Lab3_20200916 (1)

pdf

School

University Of Arizona *

*We aren’t endorsed by this school

Course

220

Subject

Electrical Engineering

Date

Feb 20, 2024

Type

pdf

Pages

Uploaded by JusticeGerbilMaster969

ECE 220 Laboratory 3 Thevenin Equivalent Circuits, Constant Current Source, and Inverting Amplifier Michael W. Marcellin Please follow all rules, procedures and report requirements as described at the beginning of the document entitled ECE 220 Laboratory 1. Always wear your safety glasses when performing your lab experiments. Prelab Part 1 Consider the circuit shown below in Figure 1. We will consider three values for the load resistor 𝑅 ௅ . These values are 1.5 𝑘Ω , 6.8 𝑘Ω , and 200 𝑘Ω . a) Calculate 𝑣 ௅ in Figure 1, for each of the three values for 𝑅 ௅ . You may use any method you like, except for Thevenin/Norton or source transformations. Figure 1. Original circuit. Now consider the circuit in Figure 1 from the point of view of 𝑅 ௅ . That is, consider the circuit with respect to the terminals labeled 𝑎 and 𝑏 with the load resistor removed. We know from class that we can compute the Thevenin equivalent of this circuit as shown below in Figure 2. b) Calculate the required values of 𝑣 ்௛ and 𝑅 ்௛ . c) Attach the three values for 𝑅 ௅ (one at a time) in Figure 2, and calculate 𝑣 ௅ for each. Figure 2. Thevenin equivalent circuit.

Prelab Part 2 All semester, we have been talking about current sources. We will now see how to make one. In this part, we consider a simple constant current source. Consider the circuit shown in Figure 3 below. Assume as usual that the op amp is ideal. Assume the power rails are set to ± 4.5 V. The left portion of the circuit uses the positive power rail, a 147 Ω resistor, and an LED to create a roughly constant input voltage (2.1 V) to the op amp circuit on the right. We could use a voltage source, but this illustrates a useful trick. You can get other such “reference voltages” (any multiple of 0.7) using regular diodes. a) Calculate the load current 𝑖 ௅ in Figure 3. Figure 3. Constant current source. Note that your calculated value of 𝑖 ௅ is independent of the load resistor 𝑅 ௅ . Thus, from the point of view of the load resistor, the circuit is acting as a constant current source. However, there is a maximum value of 𝑅 ௅ for which this will be true. Hint: Think about the power rails. b) Calculate the maximum value of 𝑅 ௅ for which 𝑖 ௅ will remain constant. We will verify your calculations using PSpice. Start a new PSpice project as you have done before, and draw the circuit of Figure 4 below. The PSpice part name for the op amp is uA741. Don’t forget that you can use the ‘v’ hotkey to mirror the op amp part vertically. This puts the inverting input (and the negative power supply pin) on the top. As before, we use three diodes to simulate an LED. We introduce a new trick this time. The right side of the circuit shows two voltage sources connected together with a ground between them. The top node is labeled +supply, while the bottom node is labeled –supply. These labels are introduced by clicking Place->Power A new window will open. From the scroll down list, select the part VCC. As always, ignore the library name. Enter a name (in the middle near the bottom of the window). In Figure 4, the names +supply and –supply are used, but you can choose any names you like. Click OK Place labels (one at a time - with different names) above and below the voltage sources as shown in Figure 4. The ‘v’ hotkey may be useful here as well. Wire the labels to the voltage

