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-   -   Suitable jfet for an zero GNFB amp? (http://www.diyaudio.com/forums/solid-state/133879-suitable-jfet-zero-gnfb-amp.html)

roender 27th November 2008 07:45 AM

Suitable jfet for an zero GNFB amp?
 
Characteristics:

low noise
Yfs > 10mS
zero Ids (Vgs) drift with temperature at any current in the range of 2...8mA

There is such device?

h_a 27th November 2008 08:44 AM

2SK170/2SJ74
2SK369

However there's no data on temperature dependence of drain current in the datasheet.

Have fun, Hannes

EDIT: I just saw your RMI-amp uses the 2SK170; so what is your particular interest, the zero tempco for ID? I don't remember that spec in any jfet datasheet, but I only have the abbreviated pdfs.

john curl 27th November 2008 08:48 AM

I don't think this will have 0 tc Too much transconductance. You need about 0.7V bias voltage to get 0 tc, and a 170 will not get you there easily.

roender 27th November 2008 08:56 AM

Quote:

Originally posted by john curl
I don't think this will have 0 tc Too much transconductance. You need about 0.7V bias voltage to get 0 tc, and a 170 will not get you there easily.
Zero Tc for k170 is at 13...14mA but at this current is has very high transconductance and it not sound good.

bogdan_borko 27th November 2008 09:50 AM

Re: Suitable jfet for an zero GNFB amp?
 
Quote:

Originally posted by roender
Characteristics:

low noise
Yfs > 10mS
zero Ids (Vgs) drift with temperature at any current in the range of 2...8mA

There is such device?

I don`t think you`ll easily find a jfet like that.
btw. I`m working on non-global-feedback amplifier and I`m using 2SK170BL/2SJ74BL in the input stage but with servo incuded.

roender 27th November 2008 10:01 AM

Linear Systems j308/j309/j310 maybe?

homemodder 27th November 2008 11:03 AM

Roender, you have mail, let me know if it works out.

Lumba Ogir 28th November 2008 12:56 AM

Roender,
Quote:

zero Ids (Vgs) drift with temperature at any current in the range of 2...8mA There is such device?
There is not (on this planet).

AndrewT 28th November 2008 10:25 AM

Feucht wrote a paper on temp coef of Jfet.
I seem to recall he said the Vgs should be 800mV above Vpinchoff for closest to temp comp.
Then the CCS is set up to ensure it too has temp comp at the required current.

roender 28th November 2008 11:36 AM

Quote:

Originally posted by AndrewT

I seem to recall he said the Vgs should be 800mV above Vpinchoff for closest to temp comp.

VGS = VP + 0.63V for zero TC
http://www.tkhifi.com/div/Erno_Borbe..._articel_1.pdf


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