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Author Topic: Over, Under, Upside Down JFETs  (Read 5681 times)

seagreen

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Over, Under, Upside Down JFETs
« on: June 23, 2011, 06:15:16 PM »
Over at some other fun forum, a nice fellow who goes by EUVL has just posted some plots he made of some N-channel JFETS connected as P-channel devices.  The curve when the device is used as an P-channel part is almost identical to when it's used as an N-channel part.

What is the horrible reason why nobody builds circuits this way?

Might be a good way to get complementary pairs...
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Gen. Dreedle

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Re: Over, Under, Upside Down JFETs
« Reply #1 on: June 23, 2011, 06:44:49 PM »
Backwards, forward, square and round.

When will it end.................?

(Inquiring minds want to know.)

The Toshiba JFETs are symmetrical. IOW, the source and drain are interchangeable. The horrible reason may have something to do with gate-source voltage. Might work on some real low voltage stuff.
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seagreen

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Re: Over, Under, Upside Down JFETs
« Reply #2 on: June 23, 2011, 07:26:36 PM »
Hey!

Phono?  Line level?

Supposedly the BF862s are symmetrical, too.  Low noise.  Cheap.  Plentiful...
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Gen. Dreedle

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Re: Over, Under, Upside Down JFETs
« Reply #3 on: June 23, 2011, 08:03:32 PM »
Yes, but............

Like all of those FETs, the noise is spec'ed at 100 kHz, or so. Which is where the curve tends to go up. The other problem is the process they use to make them is prone to 1/f problems. None of those parts are spec'ed for that. Since they are designed for mixers, switches, RF stages, stuff where 1/f usually does not matter. You may get a quiet one, you may not. If you have the time to buy a big bag, and sort them all, you may get lucky.
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seagreen

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Re: Over, Under, Upside Down JFETs
« Reply #4 on: June 23, 2011, 08:35:52 PM »
According to the experts (!) the process used for BF862s is such that you get good 1/f noise in the audio band.  It's similar to Toshiba parts.  Ask your friendly local Devil...
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Gen. Dreedle

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Re: Over, Under, Upside Down JFETs
« Reply #5 on: June 23, 2011, 09:39:28 PM »
I have some, somewhere...........

Guess I need to measure them.
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MystereoN

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Re: Over, Under, Upside Down JFETs
« Reply #6 on: June 24, 2011, 09:28:51 AM »
yes, for BF862 drain and source are interchangeable.
If my memory serves me, "Captains's Kirk cheef engineer" measured them and results should be posted somewhere in that funny diy engineering forum. No way to see them, guests can't see attachments and they won't let me register there.

Figured out few possible reasons:

a) I'm not an engineer and don't have MS in EE   :'(
b) my education is into human medicine and while I'm good enough to understand people's flaws & problems and solve them, I'm probably to dumb to understand audio circuits  :-[
c) they somewhat figured out point b) and they suffer from white coat syndrome  ;)
d) I don't like sound of opamps at unity gain, I prefer them at unity gain in power supplys  :-X
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seagreen

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Re: Over, Under, Upside Down JFETs
« Reply #7 on: June 24, 2011, 11:07:46 AM »
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MystereoN

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Re: Over, Under, Upside Down JFETs
« Reply #8 on: June 24, 2011, 09:29:31 PM »
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Gen. Dreedle

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Re: Over, Under, Upside Down JFETs
« Reply #9 on: June 24, 2011, 10:24:35 PM »
You seem to forget some of us are unable to look at their attachments, either!
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MystereoN

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Re: Over, Under, Upside Down JFETs
« Reply #10 on: June 25, 2011, 07:48:42 AM »
Mea culpa...
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MystereoN

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Re: Over, Under, Upside Down JFETs
« Reply #11 on: June 25, 2011, 09:06:50 AM »
talking about low noise... got LP5900-3.3 gizmo in LLP-6 package.
Good PSSR, low broadband noise, moderate 1/f noise. Soldered together two 1uF 0805 capacitors, attached leads... results can be seen on picture.  3mm LED and 2SC2240 are for comparision. It can do 3.3V from 5V @120mA without sweating.

And yes, capacitors should be returned to GND at better point  :)

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Gen. Dreedle

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Re: Over, Under, Upside Down JFETs
« Reply #12 on: June 25, 2011, 10:06:15 AM »
So, where is the proof? We demand measurements!
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MystereoN

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Re: Over, Under, Upside Down JFETs
« Reply #13 on: June 25, 2011, 11:50:05 AM »
Informations about low noise written from datasheet, since my E-MU 0404 USB is in digital heaven. Inputs are working, nothing on output. Probably A/D converters are fried. If anyone have schematics, it would be great.

Will order new 0204 USB this weekend and it should be here in few days.
I still need to put together low noise preamp with LME49990, sound card's gain probably won't be enough.

Anyway, I can always send you that gizmo, if you want. You have way more experience with low noise measurement and proper equipment to do 1/f measurement. I have (will have) only soundcard.
No need to send it back, I got two chips to play with them  :)
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Gen. Dreedle

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Re: Over, Under, Upside Down JFETs
« Reply #14 on: July 12, 2011, 01:49:31 PM »
Hey, when are the over-engineering crowd gonna measure a bag of a thousand? We demand proof!

And then............an ABX/DBT on whether they sound better than a 2SK170. Remember, the test has to be statistically significant, and has to be peer-reviewed. Otherwise, it is just a load of codswallop.

Just like.........well, you know! (You should, at least by now. Are you guys really that dense?)
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