[sdiy] 2SK170 FET alternative

[Jay Schwichtenberg]

They have different transfer curves (VGS) so I wouldn't consider them exchangeable.

Here are a couple of good articles on JFETS:
<http://www.borbelyaudio.com/special_articles.asp>

Check out JFETs, The New Frontier Parts 1 and 2.

Jay S.

-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Seb Francis
Sent: Tuesday, December 13, 2005 5:16 PM
To: Synth DIY
Subject: [sdiy] 2SK170 FET alternative


Hi,

I'm building a compressor based on this circuit:
http://www.thatcorp.com/datashts/dn115.pdf

It uses a 2SK170 FET as part of the adjustable attack/release bit in 
between the RMS level detector and VCA.

Neither of my 2 regular shops stock this FET, and while I'm sure I'd be 
able to track one down, I'm wondering if a 2SK30A-Y would work instead. 
They look fairly similar from the datasheet, but unfortunatly my 
experience of FETs is pretty non-existant, so I wonder if some kind 
person could cast a quick glance at these specs and tell me in what 
situations the differences would matter (or not)..

2SK170
Gate cut-off current IGSS = −1 nA (max) (VGS = −30 V)
Drain current IDSS = 2.6 to 20 mA (the FET suffix isn't specified on the 
schematic so I can't be more specific about this)
Gate-source cut-off voltage VGS (OFF) = -0.2 to -1.5V
Forward transfer admittance Yfs = 22mS typ
Input capacitance Ciss = 30pF typ
Reverse transfer capacitance Crss = 6pF typ
Noise figure NF = 0.5dB typ or 1.0dB typ

2SK30A-Y
Gate cut-off current IGSS = −1 nA (max) (VGS = −30 V)
Drain current IDSS = 1.2 to 3 mA (this is for the Y suffix that I have)
Gate-source cut-off voltage VGS (OFF) = -0.4 to -5.0V
Forward transfer admittance Yfs = 1.2mS min
Input capacitance Ciss = 8.2pF typ
Reverse transfer capacitance Crss = 2.6pF typ
Noise figure NF = 0.5dB typ

The max ratings look very similar apart from the drain power dissipation 
which is 4x higher (400mW) for the 2SK170, but I don't think I'm going 
to be running the device anywhere near its max ratings.

Seb