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Old 16th May 2012, 04:55 PM   #1
hags is offline hags  United States
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Default complementary jfet input trimmer

What is the purpose of P1 in this input stage other than for dc offset trimming?


Can I assume that when used with matched jfets, precision resistors and a dc servo in following stages that this trimmer is not necessary?



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Old 16th May 2012, 05:25 PM   #2
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It adjust both, dynamic and static symmetry simultaneously. Not pretty good.
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Old 16th May 2012, 05:33 PM   #3
hags is offline hags  United States
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I've seen this used in some Borbely designs.

Is this supposed to cancel out small inbalances that affect more than just static/DC operation?
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Old 16th May 2012, 05:51 PM   #4
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I donīt believe. Also, I donīt like such an adjustments, both dynamic and static simultaneously. In my taste, both must be separate.
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Old 16th May 2012, 06:31 PM   #5
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Because Vgs of the N-ch is different than the P-ch at the same conductance, the pot trims DC output. It would have a small effect on gain symmetry, but the AC current change of the input J-fets will be small, that current change being dependent on what the next stage requires. If the DC voltage on the gates is 0V, the DC output would be the difference between Vgs of the N-ch and P-ch J-fets times Av. Also if the J-fets are not bias at their zero temp co, the DC output may drift a bit with temperature since the change in conductance vs Vgs is unlikely to be equal between N-ch and P-ch devices. A DC servo would perform the same function and not have a pot wiper contact making up the FB loop and cancel any effects of thermal drift. Or you could match 'complementary' J-fets, but good luck with that.
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Last edited by CBS240; 16th May 2012 at 06:34 PM.
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Old 16th May 2012, 09:21 PM   #6
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The pot either nulls 2'd harmonic, OR corrects for DC offset. Usually there is a different position for each.
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Old 16th May 2012, 09:34 PM   #7
hags is offline hags  United States
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Quote:
Originally Posted by john curl View Post
The pot either nulls 2'd harmonic, OR corrects for DC offset. Usually there is a different position for each.
John, thanks.

Would you say I could do without it with the above conditions? I don't see pots used with complementary differential input feedback circuits.
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Old 16th May 2012, 09:34 PM   #8
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Sure, but you will have SOME DC offset.
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