# Exam Help, Op-Amps, Question 1/5

#### ats314

Joined May 19, 2014
22
(I changed it to ask only one question at a time) Hi everyone. At my school we are allowed to do test corrections for the midterm. The following is the five questions on my exam along with my solutions. We have to have all solutions perfect to get credit and we are allowed to get help. Could you please check my work and let me know if I have made any mistakes?

Problem 1: Solution 1:  Last edited:

#### ats314

Joined May 19, 2014
22
If there is a better way for me to format, like asking only one question at a time, or linking images instead of posting the whole image in the thread, please let me know and I will correct that.

#### #12

Joined Nov 30, 2010
18,223
I don't see anything in the photos that resembles the op-amp circuits.

#### ats314

Joined May 19, 2014
22
oh, geez, posted the wrong images, let me correct, thank you.

Ok, correct images now.

#### WBahn

Joined Mar 31, 2012
26,398
Look at the final opamp plus the three resistors connected to it. What classic opamp circuit do they implement?

#### ats314

Joined May 19, 2014
22
Look at the final opamp plus the three resistors connected to it. What classic opamp circuit do they implement?
are you referring to a summing amplifier?

#### WBahn

Joined Mar 31, 2012
26,398
are you referring to a summing amplifier?
Yep.

So write the equations that govern the behavior of each of the three circuits (include the limits) and then combine them into a single equation for the overall circuit.

#### ats314

Joined May 19, 2014
22
So write the equations that govern the behavior of each of the three circuits (include the limits) and then combine them into a single equation for the overall circuit.
I'm sorry, I don't quite understand what you mean. Can you tell me which part, A,B,C,D is incorrect that you are seeing and maybe I will understand better?

#### ats314

Joined May 19, 2014
22
ok, so I think maybe I understand. Using what I think are the correct values for V03. Does this look correct now? Thank you.   Last edited:

#### Jony130

Joined Feb 17, 2009
5,244
But how can voltage at Vo2 reach -18V ? If Op amp negative supply voltage is -15V?

#### atferrari

Joined Jan 6, 2004
4,444
All of them look inverting amps to me.

Edit / No1 is non inverting - Ooops /Edit

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#### Jony130

Joined Feb 17, 2009
5,244

#### atferrari

Joined Jan 6, 2004
4,444
I disagree with you.
And now, I agree with you! #### ats314

Joined May 19, 2014
22
thank you, I will rework it with these suggestions.

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#### Jony130

Joined Feb 17, 2009
5,244
Hmm, I show you the correct answer

Vo1 = 3V * (1 + 20K/10k) = 9V
Vo2 = 4.5V * -(40k/10k) = -18V but because negative supply voltage is -15V the Vo2 voltage cannot be lower then -15V. So this mean that op amp is in negative saturation and Vo2 = -15V.
Vo3 = 9V * -(60k/20k) + (-15V)*-(60k/30k) = 3V

Iout = Vo3/60K = 50μA

And now if we change Vin1 = 4.5V into 0V Vo3:

Vo3 = 9V * -(60k/20k) = - 27Vand again we have a op amp in saturation, but this time IC3 is in saturation, so Vo3 = -15V

#### ats314

Joined May 19, 2014
22
Hmm, I show you the correct answer

Vo1 = 3V * (1 + 20K/10k) = 9V
Vo2 = 4.5V * -(40k/10k) = -18V but because negative supply voltage is -15V the Vo2 voltage cannot be lower then -15V. So this mean that op amp is in negative saturation and Vo2 = -15V.
Vo3 = 9V * -(60k/20k) + (-15V)*-(60k/30k) = 3V

Iout = Vo3/60K = 50μA

And now if we change Vin1 = 4.5V into 0V Vo3:

Vo3 = 9V * -(60k/20k) = - 27Vand again we have a op amp in saturation, but this time IC3 is in saturation, so Vo3 = -15V
Thank you very much. This is what my final values are as well. Do you have a suggestion for part D? Some people are suggesting that Op-amp 3 is inverting, and some are suggesting summing. I was going to go with summing.

#### ats314

Joined May 19, 2014
22
But how can voltage at Vo2 reach -18V ? If Op amp negative supply voltage is -15V?
good catch, thank you, I missed that my second time through the problem.

#### Jony130

Joined Feb 17, 2009
5,244
As for the part D
1 - non inverting
2 - inverting
3 - summing

#### WBahn

Joined Mar 31, 2012
26,398
I've shown this problem to probably ten different people and seen six or so different answers now. I'm more confused then before I started. I guess it makes sense over 80% of people missed this one on the exam.
So check the answers you are given and see which ones are wrong and why they are wrong. You will likely learn quite a bit in the process.

Break the problem down into modular pieces.

You have two input signals and three opamp circuits. The two inputs are V1 and V2. As shown, V1=3V and V2=4.5V.

The first circuit, OA1, acts on V1 only: The second circuit, OA2, acts on V2 only: And the third, OA3, acts on the outputs of the first two: These three circuits are connected as follows: Take each circuit in turn and forget about the others. What type of circuit is it? What is the output of the circuit in terms of the input(s)? What are the limits?

Then combine these results to yield the overall circuit equation. At this point you have a nice generic solution for any set of input voltages. Now you can just plug in the voltages needed for a given part of the question and get your answer.

A good exercise would be to plot the active region of operation on a 2-D graph with V1 on one axis and V2 on the other axis. The shape might surprise you (at least at first). You might think about it in your head before you draw it to see if you can estimate what it will look like. Then plot it out and see how close you were. Even more valuable is to make sure you understand why your estimate was incorrect (if it was).