ELECTRONIC II : LAB 4

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ELEC 161 Electronics II

Module 4 Lab: Op Amp Applications – Comparators, Integrators and Differentiators

Introduction.- This lab continues exploring other common applications of Operational Amplifiers. We will study circuits that make up comparators, integrators and differentiators. We will also introduce a new component, (the LED) and two new types of sources: the square signal and the triangular signal.

Procedure

1.- Comparator. The Comparator is a circuit which output can only have two values: high or low. The value of output voltage is a function of the value of the input voltage, the reference voltage and the connection of these two input voltages.

1.1 Build the circuit shown in Figure 4-1.

Figure 4- 1: Basic Comparator Circuit

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1.2 Calculate the value of Vref.

1.3 For this comparator, if Vin is higher than Vref, then the output voltage will be low. For this circuit, the low voltage is approximately - 11V. If Vin is lower than Vref, the output voltage will be high. For this circuit, the high voltage is approximately + 11V

1.4 Connect a DC source to Vin. Measure Vout with the DMM and verify the behavior of the circuit.

1.5 A different way of verifying the output of the comparator is correct is by connecting a LED. Figure 4-2 shows how to find the LED in the Multisim Library. Choose the RED LED (or blue, green, yellow, etc.) and connect to the output of the circuit as shown in Figure 4-3. We will also connect an AC signal at the input that will make Vin go above or below Vref. We have chosen a very low frequency for the input signal so we can visualize the LED lightning ON and OFF.

Figure 4- 2: LED library in Multisim

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Figure 4- 3: Comparator with LED

1.6 Run the simulation and observe the output of the LED. You should see the LED flickering ON and OFF depending on the comparison between the input signal and Vref.

1.7 Feel free to experiment with different frequencies for the input signal.

1.8 Connect Channel A of the oscilloscope to the input signal and Channel B at the output of the Operational Amplifier and run the simulation. Explain the waveform at the output. Include screenshots

1.9 Modify the voltage divider made by R1 and R2 so Vref is approximately equal to 8 V.

1.10 Run the simulation again. Explain the new waveform you obtain now.

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2.- Integrator.

2.1 The circuit shown in Figure 4-4 is a practical integrator. The resistor R2 is used to neutralize the effects of the offset voltage.

Figure 4- 4: Practical integrator circuit

2.2 The input signal that we will use is a square signal. To select this signal in Multisim, we will do the following:

Fron the “Sources” Library, select “Signal_Voltage_Source” and then “Pulse_Voltage” as shown in Figure 4-5

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Figure 4- 5: Selecting Square Signal

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Double clicking on the component will bring the following dialog box (Figure 4-6):

Figure 4- 6: Parameters for the Square Signal

These are the main parameters to select:

Initial Value = - 0.75 V Pulsed Value = 0.75 V Pulse width = 0.4 ms Period = 0.8 ms

2.3 Connect Channel A of the Oscilloscope to the Input signal and Channel B to the Output of the Op Amp. Run the simulation. Comment on the signal obtained. Don’t forget to include screenshots as necessary.

2.4 Compare your measured values with your calculated results and comment on them.

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3.- Differentiator

3.1 Build the circuit shown in Figure 4-7, that is a basic differentiator.

Figure 4- 7: Basic Differentiator

3.2 In order to connect an input signal, we will use a new instrument that is the Function Generator. This is available through the menu of instruments on the right as shown in Figure 4-8. Alternatively, we can also bring the Function Generator from the following Menu as shown in Figure 3-9: Simulate  Instruments  Function Generator.

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3.3 Double-clicking the Function Generator will bring its control panel as shown in Figure 4-10.

Figure 4- 8: Function Generator from Instruments Menu

Figure 4- 9: Function Generator from Main Menu

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Figure 4- 10: Control Panel for Function Generator

As we can see in Figure 4-10, the Function Generator allows for Sine, Square and Triangular signals for which we can vary the amplitude and frequency. This is similar to the signals that we have used in the past although those have more parameters that can be adjusted, for example their phase.

3.4 Configure the Function Generator as follows:

- Signal: Triangular - Frequency: 1 kHz - Duty Cycle: 50% - Amplitude: 5 Vp - Offset: 0 V

Also, connect the terminal marked as ‘+’ to Vin and the terminal marked as ‘-‘ to ground.

3.5 Connect Channel A of the Oscilloscope to the Input signal and Channel B to the Output of the Op Amp. Run the simulation. Comment on the signal obtained. Don’t forget to include screenshots as necessary.

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3.6 Compare your measured values with your calculated results and comment on them.

Laboratory Report Create a laboratory report using Word or another word processing software that contains at least these elements:

- Introduction: what is the purpose of this laboratory experiment?

- Results for each section : Measured and calculated values, calculations, etc. following the outline. Include screenshots for the circuits and waveforms as necessary -- You can press Alt + Print_Screen inside Multisim or if using Windows 7, you can use the “Snipping tool”. Either way, you can paste these figures into your Word processor. Also include here the charts and graphs that you have created with the data you have collected.

- Conclusion : What area(s) you had difficulties with in the lab; what did you lean in this experiment; how it applies to your coursework and any other comments.