Lab 5 Op-amps III, The Op-amp Integrator

Authored by Jeremy Garrod

3/7/2017   

garrod@unlv.nevada.edu

Pre-lab work

 

Lab Work





In order to simplify the calculation, the 100k resistor can be neglected. It is only there to make sure that an offset does not cause the output to saturate at the rail voltage. It is also quite a bit larger than the other resistor, so its affect is minimal.

   In the case of an ideal Op-amp, the circuit will work perfectly fine without the 100k resistor. However, once you use a real Op-amp that has an offset it will not work without the resistor. The offset will cause the voltage to just sit at the rails.

Since the A/C current can flow through the capacitor, which has a much lower impedance than the resistor, it effectively acts like a short to the resistor. The majority of the current will flow through the capactior, causing the resistor to have little impact.

  
The wave was starting to clip on the lower half in my original experiments. I found that my voltage divider did not ouput exactly 2.5V, instead it output 2.48V. Once I set the offset of the function generator to match that of the voltage divider, that issue went away.


The amplitude is the same between the input and output with a 90 degree phase shift at 160Hz. This is what was calculated and expected.

The lower the frequency, the higher the gain of the Op-amp. This can be seen in the images below, which are at 75Hz, 125Hz, 190Hz, and 250Hz.



At 75Hz, a gain of roughly 2 was measured.


At 125Hz, a gain of roughly 1.25 was measured.


At 190Hz, a gain of roughly .83 was measured.


At 250Hz, a gain of about .63 was measured.


The hand calculations below were used to determine the required capacitor and resistor values. A capacitor value was chosen based on what was availble in the lab and the resistor value was calculated from that.
My input was a 10kHz square wave that had a peak-peak voltage of 5V.



I had initially used a capacitor value of 10nF which resulted in a resistor value of 4.166k. However, the output triangle was heavily distorted. I went to 1.5nF instead, which had a much better output. I did forget to take a picture of this though. I chose a 3MEG resistor since I wanted a couple of orders of magnitude larger than the 27.77k resistor.

   
Schematic for square-wave to triangle-wave generation circuit


Simulation of the above schematic

 

Schematic and simulation of my original design. It can be seen that it works the same when dealing with ideal models. However, once it was impleneted in the lab, this design did not work very well.


Oscilloscope output of square-wave to triange-wave generation circuit

The results of this experiment did not exactly match my simulation. I had to use a 27k resistor instead of the value 12.8k that I had calculated. My capacitor was also not exactly 1.5nF, but I was not able to get an accurate value for it, so all I have to go off of is the value that was listed on it. Having those values not be perfect could have easily have influenced the output that I had recieved


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