Did my last message go through? If not, here it is again

I didn't see a reply to my last message, so I'll repost it in case it got over looked. If someone replied, can you please repost it because I didn't see it.


Something was really weird with MultiSim, maybe I had too many programs open the night I typed this question causing it to slow my PC down. After the op-amp went to the rail, there was a delay before the RC started rising. That really threw my understanding of an intergrater circuit off.

I tend to get baffled by the "outside" of the circuit and not think about the simpliest thing "the inputs are 'equal'", that explaination you gave made perfect sense.

You never got to the question about putting a cap from the resistor/capacitor junction (the inverting input) to ground. I saw a circuit with this capacitor and wasn't sure how to "add" that into the equation. Per MultiSim, it only changed things VERY little. Keeping in mind I use an ideal op-amp (no offset voltage, bias current, etc..) so when I calculate something and run it in MultiSim, my answers should be just about identical.

The caps aren't in parallel or series because I tried both and my numbers were out in left field.

Thanks again!

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Placing a cap across the two inputs to an 'ideal' opamp doesn't 'do' anything. As long as the opamp output isn't against a 'rail', the inverting input is at 'virtual ground', while the non-inverting input is at actual ground. So the cap never sees any charge current or voltage developed across it.

Once you're against the rail, well then the cap charges as the inverting input moves away from 'virtual ground'.

Now, in a 'real' opamp, it's a different story. The limit on the output slew-rate means that sudden changes in the input signal will cause the inverting input voltage to move *temporarily* from 'virtual ground'. The addition of a cap between the inverting input and ground (or the non-inverting input if you wish, it's the same) means the opamp will not 'see' this offset as quickly as without the cap. So the slew rate of the output is slowed even further.

hope this helps.


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