sources. You can now place the same labels elsewhere in the circuit. This has the same effect as connecting wire between all labeled pins of the same name. In particular, see the labels connected to the op amp power supply pins. The purpose of this trick is to avoid cluttering your drawing with lots of voltage sources and/or having to cross wires. Set up a DC sweep for the load resistor using the same procedure we have used in previous labs. Do a linear sweep of the load resistor from 100 Ω to 3 𝑘Ω with a step size of 10 Ω. Put a current marker on the left pin of the load resistor as shown in Figure 4, and run PSpice. The resulting graph should show 𝑖 ௅ as a function of 𝑅 ௅ . The load current should remain constant at the value you calculated above until the load resistor approaches the maximum value you calculated above. The load current should then decrease as the load resistance is increased further. Note that the load current starts decreasing before 𝑅 ௅ actually reaches the maximum value that you calculated above. Think about why this occurs. Hint: Look at the output voltage of the op amp. c) Print your graph. Figure 4. PSpice schematic. Prelab Part 3 Figure 5 below shows an inverting amplifier circuit. Assume power rails of ± 5 V. a) Calculate the value of 𝑣 ௢ in Figure 5. Note that this value does not depend on 𝑅 ௅ . Do a PSpice simulation of the circuit in Figure 5. Make a graph of 𝑣 ଴ as a function of 𝑅 ௅ . To do

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

this, perform a sweep of 𝑅 ௅ from 1 Ω to 500 Ω , with a step size of 1 Ω . You will note that 𝑣 ଴ is constant for all values of 𝑅 ௅ above some minimum value. At this minimum value of 𝑅 ௅ , the output current of the op amp reaches its maximum value. The op amp cannot provide more current to hold the voltage constant for smaller values of 𝑅 ௅ . b) Print your graph. c) Use the minimum value of 𝑅 ௅ noted above to calculate the maximum output current that can be provided by the op amp (according to PSpice). Note: You would normally just look up 𝑖 ௠௔௫ in the data sheet for the op amp. Figure 5. Inverting amplifier.

Prelab Grading Part 1 a) 3 pts b) 2 pts c) 3 pts Part 2 a) 3 pts b) 3 pts c) 4 pts Part 3 a) 1 pt b) 4 pts c) 2 pt _______ 25 total pts

Lab 3 Data Sheet Please follow all rules, procedures and report requirements as described at the beginning of the Lab 1 Data Sheet. Lab Grading: Data Sheet = 15 pts; Explanation = 10 pts. Lab Part 1 𝑣 ௢௖ = 𝑖 ௦௖ = 𝑅 ்௛ = 1 pt each 𝑅 ௅ = 1.5 𝑘Ω 𝑅 ௅ = 6.8 𝑘Ω 𝑅 ௅ = 200 𝑘Ω 𝑣 ௅ = 𝑣 ௅ = 𝑣 ௅ = 0.5 pt each 𝑅 ்௛ = 1 pt 𝑅 ௅ = 1.5 𝑘Ω 𝑅 ௅ = 6.8 𝑘Ω 𝑅 ௅ = 200 𝑘Ω 𝑣 ௅ = 𝑣 ௅ = 𝑣 ௅ = 0.5 pt each Lab Part 2 𝑅 ௅ = 100 Ω 𝑅 ௅ = 470 Ω 𝑅 ௅ = 1 𝑘Ω 𝑅 ௅ = 1.5 𝑘Ω 𝑖 ௅ = 𝑖 ௅ = 𝑖 ௅ = 𝑖 ௅ = 0.5 pt each Lab Part 3 𝑣 ௜ 𝑣 ௣ = 𝑇 = 𝑣 ௜ (𝑡) = 𝑣 ௢ 𝑣 ௣ = 𝑇 = 𝑣 ௢ (𝑡) = 1 pt each Speaker and volume control work: Y / N (circle one) 0.5 pt

