Engineering Op Amp Circuit Help: Solving KCL Eq with Source Transform

[wclawson]

It was much easier scanning the pictures and problem than trying to recreate:

I began by doing a source transform, making Vs = is*Rs, and placing Rs in series instead of parallel. I then did KCL at Vn.

I tried to break the KCL eq down to il/is, but just ended up with some un-godly mess that ended up approximating to 0 for part b. Is the source transform throwing me off? Or is there just a much easier way to solve this than what I tried.

Thanks a lot.

 

[berkeman]

It was much easier scanning the pictures and problem than trying to recreate:

I began by doing a source transform, making Vs = is*Rs, and placing Rs in series instead of parallel. I then did KCL at Vn.

I tried to break the KCL eq down to il/is, but just ended up with some un-godly mess that ended up approximating to 0 for part b. Is the source transform throwing me off? Or is there just a much easier way to solve this than what I tried.

Thanks a lot.

Since they want you to calculatet Il/Is, I'd probably leave the source un-transformed. Write the 2 KCL equations (one for each input on the opamp), and see how it shakes out. And yes, the approximation when the gain is infinite helps simplify it a lot.

 

[danhamilton]

I happen to be working on this same problem right now (University of rochester, ECE 111?)

Wouldn't the currents of each input always equal zero?

 

[berkeman]

I happen to be working on this same problem right now (University of rochester, ECE 111?)

Wouldn't the currents of each input always equal zero?

For an ideal opamp, yes. But Figure 4-4 shows some of the more practical (non-ideal) model bits...

 

[wclawson]

I happen to be working on this same problem right now (University of rochester, ECE 111?)

Wouldn't the currents of each input always equal zero?

Haha yes, ECE 111 at U of R.

Thank for the help, I've got it now.