# Ideal Op Amp

Discussion in 'Homework Help' started by cheypr, Dec 8, 2009.

1. ### cheypr Thread Starter Active Member

Oct 13, 2009
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Greetings!!!

I was doing my homework and I'm not sure about this problem. Is it OK or not? If not, what I'm doing wrong?

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2. ### steveb Senior Member

Jul 3, 2008
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I didn't check the entire thing, but I stopped when I got to the point that you said Vo=3V+. This is not correct. The negative feedback path from output to the negative terminal shows a feedback of Vo=2V-, and you are assuming that V+=V-.

3. ### cheypr Thread Starter Active Member

Oct 13, 2009
34
0
in my notes i have that in an ideal op amp V+=V-. That's why I'm assuming that 3V-=3V+.

From my KCL@V- i get that V0=2V- + V-. So, Vo=3V-. What I'm doing wrong?

Am I taking the right approach?

4. ### t06afre AAC Fanatic!

May 11, 2009
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Then it comes to feedback system it is correct to assume that V+=V- But I did not quite understand why you set Vo=3V
Do you have the correct answer?

5. ### cheypr Thread Starter Active Member

Oct 13, 2009
34
0
No, unfortunately I dont have it. I'm getting Vo=3V from the KCL@V-.

6. ### t06afre AAC Fanatic!

May 11, 2009
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As I see V-=Vo*50/(50+100)

7. ### t06afre AAC Fanatic!

May 11, 2009
5,936
1,227
Also use superpostion to find the V+ that is simpler

8. ### steveb Senior Member

Jul 3, 2008
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469
I'm sorry about my mistake above. I was running out the door to go to work, and didn't think carefully.

Basically, you did the problem right.

You have Vo=3*V+ and Vs=-(4/5)*V+

Your mistake is your last step. If you combine these you should get.

Vo/Vs=-15/4