# Operational Amplifier

Discussion in 'Homework Help' started by Juss, Nov 24, 2013.

1. ### Juss Thread Starter New Member

Nov 24, 2013
14
0
I am really struggling with this topic...

(a) is it infinite input resistance in this so the I in = 0?
(b) is that "differential inputs stick together"? but I think that Vp and Vn are not an input.
(c) I have no idea how to deal with the resistors below 16k (4k & 6k). can I treat them as series and parallel to sum them up?
(d,e) can someone show me the steps and answers with some explanations please?

Many thanks!!!

2. ### WBahn Moderator

Mar 31, 2012
23,838
7,368
(a) Correct.

(b) If Vp and Vn are not inputs (to the opamp), what would you say are? Remember, the question isn't asking anything about Vp or Vn individually, it is only asking about the difference between Vp and Vn.

(c) Can you treat them as "series AND parallel"? What do you mean by that?

(d,e) How many nodes do you have in the circuit? Can you write equations relating them to each other?

Make and post your best attempt and we can proceed from there.

3. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,519
717
For d you will probably want to redraw the circuit using hybrid pi model of op-amp. Should be in your textbook, somewhere.

4. ### Juss Thread Starter New Member

Nov 24, 2013
14
0
(b) so if it is then Vp and Vn are the same and subtracting them would be zero?

(c) sorry I mean series or parallel. if I do
(16*4)/(16+20) = 3.2 k ohms
3.2+6 = 9.2 k ohms
would that be correct?

if the above is correct then using nodal analysis

(Vn-Vx)/4 + (Vn-Vout)/9.2 = 0
then I can get the Vout?

(e) really have no clue where to start from, is there any formula for the saturation?

thanks a lot for helping

5. ### Juss Thread Starter New Member

Nov 24, 2013
14
0
never seen that before, I think thats not covered in my course.

6. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,519
717
c is asking you to express Vp in terms of Vout.
Do you see any Vp in what you wrote? Do you see any Vout in what you wrote?

I don't.

7. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,519
717
What textbook are you using?

8. ### Juss Thread Starter New Member

Nov 24, 2013
14
0
I am assuming Vn and Vp are the same but Im not sure about that.
is that should be
(Vp-Vx)/4 + (Vp-Vout)/9.2 = 0
?

We just use the notes that the lecturer issued.

9. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,519
717
(c) Use voltage division to express Vp as a fraction of Vout.

The Vn is not involved. The Vx is not involved.

Vout is voltage from op-amp output to ground. 4k and 6k are resistors from op-amp output to ground. Draw this circuit. Mark Vp on the circuit. Express Vp in terms of Vout.

10. ### WBahn Moderator

Mar 31, 2012
23,838
7,368
You don't need to assume anything about Vn or Vx.

You have a voltage at the output, namely Vout.

You have two resistors, a 4kΩ and a 6kΩ, connecting Vout to ground.

There is a connection from the junction of the two resistors that goes to the Vp input of the opamp.

Q1) What is the current going into the Vp input of the opamp?

Q2) Given the answer to Q1, what is the current in the two resistors?

Q3) Given the answer to Q2, what is the voltage at the Vp input of the opamp?

11. ### Juss Thread Starter New Member

Nov 24, 2013
14
0
(Vout - Vp)/4 + (Vout - 0)/10 = 0
is this correct?

is it 0 if it is connecting to the ground?

12. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,519
717
Draw the circuit and post it.

Nov 24, 2013
14
0

14. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,692
1,321
What are you talking about? Why do we need any "model" to find Vout for Vin = 6V ?

15. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,519
717
Wrong.

The correct circuit is this:

Where does the Vp go?

16. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,519
717
Well, for one thing I assumed they had a proper textbook. You have probably noticed that the particular assumption has bitten me on my ass.

At this point I will just help with stuff I know.

17. ### Juss Thread Starter New Member

Nov 24, 2013
14
0
is it at the middle of the resistors?

18. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,519
717
In the resistive network like this one, you would say Voltage X is across Resistor Y.

So. Where is Vp?

19. ### Juss Thread Starter New Member

Nov 24, 2013
14
0
Vp is across 4k resistor?

is it Vp = (Vout*4)/(4+6)

20. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,519
717
Seriously?

Look at the pic in your problem. Work from the output of the op-amp toward the ground.

At the output of the op-amp we have 4k resistor, there will be some voltage across the 4k resistor. Is it Vp?

From the 4k resistor we have another resistor, the 6k. There will be some voltage drop across the 6k resistor. Is it Vp?