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Old 2nd October 2012, 01:07 AM   #1421
qusp is offline qusp  Australia
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Quote:
Originally Posted by john curl View Post
Where do you get the idea that there is asymmetry in the Toshiba 2SK170, 2SJ74 jfets? Are you confusing these with American jfets?
I thought I saw conversation in your blowtorch thread and perhaps Patrick may have mentioned it. not that they arent symmetrical, just that there was one or 2 parameters that wasnt exactly the same when reversed, but I cant find it again now. it IS possible i'm confusing it with another part, I actually hope I am

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Old 2nd October 2012, 01:18 AM   #1422
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I am pretty sure that, for all practical purposes, both the Toshiba N and P jfets are symmetrical and will work 'upside-down' just as well.
This has NOTHING with trying to make an N into P or anything like that, as N and P jfets have opposite 'doped' channel material, so they are forever different.
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Old 2nd October 2012, 01:22 AM   #1423
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The American N channel jfet the J113 is NOT completely symmetrical. This is important sometimes, but not too often. This is due to the wafer fabrication. The pin outs are really different and cause problems.
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Old 2nd October 2012, 02:08 AM   #1424
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The so called "short channel" JFETs you can reverse the Source and Drain and the device dosen't not know the difference. It's not that an N becomes a P. Just that you can switch the source and drain. I honestly don't remeber the structure enough to explain more. All JFETs do not have the same physical structure or channel length and gate layout? Wish I was so interested when I was working in an area I could learn all this. Now I just work with multi phase switching stuff for processor and memory server board applications. Certanly I'ld rather work in an audio function somehow but that's all filtered down to Class D or ? Mid-Fi analog Yuk
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Old 2nd October 2012, 02:26 AM   #1425
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This has NOTHING with trying to make an N into P or anything like that, as N and P jfets have opposite 'doped' channel material, so they are forever different.
'using as' P channel (acting as, not changing the atomic structure, didnt I make this correction already?)


re J113: hmm no I dont think thats the fet, i'm going to have to do some more searching to see where this was discussed, one of the many threads lamenting the lack of P-channels
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Old 2nd October 2012, 05:44 AM   #1426
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Originally Posted by bcarso View Post
No way. JFETs are symmetrical, mostly, for an interchange of the leads labeled "source" and "drain", but that doesn't make them function like P channel parts. Life would be nicer if they did, although very confusing for solid state physics.
All clear now. Thank you.
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Old 2nd October 2012, 05:53 AM   #1427
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Because JFET is Junction Field Effect Transistor. Gate is p-n junction, like diode. It is either P in respect to both drain and source, or N in respect to both drain and source.

JFETs work with reverse biased junction. Devices that work with forward biased junction are called 2-base diodes or uni-junction transistors, avalanche effect starts as soon as junctions start conducting abruptly decreasing base resistance.

http://en.wikipedia.org/wiki/Unijunction_transistor

http://en.wikipedia.org/wiki/Jfet
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Last edited by Wavebourn; 2nd October 2012 at 06:00 AM.
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Old 2nd October 2012, 07:04 AM   #1428
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Default Inductor trick

Great design(s), Scott!

You mention the "inductor trick". What is that?



/S

Quote:
Originally Posted by scott wurcer View Post
The diodes are essentially free and the 12 bipolars are 5-10 cents each. The JFET's can vary but there are some < 25 cent options. This exercise was an attempt to also "make the numbers".

The noise is just under 2nV, if you really wanted 1 nV I would entertain the inductor trick and some paralleled FET's on the input. If you wanted 30 Ohm drive I would just use a good closed-loop buffer or high current op-amp as an output stage. Those MJE devices have big Cje's to drive and two of them are bigger than the op-amp would be anyway. Notice the JE990 drops to -75dB THD at 75 Ohms.
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Old 2nd October 2012, 09:11 AM   #1429
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Just to let people know I've whipped up an LTspice version of Scott's 2nd, complimentary version so no-one really needs to duplicate this (the easiest one first!), and it works, how amazing, ! Of course, not being an amp designer, nowhere near it, took me a little bit to realise that pfets were need -- still need to insert real, reasonably accurate models of the actives, so won't post for the moment ...

Something to play with in an idle moment ...

Frank
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Old 2nd October 2012, 10:11 AM   #1430
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Maybe I've not been reading carefully but what are the voltage gains avalable with Scott's circuit? Is this a post volume control line/headphone amp or can it be used as a mic preamp? What are the best matched Jfets with unique gm to use? John mentioned the J113 and J175 or should it be 2sk170/J174. The first posted circuit had a + and - 30v power supply, why subsequent posts were + and - 15v? So, I assume that this has not been built with real parts on a bread board and what we see is an elegant circuit and a good SPICE simulation. Maybe a good parts list is in order. I'm ready to get the bread board out and start soldering. Thanks, Ray
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