# A quick op-amp analysis question

**URL:** <https://boards.straightdope.com/t/a-quick-op-amp-analysis-question/208628>\
**Category:** Factual Questions\
**Created:** [October 21, 2003, 2:40am UTC](https://boards.straightdope.com/t/a-quick-op-amp-analysis-question/208628 "2003-10-21T02:40:02Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Trigonal\_Planar](https://avatars.discourse-cdn.com/v4/letter/t/0ea827/32.png) [@Trigonal\_Planar](https://boards.straightdope.com/u/Trigonal_Planar)\
**Post date:** [October 21, 2003, 2:40am UTC](https://boards.straightdope.com/t/a-quick-op-amp-analysis-question/208628/1 "2003-10-21T02:40:02Z")

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When you’ve got an (ideal) op-amp circuit to analyse, you assume no current flows into the op-amp. Do you also assume no current flows OUT of the op-amp?

Thanks a lot.

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**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\
**Post date:** [October 21, 2003, 2:41am UTC](https://boards.straightdope.com/t/a-quick-op-amp-analysis-question/208628/2 "2003-10-21T02:41:42Z")

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I assume you mean at quiescence? If so, then yes.

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**Author:** ![Trigonal\_Planar](https://avatars.discourse-cdn.com/v4/letter/t/0ea827/32.png) [@Trigonal\_Planar](https://boards.straightdope.com/u/Trigonal_Planar)\
**Post date:** [October 21, 2003, 2:52am UTC](https://boards.straightdope.com/t/a-quick-op-amp-analysis-question/208628/3 "2003-10-21T02:52:52Z")

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Oh, I realize that “ultimately” there will be a sort of output current - whatever comes through the bypass resistor (and/or other connections to Vout). But I’m wondering is if this total current also includes current that comes out of the op-amp output terminal itself.

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**Author:** ![askol](https://avatars.discourse-cdn.com/v4/letter/a/53a042/32.png) [@askol](https://boards.straightdope.com/u/askol)\
**Post date:** [October 21, 2003, 3:16am UTC](https://boards.straightdope.com/t/a-quick-op-amp-analysis-question/208628/4 "2003-10-21T03:16:21Z")

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In my EE courses, we assumed the following regarding ideal op-amps:

Infinite input impedance (no input current)

Infinite gain (with negative feedback, the two inputs are of equal voltage)

Zero output impedance (as much current as we want leaving the output of the op-amp)

Hope that helps.

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**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\
**Post date:** [October 21, 2003, 3:57am UTC](https://boards.straightdope.com/t/a-quick-op-amp-analysis-question/208628/5 "2003-10-21T03:57:53Z")

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Yes, there is current coming out of the output terminal of an op-amp. A circuit that can’t deliver current from its output is not very useful!

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**Author:** ![vasyachkin](https://avatars.discourse-cdn.com/v4/letter/v/c5a1d2/32.png) [@vasyachkin](https://boards.straightdope.com/u/vasyachkin)\
**Post date:** [October 21, 2003, 6:16am UTC](https://boards.straightdope.com/t/a-quick-op-amp-analysis-question/208628/6 "2003-10-21T06:16:07Z")

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what scr4 said. there certainly IS a current coming out of opamp. if you disconnect the opamp from the load, it will still work, so where is the current through the resistor going? into the opamp out!

in fact, an ideal op-amp should be able to deliver infinite current from its output, of course in reality this is impossible.