# Current through op-amp feedback resistor

Image source: Fundamentals of Electric Circuits by Alexander and Sadiku. 5th Edition. Page 182

Can someone explain to me how 26.25uA is the correct answer here?

I am getting 11.25uA. If I assume virtual ground at the inverting input, then apply Ohm's Law on either the 4kOhm resistor or the 280kOhm resistor. I don't even need to apply virtual ground if I assume the output is -3.15V and input is 45mV.

(45mV-0)/(4kOhm) = 11.25uA

(3.15-0)/(280kOhm) = 11.25uA

(3.15+45mV)/(280k+4k) = 11.25uA

• I agree with your figure. Is there more information elsewhere, eg bias current of the opamp that would affect the answer? Commented Apr 30, 2022 at 17:46
• @KevinWhite nope, this is it. It's just a chapter on introducing op-amps and this is one of the practice problems that I tried. Commented Apr 30, 2022 at 17:47
• What book is it? Commented Apr 30, 2022 at 17:50
• @KevinWhite Fundamentals of Electric Circuits by Alexander and Sadiku. 5th Edition. Page 182 Commented Apr 30, 2022 at 17:56
• From the errata, "Practice Problem 5.3 The current through the feedback resistor is 11.25 µA." highered.mheducation.com/sites/0073380571/… Commented Apr 30, 2022 at 18:26

## 1 Answer

You found an error in the book. From the Errata for the 5th Edition:

Practice Problem 5.3 The current through the feedback resistor is 11.25 µA.

PS: I copied this information from GodJihyo's comment as I think it is the correct answer.