University of Portland EE 271 Electrical Circuits Laboratory. Experiment: Op Amps

Transcription

1 University of Portland EE 271 Electrical Circuits Laboratory Experiment: Op Amps I. Objective The objective of this experiment is to learn how to use an op amp circuit to prevent loading and to amplify a small signal. II. List of Needed Components This experiment requires the following components: Six 1 kω resistors One 10 kω resistor One LM741 Op Amp IC One 100 kω potentiometer III. Background An op amp is an integrated circuit that can be used to construct a variety of useful circuits. Although the op amp circuit is complicated, under normal operating conditions, it can be mathematically modeled by the ideal op amp model where the voltage between the inverting and non-inverting inputs is assumed to be 0 Volts, and the current into the inverting and non-inverting inputs is assumed to be 0 Amps. Using these assumptions, it can be shown that the output of the circuit in Figure 1 is equal to the input: VVVV = VVVV. Since the output voltage equals the input voltage, this circuit is called a voltage follower. Figure 1: Voltage Follower Circuit University of Portland - p. 1 of 8 - Exp - Op Amps.docx

2 An amplifier is a circuit whose output voltage is the input voltage multiplied by constant called the gain: VVVV = VVVV GGGGGGGG. The gain of an amplifier can be computed by dividing the peak (or maximum voltage) voltage of the output by the peak voltage of the input: GGGGGGGG = VVVV VVVV. It can be shown that the output in the circuit in Figure 2, is given by VVVV = RR 2 VVVV and RR 1 VVVV (pppppppp) GGGGGGGG = = RR2 RR1 VVVV = RR 2. Since the gain of this circuit is negative, the output of VVVV (pppppppp) VVVV RR 1 the circuit is inverted as compared to the input (that is when the input is positive, then the output is negative, and vice versa), and so this circuit is called an inverting amplifier. Figure 2: Inverting Amplifier The output of the differential amplifier in Figure 3 is given by VVVVVVVV = RR ff RR aa (VV 2 VV 1 ). This circuit can be used to amplify the difference of two input signals. Figure 3: Differential Amplifier University of Portland - p. 2 of 8 - Exp - Op Amps.docx

3 IV. Prelab Assignment 1. Find the voltage Vi in the circuit in Figure 4 for the three values of RL listed in Table Find the voltages Vi and Vo in the circuit in Figure 5 for the three values of RL listed in Table Find the gain for the inverting amplifier in Figure 8. V. Procedure Part 1: Loading In this part of the experiment, we will demonstrate a very common problem in circuits where the output voltage of a circuit changes when you connect a load to it. For example, connecting the output of a sensor to a circuit such as a measurement device, amplifier, or analog-to-digital circuit, may actually change the output voltage of the sensor, which may cause measurement errors or other problems. This undesirable effect is called loading. In order to demonstrate the loading problem, we will use a voltage divider circuit as a model for a sensor and a resistor as a model for the measurement device or other circuit. Construct the circuit shown in Figure 4 with R1 = 2 kω and R2 = 1 kω. The output of the voltage divider circuit, which is labeled Vi, simulates the voltage that would be generated by a sensor and would be the input voltage to a circuit or measurement device. Initially we will set the load resistor RL to an open circuit (or infinite resistance) by removing the load resistor. Record the measured value of Vi (see Table 1) and compare it to the value you computed in the prelab. Figure 4: Model for Sensor with Load Resistor University of Portland - p. 3 of 8 - Exp - Op Amps.docx

4 Next connect a load resistor with RL = 10 kω to the circuit and measure Vi. Then change the load resistor to RL = 1 kω and measure Vi. If the output voltage Vi changes when a circuit is connected to it, we say that the output has been loaded down. Do the load resistors load down the output of this circuit? Table 1: Vi with Different Load Resistors Load Resistance RL Vi Open Circuit (remove RL from circuit) 10 kω 1 kω If we were using a sensor to measure a quantity, such as strain, temperature, etc., we would not want the measuring device to change the output voltage from the sensor because that would cause error in the measurement. That is why most voltage measuring devices, such as the DMM or oscilloscope, are designed so that their inputs behave like very large resistances. However, for situations where the sensor will be connected to a circuit whose input does not behave like a very large resistance, we can avoid loading by adding a voltage follower circuit. Since the input of the op amp behaves like a very large resistance, it will not load down the sensor output. Part 2: Voltage Follower Circuit In this part of the experiment, we will use a voltage follower circuit to prevent loading. Connect a voltage follower circuit to the output of the voltage divider as shown in Figure 5. Note that there is a convention that a component connected to an arrow marked with +15V is connected to the +15V power supply. So Pin 7 on the LM741 op amp is connected to the +15V supply in Figure 5 (just like R1). Likewise the arrow marked -15V indicates a connection to the -15V supply. This convention simplifies the schematic diagram for some circuits. Figure 5: Model for Sensor with Voltage Follower Circuit University of Portland - p. 4 of 8 - Exp - Op Amps.docx

