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I'm trying to understand the classic "ideal power diode" circuit. An example (taken from the Raspberry Pi) was in this question.

My question concerns the left-hand PNP transistor. The function of it's EB junction is clear, but why is the collector tied... anywhere? What purpose does the collector current serve? Why wouldn't it work just as well with that collector disconnected?

The collector on the right transistor is more straightforward - it's purpose is to draw the gate voltage high to turn the MOSFET off. But what about the left one?

nsayer
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  • You need to understand that there is an equation (Shockley) that relates collector current to base-emitter voltage (which works in both directions.) The collector current of the left BJT ***sets*** the base-emitter voltage of the left BJT. The base voltage of the left BJT (a result of the input voltage and the collector current generating a base-emitter voltage) is tied directly the right BJT's base. – jonk Jan 19 '19 at 07:10

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