Gas station without pumps

2015 May 5

First op-amp lab was quick

Filed under: Circuits course — gasstationwithoutpumps @ 21:12
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On Monday I provided a little feedback on the design reports for the electrode lab.  The big issues were

  • Students not reporting the models they were fitting to the data.
  • Students not reporting the parameters of the fits after doing the fitting.
  • Students choosing overly complicated models (like R+(R||C) for data showing constant impedance)
  • Students not modeling important phenomena (like the (R||C) input impedance of the voltmeters)

Little issues included

  • Using “due to the fact that” rather than “because”
  • Omitting leading zeros before a decimal point.  Numbers should never start with punctuation.

After that brief intro, I worked with the students to develop a block diagram for an audio amplifier using the electret microphone and loudspeaker that they had already characterized.  This had been part of their homework, but I expected them not to have really grasped the point of a block diagram.

Another thing I went over in class, because I’d seen problems with it in previous reports and prelabs ,was reminding students that V=IR is not a ritual magic incantation. Reciting it doesn’t make solutions to problems right, if it is just randomly applied.  I reminded them that the voltage has to be across the resistor that the current is through—picking random voltages or currents in the circuit is meaningless.  I showed them an example taken from the prelab they were turning in at the end of class.

When I did the grading for the prelab homework Monday night, I saw that many of the students managed to copy the block diagram we had done in class, but none had appropriately labeled the signals between the blocks.  I think I need to provide some more and better examples in the book.  (Ah, I see I already have marginal notes to myself to add a couple in Chapter 2.)

The V=IR error was very common, mostly with V was taken to be the power supply voltage, rather than the voltage across the resistor that biases the microphone.

Students also had a lot of trouble with computing the AC voltage of the signal out of the microphone, based on the loudness of the sound input and the sensitivity of the microphone. I knew this was a difficult assignment, but I thought that it would be relatively easy, because they had supposedly already created a worksheet for themselves as part of Lab 4 (the microphone lab).  Either they forgot everything they learned there, or they never really got the idea of the worksheet they created.  One student asked in class on Monday, quite reasonably, for a worked example.  I’m going to have to come up with one that doesn’t just do all the work for them—I know these students can fill out worksheets, but what I need to get them to do is to solve problems when the steps aren’t all set out for them.

The afternoon lab section (many of them working together) did much better on the prelab than the morning section—the difference between the sections has been noticeable from the beginning, but it seems to be getting bigger, not smaller.  For some reason the descaffolding is working better with the smaller section.  Individuals in the morning lab are doing quite well, but there are more floundering students in that section, and I don’t know how to get them back on track.

Even though the morning lab is struggling more than the afternoon lab, I think that both are doing better than previous year’s classes at this point of the quarter.  With only one or two exceptions, everyone in both lab sections got their op amp circuit designed, wired, and demonstrated within the 3-hour class period.  That means that Thursday’s lab can be a tinkering lab for most of the students, where they can try various ways of improving the design:

  • Switching from a symmetric dual power supply to a single power supply.
  • Paralleling two op-amp chips to get twice the current capability.
  • Adding a potentiometer for variable gain.
  • Adding a unity-gain buffer to separate the loudspeaker driver from the gain amplifier.
  • Adding a tone-control circuit, like the Baxandall tone control on http://www.learnabout-electronics.org/Amplifiers/amplifiers42.  They can’t use exactly that circuit, as they have only 10kΩ potentiometers, not 100kΩ ones.  The idea can be adapted, or the students could do simple treble-cut or bass-cut circuits.
  • Using a loudspeaker as a microphone. I think that should work, as I get about a 500µV signal from my loudspeaker when I talk into it.  The don’t need any DC bias for the loudspeaker mic, and they may even be able to eliminate their high-pass filter, as the loudspeaker mic can be set up to have its output already centered at 0V.

I’ll talk about some of these possibilities in class tomorrow (plus stroking the students a bit about getting the lab done quickly). I attribute he good performance on the lab to them having put in more time on the prelabs, even if they didn’t get the answers to the questions exactly right.  Thinking about the design ahead of time (and getting a little feedback) goes a long way toward clearing up confusion they have had.

