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

Author

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

Peter

2005-08-15, 11:21 pm

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.

Thanks

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!

daestrom

2005-08-20, 1:21 pm

"Peter" <private@private.com> wrote in message
news:1LmdnX4i9rla25zeRVn-gw@comcast.com...
>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.
>
> Thanks
>
>
>
> 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.
>

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.

daestrom

> Thanks again!

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