# Are op-amps always assumed to be ideal?

Given a circuit with resistors, op-amps, independent voltage and current sources, how do I solve the node voltages? Do I always assume that op-amps are ideal?

• If it is for a real world application then you must include real-world effects or your results will be wrong. If it is for an academic exercise then you need to ask the person who gave you the question.
– Jon
May 9, 2015 at 14:40
• This could be made a better question if you put a little bit of effort into it. Explain all your doubts and why you think opamps should be always treated as ideal and give a little more context. Note: I'm not the downvoter, but improving the question could make him reverse his vote. And this could be made into a nice beginner-level question if reworded and given some care. May 9, 2015 at 18:24
• Okay, I'll do more research about this. I'm still learning op-amps so all the terms sound foreign to me. Thanks for the comments/suggestions anyway. May 10, 2015 at 2:02
• I think you should have posted a new question, as you've invalidated the answer given by @SpehroPefhany. please consider reverting the edit and posting a new question.
– JRE
May 13, 2015 at 9:59
• I have rolled back the edit -- please post your new question separately. May 13, 2015 at 10:42

No, depending on what you are trying to do you might have to use a more complex op-amp model with finite gain, input offset voltage, input bias current, etc.

If it is a real problem you can apply experience and judgment to decide what can be left out. For set problems the rule usually is that you should use all the information given. If nobody mentions offset voltage, assume it is zero. There are exceptions- sometimes they'll throw in an irrelevant number (find DC voltages and they tell you the Gain-bandwidth product, for example), and sometimes you might be expected to parametrize the unknown variable.

• Then is it okay if offset voltage is assumed to be zero? By the way, I'm still new to op-amps. May 9, 2015 at 14:49
• More context is necessary to answer that question.. if it's a textbook problem and offset voltage is mentioned nowhere, then probably yes. May 9, 2015 at 15:22
• For most applications the real and finite offset voltage is of less importance. The most important real parameters to be considered are (a) finite and frequency-dependent gain (corresponding phase shift!!) and (b) finite slew rate.
– LvW
May 9, 2015 at 16:03
• @LvW What is important is pretty domain-specific. I did many years of precision instrumentation in which Vos, TCVos and gain (sometimes Ib) were about all we cared about. For a current project, it's mostly the noise parameters (no current monolithic op-amp is good enough). I dare say every one of the numbers in the data sheet are important in some particular context. May 9, 2015 at 16:23

There are 2 books/documents on opamps and electronics that would serve you well as a student, and will give you answers and approaches to your question:

• Op Amps for Everyone, 2ed., Ron Mancini (Texas Instruments), 2003, Newnes, ISBN 9780750677011. This book is also available as a downloadable document SLOD006A from the Texas Instruments website - check for updates. The hardcopy book may be in its 3rd edition.

• The Art of Electronics, 2ed., Paul Horowitz & Winfield Hill, 1989, Cambridge Uni Press. Available for webviewing on Paul Horowitz's website. Very good for circuit analysis and analogue electronics, a bit dated for microprocessors, but still a great book to learn from. Aka The Silver Bible.

• Get the 3rd edition of AoE! Very much expanded and updated, with a lot of original research. May 10, 2015 at 3:23
• @Sphero, thanks for the heads up. Didn't know it was out. It had a very long gestation period. May 10, 2015 at 3:31