

mrmellon · Joined: Jul 25, 2010 Posts: 7 Location: up north

i just did a restock of my dual transistors.
the crown juvels are 2x mat 01
and 2x 2n2916 (they are not matched i belive but ok, just need to tweek a bit with this one so there are better options i think).

and then more dual fets but we kinda know where that will get me.

on the same board where i stripted the mat01s i also striped 2x ca3140 and
lm 308 they might be interesting if i deside to go 15v. the plan was 12v

gdavis · Posted: Tue Apr 18, 2017 11:36 am Post subject:

mrmellon · Joined: Jul 25, 2010 Posts: 7 Location: up north

ok so ill not be posting more here since i got my answer and a great advice on what to use the fed for. and y qdavis im into pedals since im a bass player doing experimental music/ soundscape ect. onetake layer mixed impro. (not for the big audience). at the moment we are again redoing the channel (it is my freind who is running the channel) so mostly quite normal styff at the moment. lousysound on youtube if u what to tjeck it out. (dont like or subscripe if u dont like).

anyhow enough ranting i will make a new topic when i get the time to finish the first module. im starting with a xr vco and wait with the ysynth vco's till i get the lm394. ill post the link here if u are interested.