op amp circuit analysis help

Discussion in 'Homework Help' started by notoriusjt2, Oct 18, 2010.

Feb 4, 2010
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im not looking for someone to give me the answer here

but what is the simplest way to analyze this circuit. what steps should i take to solve this problem?

i assume V1=9V and V3(bottom left)=5V

how do i calculate voltage drops across those resistors without a current value?

2. hgmjr Moderator

Jan 28, 2005
9,030
215
The labels on the opamp stages are not clear.

hgmjr

Feb 4, 2010
209
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i think A1 and V1 are top left
A2, V2 top right
A3, V3 bottom left
A4, V4 bottom right

4. Engr Member

Mar 17, 2010
114
5
I think the best way to solve the circuit is to use the nodal analysis. There is no given current in the circuit but the current in the circuit can be solve.

5. hgmjr Moderator

Jan 28, 2005
9,030
215
Start by acquainting yourself with the key characteristics of an "ideal" opamp.

hgmjr

Feb 4, 2010
209
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Ideally Einv = Eni

So that means that the voltage of the + and - of each op amp are equal. So the voltage at V3 = 5v

7. hgmjr Moderator

Jan 28, 2005
9,030
215
Due to the poor image readability, I am not sure which node is labeled V3 on the diagram.

You can begin by determining what voltage is present on the negative input of the opamp in the upper right position.

You then know what the positive input of the opamp in the upper right position should be.

You know what voltage is present at the output of the opamp in the lower left position. From that you should be able to determine the voltage that you need at the output of the opamp in the lower right position.

Once you know the output voltage needed at the output of the opamp in the lower right position you should be able to determine what the output of the opamp in the upper right position needs to be. That will be your answer.

hgmjr

Feb 4, 2010
209
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I stand corrected A3 is in the top right with its output facing to the right

A4 is directly underneath it

Feb 4, 2010
209
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so am i correct in saying that the voltage @ V2 = 5V?

Last edited: Oct 30, 2010

Feb 4, 2010
209
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here is a better version

11. Ron H AAC Fanatic!

Apr 14, 2005
7,049
659
No, it is not 5V.

Feb 4, 2010
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my bad, i meant V2. i edited my last post to reflect that

13. Ron H AAC Fanatic!

Apr 14, 2005
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659
Yes, V2 is 5V. A1 and A2 are voltage followers (unity gain, noninverting).

14. themindflayer Member

Oct 29, 2010
44
1
Here's a very good intro to op-amps ....

As for circuit solving ... following the golden rules in the above file .... V1 and V2 are 9V and 5V respectively ( o/p makes itself so that V diff at i/p is zero, it will be zero when the inv pin has the same volts as the non inv pins ... hence the answer)

A3 inv pin has 4.5V, voltage across 10K A4's FB res is the same as the voltage across the 30K ... which is as you might have guessed V3 (since the junction of A4's 10k and 30k is at 0V due to the non inv being at 0V)

so now it all boils down to what V are at A3 non-inv since if the diff btw A3's i/p is >0 V3 shoots to +ve rail (12V i suppose) if the diff is < 0V V3 goes to the -ve rail (-12V?)

so what is the voltage at the A3's non inv i/p ... that is what i cant figure out either and call on to help ......

15. Ron H AAC Fanatic!

Apr 14, 2005
7,049
659
As you mentioned, the -input of A3 is at 4.5V. Feedback through A4 makes the +input of A3 also equal 4.5V. Since we know V2, we can calculate V4. Once we know V4, we can calculate V3.

16. eblc1388 AAC Fanatic!

Nov 28, 2008
1,543
102
If you are still struggling, follow the steps and fill in the blanks.

• opamp_out.png
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Feb 4, 2010
209
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1. G=0
2. V=9V
3. G=0
4. V=5
5. V=4.5V
6. V=2.5V
7. i=0.5mA
8. V=0V
9. how do i find this one?

18. eblc1388 AAC Fanatic!

Nov 28, 2008
1,543
102
You have many of them wrong.

1. G=0 wrong
2. V=9V
3. G=0 wrong
4. V=5
5. V=4.5V
6. V=2.5V wrong
7. i=0.5mA wrong
8. V=0V wrong
9. how do i find this one?

You need reasoning (not calculation) based on ideal Opamp model for answer to item 6 and 7.

You also have to re-read the gain definition of common opamp configuration. Hint: gain G can be positive or negative and higher or lower than 1.

Last edited: Oct 31, 2010