Discussion in 'Electronics Homework Help' started by kamikaze1993, Jul 8, 2016.

1. ### kamikaze1993

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Mar 25, 2016
Hello I was wondering how the approach is to solve this exercise?

I wonder how to find u+ = 2.5 V?

Any suggestion ?

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2. ### Bluejets

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Oct 5, 2014
No one is going to tell you outright but a tip is...
.............where is the voltage located?
have a good look.

3. ### LvW

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Apr 12, 2014
Before solving it one must know the task.
So - what is the question?

Arouse1973 likes this.
4. ### duke37

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Jan 9, 2011
R5 and R6 are connected in series with a positive voltage at on end and a negative voltage at the other end.
You can find the current from the total voltage and the total resistance.
Then calculate the voltage across each resistor and find U+ relative to one end and then shift the reference to the common.

The values are chosen so the sums can be done in your head.

Harald Kapp likes this.
5. ### Harald KappModeratorModerator

11,662
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Nov 17, 2011
Or use the common equation for a voltage divider.

This is so obvious

6. ### Harald KappModeratorModerator

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Nov 17, 2011
The important thing to consider is the impedance of the Opamp's input in comparison to the resistors' values...

7. ### kamikaze1993

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Mar 25, 2016
When I used the voltage divider law:

(7,5+12,5)*3/20+10 = 2 V

I know this is wrong, could someone explain too me why?

8. ### Herschel Peeler

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Feb 21, 2016
Break it down into simple steps.
Total resistance of R5 and R6.
Total voltage across R5 and R6.
Current through R5 and R6.
Voltage drop across R5. subtract that from +10 V.

9. ### Herschel Peeler

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Feb 21, 2016
Why? It doesn't tell you what the op amp is so there is no way to know. Given an ideal op amp, the impedance of the op amp is of no concern.

10. ### kamikaze1993

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Mar 25, 2016
Thank you for a good answer. Maybe this question is very easy anyhow, how do you find "Total voltage across R5 and R6". Why do you need solve only the voltage drop across R5, why not R5+R6?

Rotal = 7,5+12,5 =20 ohm

11. ### Herschel Peeler

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Feb 21, 2016
See where R5 and R6 go. One side goes to +10 V the other side goes to -10 V, so a total of 20 V across a total of 20,000 ohms. Easy calculation, 1 mA. Your point of interest is 7,500 ohms down from +10 V, at 1 mA that puts it at+2.5V. It works out the same if you use the other resistor.

12. ### Harald KappModeratorModerator

11,662
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Nov 17, 2011
The input ompedance of an ideal opamp is infinity, that's why it is of no concern and that was where I was hinting at.