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enter image description here

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

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  • \$\begingroup\$ I agree with your figure. Is there more information elsewhere, eg bias current of the opamp that would affect the answer? \$\endgroup\$ Commented Apr 30, 2022 at 17:46
  • \$\begingroup\$ @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. \$\endgroup\$ Commented Apr 30, 2022 at 17:47
  • \$\begingroup\$ What book is it? \$\endgroup\$ Commented Apr 30, 2022 at 17:50
  • \$\begingroup\$ @KevinWhite Fundamentals of Electric Circuits by Alexander and Sadiku. 5th Edition. Page 182 \$\endgroup\$ Commented Apr 30, 2022 at 17:56
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    \$\begingroup\$ From the errata, "Practice Problem 5.3 The current through the feedback resistor is 11.25 µA." highered.mheducation.com/sites/0073380571/… \$\endgroup\$
    – GodJihyo
    Commented Apr 30, 2022 at 18:26

1 Answer 1

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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.

Link: https://highered.mheducation.com/sites/0073380571/information_center_view0/errata.html

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

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