EE 210: CIRCUITS AND DEVICES

Transcription

1 EE 210: CIRCUITS AND DEVICES OPERATIONAL AMPLIFIERS PART II This is the second of two laboratory sessions that provide an introduction to the op amp. In this session you will study three amplifiers designs: (1) the voltage follower/buffer, (2) an inverting summing amplifier, and (3) a voltage level display using the op amp in comparator mode. Experiment 1: Voltage Follower/Buffer Loading Effect The purpose of this experiment is to study how the buffer is used to overcome the loading effect for a practical source such as the Function Generator. Function Generator v s R s i o v o i L v L Load Resistor R L Figure 1: Network to evaluate loading effects Procedure: 1. Construct on your prototyping board the voltage follower network shown in Figure 2; use a 68 kω load resistor. a. Set up the Function Generator to serve as v in, which should be a sine wave with frequency of 1 khz, amplitude of 5 V pp, and dc offset of 1 V. b. Set up the oscilloscope to observe v in on CH 1 and v out CH 2 as waveforms. 2. Create a table in your laboratory notebook that has the following column headings: nominal R load, actual R load, v L ac amplitude, v L dc offset, i L ac amplitude, and i L dc offset.

2 3. Measure and record the value of the ac amplitude and dc offset of v L for the following values of load resistor: 68 kω, 680 Ω, 360 Ω, 100 Ω, and 51 Ω. a. Measure and record the value of each load resistor before inserting it into the prototyping board. b. Measure and record the value of the ac amplitude and dc offset of v L. c. Calculate the corresponding ac amplitude and dc offset of i L using Ohm s law. 4. Discuss the effect/purpose of the op amp buffer circuit. 5. Experiment 2: Inverting Summing Amplifier The purpose of this experiment is to design an inverting summing amplifier. This can be done by applying the concept of superposition to the standard inverting amplifier design as discussed previously. In this application, two voltage signals from two separate sources are added to each other, resulting in a single signal at the output. An example of where this circuit is useful is for converting stereo audio signals to mono for playback through a single speaker. This circuit can also be used to average signals together from multiple sensors in order to get a more accurate reading of whatever property you are measuring. Figure 2: Inverting Summing Amplifier Procedure: 1. Design the values values of R 1, R 2 and R 3 so that: a) the gain Vout/V_left = Vout/V_right, and b) for V_left = V_right = 1 Vpp, the voltage amplitude of the output is 10 Vpp. Since this is a linear circuit, you can compute the gain Vout/V_left and Vout/V_right via superposition. For these resistor values chosen, compute Vout with V_left = 1Vpp and V_right = 0V. EE210 Lab Op Amps 2 page 2

3 2. Construct the circuit in Figure 2 with the values of R 1, R 2, and R 3 Measure and record the actual values of R 1, R 2, and R 3 before inserting them into the circuit. 3. Since this is a linear circuit we can apply the property of superposition. This means that we can experimentally measure the output of the circuit for V_right with V_left set to zero, and V_left with V_right set to zero. The actual output will be the sum of the two responses. a. Set up the Function Generator to serve as V_right, which should be a sine wave with frequency of 440 Hz, amplitude of 1 V pp, and dc offset of 0 V. Connect the function generator to the V_right input terminal. b. Short circuit V_left to ground. c. Confirm that V_out is as predicted. This step is important in order to determine if your circuit is operating properly. d. Disconnect the function generator and open-circuit V_left. e. Set up the Function Generator to serve as V_left, which should be a sine wave with frequency of 3520 Hz, amplitude of 1 V pp, and dc offset of 0 V. Connect the function generator to the V_left input terminal. f. Short circuit V_right to ground. g. Confirm that V_out is as predicted in the prelab. This step is important in order to determine if your circuit is operating properly. h. Disconnect the function generator and open-circuit V_right. 4. We will now test this circuit by feeding it with separated L and R channels from two function generators one of then configured for V_right and the other one configured as V_left. 5. Sketch the waveforms for V_left, V_right and Vout Comment on your results. Experiment 3: Voltage Level Display The purpose of this experiment is to use opamps in comparator mode, along with light emitting diodes (LEDs), to create a simple voltage level meter. As seen in a previous experiment, when an opamp is employed without negative feedback, its output saturates to the positive or negative voltage rail depending on whether the voltage level at the inverting input or the non-inverting input is greater. (Comparators compare the voltage level at each of the inputs!). In this experiment, we ll use comparators to compare a voltage signal with a series of reference voltages and use the output of the comparators to light a string of LEDs. Specifically, we will design a simple 2-level voltage level display that will light one LED when the input signal reaches 2.5 V and light a 2 nd LED when the input reaches 5 V. EE210 Lab Op Amps 2 page 3

4 Procedure: 1. When we build our level meter, we will need some fixed voltages to serve as references of comparison. We can produce these voltage references by dividing down the positive voltage rail used to power the op amps. See Figure 3. Choose values for R1, R2, and R3 such that V1 = 5V and V2 = 2.5V. 15V R1 V1 R2 V2 R3 Figure 3 Voltage References 2. Using your results in step 1, design and build the 2-level voltage level meter in Figure 4. Use 1 kω as the initial value for Rlimiting. +15 R1 Vin 3 8 U1A 1 2 R2 4 R limting -15 LED U1B 7 6 R3 4 R limting -15 LED2 Figure 4 Two LED Level Meter EE210 Lab Op Amps 2 page 4

5 3. With the circuit powered, but without applying any input signal, measure the voltage level at the inverting input of each of the comparators and record these values in your Lab Notebook. Verify that these reference voltages are identical to the desired references from step Supply a voltage at Vin. Start at 0V and increase the voltage slowly until the first LED comes on. Measure this input signal voltage and record the value in your Lab Notebook. Continue to increase the voltage until the second LED comes on. Measure the signal voltage again and record the value in your lab notebook. Compare the operation of this voltage level display with the desired operation. 5. Now use the function generator to change Vin to a 440 Hz sinusoid with amplitude of 1V. This is a reasonable mathematical model of a typical voice/music signal. Slowly increase the amplitude of this sinusoid and observe what happens to the LEDs as the input signal s amplitude increases beyond the 2.5 V and 5.0 V thresholds. When the LEDs are on, do they appear to be steady or blinking? Are they actually steady or not? Explain in your lab notebook. WRITE A LABORATORY REPORT FOLLOWING THE GUIDELINES FOR LABORATORY REPORTS IN THIS COURSE. EE210 Lab Op Amps 2 page 5