Lab XI: Operational amplifier (Op Amp)

The purpose of this lab  was to learn about the workings and brief potential applications of the operational amplifier (op amp).

Started out by  forking the lab . The first part of the lab consisted of building a voltage follower circuit. Then we had the option to choose whichever circuit we wanted to apply the op amp. I chose to redo the NOT NOT circuit from last week and use the op amp to supply a constant voltage which was not affected by the LED.



Op amp
Function generator
Breadboard power adaptor


Voltage Testing
Although the op amp is not delicate, we had to measure the voltage of our power supply to ensure it was working properly so that it wouldn’t damage the op amp. Since I decided to use the protoboard instead of the heathkit, I was kind of lost on what my power supply was going to be; previously I had used the DAQ card, but the voltage follower circuit I needed to build later on required +12V and -12V. Then PhD. Koch introduced me to the breadboard power adaptor, I hooked it in and measured the voltage; everything ran smoothly.

Voltage Follower
First, we were instructed to go through these applets:
Voltage follower (hyperphysics)
Voltage follower (falstad) 

Question: What’s the advantage of using a 1:1 amplifier? The advantage is that the golden rules can be applied which are:

(i)The output does everything it has to
in order to keep the difference in voltage
between the inputs to equal zero

(ii) The input draws no current

We were asked to compare and see if there was anything different between the circuit in image 1 and in image 3. I didn’t verify this with PhD. Koch or Anthony or on the protoboard, but I would guess there’s no difference between the two since the golden rules can still be applied to both.

                Image 1:Unity-gain follower circuit (from the Analog Devices data sheet)


I took me a while to figure out how everything was connected, but after Anthony showed me the diagram of how everything was wired up inside the op amp, I began to understand how things were supposed to be wired in the circuit.

                   Image 2: Op Amp inputs and outputs explained


                           Image 3: Voltage Follower Circuit


An error I made when wiring the circuit was leaving out the connection from the input to the output. When I didn’t have this connection, PhD. Koch said that the voltage became amplified between the + and the – inputs. I asked if it was amplified enough to break  the op amp, but it’s not.

                             Image 4: Voltage Follower circuit (white=input, purple=output, yellow=function generator sine wave)


I wondered whether it was necessary to use both channels or not. PhD. Koch said I needed both so that I could see the wave function from the output and the input and verify that they’re the same since I’m applying the golden rules, but my graph was not looking like it needed to. Then, he said that I needed to connect the leads of the oscilloscope to ground and my function generator was not wired into the circuit. Fixing those two errors, the oscilloscope showed two identical waves, meaning that the input and the output were the same, which is what we wanted.

                         Image 5: Voltage follower circuit; Input signal = output signal (they’re overlapping each other)

Question: At what frequency does the gain start dropping below one?

The gain=one when the input=output.
When the frequency got really high, the output was a lot lower than the input. I didn’t see a difference until the frequency got to 100kHz. At this frequency, the output was lower than the input. I asked if the same would happen if the input was lower than the output and PhD.Koch said  that if I had a circuit wired up so that it did that I would.

                         Image 6: High frequencies cause the gain to drop below one



NOT NOT with LEDs Again
The last part of the lab contained three options. The one I chose included the op amp into the NOT NOT circuit from last week. The problem with this circuit  was that the LEDs drew so much current that it reduced the drain voltage from +5 V to around 2.06V.
I kept thinking the op amp needed to be incorporated into the circuit with the LEDs somehow. However, Skylar and Anthony explained to me that all the LED needed to do was to act as an indicator for the NOT NOT gate. Hence, the LED can branch off from the circuit, just so long as it displays the correct TRUE/FALSE value. So I wired it up by combining the voltage follower circuit and the NOT NOT circuit, while having the LED branch off. I connected the gate voltage of the first NOT gate into the input of the op amp. Then,connected the output of the op amp to the LED. I verified that the NOT gate values were correctly indicated by the LED, and most importantly I verified that the drain voltage= +5V (!!).

          Image 7: Voltage follower + (NOT NOT+ LED) circuit


When I was first wiring this circuit I kept having errors with the voltage reading, then Skylar proposed that I measure the voltage of the left side of the protoboard. There was no voltage!! I remembered PhD. Koch mentioning this at the beginning of lab, and I kept it in mind for the first part of the lab but forgot it for this one. Skylar said this could be fixed by just connecting the little orange wires across so that the voltage continues on from the right side of the protoboard to the left side.