ECEG 350L Electronics I Laboratory Fall 2018

Transcription

1 ECEG 350L Electronics I Laboratory Fall 018 Introduction Lab #: The Basic Difference Amplifier [Lab Scores section revised 9/7/18] A common problem in the design of many communication and monitoring systems is that the cables used to carry signals from one location to another pick up interfering signals and noise radiated by other devices. For example, the output from a medical sensor might be transmitted to a monitor via a cable, but that same cable might also pick up 60 Hz common-mode energy from the clinic s AC wiring and electromagnetic radiation from nearby cell phones. These unwanted signals could corrupt the desired sensor signal, leading to misleading displays on the monitor. In this three-week lab exercise you will build a difference amplifier using the 741 op-amp and investigate its ability to amplify differential-mode signals while at the same time rejecting common-mode signals. Group assignments are listed at the end of this handout. Theoretical Background A circuit diagram of a simple difference amplifier is shown in Figure 1. The circuit amplifies the difference in the voltage values between inputs and v, giving the amplifier its name. v 3 v VPOS VNEG vo 4 Figure 1. A simple difference amplifier. The nodes marked VPOS and VNEG are the connections to the positive and negative power supplies, respectively. The small triangle represents a connection to the ground node. Power supply bypass capacitors are not included in the figure but should be added to the actual circuit. The analysis of the circuit is most easily accomplished using the principle of superposition and leads to an expression for the output voltage given by v o 4 = 1 v If the resistor ratios obey the relationship 4/3 = /, then the output voltage expression reduces to 1 of 7

2 v o = ( v ). 1 This ideal result depends on a perfect match between the two resistor ratios, a condition that cannot be achieved in practice with fixed resistors. As we have seen in the lectures, difference amplifiers are widely used to boost signals arriving via long cables that are susceptible to common-mode noise pick-up. Analysis of the circuit is therefore aided by defining the differential and common-mode input signals as v Id = v v 1 and v 1 v v Icm =. Solving these equations for and v yields the equivalent relationships = v Icm 0. 5v Id and v = v Icm 0. 5vId. The latter set of expressions suggests a way to model the differential and common-mode voltages applied to a difference amplifier. Figure shows a commonly used approach. VPOS vicm 0.5vId 0.5vId v 3 4 v VNEG vo Figure. Circuit model of the differential-mode and common-mode signals at the input terminals of a difference amplifier. The electrolytic capacitors connected to the power supply terminals help eliminate noise that enters the circuit through the power leads. The two voltage sources labeled 0.5vId represent a single differentialmode input signal; the split voltage sources are merely a modeling convenience. The common-mode signal is modeled as a single voltage source vicm referenced to ground, and the differential-mode signal is modeled as split voltage sources each of value 0.5vId connected to the two input terminals. The common-mode gain can be determined by finding the output voltage with the two 0.5vId sources set to zero (in which case the inputs and v are connected together). Similarly, the differential-mode gain can be determined by finding the output voltage with the vicm source set to zero. In the latter case, the connection to ground between the two 0.5vId sources should be preserved to facilitate the analysis. of 7

3 The complete expression for the output voltage in terms of the differential and common-mode input signals is (as derived in a separate set of course notes) v = v v 1 = 1 v 1 v 4 4 o od ocm Id Icm where vod and vocm are the components of the output voltage due to the differential-mode input voltage and the common-mode input voltage, respectively. If the relationship 4/3 = / is satisfied exactly, the first coefficient (the differential-mode gain) reduces to /, and the second coefficient (the common-mode gain) reduces to zero. In a practical amplifier, the variations of the resistor values from their nominal values and other imperfections prevent complete cancellation of the common-mode signal. Design Specifications Your assignment is to design, build, and test a difference amplifier like the one shown in Figure that meets the following specifications as closely as possible: Differential-mode gain (vod/vid): Ad = 1 V/V Min. differential-mode input resistance: id = 40 kω Power supply voltages: not critical, but at least ±1 V to avoid clipping Input bias current effects and input offset voltage effect minimized as much as possible Each resistor in your circuit must be a single fixed unit. That is, you may not combine multiple resistors in parallel or in series in an effort to obtain a total resistance close to the exact value you need. This restriction is typical in real-world design situations. Using more resistors than necessary adds cost, weight, and volume to a circuit. You should also connect 10 µf electrolytic capacitors between the op-amp s power supply pins and ground to suppress noise arriving through that path. (Watch the polarities!) Bypass capacitors should be mounted as close to the IC as possible using short leads. The use of power supply bypass capacitors is a good design practice and should be employed in almost all circuits. One of the tasks you will be required to complete will be to measure the output voltage of the amplifier when the inputs are driven by a purely common-mode signal. The resulting output voltage should be very low, possibly as low as 1-10 mv. The measurement could be corrupted by the presence of an error voltage at the output of the op-amp due to the combined effects of the input bias currents and the input offset voltage, so you will need to take steps to minimize it. The output voltage error can be measured using the test configuration shown in Figure 3. Note that you will need to add some components to mitigate the voltage error. Those components are not shown in Figure 3. Consult the op-amp s datasheet for details on how to mitigate output voltage error., 3 of 7

