# AOp

Discussion in 'Homework Help' started by omayma, Dec 2, 2014.

1. ### omayma Thread Starter New Member

Dec 2, 2014
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The circuit is a current voltage converter. 1. Establish the gain expression G= vout / iin : a) by considering the op amp ideal (has infinite open-loop gain A0), the gain is then written G0; b) by considering the op amp real (has very high -but finite- open-loop gain A0): give G as a function of G0 and A0. 2. Calculate the input resistance Rin and the output resistance Rout of the real converter.

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2. ### crutschow Expert

Mar 14, 2008
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We don't do your homework here for you but someone will try to help you if you show what you have already done in trying to answer the question.

3. ### omayma Thread Starter New Member

Dec 2, 2014
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i need help and to check if what i did is correct or not

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4. ### WBahn Moderator

Mar 31, 2012
20,267
5,762
Your work is hard to follow. You have three parts to the problem and three attached images titled 1, 2, and 3. This would seem to imply that each is the attempt to answer the corresponding part of the problem. Yet this obviously isn't the case. Don't make it hard for the people that you are asking free help from to follow what you are doing. A corollary to this is that it's not a good idea to make it hard for the person grading your work to follow what you are doing. Presenting your work in a clear and coherent way will help prevent you from making mistakes and make it far more likely that you will get maximum credit for it when it is graded.

Whenever you work a problem you should always do two things: Always, always, ALWAYS track your units; Always, always, ALWAYS ask if the answer makes sense.

In your solution in 1.jpg you have

$
G \; = \; G_0 \; + \; A_0
$

Does this make sense? You know that G0 is the gain that you get when A0 goes to infinity. Does your expression for G above match this behavior?

What are the units of G? Its a voltage over a current, so it has units of resistance (and why this is called a transimpedance amplifier). What are the units of Go? You got that Go is -R, so it has units of resistance as well. But what are the units of Ao? They are volts/volt and so it is dimensionless. Can you add a dimensionless value to a resistance? No. So you KNOW this answer cannot be correct.

5. ### omayma Thread Starter New Member

Dec 2, 2014
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1)Vout= -R.in => G=Vout/In = -R
2) Vout-VR-Vin=0 => Vout= VR-Vin => (Vout/in)= (VR/in)+(Vin/in) => G=G0+A0

6. ### WBahn Moderator

Mar 31, 2012
20,267
5,762
Again, does this make sense? What is G as A0 goes to infinity?

You appear to be saying that Vin/Iin is A0. Is that what Ao is?

Again, do the units work out?

What is the definition of Ao?