How to Measure the Output of an Op Amp Circuit?

[slain4ever]

Homework Statement

i have this circuit
http://screencast.com/t/6rgXUvZW
but I am not sure if i have set it up correctly,
firstly is where I've put the multimeter V out? I am not quite sure
and are the Vins supposed to be in AC or DC? and are they supposed to be connected to ground like i have done

The Attempt at a Solution

http://screencast.com/t/6rgXUvZW

 

[lewando]

For output voltage, the multimeter should be between pin 6 and ground. You are set up to sum DC voltages. Was that your task? How is the 741 getting powered-up?

 

[slain4ever]

well basically it has a circuit of the same thing i have but is cut off before the power supply and just says v1 and v2, the multimeter isn't there and we have to find Vout when v1 = 1V and v2 = 0.5V

 

[lewando]

Relocate the voltmeter, as discussed, and energize your 741. To do that, attach +12V to pin 7 and -12V to pin 4. I picked +/-12 V because it is a common choice. Also R1 and are R2 are different. If your tasks is to build a summer, the two different resistances will affect your expected results. If your task is to build it as shown, then just energize the 741 and you should be fine.

 

[Quinzio]

What is this circuit supposed to do ?

 

[slain4ever]

im sorry i don't understand, you have +- 12V but the circuit in my book asks for two different voltages of v1 = 1V and v2 = 0.5V. and also is this ac or dc?
and the book says that the v1 is connected to r2 and v2 to r1, not to the 7 and 4 pins

 

[lewando]

Whatever your circuit is intended to do, it will not operate properly of the 741 is not "powered up" or energized. Connecting the 741 to a DC power supply capable of generating +12VDC and -12VDC will help in this regard. I did not say to connect V1 or V2 to pins 7 and 4. V1 and V2 are your input voltages to your circuit connected via R1 and are R2. Let's assume that these input voltages are DC. It might be worthwhile for you to review some common application circuits using the 741. http://www.ece.ucsb.edu/courses/ECE002/2A_S07Banerjee/ECE2A Lab 4.pdf"

 

[willem2]

If you have and opamp with negative feedback, both imputs will be at the same
potential. (because if the potential at the negative input was a bit more positive, a large negative output would result, making the negative input less positive)
So the negative input wil be at 0V, and it shouldn't matter if you measure the potential
difference across R3 or between the output of the opamp and the ground
You can now compute the currents through R1 and R2, and use Kirchhoffs's current law to calculate the current through R3, and from that the voltage.
2.173 can't be right.

 

[slain4ever]

Thankyou, i put two batteries to the pins as you said and the other end to ground, and now i get 5v (4.99) which is correct so thankyou for your time and assistance.

 

[Mindscrape]

Um no, you should get a negative voltage because the output is creating a voltage and sending current to the inverting input so that it will be at the same potential as the noninverting input. Even though I am very guilty of wrongdoing myself, signs are important!

This configuration is often called a summing amplifier, and it's really a special case of the inverting amplifier.

 

[lewando]

You may want to check your polarities. With positive voltages applied to R1 and R2, you should be getting a negative voltage at the output of the op amp (pin 6) with respect to ground. Your circuit is an inverting weighted summer.

edit: Mindscrape beat me to it :(

 

[slain4ever]

oh yeah, the multimeter is the wrong way around, i forget it flows from right to left through r3

 

[lewando]

If you are trying to measure the output of the circuit, you can do it directly by measuring the voltage at pin 6 (the "output" terminal) with respect to ground. That way you don't have to risk any confusion about which way the current is flowing through R3.