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I though I found an error and it seems that it is not really an error as it works in simulation... I am completely confused and I do not understand how it works. What is the math behind this scheme? How to calculate the current through the diode with its PNP set wrongly on the schematic?

enter image description here

And here is the current through the diode and the voltage input of the source voltage:

enter image description here

Could this work in practice?

jsotola
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Jess
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1 Answers1

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A PNP transistor (or NPN) can work in reverse. The gain (β) won't be so high but, it'll work in this circuit. The device is PNP and ostensibly has a symmetrical topology. OK, the doping is different between emitter and collector so, it'll operate as a low-gain transistor. In fact, in some circuits I've seen it used when a low saturation voltage is required.

What you can't rely on is operating this circuit at supply voltages higher than 6 to 10 volts (a hand-wavy value) because the base-emitter reverse voltage is usually less than 10 volts.

Andy aka
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  • Thank you for your answer :) By low gain what do you mean ? A gain of 10 ? Also do you think that it will be reliable over time ? I mean that beta will probably become even lower ... Do you think that the simulation is considering the correct gain of the transistor ? – Jess Dec 13 '22 at 07:35
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    I'm sure the simulator is doing it's best @jess but, there is no real spec to compare it against. The regular data sheet for the device doesn't consider using it in reverse so there are no guarantees of anything really. – Andy aka Dec 13 '22 at 10:15
  • It sounds bad ... – Jess Dec 13 '22 at 10:23
  • I wouldn't use it in reverse. I know one guy who did it but I was convinced he did it just to appear clever. I had to ask him to mark his schematic up with a note saying it purposefully used in reverse to get a lower saturation voltage. – Andy aka Dec 13 '22 at 10:27
  • Well thank you for your opinion... I will try to convince my superiors that it is not reliable... – Jess Dec 13 '22 at 10:31
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    These days you would use a MOSFET to obtain a very low saturation voltage between drain and source so, time, in its passing, has made the reverse transistor redundant in my opinion. – Andy aka Dec 13 '22 at 10:33
  • I agree with you. But in this circuit it would not work as if we place a PMOS, the "parasitic body diode" will always conduct – Jess Dec 13 '22 at 10:42
  • Have you looked up how AC solid-state switches use back-to-back MOSFETs. OK, it's more complex so maybe not useful. – Andy aka Dec 13 '22 at 10:45
  • I will take a look on it. Thank you for everything :) – Jess Dec 13 '22 at 10:48