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I am creating an Op-Amp Integrator circuit in Multisim. It seems to work for sinusoidal voltage inputs, square wave inputs, etc but when I connect it to a DC source the output voltage (channel A in the diagram below) just immediately goes to -11V. Based on the integrator equation (-1/RC)*integral(v(t)dt), the output voltage should be equal to -0.4t (-0.4 volts per second). I have tried using a bunch of different values for R, C and Vin. The Op-Amp is a 741 Op- Amp. Thank you for any advice.

Integrator

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    \$\begingroup\$ Add a suitable resistor across c1. The capacitor blocks DC. \$\endgroup\$ – Plutonium smuggler Feb 18 '15 at 2:33
  • \$\begingroup\$ Thank you, now the voltage isn't -11V but unfortunately it still seems to remain constant with time instead of having an increasing slope. (Tried many different R values from 10 to 1M) \$\endgroup\$ – user1725855 Feb 18 '15 at 2:40
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Upon initialization, MultiSim attempts to perform a DC balance. Since for DC a capacitor is the equivalent of an open circuit, that's what you get.

Instead, replace V1 with a step function, with the output at 0 for t = 0, and an output of 1 for t > 0. Alternatively, replace V1 with an offset square wave with amplitude 0.5 and a 0.5 offset - that is swinging from 0 to 1. Make the frequency 1/1000 Hz, and as long as you're only interested in t < 500 you'll be fine.

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  • \$\begingroup\$ I thought you couldnt really amplify DC using integrator. And then you asked the user to use a step input ( which has me confused instead ) . \$\endgroup\$ – Plutonium smuggler Feb 18 '15 at 2:57
  • \$\begingroup\$ Thank you very much, this solved the problem! I was intending on using the voltage across a resistor as the input to the Op-Amp, so the step function won't work, but I imagine using a switch instead should work for simulation purposes. \$\endgroup\$ – user1725855 Feb 18 '15 at 3:02
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    \$\begingroup\$ @Plutoniumsmuggler - It's just a way of setting the output to zero at t = 0. Another way would be to use the existing setup with a switch across the cap, with the switch closed until after the simulation starts, then opening it. \$\endgroup\$ – WhatRoughBeast Feb 18 '15 at 3:49

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