Operational Amplifiers and Wheatstone Bridge Circuits - Experiment | MAE 170, Lab Reports of Mechanical Engineering

Download Operational Amplifiers and Wheatstone Bridge Circuits - Experiment | MAE 170 and more Lab Reports Mechanical Engineering in PDF only on Docsity! Experiment 4: Operational Amplifiers and Wheatstone Bridge Circuits Having prepared for the lab experiment (including reviewing Experiment 4, reading chapter 3 in the textbook, demonstrating that your LabVIEW vi for this week is working, reading the files on this week’s web page) you should be able to answer the following questions. Pre-Lab Questions 1. Consider an inverting 741 amplifier with Ri=1 k and Rf=50 kSee Figure 2.) Given an input voltage of 10 mV, what is the output voltage? If the output voltage were connected to a computer, what voltage would the computer read? 2. Repeat question 1 for an input voltage of 1 V. 3. Repeat question 1 for a non-inverting amplifier. 4. Sketch the circuits for: (1) a non-inverting op-amp, (2) an inverting op-amp, and (3) a Wheatstone bridge. 5. What are the two main properties of an ideal amplifier, and why are they desirable? 6. What is the relationship between the values of the four resistors of a Wheatstone bridge when the bridge is in balance? Objectives 1. Construct an inverting op-amp, measure its frequency response, and understand its notable characteristics (i.e. clipping, drift). Explore another op-amp circuit of your choice. 2. Use a Wheatstone bridge to measure an unknown resistance. Lab Procedure Figure 1 shows the pinout for the 741 amplifier that you will use in constructing the operational amplifier circuits. 1. Note: Make sure that the DAQ is set for Reference Single Ended Mode. To change NI-DAQ settings: From the start menu (or desktop shortcut) open the "Measurement and Automation Explorer" program. In the menu on the left, expand the "Devices and Interfaces" option, then expand the "Traditional NI-Dac Devices" choice. Right-click on the "6024E" card, and select "properties". In the new window click on the "AI" tab, and select Mode “Reference Single Ended.” Note: you will need to change this back when you are doing your Wheatstone Bridge circuit. 2. Note: There is a small circle left side of the opamp, above pin one. Wire color codes; White wire – output from Opamp pin 6 to “O” scope and ACH1+ Blue wire - input signal at R1 to ACH0+ Red wire - input signal positive from Signal Generator Red wire - +15V to Opamp pin 7 Green wire - -15V to Opamp pin 4 Black wire - Signal Generator, “O” scope and Opamp ground (inverting Opamp pin 3) Brown wires - trim pot pins 1,2,3 to Opamp pins 1,4,5 LM 741 7 +V 8 N. C. 6 OUTPUT 5 OFFSET NULL OFFSET NULL 1 INVERTING INPUT 2 NONINVERTING INPUT 3 -V 4 - + Figure 1. Pinout for a 741 operational amplifier 3. Construct the inverting amplifier circuit shown in Figure 2 using Ri =1 k ,Rf =47 kand the blue bodied resistance trim potentiometer Rt . (Measure actual resistance values using the DMM.) For the input output signals and measurements you will use the Frequency Function Generator and the benchtop oscilloscope. a) Pin 1 of Rt connects to pin 5 of the Opamp, pin 2 ( center tap) to pin 4 ( -15V) of the opamp, and pin 3 to pin 1 of the opamp. 2 gainV V io   . ii. Again, make sure Vi goes to analog input channel 0 and Vo to channel 1. Insure that the vi dial is set for “Input channel 1”. iii. Launch the Frequency Response vi (from last week) Note: Multiply your cutoff freq by .1, .2, .4, .6, .8, 1, 4, 6, 8. iv. Plot the output, include proper scaling, labels, and comments. Consider the cutoff frequency you calculated. 10. Measure the DC offset drift. i. Turn off the power to the proto-board. Zero the 741’s input signal (as in step 4), restore power to the proto-board, and measure the output signal (without adjusting the trim pot). ii. Calculate the DC offset drift of the amplifier using this measurement, the initial measurement in step 4, and the time between measurements. Comment on the result. 11. Construct the Wheatstone bridge circuit of Figure 3. Use the decade resistance box in place of the unknown resistor, RX1. You will use the computer (via analog input channel (ACHO+ and ACHO-) to read the differential voltage across the “bridge voltage.” To do so, make sure the DAQ is in differential mode. Comment on why this is necessary. To change NI-DAQ settings: From the start menu (or desktop shortcut) open the "Measurement and Automation Explorer" program. In the menu on the left, expand the "Devices and Interfaces" option, then expand the "Traditional NI-Dac Devices" choice. Right-click on the "Elvis" option, and select "properties". In the new window click on the "AI" tab, and select “Differential.” 120  120  5 VDC RX1 RX2 = 120  To Computer And Digital Multimeter Decade Resistance Box Figure 3. Experimental Wheatstone bridge circuit 12. Use the “Calibrate for Wheatstone” vi to measure and record the bridge voltage as a function of RX1. i. Find and record the balance value of RX1 (where the bridge voltage is approximately zero). Record at least five data points on each side of the balance value. Do not exceed 10  from the balance value. Would your data vary if you selected points far from the balance value? ii. When you have finished taking data, select “plot all points,” and print the screen when the plot looks acceptable. Please do not save the data to a file. Note: Once you hit the “Done Totally” button, you will no longer be able to plot data. iii. Print the graph, label the axes, and include pertinent comments. Note that the graph is a plot of the bridge voltage versus 1/(RX1+120). Why is the independent variable 1/(RX1+120) instead of RX1? 13. Choose one of the following activities: Note: Change the ELVIS back to Single Ended Mode in the "Measurement and Automation Explorer". a. Construct a non-inverting amplifier circuit with a gain between 2 and 100. Measure and plot its characteristics (-3dB frequency and frequency response of gain and phase) and compare them to those of the inverting amplifier. What are some applications of inverting and non-inverting amplifiers? b. Construct and test an integrator, differentiator, or comparator circuit of your own design. (Literature on these circuits can be found at http://www.allaboutcircuits.com, and you are welcome to use additional/other sources.) Demonstrate the circuit(s) to a TA, and sketch the input and output signals. (Consider using an input signal other than a sine wave.) Include descriptions of the circuit’s behavior and comments on its comparison with theory. In what applications might the circuits be useful? c. Design and build either a Butterworth low-pass filter or a Butterworth high-pass filter. (Circuit diagrams are in chapter 3 of the textbook.) Using circuit analysis, calculate the -3dB frequency. Measure and plot the frequency response of the filter’s gain, and comment on its comparison with theory. (Consider -3dB frequency and gain rolloff.) In what ways are active filters better than passive filters? Applications The utility of op-amps extends far beyond simple amplification; they are an indispensable tool in creating circuit elements with complex functions. They are also widely used as buffers to protect output signals from loading. Below are some links to examples. Horowitz, Paul and Hill, Winfield. The Art of Electronics. Cambridge, UK: Cambridge University Press, 1989 http://www.allaboutcircuits.com http://web.telia.com/~u85920178/begin/opamp00.htm http://www.national.com/an/AN/AN-20.pdf