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Source follower thermal cycling?
Source follower thermal cycling?
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Old 25th October 2009, 08:16 PM   #1
wildswan is offline wildswan  Ireland
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Default Source follower thermal cycling?

I'm using the simple source follower below as a output driver for a tube preamp, but the output voltage seems to be wandering up and down a few volts (With a load of 50k or so)

Now it seems that this might be caused by thermal cycling (the fets and resistors are a couple of centimeters apart....but it seems very severe for a follower!

So far I've tried lowering the gate-souce resistor to 100k and put in new fets, but nothing changes. I was expecting some offset but the wandering bias is surprising me........Can't seem to find any thermal data on ss jfets.

Very confused

Has anyone had this problem before?
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Old 25th October 2009, 08:37 PM   #2
peranders is offline peranders  Sweden
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Source follower thermal cycling?
R3 should be connected to signal ground.
/Per-Anders (my first name) or P-A as my friends call me
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Old 26th October 2009, 02:39 PM   #3
wildswan is offline wildswan  Ireland
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Thanks Peranders!

Pretty dumb when I think about it :-) Of course any offset will produce offset at the gate too!

Down to about 10mV or so for the record now, using 2sk369
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