Gas station without pumps

2013 February 28

Rethinking the power-amp lab again

Filed under: Circuits course — gasstationwithoutpumps @ 22:30
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I took the breadboard that had been working for me at home for the class-D power amp to the lab today, and tried getting it to work in the way that I expected the students to do it, with 3 power supplies.

It failed miserably.

Even after tinkering with the circuit a bit, the FETs kept getting hot (indicating that I was not successfully having only one on at a time).  I’ve already released the handout for the power amp lab,  but the students will not be able to get the amplifier working from those instructions.  I have several choices facing me:

  • Get rid of the power-amp lab entirely, and take 2 weeks for the EKG lab.  Originally, I had planned 2 weeks for the EKG lab, as it is slightly more difficult than the pressure sensor lab, but the difference in difficulty is not 2-to-1.
  • Modify the class-D lab to use a single power supply, as I’ve been doing at home.  I think that the problem I was facing was that the larger voltages of the dual supply made the overlap range where both FETs were on much larger, and the simple cMOS-inverter output stage could not be driven fast enough to pass rapidly through the range.  It may be enough to use two 3-v supplies, with everything except the loudspeaker running from +3V to -3V, and the other end of the loudspeaker at 0V.  I’d be limited to the voltage range of the MCP6004 chip, which is 6V (the absolute max is 7V, and I’ve been running them at 6.6V at home without much trouble, though I’ve probably shortened their lifetime a lot).  That would limit the power to the loudspeaker to around 0.6W, which is still a lot more than the op amps can deliver.  Dual 3V supplies (and no extra 6v supply) would be a simpler design that what I have in the handout, but it should be very close to what I debugged at home.  I should probably try it out in the lab tomorrow.
  • Use bipolar transistors to drive the FET gates with more current so that they switch faster.  We haven’t talked about bipolar transistors (except very,very cursorily in the context of the phototransistor, and the lab reports indicated that only one or two people had followed through to understand how the phototransistor works).
  • Give up on class-D and do a simpler class-A amplifier with the loudspeaker as the load resistance.  This is not the right way to use the loudspeaker, since it will have a large DC bias (pushing the cone out or pulling it in, rather than having it rest in the middle), but is a very simple circuit, and can use negative feedback from the loudspeaker to correct for any nonlinearity in the the circuit.  It is also horribly inefficient, and whatever FET we use is almost sure to get warm. We could solder a heat sink onto the transistors, if needed, but that adds a different sort of complexity to the lab.
  • Do a class-A amplifier with a power bipolar transistor.

I don’t really like any of these solutions, but I’ll have to pick one this weekend and write it up for the students.  If we start the EKG lab a week earlier than planned, I’ll have to try building an EKG amplifier on the protoboard this weekend, to make sure that it works well enough, and get the handout for it written. Doing a lower-voltage class-D amplifier would require the least modification to the handout.  Bipolar transistors would require not only acquiring the transistors, but debugging the lab with bipolars (and I might want to switch back to a class-AB amplifier if I use bipolars).




1 Comment »

  1. […] Rethinking the power-amp lab again, I described the failure of my class-D power amp on Thursday afternoon, and the dilemma I faced in […]

    Pingback by Rethinking the power-amp lab yet again | Gas station without pumps — 2013 March 2 @ 14:06 | Reply

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