# Solving for steady state voltages (Op-Amp)

simulate this circuit – Schematic created using CircuitLab

so I have this Op-Amp problem. I need to find for the values of V1, V2 and Vout for t > 0. I have this solution but I'm not sure if I got it right. If not, what part did I have an error?

It's not in the picture but the continuity variables V1 and V2 both have a value of 1 volt. Also, I don't know how to interchange the polarities but the positive terminal of the Op-Amp should be on top.

• Do you realize that there is a schematic entry and simulator tool built into the editor here don't you? Rather than posting a photo of a sketch how about posting a neat schematic. Who knows the simulator may even tell you if your answer is correct or not. Commented Feb 7, 2017 at 14:14
• Just found out about it 'cause I'm a newbie. I made one already. Commented Feb 7, 2017 at 14:47
• Why do some home work problems give such unrealistic values for R's and C's. I would think it gives students the wrong idea. Commented Feb 7, 2017 at 14:57
• Why do your reference designators for your caps begin with "V" instead of "C"? Shouldn't the junction of R2 and V2 be connected to the junction of R5 and V1?
– Tut
Commented Feb 7, 2017 at 15:09
• You notice V+ and V- on the op amp are reversed between your schematic and your image? Commented Feb 7, 2017 at 15:33

No need for all that maths....

A TL081 does not have much output drive and with only 10 ohms to ground the thing is going to struggle to move the output anywhere, I would guess the thing will sit with the output pretty much at ground with the output stage hard in current limiting no matter what you do to the inputs.

Note also that your two schematics are NOT the same, the one on paper has R2/R4 joined at Va, the other one does not, and the inverting/non inverting inputs are swapped between the two if you were to assume an ideal opamp instead of the 081 it would make a difference.

R4 can be replaced with a short circuit without changing anything, and Vout = Vb * (1 + R7/R6) = 2 * Vb.

• I fixed the schematic - the site peer maybe publishes it
– user136077
Commented Feb 7, 2017 at 16:05

Your input 4exp(-t) diminishes to zero as the time goes on. The voltages of your capacitors V1 and V2 have in your final equations a limit 0,14V. That's not possible, because the capacitors get finally discharged through the resistors.

ERRORS: Seemingly you have not understood that V1 and V2 are the given initial voltages of the capacitors. The nodal equations should be written with Va and Vb; the rightmost capacitor should have their difference as its voltage. Vout should be written as amplified Vb.

You should solve the equations by using the Laplace transform - it has the places for the initial values as soon as you apply it to the derivatives. You will get Va and Vb at first as their Laplace transforms, but the inverse should be easy.

Solving for steady state voltages (Op-Amp)

I need to find for the values of V1, V2 and Vout for t > 0.

those are two different questions.

for the steady state solution, Vout = 0 for ideal opamp;

for the steady state solution, Vout needs to be measured empirically with a real opamp.