What characteristics have a good small signal FET ???

[aldovan]

Friends

Happy New Year !!!
My question is: What electric characteristics a good small signal j-FET have ?
High transcondutance ? Small Ron ? Other ?
What is more important for use in HIFI ?

Thanks,

Aldovan

[Lumba Ogir]

Hi,
good sounding characteristics.

[Bigun]

I've considered them for input devices, as part of a CFP input, but they have such low transconductance that I've never gone much further than thinking about the idea. My fear about low transconductance is insufficient gain, which means it's harder to linearize the input - often also the error amplifier - with local or global feedback. So whilst the transfer curve of the JFET might be 'nicer' on paper than the exponential curve of a BJT I"m not sure if you get the full benefit of it in an error amplifier.

My current project uses a PNP input device, so I'd need the less common p-JFET too. If somebody wishes to suggest a particular device that I can order from DigKey (unless you want to send me some free samples...) then I could at least do the listening test at some point.

[Mooly]

It depends what you want it for... switching/constant current source/audio switching etc

I use a lot of 2SK152 for audio switching

[akis]

I use the 2N5459 just as a buffer when I need very high input resistance.

[Nrik]

Originally Posted by aldovan

My question is: What electric characteristics a good small signal j-FET have ?

It depends on where in the circuit you use it.

For use in input-stage and VAS (voltage-amplification-stage) you would want high transcondutance.

For buffer (line-level outputstage or headphone buffer) you would want small Ron and/or high Idmax.


Check these articles:
http://www.borbelyaudio.com/adobe/ae599bor.pdf
http://www.borbelyaudio.com/adobe/ae699bor.pdf

...and go for a tested and ready-to-make conctruction:
http://www.borbelyaudio.com/pics/305borbely-new.pdf

[SQLGuy]

My PS Audio 200c came stock with 8-pin dip plastic NPD5565 dual JFETs at its inputs. I replaced those with LSK389's as part of some troubleshooting of some noise problems. The JFETs were not, it turned out, the source of the noise, but I still left the LSK389's in place, as they provide better matching and even lower noise than the NPD's.

I'd say low noise and good matching are things to look for in these devices. The LSK389's aren't cheap, but they are relatively easily available (you can buy small quantities from Richardson Electronics), and they do sound good. They also come in metal cans, which I like.

[aldovan]

What types is recommended for voltage amplification ?

Aldovan

[Lumba Ogir]

aldovan ,
BJTs.

[john curl]

The NPD5565 is a pretty good device that we could get cheap. We have used it for decades, as a buffer, first stage differential amplifier and in many other places. It is cheap, rugged, and it used to be made in quantity. It is equivalent to the American J113, which I have used by the thousands.
The 2SK389 is actually mostly a better part: quieter, more Gm, even better matching, BUT it has MORE input capacitance, and this can be a problem with hi Z microphones, etc. There are always tradeoffs.