Gas station without pumps

2014 May 30

Class-D amplifier lab done, EKG block diagrams begun

Filed under: Circuits course — gasstationwithoutpumps @ 21:08
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Yesterday’s lab ran long (as expected), because students had not gotten enough done in Tuesday’s lab.  But everyone in the lab did get a working class-D power amplifier.  Several also managed to measure the turn on and turn off times for the comparators driving the FETs, though that required some hands-on guidance in using the digital scopes (setting the trigger level to the FET threshold voltage, then looking to see how long the rise or fall was before reaching the trigger level.  As expected, everyone had chosen values that made the pFETs turn on and off quickly, but it was difficult to get the nFETs to turn off quickly.  I don’t know whether anyone managed to equal turn on and turn off times for the nFET (they turned on fairly fast), but several groups managed to keep their FETs cool.  Even those with warm FETs did not dissipate so much in them that they got dangerously hot.

I’ll be reading the design reports over the weekend, and I’ll see whether the students really understood PWM or not.  I suspect that about half the groups understood what they were doing well enough, and the other half got part of the ideas.  There should be time on Monday to review the idea of PWM and to explain again why it is a good choice for efficient power delivery, particularly for inductive loads.

Today, I returned the quiz 2s redone as homework.  Students did fairly well on them as homework (range 18.5 to 31.5 out of 36, up from 7 to 17 on the timed quiz).  The biggest difficulty was with the last problem, which asked them to design a simple amplifier, giving both a block diagram and a schematic.  A lot of students did not understand the question as I phrased it, perhaps because I had not been clear enough earlier in the quarter about what a block diagram means and how to use it.

Students have not yet internalized the idea of something having inputs and outputs, and a block diagram being a refinement of an I/O spec into I/O specs for subunits.  I may need to use that language more explicitly earlier next year. I’m thinking also that I need to add more text to the lab handouts next year and refer to them as a draft textbook rather than as lab handouts.  How many pages do I have so far?

handout pages
01-thermistor  11
02-microphone  9
03-hysteresis  11
04-electrodes  7
05a-loudspeaker  8
03b-sampling  7
06-audio-amp  6
07-pulse-monitor  11
08-pressure-sensor  8
09-power-amp  13
09-power-amp-addendum  6
10-EKG  5
total  100

One hundred pages is a bit short for a textbook, but there is a lot of explanatory material still missing (most of which I provided in class or in lab). If I worked on it diligently over the summer, I could probably create a book with most of what the students need that would be around 200 pages. Do I have the energy to turn this into a textbook? Is it worth the effort?

After going over the block diagram of the quiz problem, I helped the students develop an EKG block diagram.They did get to the realization that the unknown but potentially large DC offset from the EKG electrode half cells limits the gain that they can ask from the first stage of the EKG, and that they’ll have to high-pass filter and add more gain.  The design is similar to their pressure sensor instrumentation amp, but the gain needs to be higher (1000 to 1500, rather than 100 to 250), and the pressure sensor amplifier had to go down to DC, so did not include a high-pass filter.

I was a little worried that I may have suggested too high a lower end for the passband (0.1Hz to 40Hz).  They’ll get less baseline drift with a 0.5Hz cutoff instead of 0.1Hz.  My EKG designs have used 0.05Hz—88.4Hz and 1.0Hz–7.2Hz for the blinky EKG.  Both worked ok, but I now think that the 7.2Hz cutoff is too low (it was adequate for blinking an LED, but not for recording the waveform).  Since I did not have much problem with a 0.05Hz corner frequency, I think they’ll be ok with a 0.1Hz one.  The blinky EKG circuit has an adjustable gain (needed to make the R spike large enough to light the LED), but it is probably better to have a fixed gain.

It would be really nice if they could finish the EKG on Tuesday, since the annual undergraduate poster symposium is scheduled for the same time as the Thursday lab, and I always like to spend an hour or so looking at the posters.

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