There are 4 more amplifier labs coming:

  • Instrumentation amps with a strain-gauge pressure sensor (measuring breath pressure and blood pressure using an arm cuff).  Will need to be 2-stage, since the INA126PA chips we are using aren’t rail-to-rail amplifiers.
  • Transimpedance amplifier fora photodiode to measure pulse.  This will also need to be multistage, since the first stage will have to have limited gain to avoid saturation.  After high-pass filtering much more gain will be needed.
  • Class-D power amplifier.  This is always the toughest lab of the year.  Even small mistakes can result in shoot-through current that gets the FETs hot enough to melt the breadboards (I have two breadboards that I’ve melted holes in).
  • EKG using only op amps (making their own 2-op-amp instrumentation amp, plus high-pass filtering and a second gain stage.  They’ll be using all 4 of the op amps in the quad op amp package for this amplifier.

I’m about a week behind on grading redone assignments—weekends are spent grading design reports, Monday nights grading prelabs, weekends plus Tuesdays adding to the book a chapter ahead of the students, and I squeeze in the redone assignments Wednesday or Thursday night, if I don’t crash too early.

4 Comments »

  1. Your comment on leading zeroes caught my eye. I try to impress the importance of this to them, but don’t push it to the level of taking off a point on exams or lab reports. Do you “grade” that or just comment on it? Or do you comment on it the first few weeks and then start taking off points?

    A colleague’s pet peeve (math prof) concerns students who use a 4H pencil to make a tiny dot for a decimal point. When the number doesn’t start with a zero, you might have to pull out a magnifying glass to find a dent in the paper.

    My pet peeve is writing e-5 (or E-5) for a power of 10 rather than writing a power of 10 or (drum roll) using SI prefixes. Now if we could figure out why calculator makers and math classes use “scientific notation” to describe something that scientists use less often than “engineering notation”.

    Comment by CCPhysicist — 2015 May 6 @ 06:10 | Reply

    • Actually, I prefer to have students write 150E-9 to having them write with explicit exponentiation of 10—by the time they are in college they should learn floating-point notation. I don’t want any of my students to make the mistake I’ve seen in biology grad students of thinking that the E-9 on the end of a number means e^(-9). I use a capital “E” in the floating point notation, to reduce the chance of students making this mistake.

      The 10^(-9) convention also requires careful spacing and baselines: “150 10-9“, and in handwriting often gets mangled to “15010-9”, which looks like a subtraction operation.

      Another pet peeve of mine (not mentioned on Monday I think), is students using the letter “x” as a multiplication symbol. Even × and ⋅ are wrong, since they are for cross-product and dot-product. Scalar multiplication is done just with juxtaposition in formulas, and with “*” in computer programs and gnuplot scripts. Some students even asked in class earlier this year what symbol they should use for multiplication, as if the juxtaposition convention was new to them. High-school and community college classes seem to be perpetuating the mistake of using the wrong multiplication symbol, which I thought was suppressed after middle school.

      Comment by gasstationwithoutpumps — 2015 May 6 @ 06:53 | Reply

  2. Point well taken about V=IR. One of our very first labs involves using one resistor in series with an ammeter (multimeter) and measuring both the power supply voltage and the resistor voltage along with cable voltages. With a careful choice of conditions, it is easy to get a very significant difference and establish that only one way gives the nominal R.

    From what I know about the labs for our trig-based physics labs, your students are unlikely to have explored the “same V, currents add” etc rules in their previous classes. I find that hands-on experience with that beats lecture any day.

    Comment by CCPhysicist — 2015 May 6 @ 06:16 | Reply

    • I might have to teach addition of voltages more explicitly in lab in future, since some students apparently haven’t grasped it yet. That might be something to work in the week 1 labs, when they are learning how to use the voltmeter and ohmmeter. I had attributed the problem to innumeracy on the part of those students (which is also a problem for them), but it may be that a demo lab in the first week to measure voltages across each of the two resistors in a voltage divider will help them grasp both addition of voltages and the ratios of voltage dividers.

      That would require getting our parts kit on the first day of lab (which didn’t happen this year, but which I think I can get to happen in future years), and it would take time. The first lab was a bit full with students learning to solder the headers onto their KL25Z boards.

      Comment by gasstationwithoutpumps — 2015 May 6 @ 07:03 | Reply


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