# OP AMP

Discussion in 'Homework Help' started by Abdel Alsnayyan, Jun 19, 2015.

1. ### Abdel Alsnayyan Thread Starter New Member

Jun 19, 2015
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Assume 0V-0I. I am having trouble understanding this diagram.
Are R1 and R2 in parallel since they share the same voltage? A closed loop around R1 and R2 has the voltages across the resistors and voltage across the OP amp which is zero, thus one can conclude that the voltage across R1 and R2 are equal. Also R2 and R3 share the same current, correct?

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2. ### #12 Expert

Nov 30, 2010
16,663
7,307
It depends on the point of view. R thinks R1 and R2 are in parallel because they both represent a load to a zero volt point. The op-amp does not see R1 and R2 in parallel because only one of them is delivering current to the amplifier, and, yes, R3 has the same AMOUNT of current as R2.

3. ### WBahn Moderator

Mar 31, 2012
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Really? 1.2 MB for this when 24 KB will do just fine:

Please try to make images a reasonable size. That way you can embed them in the post and people don't have to jump through hoops or download large files to help you.

4. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
#12 answered your questions quite well. The only thing I will add is that this is why we call it a "virtual ground" or a "virtual short". It has some of he behavior of a ground (or a short) but not all of them. In particular, the voltage at that point will be driven to the same voltage as the non-inverting terminal. But no current can flow to this ground or through this short.

5. ### epsilon58 New Member

Jun 19, 2015
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Are you implying that both are correct? Which point of view should I choose and why? Thank you!

Jun 19, 2015
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7. ### #12 Expert

Nov 30, 2010
16,663
7,307
Each answer is correct for one point of view. To be a good designer, you have to consider each point of view, one at a time, in order to insure your circuit will operate in the way you intended. In the course of a real product, you will consider how the surrounding circuitry appears to dozens of nodes in both directions. Get over the idea that there is only one answer.

8. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
One of the most common phrases you will see (and use) is "... as seen by ..."

As seen by the person on the ground, the top of the Empire State building is a long way up. As seen by the person in the airplane flying over, the top of the Empire State building is a long way down.