# Problem with the output of an integrator

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#### abbey

##### Junior Member level 1
I am trying to run this circuit on pspice.I use a square wave as input.The output should be a triangular pulse.Does anyone know if there is a mistake? Thank you

#### LvW

##### Advanced Member level 5
Re: Integrator

The LM741 model is not offset-free !
Thus, there is a dc voltage at the output which charges the feedback C continuously until it reaches the limits (power rail). You must provide a certain degree of dc feedback.
By the way, the input R is too large.

johnson933

### johnson933

Points: 2

#### srizbf

##### Advanced Member level 5
Re: Integrator

make pw=100ms to 500ms and change the period appropriately.

see the output.

if it is testing only change r and c values for integrator.

srizbf
7thmay2010

#### WimRFP

##### Advanced Member level 5
Integrator

Hello,

When set V(C1) = 0, you will see that the output voltage drifts away to negative or positive supply. I think you can set the initial voltage voltage when you go into the component properties.

As what LvW said, you have offset, and this will be integrated giving a slope toward the positive or negative supply.

#### abbey

##### Junior Member level 1
Re: Integrator

WimRFP said:
Hello,

I think you can set the initial voltage voltage when you go into the component properties.

Thank you!!!I did this and it works fine.

#### WimRFP

##### Advanced Member level 5
Integrator

Hello,

Maybe you saw the output drifting (depending on the total time of simulation).

To avoid too much drift, you can use the feedback as suggested by LvW. Play with the value of the resistor. A lower value will give more departure from straight slopes, but less DC offset at the output.

By adding the resistor, in fact you make a low pass filter, but with a corner frequency far below the frequency of your signal. The DC gain will be
DC gain = -(resistor parallel to C1)/R1

You may add a resistor (R = R1) in the + input to give some reduction in offset due to input bias current. Of course you can also take a better opamp with less input current, less input offset current and less offset voltage.

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