(4/4/17) Day 10: Op Amps and Inverting Voltage Amplifier

Day 10

Lecture:
Today's Lecture was primarily focused on Operational Amplifiers and their many different configurations.

This particular problem was dealing with what an op amp actually acts like internally.

Then we went through an algebra practice exercise by calculating what the values will be in symbolic fashion. We used Nodal Analysis.

This is another problem in which we had to use the internal effects from an op amp into consideration to find the closed loop gain.

Lab:

Inverting Voltage Amplifier

Prelab:

We began by creating a design for an inverting op amp that would satisfy the necessary criteria. The parameters were that the op amp would have a Gain of -2 and an input resistance of 2k Ohms. Unfortunately, in the real world there are no 2k or 4k resistors. So even conceptually we had to settle for standard e12 values.

After the planned out circuit was approved, we grabbed live components and measured their actual resistances. The blueprint was also updated to reflect this.

During the prototype stage of the lab, we made sure to use the pin diagram of our designated chip appropriately.

Finally, attaching the DMM to read the input voltage (over the 2.15K resistor), we had a negative reading (which makes sense, since we had input a negative voltage).

Then the output was examined to be the opposite, and almost lined up with our desired Gain! 

Using Gain = V(out)/V(in) = 4.22V/-2.19V = -1.93

(If you're confused about the sign, reread the title of this lab)

After gaining confidence in the function our prototype we set out to test it with a range of input voltages to analyze and predict future outputs. This was proudly done using the technologically advanced materials Mt SAC labs is best known for.

Whiteboard/Marker

And Excel

Summary:

To begin, our goal for a Gain of 2 was off because of realistic values for resistors. The values we used conceptually would have lead to:

Gain = -Rf/Ri = -4.7k/2.2k = -2.14

and using the actual values...

Gain = -Rf/Ri = -4.62/2.15 = -2.15

Using our actual gain vs the desired, a percent error can be obtained:

[| -2.15+2 | / -2]*100% = 7.5% deviation from desired result

(Hopefully this particular design never goes into medical equipment...)

Aside from the deviation in the output, there is an interesting behavior of op amps that should be put into consideration. Looking back at the graph of input/output it is clear to see that there is point in which the output is at a standstill. At approx. -2V (4.23V) and 2V (-3.42V) the op amp enters what is commonly known as the "saturation zone."

To further probe into this matter, when we put 3 Volts into the op amp we expected to see an output of approximately -6.45V. Instead we get a measly -3.41V, a telling product of the saturation tendencies of this particular element.