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Old 4th February 2010, 10:26 AM   #1
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Default jfet distortion question

When used in a circuite like this does the drain resistor value have a large
effect on distortion. The reason I ask is I would like to run a 2sk170 in a
circuite like this but at 5ma with a 200ohm drain resistor. I realize the gain
will be a lot less but would the distortion be higher as well?
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Old 4th February 2010, 11:03 AM   #2
AndrewT is offline AndrewT  Scotland
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5mA passing 200r source resistor will give Vgs=1V.
You might find it difficult to source a k170 that passes 5mA @ Vgs=1V.
I think the highest Vgs a BL will hold will be about 0.2mV @ 5mA.
I somehow doubt a V grade will give you what you require.
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Old 4th February 2010, 12:07 PM   #3
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I was asking about the drain resistor not the source resistor. Let me ask it a little differntly. If I have a jfet with the source grounded and only need .01v output
would I have more distortion if I used a 100ohm drain resistor or a 10kohm drain
resistor.
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Old 4th February 2010, 12:45 PM   #4
AndrewT is offline AndrewT  Scotland
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a 200r drain resistor will drop 1V from supply to drain.
This limits your maximum +ve voltage on the output to a bit less than 1V, you might get a maximum of 0.6Vpk just before the signal clips.
That would be an output of ~400mVac with large distortion.

I don't think that is what you want.
That's why I thought, mistakenly, that you meant source resistor = 200r.

If you have the source grounded and the gate referenced to the same ground, then the FET will try to pass Idss. This will cause a voltse drop across the drain resistor. It would be normal to adjust the drain resistor to get a DC output voltage of about half the supply voltage.
Not many designers run their FETs ~ 100% of Idss. They are more usually run between 20% and 90% of Idss.
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Last edited by AndrewT; 4th February 2010 at 12:49 PM.
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Old 4th February 2010, 12:57 PM   #5
wahab is offline wahab  Algeria
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supply voltage must be increased to 15v....
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