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?
Can I assume that when used with matched jfets, precision resistors and a dc servo in following stages that this trimmer is not necessary?
An externally hosted image should be here but it was not working when we last tested it.
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.
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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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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