High Idss jFETs

[AndrewT]

picked three FETs at random

Idss . Vpinch ... Yfs
20.5 .. 0.99 ... 41mS
26.5 .. 1.24 ... 43mS
28.9 .. 1.28 ... 45mS

using Rpinch off =4M7, ~0.2uA
Yfs=2 * Idss / Vpinch

seems like these high Idss lsk170c are measuring up similar to 2sk369V.

 

[h_a]

This kind of explains why k369 is useful for low noise amps and why k170/j74 & k389/j109 are often used for front end amplification duty.

Don't forget that noise does not add up simply - it's added geometrically - so 2x 2SK170 are indeed also noise-wise the same to a single K369 (with same summed currents).

Have fun, Hannes

 

[Mooly]

Noise is ( was) always mentioned as a problem with junction FET's. "Flicker noise" it was called.
Isn't this similar to the debate on opamps and why the old NE5534 etc is capable of lower noise performance than FET types. At the impedances we are talking about for audio the bjt is quiter.
When you parallel individual devices up the noise is reduced by 1/ square root of number of devices. Very much the law of diminishing returns.

 

[jacco vermeulen]

Noise, you said ?
What noise ?

 

[h_a]

Noise is ( was) always mentioned as a problem with junction FET's. "Flicker noise" it was called.

Well, maybe I remember wrong, but Scott Wurcer mentioned once that flicker noise is no longer an issue for jfets (seems a production issue). But don't quote me on that.

Have fun, Hannes