Some related examples can be found on my website: (Indeed, we might even use that clamped current to power the supply. Note that the zener diode can go away, or perhaps be moved to the supply instead, in this case: the complementary emitter follower can be driven at most V BE outside of its supply rails - it's self-clamping. a 12V supply with its positive tied with +80V. This is easily solved by introducing a "bootstrap" rail, "hanging down" from the supply, i.e. That's already a huge liability at 80V, even with larger packages (say SOT-223 or DPAK) and it's just poor efficiency. The pull-down emitter follower (PNP) meanwhile has to sink full load current from GND. That's easily some 100mW, which still isn't game-over for a SOT-23 for example, but one will definitely run hot, and it's not very scalable - it's not going to work at 600V, or if you need 10A peak, say. So, easily a few mA for typical applications. The pull-down transistor has to sink full drive current divided by emitter follower hFE. And they need to be driven inversely, whether from one inverted, or perhaps by making one common-emitter and the other common-base.Īnother option is a differential pair, which has different limitations on voltage range (not a problem here, but might be important if the supply voltage is highly variable, say it should work from 5 to 80V?), but also offers a more precise/customizable input threshold (it can be used as a comparator).
So you will need two level shifters (like the NPN with emitter resistor), one which drives a pull-up switch (common emitter PNP). Something to hold it high (that can be switched off, not a static resistor), and something to hold it low.
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