Paraphrase a report

Objective

The purpose of this experiment was to become familiar with operational amplifiers (op amps) and their function in various circuits.  The role of op amps will be important to understand because they are a commonly used piece in modern-day circuits.

Background:

Operational amplifiers are a DC circuit high gain electronic voltage amplifier with differential inputs and a single output. They are widely used in electronics today. Op amps are the basis of the electronic equipment analog computer, where op amps were used to model mathematical computation such as addition, subtraction, multiplication, differentiation, integration, etc. In this lab, we will work with an operational amplifier (op amp) chip. We will study its most important input-output properties. We will then use the op amp as a simple comparator. Understanding op amps will greatly assist us in being able to design and build useful and interesting circuits.

Step 1:

In this step, we needed to set the triple output power supply to 8 Volts, and -4 Volts, and used COM as our circuit ground. We set the power supply as close to the needed values as possible, notably we set them to 8.02 Volts and -4.01 Volts respectively. Then, we carefully adjusted the 10k trim pot from one extreme to the other to find and values at the given voltages.

Using the DMM, we measured to be 7.969 Volts, and to -4.054 Volts.

-Do and differ from V1 and V2 respectively?

As we can see from the results, they differ very slightly, there is not much of a difference between them. Between V1 and , we calculated a 0.39% difference. While between V2 and we had a 1.33% difference.

Step 2:

In this step, we changed V1 and V2 of the triple supply to +5V and -5V, respectively. We then adjusted the 10k pot to the middle so that is between -1V and +1V, connected +5V to pin 7, and -5V to pin 4 of the op amp.

Using two DMMs, we monitored and and recorded the following voltages:

Step 3:

In this step, we adjusted the 10k potentiometer so that varies between -2V and +2V in 200mV steps and recorded the . Our is 4.438 Volts while is -3.726 volts.

Using two DMMs, we monitored and and recorded the following voltages:

Steps 4 & 5:

In this step, we adjusted the 10k potentiometer so that varies between -50mV and +50mV in 10mV steps, and used two DMMs to measure the values of and .

-What value of VID should produce 0V at Vout? What was the estimated value of VID that would make Vout = 0?

By looking at the graph above, we can probably say that around 0.1 Volts for VID would result in a Vout of 0 volts.

Step 6:

The slope looks like it would be very closed to being “undefined” by looking at the graph, so the closest value we used was 0.2 Volts for VID, and 4.39 Volts for Vout, which resulted in a slope value of 21.39v.

Step 7:

-Based on your plot from Step 3, what is Vout if Vin > VREF and Vin < VREF?

When Vin > VREF we see that Vout is positive, usually the Vmax, where as if Vin < VREF, our Vout would be our negative VMIN value.

Step 8:

For this step, we built the circuit drawn below using 1VDC as the reference voltage. We connected a second power supply to serve as VIN, making sure both power supplies had the same ground. We increased VIN slowly from 0VDC to 3VDC, taking note of when VOUT switched between the max and min. We did this with a second VREF, which is 2VDC.

-Are your observations consistent with your expectations from step 7? What impact does VREF have on the transition voltage for Vin? Describe a comparator.

Yes, our observations clearly show that this is what we expect. Here we have the small amount of data needed for step 8. VREF is where the op amp operates at, its operating point so to speak. If Vin is less than the VREF, we will have our negative output, where if Vin is greater than VREF, we will have a positive output.

A comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. It has two analog input terminals v+ and v-, and one binary digital output Vo.

Step 9:

Next, we connected our resistor-LED combination to the output of our Op Amp circuit. We chose a resistor value of 2k and measured resistance of 1.983K.

We noted that the LED turned on as VIN rose above 2VDC and turned off when it fell below that threshold. It did this because VREF was set to 2VDC, establishing the baseline for the comparator. So, as the input voltage exceeded the reference voltage, the op amp output was high, as visually indicated by the LED illuminating. When the reference voltage was higher, the output was low and therefore the LED was off.

- Explain why the LED turns on and off when it does. How do VID and Vout determine when the LED turns on and off?

Well, we know diodes only work when they are forward biased, meaning the positive voltage enters their positive anode side. So, when the output voltage of the op amp is negative, it is obvious the LED will not turn on, but when the output voltage is positive we see that the LED does turn on. The LED turns on because the op amp acts as a comparator. This is because the voltage is greater than the VREF causing the op amp to release voltage. The opposite occurred, when it’s less than the VREF, resulting in no voltage out.

Step 10:

Next, we modified the circuit so that the LED would turn work in the opposite manner, on when VIN < VREF and off when VIN > VREF. To do this we simply flipped the leads of the diode, switching the direction of the biases.

The LED turns on at 2.00 Volts and turns off at 2.01 Volts.

-Explain why the LED lights up differently from Step 9.

The reason it lights up differently is because now the LOW output completes the circuit, illuminating the LED. By placing the cathode first, the biases have been reversed and therefore the conditions that satisfy the completion of the circuit have been reversed.

Step 11:

We selected a new resistor value of 510and measured value of 502. Then, we repeated the previous step. While varying , we determined that the LED switches on at 2.00 Volts and off at 2.01V. Changing the value of the resistor had no effect on the voltage required to operate the comparator. The smaller resistor value did make the LED brighter. But that seemed to be the only impact on the circuit, which follows Ohm’s Law. Since I=V/R, as R decreases, I increases. More current would mean more light generated by the LED. We measured the current flow while the LED was on and found it was 3.09mA.

Step 12:

At this final step, we removed our resistor-LED combo and attached the function generator to Pin 3 of our Op Amp. Then, we set VREF to 0.5VDC and FG to 2VPP and a 100Hz Sine Wave. Then we set the offset to 0VDC and set both channels of the oscilloscope for DC Coupling. Finally, we set the frequency generator to Hi-Z output and measured VOUT.

The output was actually a square wave. The op amp is operating as a comparator, meaning it is off until the reference voltage is exceeded. But once it turns on, the output is a constant high until the voltage falls back below the threshold. So, the output should be big gain, steady on, big drop, just like the square wave we got.

We measured VOUTmax to be 4.176V and VOUTmin to be -3.975V, keeping consistent with our results thus far. We used the cursors on the oscilloscope display to measure where the switch happened at and determined it was around 0.5V. The output is a square wave because once the thresholds are crossed the output remains a constant high or low. There is no gradual rise of fall, just steep cliffs.

Conclusion:

We feel that this experiment was a success for our group.  We are confident that we learned how a basic op amp functions in a circuit, and have learned what aspects of them causes their VOUT to actually be amplified from their VID.  We also believe that our results and data are accurate, because we were able to answers all the questions confidently.