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Old 27th November 2008, 07:45 AM   #1
roender is offline roender  Romania
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Default 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?
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Old 27th November 2008, 08:44 AM   #2
h_a is offline h_a  Europe
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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.
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Old 27th November 2008, 08:48 AM   #3
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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.
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Old 27th November 2008, 08:56 AM   #4
roender is offline roender  Romania
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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.
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Old 27th November 2008, 09:50 AM   #5
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Default 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.
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Old 27th November 2008, 10:01 AM   #6
roender is offline roender  Romania
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Linear Systems j308/j309/j310 maybe?
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Old 27th November 2008, 11:03 AM   #7
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Roender, you have mail, let me know if it works out.
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Old 28th November 2008, 12:56 AM   #8
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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).
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Old 28th November 2008, 10:25 AM   #9
AndrewT is offline AndrewT  Scotland
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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.
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Old 28th November 2008, 11:36 AM   #10
roender is offline roender  Romania
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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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