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

Lab Experiments Note: You may need to use more than one resistor to get a desired resistance in this lab. Lab Part 1 Build the circuit of Figure 1. Leave the load resistor off. Use the DMM to measure 𝑣 ௢௖ and 𝑖 ௦௖ . Record these values in your data sheet. Remember, an ideal volt meter looks like a resistor with value 𝑅 = ∞ (open circuit), while an ideal current meter looks like a resistor with value 𝑅 = 0 (short circuit). While the DMM is not ideal, it is pretty good. Thus, you can connect it directly to terminals 𝑎 and 𝑏 to measure the relevant quantities. Use your measured values to compute 𝑅 ்௛ . Think about Method 1. Record your calculated value of 𝑅 ்௛ on your data sheet. Connect 𝑅 ௅ having values 1.5 𝑘Ω , 6.8 𝑘Ω , and 200 𝑘Ω (one at a time). Use the DMM to measure the value of 𝑣 ௅ for each value of 𝑅 ௅ . Record these values on your data sheet. Now, remove the voltage source (and load resistor) from your circuit. Replace the voltage source by a piece of wire (short). Use the DMM to measure 𝑅 ்௛ as the resistance between terminals 𝑎 and 𝑏 . Think about Method 2. Record this value on your data sheet. Build the circuit of Figure 2. Connect 𝑅 ௅ having values 1.5 𝑘Ω , 6.8 𝑘Ω , and 200 𝑘Ω (one at a time). Use the DMM to measure the value of 𝑣 ௅ for each value of 𝑅 ௅ . Record these values on your data sheet. Have your TA initial your results. Online students: take photos of your circuits and DMM showing each of your recorded values. Lab Part 2 Build the circuit shown in Figure 3. Use ±4.5 V for the power rails. We will use an ADTL082 op amp for this circuit. We used µA741 in the prelab because there is no model for the ADTL082 in the “lite” version of PSpice. For your convenience, the pin diagram of the op amp is shown below in Figure 6. Some discussion about the voltage sources in Figure 3 may be helpful. The power supply in the on-campus lab has two sets of outputs which can be used as two different voltage sources. Each set of outputs has a black terminal, a red terminal, and an (earth) ground terminal. As discussed in class, the “triangle” symbol that we use to mark the reference node (0 ground) is different than earth ground. We do not need an earth ground in this (or any other) 220 lab. Thus, make sure that the metal straps attached to the earth ground terminals on the power supply are not connected to any of the red or black terminals. Then use

a cable to connect the black (negative) terminal of one voltage source to the red (positive) terminal of the other voltage source. These two terminals that are connected together then form your reference node, NOT the earth ground terminal. This is illustrated in Figure 7. Online students: Your power supplies are already connected appropriately, with the black wire being 0 ground. Use the DMM to measure 𝑖 ௅ for four different values of 𝑅 ௅ = 100 Ω , 470 Ω , 1 𝑘Ω and 1.5 𝑘Ω . Record these values on your data sheet, and have your TA initial it. Online students: take a photo of your circuit and DMM showing each of your recorded measurements. Figure 6. ADTL082 op amp pin diagram. Note: The chip contains two op-amps. We will use only one of them for this part. Figure 7. On-Campus Power supply connections. Lab Part 3 Build the circuit shown below in Figure 8. Use ±5 V for the power rails. Again, use the ADTL082. Note the changes in this circuit as compared to the one in Figure 5 of the prelab. Connect a 10 𝑘Ω pot as a variable resistor as shown. Initially, set the pot so that the feedback resistor is 10 𝑘Ω . Use the function generator for the sin wave. Use the scope to view 𝑣 ௜ on channel 1. Set the scope to trigger on channel 1 with positive slope and 0 level so that 𝑣 ௜ looks like a sin (not cos, or other). Now, connect channel 2 of the scope to 𝑣 ௢ . Measure and record the amplitude and period of both 𝑣 ௜ and 𝑣 ௢ . Note that 𝑣 ௢ is inverted with respect to 𝑣 ௜ , as expected. Write an equation for both of these voltages.

Now, vary the pot setting to see its effect on 𝑣 ௜ and 𝑣 ௢ . Make sure to investigate pot settings below 1 𝑘Ω . Replace the 20 𝑘Ω resistor with the speaker. Note that this is an 8 Ω speaker. For high gain settings (large 𝑅 ௙ ), the op amp will not be able to supply enough output current to yield the desired results (see Prelab Part 3). So, keep the pot setting below half or so. Observe the effect of varying the pot setting. Observe the effect of varying the frequency of the sin wave. Have the TA initial your results. Online students: make a video (with audio) for the sin wave that shows the speaker getting louder and softer, as well as the frequency changing higher and lower. Figure 8. Inverting amplifier with volume control.

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version