One of the student suggestions from the “bar exam” was to design a protoboard for the class D amplifier, so that they could keep it as a permanent object (a more useful one than the pressure-sensor amplifier, since they don’t have any strain gauges).
I’ve been thinking about that and see a few problems:
- The block diagram the students came up with this fall (with my guidance) calls for 3 power supplies, which makes the amplifier a bit impractical to power from a battery or wall-wart power supply.
- The students used an external triangle-wave generator.
- The protoboard would have to be fairly large to accommodate the FETs and inductor and enough wiring space, which would probably result in a larger board than 5cm × 5cm, and so $2/board rather than $1/board.
I’ve not given much thought yet to the power-supply problem, but I did think a little about the triangle-wave generator. Since they only need to generate triangle waves at a single frequency (in the 60kHZ–100kHz range), it should be possible to use a hysteresis oscillator with an integrator rather than a simple RC timing circuit. Something like the following might work:
The slew rate of the MCP6004 op amps is only 0.6 V/µs, but the triangle wave only swings from VIL to VIH, or about 1V, so the minimum period is about 3.3µs, for a maximum frequency of 300kHz. If we use another op amp to amplify the signal to make it swing almost 5v, instead of 1V, the maximum frequency would be only 60kHz.
I might want to try this circuit out this week, to see if it is worth having the students play with next year.