Kevin Aylward
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Speed may be an issue in that slower devices may be harder to stabilise in a feedback loop. In a CMOS process I would expect that the npn will be much faster than any pnp. Often the pnp is not a real one. The best bandgaps use bipolars as transistors, not merely as diodes, and use both types throughout. A key issue when Mosfets are used, is the difficulty in trimming to a universal “magic” voltage. If all bipolars are used, all offsets are tuneable ptats. That is, if the bg is trimed to the magic voltage, it will have the correct square shape temperature curve, irrespective of individual errors. This is important as it allows the band gap to be set with one trim at, say, 27 deg. Unless all Mosfets are relatively, absolutely huge, say 200u/40u, a trim to what should be a magic voltage won’t be. In which case production trimming becomes a nightmare.
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