4 VPOS vo 3 4 VNEG Figure 3. Test configuration for measuring the effects of the input offset voltage and input bias currents on the output voltage. Both inputs of the amplifier are shorted to ground. Note that bypass capacitors have been added to help reduce noise entering the circuit through the power supply leads. Experimental Procedure After you have completed your design, measure the exact values of resistors through 4, record their values, and calculate accurate expected values for the differential-mode gain (vod/vid) and common-mode gain (vocm/vicm) using the appropriate information from the Theoretical Background section. Carefully record your results. In the documentation you submit after the lab exercise, you will be required to include a table that presents all of your calculations and measurements. All of this data must be complete and well organized. After the difference amplifier has been assembled and the effects of the input bias currents and input offset voltage eliminated, apply a purely common-mode DC voltage to the amplifier, and determine the common-mode gain (vocm/vicm) from the measured voltage values. Use a common-mode voltage level that will allow you to make the most accurate measurement possible. You may assume that the bench-top power supplies act as ideal voltage sources (i.e., each has nearly zero output resistance). Carefully record your measurements and calculations. You will need to include them in your submitted documentation. Compare the measured common-mode gain to the calculated common-mode gain determined earlier from the actual values of resistors through 4. If the two gain values do not match relatively closely (within 0-30% or so), examine your circuit carefully. If you are confident that the circuit is wired correctly, think of plausible reasons why the discrepancy might have occurred. Now apply a differential-mode DC input signal to the amplifier and verify that the amplifier has the specified differential-mode gain. As shown in Figure, to do this properly you will need to simulate two equivalent voltage sources with the positive terminal of one and the negative terminal of the other connected to ground. You cannot do this using the bench-top power supply because the bipolar part of the supply is being used to supply the operating voltages for the op-amp. However, you should be able to devise a simple circuit to do the job using one or two op-amps and a few resistors. Other options could also be used. emember 4 of 7

5 to use modest resistor values in the new circuit to avoid output voltage errors due to the input offset voltage and input bias currents, but make the resistors large enough to keep the opamps output currents within the rated limit. Alternatively, you may use the 6 V power supply as the differential input if you can correctly apply it as a truly floating voltage. Before proceeding, ask the instructor or lab TA to check your differential-mode test circuit. Calculate the differential-mode gain (vod/vid) from your measured data, and verify that it is close to the design value. Carefully record your measurements and calculations so that you can add them to the table you will present in your documentation. Demonstrate your operating differential-mode and common-mode test configurations and the differential and common-mode gain values you calculated and measured to the instructor. A successful demo must include well organized and legible circuit diagrams with clearly labeled component values and a well organized presentation of results. Be prepared to explain how you verified that your test circuits are operating properly and that they provide the most accurate measurements possible. Also be ready to explain how you eliminated the effects of the input bias currents and the input offset voltage. Next you will demonstrate how matching the resistor ratios 4/3 and / as closely as possible leads to a significant reduction in the common-mode gain. eplace one of the fixed resistors in the diff amp with a potentiometer with a range that spans the fixed resistor s value. For example, if one of the fixed resistors is 10 kω, then you could replace it with a 0 kω potentiometer. You may also replace it with a fixed resistor and potentiometer in series to obtain finer control; for example, a 9.1 kω fixed resistor in series with a kω potentiometer. Next, set the function generator to produce a sinusoidal voltage in the low audio frequency range (say, around 1 khz) with an appropriate peak amplitude. Apply the sinusoidal voltage to both inputs of the diff amp as if it were a common-mode signal (like vicm in Figure ). Adjust the potentiometer while monitoring the output voltage of the diff amp on the oscilloscope. There might be some interaction with your hand due to stray capacitance effects. Make sure you understand what you observe. When you do, demonstrate your results to the instructor. For your documentation, calculate the common-mode rejection ratio (CM) of the original amplifier (the one with all fixed resistors) using your measured data, and include it in the table submitted with your documentation. In the table caption, comment on whether or not your results are consistent with the worst-case CM given by CM min nom 1 nom =, 4ε where ε is the resistor tolerance expressed as a fractional quantity, and nom and nom are the nominal values (i.e., the marked values) of and. Express all CM values in db units. Be sure to note whether the measured CM is barely or vastly greater than the worstcase value, and briefly discuss the implications of that observation. 5 of 7

