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Input impedance of a (generic) IC? LM 339?
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magic:

--- Quote from: duak on March 19, 2020, 11:45:45 pm ---Bringing the diff amp Q2 & Q3 back in, the common emitters are biased to V+ with a 100uA current source.  Depending on the relationship between the input voltages, the base currents of Q2 & Q3 can vary from 0 to 100 uA / HFE.  If the HFE is 100, the bias current to the emitter followers can vary by 1 uA from 3.5 to 4.5 uA.  This will manifest as a variation in the input bias currents but I can't see it with my setup.

--- End quote ---
You see it as the dip in impedance near zero amplitude, when both inputs are at 2.5V and the diffpair transitions from one branch to another and base currents of Q2 and Q3 vary significantly with input voltage. At this point the comparator behaves essentially like an opamp and its input characteristics should be similar.

And yes, I was wrong before. I didn't realize that emitter current is so much higher in Q2,Q3 than in Q1,Q4.

--- Quote from: magic on March 18, 2020, 05:47:29 pm ---Good news is that it should be largely independent of what the other input pin is doing.
--- End quote ---
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