Elec 161 Electronics II Module 1

ELEC 161 – Module 1 Laboratory - Page 1

ELEC 161 Electronics II Module 1 Lab: Basic circuits based on Operational Amplifiers

Introduction.- This laboratory experience introduces students to the basic circuits based on Operational Amplifiers as well as the common instrumentation and measurement practices for these circuits.

Being this the initial lab for ELEC 161, we will spend extra time with the basics for setting up the circuits and instruments in Multisim. Make sure you understand these concepts as you will be required to do those on your own in upcoming experimental exercises.

Procedure

1.- Inverting Amplifier

1.1 Procedures for gain calculation

Build the circuit shown in Figure 1-1 that is a basic inverting amplifier. Note that we will not connect Pins 1 and 5 (these pins are used for internal offset adjustments if necessary).

Figure 1- 1: Circuit for Inverting Amplifier

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Note the two voltages used to power the Omp Amp at Pins 7 and 4. The voltage at Pin 7 is + 12 V respect to ground while the voltage at Pin 4 is -12 V respect to ground.

The uA741 Operational Amplifier is available in the Op Amp family of Multisim as shown in Figure 1-2. You can see how there are several other Op Amps, each one with their own specific characteristics for determined applications.

Figure 1- 2: Library for Operational Amplifier selection

The next step is to include a voltage source that will be used as an input signal. Make sure to use the “AC Voltage” source instead of the Power Source as shown in Figure 1-3. We will select a sine signal with an peak amplitude of 10 mV and a frequency of 2 kHz. The parameters of the signal can be adjusted by double clicking the voltage source.

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Figure 1- 3: Selecting input signal

The resulting circuit is shown in Figure 1-4.

Figure 1- 4: Inverting Amplifier

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We will use the oscilloscope to measure the gain of the circuit. We will connect Channel A at the input source and Channel B at the output node as shown in Figure 1-5.

Figure 1- 5: Connecting the Oscilloscope

To facilitate distinguishing Channel A and Channel B on the oscilloscope, we will change the color of the trace. . By default both wires are red. We will leave this color for Channel A and will change Channel B to Blue. Right click with the mouse on the wire for channel 2 and change the color of the wire to blue (or any other color different than red).

Run the simulation and double click on the Oscilloscope Icon to open it. If you are using the default values in Multisim, it will be difficult to read and visualize the output. You will get something similar to Figure 1-6:

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Figure 1- 6: Oscilloscope view with default settings

To better visualize the signal on the oscilloscope, we can do the following:

- Click on the Reverse button, so the background becomes white. - Change the Channel Amplitude for Channel A and Channel B until you can see the amplitude of the

signals. - Change the Timebase until you can see approximately 6 to 10 signal periods on the screen.

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After changing the settings you should see the output of the oscilloscope similar to the image shown in Figure 1-7.

Figure 1- 7: Resulting signal after adjusting settings

- If the signal appear unstable on the screen, change the Trigger Options to either Normal or Auto. You may have to experiment for the best options.

- You will notice that the blue signal (output) appears shifted upwards. We can solve this by pressing the AC mode in the AC/DC coupling.

Feel free to experiment with the other settings of the oscilloscope. These can help you in future experiments.

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The gain of the amplifier can be measured as 𝐴𝑣 = 𝑉𝑜𝑢𝑡 𝑉𝑖𝑛

To measure Vout and Vin, we can use the cursors in the oscilloscope that are initially located at the right and left corners of the screen as shown in Figure 1-8.

Figure 1- 8: Initial cursor locations

Drag the red cursor until it gets lined up with the peak of the red signal (Channel A or Input) and drag the blue cursor until it lines up with the peak of the blue signal (Channel B or Output) as shown in Figure 1-9.

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Figure 1- 9: Using cursors to measure amplitude

The cursors indicate the following values:

- Channel A (input) : 9.937 mV peak (note that it is very close to the 10 mVp signal at the input). - Channel B (output): 49.698 mVp

Therefore the gain of the circuit can be calculated as: 5.0013. Note that this is very close to the gain given by the formula for the Inverting Amplifier for these values of resistors. Also note that input and output signals have a 180® phase shift.

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1.2 Student work for Inverting Amplifier.

On the previous circuit, change the resistor of 100 kΩ with a resistor of 470 kΩ and change the resistor of 20 kΩ with a resistor of 33 kΩ.

Configure the input voltage source for a peak voltage of 200 mVp and a frequency of 500 Hz.

- Calculate the theoretical gain of the circuit. - Measure the gain of the amplifier using the procedure developed in Section 1.1. You will have to include

screen shots in your lab report, so make sure your circuit is working correctly. - Comment on the differences between theoretical and real gain of the amplifier.

2.- Non-inverting Amplifier

2.1 Circuit design

Consider the circuit shown in Figure 1-10 that is a basic Non-Inverting amplifier.

Figure 1- 10: Non-Inverting Amplifier

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- Design the value of the Resistor R2 so the overall gain of the circuit is equal to 5. R2 = _______

- Build the circuit and measure the gain of the circuit using the procedure shown earlier. Include screenshots as necessary.

- Compare the actual and theoretical gain of the circuit and extract conclusions.

3.- Buffer or Unity Follower

We have studied the Buffer or Unit Follower as a circuit in which Vout = Vin.

3.1 Create in Multisim a Unity Follower circuit. Attach screenshots to your lab report.

3.2 Connect a voltage source at the input. Choose appropriate values for amplitude and frequency for the circuit to work correctly. Again, attach the appropriate screenshots.

3.3 Increase the amplitude of the input signal until the output signal starts to distort or clip. Distortion or clipping occurs when the output signal starts to look different form the input signal. Figure 1-11 shows an example of distortion or clipping (this figure only shows the output signal for clarity)

Figure 1- 11: Clipped signal

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What is the value at which distortion or clipping starts to appear?

What do you think originates the distortion or clipping?

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.