Intergrator/Differential Op-Amp Frequency

[big_tobacco]

Hello,

I'm baffled as to why different frequencies affects the output (amplitude) of a integrator and differentiator op-amp circuit.

I'm using a variety of frequencies (100 Hz - 10Khz), with a fixed 1V square wave input? Why does the frequency cause a change in the amplitude?

This isn't a homework question, I work with simple op-amps all the time and it's about time that I should know the principles.

Any help would be greatly appreciated.

Regards

 

[dlgoff]

Integrators and Differentiators are filters circuits (hence the frequency response you are seeing). They are low-pass and high-pass respectively. Check out this wiki page to see the circuits and their "input to output" voltage equations.
http://en.wikipedia.org/wiki/Operational_amplifier_applications#Integrator"

 

[waht]

Also another factor could be the op-amp's frequency range.

For example, a 741 works up to 20 KHz,

but a square wave is composed of odd order harmonics. So if you apply a 10 KHz square wave, then there is a frequency component at 30 KHz, which would likely be killed by the op-amp like the 741 - and hence signal amplitude is reduced.

 

[Adjuster]

Another way to look at this is that with a square-wave input, an integrator should give a triangular-wave output. Thus the output amplitude is inversely proportional to frequency. Provided that the fundamental frequency of the square wave lies well within the bandwidth of your op-amp, the practical result should be a fair approximation to this.

The situation for a differentiator is more complicated. In fact, a perfect square-wave input applied to a perfect differentiator would give an output consisting of infinitely narrow and high pulses. In the real world, a square-wave will have limited edge speeds, and an integrator will have limited maximum output amplitude and limited slew rate.

If the frequency of a square-wave input to a practical differentiator is increased from a low value, the output will most likely be a series of alternating narrow pulses of roughly constant amplitude, which get closer in time as the frequency increases. If the frequency is increased sufficiently, there will not be enough time for each pulse to reach full height, and the output amplitude will begin to drop off if the frequency is increased further.