LM3900 integrator. Help me understand this

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devinw1
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Post by devinw1 » Thu Dec 06, 2018 12:55 pm

LOL, I'm with you. I'm just to stage of understanding regular op amps, but these Norton amps are kind of black magic to me still. :hmm: :hmm:

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Post by guest » Thu Dec 06, 2018 1:50 pm

if it helps, here is how i think of them: they are like regular opamps, but the inputs are currents rather than voltages. so, on a normal opamp no current flows into the input pins, and the output does whatever it has to to keep the two input pins at the same voltage (assuming some form of negative feedback). with the LM3900, the input pins are always at 0.6V, so both are always at the same voltage, and the output tries to keep the input currents equal, rather than the voltages.

I_positive - I_negative = I_feedback

this analogy mostly holds up, except that a regulat opamp can have both negative and positive voltages at its inputs, but the LM3900 is single sided, so it can only source positive currents (for the most part, there are a few edge cases). this means I_positive also can only be positive as well (no exceptions here). I_negative can be both positive or negative, so long as its less than I_negative at its max positive value.

integrators sort of cheat here, as the voltage at the output isnt directly related to the feedback current, as it is with a resistor, so you can get negative currents for a short period of time, until the capacitor completely fills up and the amp hits the rails.
openmusiclabs.com

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devinw1
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Post by devinw1 » Thu Dec 06, 2018 3:50 pm

Thanks guest. That does make a bit more sense!

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