6 Lab Documentation After the lab sessions are over, but BEFOE preparing your documentation, carefully read Subsection B ( Tables and Their Captions ) of the Lab Documentation Formatting Guidelines. Also read Subsection A if you have not already. It would not hurt to re-read it if you have. A copy of the guidelines is available at the Laboratory web page. Compile the following items into a single electronic document: a. A professional-quality table that contains your measured input and output voltages for the differential-mode and common-mode tests, your calculated and measured differentialmode and common-mode gain values (and their percentage differences), and the associated calculated and measured CM values (in db) in proper type-set form. The table should be prepared using Microsoft Word. b. An appropriate and descriptive caption for the table that includes the calculated worstcase CM and a brief comment on the relationship between it and the calculated and measured CM values. The table and caption together should clearly summarize all of the measurements and calculations you made and briefly explain their implications. c. The measured values of resistors through 4. These do not have to be included in the caption, but they should be recorded somewhere to allow reviewers to verify the various calculated gain quantities. The documentation must be in MS-Word (*.doc or *.docx) format using 11-point or larger font. The file size must be less than 5 MB. Include your group members names, the course number (ECEG 350), the lab session dates (Sept. 17-Oct. 1, 018), the lab meeting time (1 pm or 3 pm), and the lab number on the first page. A cover sheet is not required. Use the file naming convention described at the lab web site. One copy per group must be submitted via the course Moodle site by the deadline posted on the lab web page. The documentation must be thorough, well organized, clear, legible, concise, and professional in tone and style. It must also exhibit good writing mechanics, spelling, and grammar. All four margins should be at least one inch. Single line spacing is acceptable. Keep a copy of your documentation if you wish to use it to prepare for the next exam. Peer eview After the lab documentation has been submitted, each lab group will be randomly assigned documentation from two other groups to review. Detailed instructions for the review will be provided later. After the peer review period is over, each lab group will have the opportunity to incorporate the reviewers suggestions into their own lab documentation and then resubmit it for a final score. 6 of 7

7 Lab Scores [revised 9/7/18] Each group member will receive the same overall score according to the following criteria. Scores will be quantized at the indicated percentage levels. 0, 10, 0, 30, 35% Demo: effectiveness and accuracy of test procedures 0, 5, 10, 15% Demo: full group understanding of test procedures 0, 5, 10% Demo: organization/legibility of diagrams and results 0, 5, 8, 10% Doc: peer review participation 0, 5, 10, 13, 15% Doc: quality and organization of data table 0, 5, 10, 13, 15% Doc: quality of table caption If the demonstration is completed after the deadline, a 10% score deduction for every 4 hours or portion thereof that it is late will be applied (not including weekend days and breaks). No demonstration credit will be given four or more days after the deadline, but credit for the lab documentation can still be obtained. Lab documentation submitted after the deadline will have a 10% score deduction applied for every 4 hours or portion thereof that it is late (not including weekend days and breaks), but the demonstration score will not be affected. Group Assignments The randomly generated groups for this lab exercise are listed below. 1 pm section Houldin-Migliuolo-Vassar Weigel-Sedig-Hoyt Brett-Tamim-Adulami Walters-Zanardi-einer 3 pm section Eckermann-Gannon-Tauber Song-Nance-Turconi McLaughlin-Meng-Ubah LaCapra-Hawk-Sullivan Jinno-Poku-Wang David F. Kelley, Bucknell University, Lewisburg, PA of 7