Class-D lab revision didn’t work

In Long weekend, I discussed what I was planning to do about anticipated problems with the class-D amplifier lab, specifically

• Replace the AOI518 nFET with one that has a lower input capacitance, such as the PSMN022-30PL,127.  The gate charge at V_GS of 4.5v is 4.4nC, about half that of the AOI518.
• Replace the open-collector comparator with one that has push-pull output, like the TLC3072, which can provide ±20mA current (more than the LM2903, even before we allow for the current through the pullup resistor).

I did a neat version of the schematics last night using the TLC3072 comparators and the AOI518 nFET. This year I remembered to include an adjustable gain stage in the preamp, so that I could more easily control the volume. Today in the lab, while the students were soldering up their instrumentation amps for the pressure sensor, I wired up the class-D amplifier, one stage at a time, confirming that each stage worked using the oscilloscope before moving on to the next. The build took me longer than I had expected—almost 2 hours.

Everything worked fine until I connected the drains of the two FETs together.  Initially it worked ok, but after about 20 seconds the shoot-through current increased, causing the current limitation of the bench power supply to kick in.  Then the voltage on the lower power rail moved up  close to ground, and the input voltage on the comparator was swinging below the negative rail.  I think that this damaged a couple of my TLC3072 chips—I’ve marked them, and I’ll have to test them before using them.

Replacing the AOI518 transistor with the smaller  PSMN022-30PL,127 did not help.

I finally borrowed an LM2093P chip from one of the students (I’d left mine at home, by mistake) and tried replacing the TLC3072 chip with the LM2093P. They have the same pinout, but the LM2093P is an open-collector output, so I had to add pullup resistors.  I guessed a couple of values, based on vague recollections of last year’s design, and the amplifier worked.

Initially I could only run the amplifier up to ±7v on the power supply, without the FETs getting too warm—there was still too much shoot-through current during transitions.  I switched to a lower resistance for the pullup on the pFET gate, to make the voltage swing less and the turn-off faster.  At that point the amplifier worked quite happily with a ±8v swing without the transistors getting warm.The circuit worked with either of the nFET transistors, so I’ll just have the students stick with the AOI518 in their parts kit.

I couldn’t crank up the volume on the speaker, though, because I got feedback squeal whenever the gain got too high.  Perhaps I should make a long speaker extension cable for students to do testing next week.  I seem to be out of speaker wire, though.

The class-D amplifier design will be a tough one for the students, and I’ll need to do a supplemental handout on open-collector outputs (I’d cut that material from the handout when I thought that we would be using the TLC3072 comparators).

Last week I thought that the students could start on the class-D amplifier in lab today, having finished the soldering for the pressure-sensor amp on Tuesday, but it took almost the whole lab time today for students to finish the soldering, even though everyone had working breadboards on Tuesday.  The lab ran over by almost 2 hours for one group, taking a total of 8 hours instead of 6 (last year the same lab took only 4 hours for the slowest group, probably because last year’s class came to lab more prepared).

The layout took longer than students expected, as did the soldering.  Everyone did (eventually) get working soldered instrumentation amps, though for a couple of groups I had to point out that their wiring did not match their schematic (they had called me over to help debugging).

In one case they had connected a resistor to the wrong point in their circuit.  I found the bug by tracing where their virtual ground was connected, and asking them to identify each component. Even after I showed them both resistors connected to their virtual ground, where only one was supposed to be, it took them a long time to realize what the discrepancy was. They had wired exactly what they had laid out, but the bug was in their layout, and they had not done a thorough enough job of checking against their schematic.

Another group had a working circuit but with too little gain. After checking a few of the DC voltage levels with them, I compared each of their resistors to the schematic.  In one place they had wired in a 1kΩ resistor where the schematic called for a 10kΩ resistor.  They unsoldered the incorrect resistor and soldered in the resistor from the design, which salvaged the circuit.

I also returned Wednesday’s quiz in lab today—pretty much like last year, the scores were much better on the second quiz than the first one, though still only half what I think the students should be able to do at this point of the quarter.  I’m once again assigning the students to redo the quiz as homework.  I need to decide soon whether to give them another quiz during the final exam time.