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posted by  ieee on 10/29/2009 7:42:10 PM  |  status: Live  |  Earned Karma: 25

confused about polarity

Course Textbook Chapter Problem Needs by
Analog Circuits Fundamentals of Electric Circuits (4th) by Alexander, Sadiku 5 1PP 11/2/2009 at 11:00:00 PM
Question Details:
I have a question about the solution from cramster on this problem.  You don't need to look in the book because the solution shows the original circuit.  Vs is plugged into the positive side in the original circuit.  Why was vs plugged into the negative side in the circuit that was redrawn?

vd is supposed to be:
(the noninverting (positive polarity) amp) minus (the inverting (negative polarity) amp)

But for the solution in cramster, it said vd was
inverting minus noninverting

I'm confused about this.  Someone please explain.
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Novice
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posted by ieee on 10/31/2009 7:24:02 PM  |  status: Live
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posted by Mike Hawk on 11/2/2009 7:41:36 PM  |  status: Live
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First off, your right it does say Vs, however instead it should be Vo, so don't worry too much about that its just a spelling error.

Secondly, if you look at the picture redrawn on Step 2, we see that V1 is at the positive polarity, and that Vs is at the negative polarity. Thus in step 4 they state that Vd = V1 (noninverting) - Vs (inverting).

I think it is a matter of knowing how to redraw the circuit with the inside of the OpAmp exposed.

Hope you understand
cheers mate ;)
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posted by ieee on 11/3/2009 11:35:17 AM  |  status: Live
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But I still do not understand how they completely changed the polarity of the circuit from the original.
In the original circuit, vd is (vs - v1).  How is it possible that they just changed the polarity of the circuit and said vd is now (v1 - vs).
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posted by ieee on 11/7/2009 2:56:42 AM  |  status: Live
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