Analysis of RC Integrator Circuit Practical vs Theoretical

Thread Starter

Lod

Joined Apr 16, 2020
2
Hi guys,

Just started learning about RC Integrator op-amp circuits and currently doing a lab report for university about it. The point of the lab is very basic, as it is just for us to see the difference between practical and theoretical results.

I was noticing differences between the two and realized for that for the practical results, the output signal was flat at the top? I'm having trouble thinking of the reason. Maybe it's the time it takes for the capacitor to charge then discharge? Ill attach them so you can see.

Sorry, if this is in the wrong subforum, thought it looked right as it relates to oscilloscope analysis.

Thanks.
 

Attachments

drc_567

Joined Dec 29, 2008
1,069
... It seems that usually the first item that is requested for an inquiry such as this is a Circuit Diagram with the key components labeled. Datasheets for op-amps, transistors, and any active device must be examined to insure that any voltage or current transient values, as well as power supply settings, do not violate the published limits.
 

LvW

Joined Jun 13, 2013
1,188
Lod - it is not easy to give you a substantial answer because lack of information:
(1) What does "theoretical" mean: Ideal FUNCTION of an (virtual) integrator? Or ideal opamp (with large but finite gain) ?
(2) What is shown in the attached graphics? Simulation or measurement?
(3) Corresponding circuit? Non-inverting or inverting integrator?
 

Thread Starter

Lod

Joined Apr 16, 2020
2
My apologies, I did not provide enough information, I am quite new to electronics as a whole so I do not know much about the real specifics in drc_567's answer and I thought that this would be a general difference between practical and theoretical and could be answered without those factors. However, I will try to answer.

To answer drc_567's points:
- R = 1k ohms
- C = 10 uF
- Rload = 1k ohms
0.1 V input signal square wave with a 500 ms width and a frequency of 1 Hz.
The device used to measure the practical value was Analog Discovery Kit 2. The Multisim values mimic those above.
However to answer LvW's points:
1) Theoretical meaning an ideal op amp with (infinite gain)
2) As I implied in my post, the graph with the square flat peak instead of the triangular peak (so the one labelled 1hz lab - tested in Waveforms) is the practical. Hence, the other one (labelled Transient Ex2) is the theoretical (tested in the Multisim software).
3) As for the circuit, it is an inverting RC Integrator op amp circuit
I will attach the circuit as well.
Also, how would I calculate the gain too? I was using the equation attached but am unsure if it is correct to use it this way as I thought it took frequency into account.

Sorry for the inconvenience.
equation.png
1hz practical.pngTransient Ex2.pngcirc ex2.pngCircuit Ex2.png
 

DickCappels

Joined Aug 21, 2008
6,979
Look at how close the top of the waveform gets to the positive power supply then check the datasheet for the LM741 and see whether it is trying to output a voltage that exceeds the maximum output voltage with that power supply.
 

LvW

Joined Jun 13, 2013
1,188
Lod - the main problem of the practical circuit (hardware or simulation with a real opamp model) is the fact that you have no DC negative feedback (necessary for a stable DC operating point.)-There is always an offset voltage which brings the output to saturation (unwanted continous loading of the cap).Hence, you need a large R across the C.
 

crutschow

Joined Mar 14, 2008
26,436
What's all the fuzz on the yellow trace.
Looks like something is oscillating.

As LvW noted, the circuit needs DC feedback for a stable operating point.
Without that, the output will tend to drift to either positive or negative saturation at the rail.
 

DickCappels

Joined Aug 21, 2008
6,979
What's all the fuzz on the yellow trace.
Looks like something is oscillating.

As LvW noted, the circuit needs DC feedback for a stable operating point.
Without that, the output will tend to drift to either positive or negative saturation at the rail.
Some people are more observant than others. That got completely past me. Try putting 1 Meg across your 10 uf capacitor. Of course the ±15 mv offset voltage (1970's technology, amazingly this is still being made) will show up as ±15V on the output. You need an opamp with much lower offset voltage and low input leakage current.

An LM358 or LM324 would be ok (also terribly old and behind the times, but it may work).

As long as you are keeping the frequency low an OP-07 with its 150 uV offset (max) should be fine.

(Yeah, I know there are better opamps out there. This sticks to those introduced during the Cambrian Explosions of Op Apamps in the 1970's)
 
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