# Finding Current and Voltage on Ideal OP-AMP! Calculated vs Simulated. HELP!

Discussion in 'General Electronics Chat' started by mrvaledon, Oct 16, 2016.

1. ### mrvaledon Thread Starter New Member

Mar 1, 2016
22
0
Hey Guys! I calculate the i1 and i2 currents and Vout. They gave me a different value from the simulated one. I simulate them on Falstad Circuit and they said that the Vout is -13.75 and i2 is 2mA.

See attachment.

Thanx.

Juan.

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2. ### ScottWang Moderator

Aug 23, 2012
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Is this your homework for school or personal study issue?

3. ### mrvaledon Thread Starter New Member

Mar 1, 2016
22
0
A practice excercise for test.

4. ### DGElder Member

Apr 3, 2016
348
87
Your solution is wrong, the simulation is a possible solution. How did you simulate this without an input signal?

Ambiguously, you show 0.5mA at the inverting input without indicating a direction. Is that how the problem was presented?
What in that schematic and problem statement are your assumptions and what are the givens in this problem?

5. ### mrvaledon Thread Starter New Member

Mar 1, 2016
22
0
They give me i1 and i2 directions. i2 go to node Va and i1 is exiting Va.

6. ### DGElder Member

Apr 3, 2016
348
87
If that were true, how could the output be negative? Please show the complete problem without your additions. Also a screen capture of your simulation circuit if you want to know why it got -13.75V

7. ### mrvaledon Thread Starter New Member

Mar 1, 2016
22
0
Here it is. See attachment

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8. ### DGElder Member

Apr 3, 2016
348
87
Now that we know the actual problem: the direction of the 0.5mA current means that Va has to be negative and this can only happen if Vo is negative. Which means that current and the current from the ground through the 2.5K resistor combine through the 5K resistor. So I1 and I2 are both negative. The voltage drop across the 5K adds to Va for Vo = -13.75V.

Your voltage and current directions are reversed at the Va node. Current flows into the positive terminal of a passive device.

Last edited: Oct 17, 2016