# Op-amp circuit analysis 2

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1. Mar 10, 2015

### ichabodgrant

Hi. I have moved on to the next question. There is something I am confused.

The question wants us to express Rf in terms of R such that vo = -15vi.
After doing the last task, I think I understand the flow of solving it. But here still exists a problem.

My attempts and question are listed in the pic.

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2. Mar 10, 2015

### Staff: Mentor

Why do you say voltage at C = voltage at D? It doesn't.

The signal at C is an amplifed non-inverted version of vi.

3. Mar 10, 2015

### ichabodgrant

Sorry.. I want to say V at E = V at D
a typo

4. Mar 10, 2015

### Staff: Mentor

Analysis of the second amplifier stage hinges on this relationship:
current from A to D + current from C to D = -(current from F to D)

5. Mar 10, 2015

### ichabodgrant

So are node D and E having same potential?

6. Mar 10, 2015

### Staff: Mentor

Yes. You have two labels on the one node. Be consistent, choose one label to save getting confused.

7. Mar 10, 2015

### LvW

Of course, you can start again at "Adam and Eve" (current-voltage relationships, KVL and KCL) - or you can make use of gain formulas which were derived from all this basic stuff:
* Non-inverting gain (1+RA/RB)
* Inverting gain (-RA/RB),
* Two inputs: Superposition theorem.

Following this approach, you immediately can write down the final formula with no calculations at all.
(If you have two resistors in series - connected to a voltage source - and if the voltage across the grounded resistor is to be found. Do you start with voltagew-current relations or do you start with the voltage divider rule?)

8. Mar 10, 2015

### ichabodgrant

I think I was taught the superposition theorem...
I just simply get confused at the line joining node D and E...
As no current flow through these two nodes (or just 1 node as their potentials are the same), so current from C to D means from C to E?

9. Mar 10, 2015

### LvW

Yes - of course. D and E form one single (common) node.

OK - you know the superposition rule. Do you also know the mentioned gain formulas? (I have used both formulas in an answer to your first post).
If not, I recommend to derive it by yourself for (a) a simple non-inv. amplifier and (b) for a simple inverting amplifier (both for ideal opamps).
Then - for future calculations (like this one) - you always can make use of these formuulas without the necessity to start always again at zero (KVL, KCL).

10. Mar 10, 2015

### ichabodgrant

I know all these...
the non-inverting and inverting op-amp... I simply can't distinguish them easily...so I prefer deriving the equations every time...

11. Mar 10, 2015

### Staff: Mentor

That is to be encouraged, so that you can accommodate arrangements which do not neatly fall within those memorized.

12. Mar 10, 2015

### donpacino

In addition the inverting stage does not fall under a typical inverting stage (Av=Rf/Rin)

13. Mar 10, 2015

### LvW

It is an inverting stage with two inputs. Indeed, one of the classical opamp applications.

14. Mar 10, 2015

### donpacino

you said it right there. two inputs. The system must be evaluated as such.

in general taking shortcuts is all well and good but it leads to confusion if you do not understand the underlying concepts. if you try to use them when you reach harder problems, where they are no longer valid. a much better way to do this problem for understanding is nodal analysis. 2 equations and very quick to do

15. Mar 10, 2015

### donpacino

I just saw part of your attempt that was not answered. op amps can supply and recive power. The current will come from the op amp output. When you are dealing with ideal components, you can almost think of the op amp output as an ideal dependent voltage source.

16. Mar 10, 2015

### psparky

The voltage at D and E are the same.....0 volts. D and E are same node and since V+=V-........which is grounded, voltage must be zero.

I give a small warning about using the basic inverted and non inverted basic formulas when you are learning op amps.

The reasons are this....the op amps your teacher gives you on the test are never simple enough to actually use these. Also, if your amp isn't exactly set up just right, these formulas don't work. (-RF/RA) and (1 + RF/RA).

Stick with nodal analysis on op amps and you will NEVER go wrong.
Once you get used to nodal analysis, you can see that you can easily derive the basic formulas.

That being said....learn the nodal first, then lean back on the basic formulas.
If I were teaching a course on this, I would insist on this method for your success.