# Why there is a DC shift in the output of the voltage wave form?

#### Murdock39

Joined Apr 21, 2021
11

#### Murdock39

Joined Apr 21, 2021
11
I have made this integrator circuit. My question is why there is a DC shift in the output of the the intergrator? I think the waveformof the output should have been strate dfrom the zero level

#### AlbertHall

Joined Jun 4, 2014
12,346
What does that note at the bottom say about C1?

#### hrs

Joined Jun 13, 2014
397
Maybe the floating point value for 0 is non-zero. The integrator will then integrate this non-zero value until it reaches one rail or the other.

#### crutschow

Joined Mar 14, 2008
34,406
You need to add a -1.25V offset to the input so the average value is 0V.

You also need to simulate without doing the initial conditions bias calculation (UIC command), since that calculation will cause the integrator to saturate before starting the transient response.

Below is my LTspice simulation of the circuit with the noted changes:

Note: To make your schematics easier to read, use the ground symbol at each of the ground connections to avoid the long, distracting ground wires.

#### Audioguru again

Joined Oct 21, 2019
6,688
The 53 years old 741 opamp has an input bias current but your simple circuit is missing a resistor on the + input to provide it. The - input has a 10k resistor so the two inputs are unbalanced producing the DC offset in the output.

Try adding a 10k resistor from the - + input to ground to balance its inputs.

A simulation does not produce the input offset voltage that a REAL opamp has that is amplified by the opamp.

#### Ian0

Joined Aug 7, 2020
9,798
The 53 years old 741 opamp has an input bias current but your simple circuit is missing a resistor on the + input to provide it
Apologies for being pedantic here, but there is a resistor on the + input through which the bias current can flow. It is a very low value of resistance!
10k in series with the + input will ensure that the two opamp inputs are both at the same voltage when the circuit input is at ground, by ensuring that the bias current creates the same voltage drop on each resistor.
I think it’s a pointless exercise, as the offset voltage of the 741 will soon make the output drift all the way to one or other supply rail.

#### Audioguru again

Joined Oct 21, 2019
6,688
Opamps in a simulation have NO input offset voltage, and also have NO voltage or current overload damage.

#### Enochs

Joined May 27, 2021
1
The 53 years old 741 opamp has an input bias current but your simple circuit is missing a resistor on the + input to provide it jcpenney kiosk. The - input has a 10k resistor so the two inputs are unbalanced producing the DC offset in the output.

#### crutschow

Joined Mar 14, 2008
34,406
Opamps in a simulation have NO input offset voltage, and also have NO voltage or current overload damage.
Depends upon the op amp model.
Some do have input offset modeled.