Estimating the impedance on OP-AMP - how to do it?

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bws

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impedanz on OP-AMP

hey !

if i need to know the impendanz for a voltage-follower configuratet OP-AMP, for example a LM324 or LM258, how do i do that ?? is it not somthing that is normally given in the Datasheet ?? if not not, how do i find it ??

hope some can help
 

impedanz on OP-AMP

I understand, that you mean the closed-loop output impedance. For the said OPs, it's not given in the datasheet. One possible reason is, that with their class-B output stage, the impedance isn't only frequency dependant (that's normal) but also strongly depending on the output current unless you force the OP into class A operation by an external load resistor.

Generally, for a standard OP, with a more or less defined open loop output impedance, you can estimate the closed loop impedance by applying a 1/G factor, where G is the loop gain. Because the loop gain is decreasing with frequency, you get an frequency increasing open loop impedance. It can be represented by an inductance with a series resistance in a wide frequency range.
 

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