5 It is important to insert the op amp with the correct orientation to avoid burning it out. The op amp has a U-shaped mark and/or a dot on the top of the package near Pin 1. Place the op amp so that it straddles the gap in the center of the breadboard with Pin 1 toward the top of the breadboard as shown in Figure 6. The pins on the op amp are numbered as shown in Figure 7. Figure 6: Placement of the Op Amp Figure 7: Pinout for the 741 Op Amp Measure Vi and Vo in Figure 5 when the load resistor is an open circuit, 10 kω, and 1 kω (see Table 2). Compare your results with those from Part 1. Does the voltage follower circuit reduce the amount of loading? Explain how a voltage follower could reduce measurement error when a sensor needs to be connected to a circuit whose input does not behave like a very large resistance. Table 2: Vi and Vo with Different Load Resistors Load Resistance RL Vi Vo Open Circuit (no load resistor) 10 kω 1 kω University of Portland - p. 5 of 8 - Exp - Op Amps.docx

6 Part 3: Inverting Amplifier Construct the circuit shown in Figure 8 where R1 = 1 kω and R2 = 10 kω. Set the function generator to produce a 1 khz sinewave with peak amplitude of 1 Volt and no D.C. offset. Set up the oscilloscope so you can observe both the input and output signals at the same time. Sketch the oscilloscope display and label the maximum and minimum voltages and the period. Also, compute the fundamental frequency. Measure the gain of the circuit in Figure 8 and compare it to the theoretical value that you calculated in the prelab by computing the percent error. Slowly increase the peak amplitude of the input signal until the shape of the output signal becomes distorted. Sketch the oscilloscope display and label the maximum and minimum voltages and the period. Describe what happens to the output signal when the input amplitude is too big. Figure 8: Inverting Amplifier University of Portland - p. 6 of 8 - Exp - Op Amps.docx

7 Part 4: What does eeeeee look like? In addition to producing sound, the speaker can also be used as a microphone to convert sound waves into a voltage. Connect the oscilloscope to measure the voltage across the speaker in the Proto-board. The speaker is not polarized, so you can connect the speaker to the oscilloscope with either polarity. Tap on the speaker or talk into it. Can you see the signal on the oscilloscope? It is likely that the signal from the speaker is too small to see it on the oscilloscope. Disconnect the function generator in the circuit in Figure 8, and replace it with the speaker. Then observe the output Vo on the oscilloscope. Can you see the signal when tap on the speaker or talk into it? It is likely that the signal from the speaker is still too small to see it on the oscilloscope. Modify the circuit so that its gain is high enough so that the signal from the speaker be easily measured on the oscilloscope. Draw a schematic of the circuit that you designed and show how it was connected to the speaker. Make a sketch of the signal from the oscilloscope while making the eeeee sound into the speaker. Label the maximum and minimum voltages and the period. Calculate the fundamental frequency. Part 5: Differential Amplifier In this part of the experiment, we will use a differential amplifier to add a D.C. offset to a sinewave. Build the circuit in Figure 9 with Ra = 1 kω and Rf = 2 kω. Be careful to connect the power supply and ground to the outside pins on the potentiometer (pins 1 and 3), NOT the center pin (pin 2). Set the function generator to produce a 1 khz sinewave with peak amplitude of 1.0 Volt and no D.C. offset. Use the DMM to monitor V2 and set the potentiometer so that V2 is 1 Volt. Set up the oscilloscope so you can observe both the input V1 and output Vo at the same time. Make a sketch of the output Vo and compare it to the expected output using percent error. Adjust the potentiometer and observe the effect on Vo. Now switch the oscilloscope to AC coupling and observe Vo while adjusting the potentiometer. If we wanted to know the actual voltage of the output including the DC offset, which mode should we select, AC coupling or DC coupling? University of Portland - p. 7 of 8 - Exp - Op Amps.docx

8 V 1 R a R f +15V +15V Function Generator ~ 100k Pot V 2 R a R f 2 _ 3 + LM V V o Figure 9: Differential Amplifier VI. Conclusion Write a paragraph that summarizes what you have learned in this lab. What does an op amp do? What are some of the uses of an op amp? What happens to the output of the op amp if the input is too big? University of Portland - p. 8 of 8 - Exp - Op Amps.docx