Solve OPAMP Circuit Problem: Find Vo with Vin1=3V, Vin2=-2V

[etf]

Hi!
Here is my task:
Find output voltage Vo if Vin1=3V, Vin2=-2V, R1=R2=R3=100KΩ, R4=300KΩ, R5=150KΩ, VCC=15V.

Applying KCL at node 2 we get:
$$-Iin2+I5-Icc=0$$
$$-\frac{V3-V2+Vin2}{R3}+\frac{V2-V3}{R5}-\frac{V3-V2+Vcc}{R4}=0$$
$$-\frac{Vin2-V2}{R3}+\frac{V2}{R5}-\frac{Vcc-V2}{R4}=0$$
$$V1=V2=\frac{Vin2/R3+Vcc/R4}{1/R3+1/R5+1/R4}=1.5V$$
But Multisim give me different voltage V2:

As you can see, Multisim measure 3.226V.
Here is OPAMP setup (since it's ideal, I put some huge numbers for A and input resistance, output resistance is 0):

Any idea what's wrong?

 

[gneill]

In your Multisim circuit you have set R3 to 300k, but the problem statement says it is 100k.

 

[etf]

I can't figure out how I didn't see it :) Thanks!

 

[gneill]

I can't figure out how I didn't see it :) Thanks!

Sometimes a fresh pair of eyes on the problem can pick out the things you miss repeatedly due to familiarity!


You might want to take note that in your simulation the "voltmeters" have an impedance of 10MΩ, and will influence the circuit behavior to some extent (it'll show up in the second or third digit of the values I'm guessing). So don't be surprised if your calculated values are almost but not quite the same as your "measured" ones!

 

[rezeew]

There are a few possible reasons why the measured voltage V2 in Multisim may differ from the calculated value of 1.5V. The first thing to check would be the values of the resistors and the power supply voltage in the circuit. Make sure they are entered correctly and that there are no typos or mistakes in the values.

Another factor to consider is that the op amp in Multisim is an ideal op amp, which means it has infinite gain and infinite input resistance. In reality, op amps have limited gain and input resistance, so this could affect the accuracy of the simulation.

Also, make sure that the op amp in the circuit is correctly connected and that the positive and negative inputs are not accidentally reversed.

If after checking all of these factors, the measured voltage still differs significantly from the calculated value, there may be a problem with the simulation itself. In this case, it may be helpful to try the simulation with different values for the resistors and see if the results are consistent.

Overall, it is important to remember that simulations are not always 100% accurate and may have some discrepancies from the theoretical calculations. It is always a good idea to double check the values and connections in the circuit before assuming there is an error in the simulation.