ECE 2006 University of Minnesota Duluth Lab 5 ECE Department Page 1 February 26/28, 2007 Figure 1: Operational Amplifier Diagram Operational Amplifiers 1. Introduction The student will be introduced to the application and analysis of operational amplifiers in this laboratory experiment. The student will apply circuit analysis techniques to study circuits containing operational amplifiers. 2. Background Operational Amplifiers, or Op Amps, are undoubtedly the most versatile analog device in common use. In addition, circuit analysis of Op Amp circuits is a straightforward endeavor. It has become common practice therefore to introduce Op Amp circuits to beginning engineering students as a means to reinforce their newly acquired analysis skills. Without getting into the details of design and construction, an Op Amp can be modeled as shown in Figure 1. It can be seen in Figure 1 that the difference in voltage across the input terminals, v+(t) and v-(t), is multiplied by the gain, A, and is available at the output terminal as vout(t) (with respect to ground). The ideal Op Amp is characterized by the following parameters: • Ri (the input impedance) is infinite. • Ro (the output impedance) is zero. • A (the open loop gain) is Infinite. From this idealization, it is possible to make the following assumptions: • iin(t) (the input current to the Op Amp) is zero. • vd(t) = v+(t) – v-(t) = 0 • Thus, v+(t) = v-(t) These conditions make Nodal Analysis of an ideal Op Amp circuit very simple.ECE 2006 University of Minnesota Duluth Lab 5 ECE Department Page 2 February 26/28, 2007 Figure 2: DC Voltage Divider Figure 3: LED Circuit 3. Procedure 3.1. Equipment • PSpice® on Personal Computer • Agilent E3631A DC Power Supply • Fluke 8050A Digital MultiMeter (DMM) • (1) 100 kΩ, (1) 390 kΩ, (1) 10 kΩ, and (2) 1 kΩ Resistors • (1) LM 741 Op-Amp • (1) Light Emitting Diode (LED) 3.2. PRELAB – Theoretical Procedure and Datasheet Using circuit analysis techniques, analyze the circuit in Figure 3 to solve for Vo. Assume that the effective resistance of the Light Emitting Diode (LED) is 50 kΩ. Record your result in Table 2. Open the datasheet for the LM741 (found under the reference material on the ECE 2006 web page). In addition to noting the pinout diagram, find the following and record your answers in Table 3: • The maximum (in magnitude) supply voltages • The maximum (in magnitude) input voltage • Slew Rate (the maximum rate of change in the output voltage) • List all the types of elements present in the schematic diagram (actual circuit) Obtain your instructor’s signature for completing the prelab. 3.3. Experimental Procedure Connect the DC circuit shown in Figure 2. Power up the adjustable DC power supply and set it for an output voltage of 6.00 Volts. Turn ON the output of the power supply. Measure Vo, the voltage drop across the 390 kΩ resistor, using the DMM. Record this result in Table 1. Turn OFF the output of the power supply. Now connect a Light Emitting Diode (LED) across Vo as shown in Figure 3.ECE 2006 University of Minnesota Duluth Lab 5 ECE Department Page 3 February 26/28, 2007 Figure 4: Voltage Follower Turn ON the output of the power supply. Measure again the output voltage, Vo, using the DMM. Record this result in Table 2. Why is the value of Vo different? Answer this question in your lab report. Turn OFF the output of the power supply. Calculate the “effective resistance” of the LED by performing nodal analysis at the output node (between the 100 & 390 kΩ resistors) and record your result in Table 2. A Vo of approximately 2.0 Volts or above is sufficient to make the LED glow, provided that it receives enough current. Does the LED turn on (light up) in this circuit? Answer this question in your lab report. Insert an Op Amp into the previous network in order to produce the circuit shown in Figure 4 (note that a minus-six volt source is needed to prevent saturation). The Op Amp circuit in Figure 4 is called a “Voltage Follower” circuit (also “buffer” or “unity gain” circuit), denoted by the unity feedback loop to the inverting input (i.e. vOut is short-circuited to v-). Similarly to the lab with the transistor, a voltage follower is useful when the same voltage is needed with a greater amount of current (and therefore power, too). As stated in the background, the theoretical difference between v- and v+ is zero. Thus, since there is a direct connection between v- and vOut, the positive input voltage would also be equal to vOut. Lastly, since the positive input voltage is directly connected above the 390 kΩ resistor, this produces vo ≈ vOut.ECE 2006 University of Minnesota Duluth Lab 5 ECE Department Page 4 February 26/28, 2007 Figure 5: Inverting Amplifier Turn ON the output of the power supply. Measure Vo and VOut with the DMM. Record these results in Table 4. Turn OFF the output of the power supply. Remove the LED. Turn ON the power supply and measure VOut now that the load (LED) has been removed. Record this measurement in Table 4. Turn OFF the output of the power supply. Compare Vo for Figure 4 with Vo for Figures 2 and 3. Which is it closer to? Does the LED turn on (light up) in this circuit? Answer these questions in your lab report. Describe the impact of putting the Op Amp Voltage Follower between the output voltage, Vo, and the load (the LED). Include this discussion in your lab report. The Op Amp is probably the most versatile analog chip available. It has a host of applications in a broad range of circuits. The key to making Op Amps do different things is to understand the impact of feedback on Op Amp performance. The first step to such understanding is to analyze the Inverting Amplifier circuit. Connect the Op Amp circuit shown in Figure 5. Turn ON the output of the power supply and verify Vs and VOut with the DMM. Record these results in Table 5.ECE 2006 University of Minnesota Duluth Lab 5 ECE Department Page 5 February 26/28, 2007 Turn OFF the power supply output. Now exchange R1 and R2 resistors, such that the circuit is the same, but R1 = 390 kΩ and R2 = 100 kΩ. Turn ON the output of the power