# Op-amp calculations

Discussion in 'Homework Help' started by Dritech, Oct 10, 2015.

1. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi all,

This is not HW, but I think this is the best section for my question.
I was reading the pdf below, where I cannot full understand what are the steps followed in Q4 a and b.
For Q4a I can see that KCL was used, but what are the steps followed from vi/R1 + (R2/R1 vi) 1/R3 till the end of the derivation?

Also, in Q4a, for which resistor is he referring to when he wrote Ri and what are the steps he followed for this same question please?

http://ece.uprm.edu/~mtoledo/5207/S2013/hw1/solhw1.pdf

Feb 19, 2010
3,506
512
3. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,993
1,116
Have you try to do the math yourself ?
First step is this
vi/R1 + (R2/R1 vi) 1/R3 = vi/R1 + v1*(R2/(R1+R3)) =

But notice that the op amp work as ordinary inverting amplifier and this is why the voltage at R3 is equal to - R2/R1 *vi.
And because we have a "virtual short" her Ri for inverting amplifier is R1.

4. ### RBR1317 Active Member

Nov 13, 2010
267
54
My approach is to always write the node equations because it is difficult to get them wrong. The two nodes here are the virtual ground at the '-' input, and the voltage across R3. Note: my preference is to write the node equations such that current leaving the node is positive, since every term in the node equation will begin with that node's voltage, meaning there is less chance to make a mistake.