Day 10 - Op-Amps

Topics Discussed
On day 10 of the ENGR 44 course, we were introduced to the concept of operational amplifiers. In the context of the classroom, we referred to voltage controlled voltage sources that essentially amplified some in-voltage to be adjusted mathematically to a new out-voltage. We discussed the process by which we could redraw an equivalent circuit from the operational amplifier. After the initial introduction into the topic, we performed a series of problems which focused on determining the relationship between the input voltage and output voltage (Fig. 1). We discussed the linear range of an op-amp as well, noting that any output voltage could only vary between the values of the positive and negative power supplies of the op-amp (voltages).

Fig. 1

Inverting Voltage Amplifier Lab
In lab, we created an op-amp based circuit using a general model idea from previous practice problems. For a certain circuit we worked on previously, we found that the out-voltage could be related to the ratio of the input resistor to the parallel op-amp resistor multiplied by the in-voltage. We wanted to get the ratio close to 2, so we decided to use a 4.7k ohm and a 2.2k ohm resistor respectively. We created the schematic for the circuit as well as the corresponding resistance values we would be using. (Fig. 2)

Fig. 2

We proceeded to actually create the circuit. (Fig. 3) We then measured the corresponding input and output voltages of the op-amp, seeking a gain of negative two across the voltage values. We also wanted to find the saturation points of the op-amp, where the voltage would not go above or below. We acquired our values and created a data table and graph. (Fig. 4)

Fig. 3

Fig. 4

Summary
In this lab, we were able to construct a circuit using an OP27 op-amp and acquire values for input and output voltages for the circuit. We constructed a graph of the relationship between the input and output voltages and found that the relationship was linear up to the points of saturation. This led us to conclude that our gain was in fact what we had hoped for. In the future, we will now be able to use these inverting amplifiers more effectively and with better understanding.