21 - Inverting Voltage Amp & Op - Amp Relaxation Oscillator

INTRO: Today we return back to the familiar category of Op-amps, however this will be with an AC voltage source. Op-amps of course are circuit elements that amplify the input signals feeding into the amplifier. These are ideal devices because they allow a large amount of current/voltage to be changed with simply making changes to the much smaller input voltage/current feeding into the amplifier. The all-familiar guitar amplifier is a common example that many people have experience with.


Below is a voltage amplifier. Just like stating above, this device is capable of controlling the voltage/current through manipulating the smaller voltage/current that is feeding into it.



LAB: INVERTING VOLTAGE AMPLIFIER
In this lab we will build a an inverting voltage amplifier. We will do so with an AC source, resistors, an op-amp, and a capacitor. Like the prior labs, we will calculate circuit values prior and compare them to the experimental values at the end of the lab.


Below is the  circuit diagram that we will build. It is very similar to our prior op-amp setups, but this one has a capacitor in the top branch.


Below we completed the pre-lab for the lab. We calculated values using given values of frequency, resistance, and capacitance. We completed calculations for amplitude gain, as well as the phase change for all three of the frequencies we ran the circuit at.

Here we set up the diagram according according to the above schematic. We have our op-amp, resistors, as well as the capacitor hooked into our breadboard. We also have an analog discovery connected at the designated points so we can send a varying voltage through the circuit.


Here we had an input voltage of 100HZ


Here we had an input voltage of 1KHZ


Here we had an input voltage of 5KHZ


Conclusion: We then measured the difference between our calculated and measured amplitude gain as well as the phase shift. The percent difference between the amplitude gain was 3.5%, while the phase shift percent difference was 7.8%.


LAB: OP-AMP RELAXATION OSCILLATOR
In this lab we need to design an oscillating circuit that oscillates.. We will do so by using an op-amp, capacitor, and a resistor.


The following schematic is what we will model our circuit after. It is a bit more complex then the prior lab, however we are still using familiar circuit elements.

Below is the pre-lab where we calculated circuit element values. Since we know we want a frequency of 99Hz, we calculate a required resistance of 8371 ohms.

Below is a picture of our circuit, which is modeled after the aforementioned diagram. Also, the output of the circuit is below.



Conclusion: Plugging in numbers from the measured values, our percent error of the measured vs. theoretical frequency was 1.9%. I would say this is within the realm of acceptable error. Also, we learned how to construct a op-amp relaxation oscillator. I've been waiting my whole life for this moment. This is better then the time I met Falcor and saved the Princess, the Rock Monster, and Atreyu from the nothing. But alas, that is a story for another time.