# integrator op amp questions

#### dmctaggart

Joined Oct 14, 2007
3
i have a few questions for you guys. when connecting up a integrator op amp circuit i know the wave form changes im just wondering why. also wondering why the amplitude changes with the frequency change? i have a base idea of both of these. any help would be great.

#### Distort10n

Joined Dec 25, 2006
429
Think of a square wave for an input. The summing junction of the inverting amp will force a constant current through the feedback capacitor during the positive 'peak' of the square wave. So the feedback capacitor integrates this current to produce an inverted voltage at the output.
At the negative 'peak' of the square wave, the feedback capacitor again integrates the constant current to produce a positive output voltage. It is always easy to think of it this way.
With sine/cosine, well that is just calculus. In other words, what is the integral of a sine wave...of a cosine wave?

The amplitude will change because the op-amp integrator is ALSO a single pole low pass filter. You can think of it in the frequency domain as in the critical frequency or cutoff. In the time domain a larger capacitor cannot charge/discharge quickly so higher frequencies are not sustained.

#### dmctaggart

Joined Oct 14, 2007
3
Wow that is very technical. i dunno if im completely grasping the concept here. sorry sir. i understand sort of. just need another angle at it to completely understand the concept.

#### dmctaggart

Joined Oct 14, 2007
3
nevermind i just read it over and over. i get it perfect now. thanks so much.

#### manstard

Joined Mar 1, 2008
3
Think of a square wave for an input. The summing junction of the inverting amp will force a constant current through the feedback capacitor during the positive 'peak' of the square wave. So the feedback capacitor integrates this current to produce an inverted voltage at the output.
At the negative 'peak' of the square wave, the feedback capacitor again integrates the constant current to produce a positive output voltage. It is always easy to think of it this way.
With sine/cosine, well that is just calculus. In other words, what is the integral of a sine wave...of a cosine wave?

The amplitude will change because the op-amp integrator is ALSO a single pole low pass filter. You can think of it in the frequency domain as in the critical frequency or cutoff. In the time domain a larger capacitor cannot charge/discharge quickly so higher frequencies are not sustained.
i have read in a book that there is a virtual ground at the inverting terminal of the op amp.i don't understand what is that virtual ground and why and how it happens.

#### manstard

Joined Mar 1, 2008
3
another thing is that why only the inverting terminal of the opamp is given the supply and the non-inverting grounded.?
cant we do the alternate,with the non-inverting connected with a supply and the inverting grounded.

#### Audioguru

Joined Dec 20, 2007
11,249
The integrating capacitor is connected from the output to the (-) input for negative feedback. If the (+) input is used then the feedback will be positive and the opamp's output will be instantly driven to a power supply rail.

Only the (-) input is a summing input.

#### manstard

Joined Mar 1, 2008
3
thank you.but,what is power supply rail.?
why only (-)input is a summing input.
explanation would be of great help.
THANK YOU.