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 R₁. These values are 1.5 km, 6.8 k, and 200 kn. a) Calculate vд 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. 2.2 ΚΩ w 300 Ω w a + 5 V 3 ΚΩ RL VL - b Figure 1. Original circuit. Now consider the circuit in Figure 1 from the point of view of RL. 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 vãn and Räh· c) Attach the three values for RÅ (one at a time) in Figure 2, and calculate vɩ for each. VTh RTh ww a b 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 £ 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 i̟ in Figure 3. 147 Ω 4.5 V 1068 Ω w RL www iL LED ✓ Figure 3. Constant current source. Note that your calculated value of i̟ 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 i̟ 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 to 3 k with a step size of 10 2. 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 i̟ 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. +Supply w R3 147 D1 R1 w 1068 D1N4002 R4 w {RL} -supply U1 4 UA741 V- 6 OUT 05245 V+ D2 D1N4002 +supply D3 D1N4002 PARAMETERS: RL = 1.5k +supply V6 4.5Vdc V7 4.5Vdc -supply 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 vo 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 this, perform a sweep of RÅ from 1 to 500 £, with a step size of 1 N. You will note that vo 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. L 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 imax in the data sheet for the op amp. -1 V Vi 1 ΚΩ ww 3 ΚΩ W Figure 5. Inverting amplifier. Vo Prelab Grading Part 1 a) b) c) 3 pts 2 pts 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