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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Passive pre amp - 50k or 100k pot?
Passive pre amp - 50k or 100k pot?
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Old 24th March 2020, 11:31 AM   #11
sgrossklass is offline sgrossklass  Germany
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Quote:
Originally Posted by rsavas View Post
On the bjt IPS example, say with a unipolar input and not complementary with some degree of bias current cancelation, is the base bias current not isolated by the input coupling cap?
DC - yes.
AC - no.

Any dynamic cancellation of input bias current nonlinearity requires matched impedances.

If you look at Samuel Groner's measurements of BJT input opamps, the input bias current related effect shows up as the flat area at lower frequencies in the input impedance distortion graph; higher up, input impedance starts to dominate.
Their JFET colleagues show almost none of that shelf (it is eventually limited by common-mode distortion) but the capacitive part tends to be stronger by about a factor of 3, with very few exceptions like the OPA627 and its bootstrapped cascode input.
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Old 24th March 2020, 11:34 AM   #12
adason is offline adason  United States
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Quote:
Originally Posted by Asterix2k10 View Post
I built a passive pre amp with a 100k ohm pot - should I have used a 50k instead?
25k
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