# Op Amp problem

Discussion in 'Homework Help' started by DYLH, Oct 28, 2013.

1. ### DYLH Thread Starter New Member

Aug 13, 2013
28
1
Refer to the problem documented here:
https://www.circuitlab.com/circuit/cju6v8/practice-problem-5_3/

Assume idea op-amp.

The textbook is Fundamentals of Electric Circuits by Charles Alexander and Matthew Sadiku.

The textbook answer for the current through the feedback is 26.25uA.

I'm stumpped as to how they get this.

For vo, I get -3.15V just like the text.

By hand, I get 3.15 v / 280,000 kOhm = 11.25uA. This is the same answer I get from Circuit Lab.

2. ### WBahn Moderator

Mar 31, 2012
20,076
5,666
Textbooks have errors in them, just like everything else. I can't tell if this is a problem from the text or a problem from a companion resource. If it's from the text, then one thing you can do is look at the text's errata sheet (available on the publisher's or authors' website, usually) and see if this is a known problem.

As to whether 11.25μA is correct, that depends on the direction assigned to the current through the feedback resistor.

3. ### DYLH Thread Starter New Member

Aug 13, 2013
28
1
You're preaching to the choir. As a licensed engineer and a published author, I am familiar with the human condition . To brush up on electrical theory, I've purchased this text, and am going through it (I never had a strong foundation in circuit theory because the professor of the the course got sick and died when I took it nearly 20 years ago).

There is no errata (or, at best it is very weak... the prior errors I've found haven't been documented).

I was hoping to get some insight into *why* the text made the error... there is usually some mistake, some other concept that may be valid in some other case, that one can learn from.. more so than having the correct answer.. looking at your 'About Me', I'm sure you know that . If you see something, great, if not.. no big deal, and thanks for the reply.

4. ### WBahn Moderator

Mar 31, 2012
20,076
5,666
Sometimes it's possible to make a reasonable guess as to how/why the wrong answer came about. One thing to remember is that the solutions manual is often written by grad students and/or undergrad students that are getting paid to work the problems. Sometimes that gets reflected in the quality of the result.

In this case, it is hard to say. One thing that many authors do from one edition of the text to the next is keep the same problems but change some of the numbers so that people that have copies of old problems or have the old solutions manual can't just blindly copy without it being obvious. By just changing the parameters, it not only keeps the cost associated with updating the problems down, but it keeps the type of problems and the concepts they cover very similar from one edition to the next.

What would the answers to the problem have been if the voltage source as 105mV and the feedback resistor was 120kΩ?

5. ### DYLH Thread Starter New Member

Aug 13, 2013
28
1
good points!

Thanks for the replies!