I was worried about the class-D power amp lab possibly being too complicated for a first circuits course, so I provided more scaffolding for this lab than for previous ones (see the handout and addendum). We also spent a bit more time in class before the lab working as a group on the block diagram—though that was largely because the students seemed to be a bit clueless about the point of a block diagram on the last quiz. I wanted to show them how to use the block diagram as design tool, showing how decisions in one block (like what power to deliver to the loudspeaker or what power voltages to use) propagated through the block diagram to other blocks.
In the lab yesterday, everyone came prepared with schematics, and I even was willing to look over them for obvious problems before the lab (in previous labs, I just said “try it and see”). I still have a few students coming in with unreadable scribbles for their schematics—they see to think that the schematic diagram is something that they do “for the teacher” rather than for themselves as a working tool. I refuse to help a student debug a circuit if they don’t have a schematic I can read, and I’ve had to do that refusal repeatedly with a couple of the students. Students with unreadable or incomplete schematics are generally unable to debug their circuits—most of the debugging is finding differences between the wiring on the board and the drawn schematic, with a much smaller part being finding errors in the schematics. The students who have been most productive are those who have been doing neat schematics (often using the drawing tool that they use for the final report) before coming to lab, and making changes to the schematics during lab as they debug, so that they leave the lab with a neatly done schematic ready to put into the report that accurately reflects what was actually done in lab.
In the three hours of the lab, everyone had their circuit working, or very close. Both of the two groups that were close had amplifiers that produced gain, but had flaws: one was still making the output FETs too hot (probably a problem with the pull-up resistor sizing, possibly trying to run at a higher voltage or PWM frequency than was feasible for the design) and one had a lot of 60Hz hum being picked up (probably from having very loose wiring). I think that the group with the loud hum just accepted that their design or construction wasn’t quite as good as hoped, and are just going to write up the problems. I think that the group with too-hot FETs is going to come in on Monday to try to finish the lab—they would have stayed later yesterday to finish, but they had to present posters in their tech writing class.
Since the poster session for the tech writing class was in “Jack’s Lounge”, the study area immediately next to the labs (and probably the most intensive group-study areas on campus—I’ve never seen it empty and there are usually 3 or 4 group study or TA office hour sessions going on), I stayed after the lab to look at the posters. There was quite a range, from grade-school-like reports on “something cool in engineering” to class projects to detailed senior thesis presentations.
I finally got home around 9pm, had some cold leftover pizza for dinner, and finished off the lab handout for next week’s lab on EKGs before midnight.
I had originally thought that the EKG lab would be the hardest one of the quarter, because of the low signal levels (interesting parts of the EKG signal are less than 0.1mV), but now I think that it is not much harder than the pressure-sensor lab, which did a lower-gain amplifier using the same chips and protoboard. The use of the solderable protoboard does force the students to be more careful in their schematics and wiring, and they don’t have the problem of wires and components making poor contacts or falling out (like they do with the breadboards). They are also much more likely to use short wires, which greatly reduces the problems with noise pickup. I’m pretty pleased with the protoboard design I came up with, which is useful for both the pressure-sensor lab and the EKG lab:
One important tool for the prototyping board is the PDF file that can be used for doing layout, either by PDF markup tools or with a pencil. I might make some changes for next year, as I realized that it would be useful to have more access to the Rgain pins, particularly for EKG circuits that use them to get the common-mode signal to feedback to the reference electrode. (We’ll probably just put a constant voltage on the reference electrode, rather than doing a feedback circuit to reduce the common-mode signal.)
After finishing the handouts last night, I spent some more time finishing reading the PhD thesis that is being defended today (I’m on the committee, but not the adviser). I finally got to bed around 1am—not much later than usual, and not the all-nighter I was expecting to have to do to catch up (I’m still behind on several things, so there may be an all-nighter coming up).