# Op Amp Equations:Impedance, input/output current & Output impedance

Discussion in 'Homework Help' started by Wizzkid2, Sep 7, 2016.

1. ### Wizzkid2 Thread Starter New Member

Sep 7, 2016
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0
I'm an electronics apprentice, I have an assignment due on op amp. I'm stuck on 2 questions I just can't wrapped my head around them, Can anyone help me solve the questions?!?!? I've attempted the second question but I don't know if it's correct.

2. ### WBahn Moderator

Mar 31, 2012
17,777
4,804
I don't see how you can answer the first question without more information.

Is Vin the voltage of the source, or is it the voltage across the input of the opamp?

It's very helpful if your diagram clearly shows what ALL of the terms in your work mean. Don't make us guess. Engineering is NOT about guessing.

Are you given the open loop gain of the opamp, by chance?

3. ### Wizzkid2 Thread Starter New Member

Sep 7, 2016
2
0
No I wasn't given the open loop gain, this why I'm having a lot of trouble with it.

4. ### WBahn Moderator

Mar 31, 2012
17,777
4,804
Is it the same opamp in both problems?

5. ### MrAl Well-Known Member

Jun 17, 2014
2,440
492
Hello,

It looks like it is possible to solve these depending on what you need to find. In the second slide for example the key to solving it is to recognize that the voltage just to the left of the Zout 'impedance' element is CONSTANT with load, while the output voltage VARIES with load. Thus, you can solve Zout (because variables cancel) and then you can solve for Zin in terms of constants and A. This means Zout will be a number but Zin will be a formula with A in it, the internal gain.
The problem with getting Zin to a single number is that Zin and A are not linearly independent. If you raise one you can lower the other and get the same results. Zin can be made into a formula though so once A is given it can be calculated out to a single number.

Unfortunately i cant read all the text either because it is hand written and the author did not realize that technical papers need to be written very clearly. There are tiny tiny numbers in various places that makes it take MUCH more time to read then should ever be necessary.
For example, what is inside that tiny circle that looks like the internal voltage source. IS that showing the gain or what?

6. ### WBahn Moderator

Mar 31, 2012
17,777
4,804
The tiny circle almost certainly is the model for the ideal voltage source within the opamp. The symbol inside it is just a V. The bottom of it should be connected to the circuit common, but it isn't (and that is just some sloppiness, nothing more).

I also have no clue what the tiny numbers are in the equations. I can't see anything obvious that would make them make sense. Perhaps node numbers from some other diagram?

He uses V1 and V2 in the equations but gives no hint as to which voltages these refer to.

I'm not going to waste any more time on it until the TS provides the information that has been requested -- if they don't care enough to do so, then there's no reason to.