Inverting Non Ideal Op Amp

1. Jul 22, 2015

GRW

[ mod note: thread title corrected ]

Hi All,

My first post in the forums, hopefully I'll be able to give out as much help as I receive. I'm currently stuck on an Op Amp gain derivation.

Attached is the question and diagram, the derivation I am struggling with is part B.

I understand the gain for this Op Amp were it ideal would be 1+ Rf/R1 but I cannot seem to derive the equation if the input resistance is not ideal.

George

Last edited by a moderator: Jul 22, 2015
2. Jul 22, 2015

Staff: Mentor

Hi GRW.

You won't be able to derive the equation in (b) until you have completed part (a), drawing the equivalent circuit with ideal components.

3. Jul 22, 2015

GRW

I have already completed part A but I can't see how is it that relevant to part B? I know the equivalent circuit and that Rin is in the Op Amp internally and that A is the Op Amp gain.

Any further guidance on how the two relate / where to start with part B would be much appreciated.

4. Jul 22, 2015

Staff: Mentor

Can you attach your completed circuit diagram for (a), with RI and RF arranged around it as in Fig Q1.

5. Jul 22, 2015

GRW

Sorry but I don't have any way of uploading my paper working to a computer. It is as in Fig 1, with Rin linked between the + and - terminals of the Op Amp.

6. Jul 22, 2015

Staff: Mentor

That is the circuit you analyze, but this time without a reliance on the dodge of a "virtual earth".

7. Jul 22, 2015

GRW

It's the circuit analysis for the derivation here that I am struggling with. Sorry if my initial explanation was poor.

8. Jul 22, 2015

Staff: Mentor

You draw a large circuit diagram. Mark vin. Label the voltage at the input to the amplifier proper, at the junction of the 3 resistances, I called it va.

How will you label the output of the amplifier?

9. Jul 22, 2015

LvW

Question to you: The input voltage is connected (via a resistor) to the inverting terminal of the opamp. Wouldnt you expect a negative gain in this case?
(Your ideal gain value (1+Rf/R1) is not correct!)
You have stated: "I understand that the gain ....".
Did you really "understand"? I dont hope that you understood a false result! That would be fatal!.

Last edited: Jul 22, 2015
10. Jul 23, 2015

Zondrina

a) This you must draw on your own. If you could somehow take a picture and upload it, we could verify if it is correct. What do you think the ideal values for $A$, $R_{in}$, and $R_{out}$ are? Hint: Think about preserving the signal. Do you need a large input impedance or a small one?

b) For the closed loop gain, what equation is relevant to op-amp operation? Use this equation, and the virtual ground between $R_1$ and $R_F$.

11. Jul 23, 2015

GRW

Thanks for all your help guys, after a decent sleep last night I woke up this morning and after re reading the suggestions I was able to derive it no trouble. Thanks again and I look forward to being part of the forum!
GRW

12. Jul 24, 2015

LvW

GRW, to be honest - did you derive the correct result really without any help?