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I have a circuit which requires a PNP transistor. I only have an NPN transistor.
Is there a way to convert one to the other given that they have the same characteristics (apart from the npn/pnp part?)
And for others after me... can the reverse also be done? e.g. how does one convert a PNP transistor to its NPN equivalent?
If it's the only type of transistor in the circuit, the translation is straightforward; build the circuit as designed, reverse the power connections and any other polarised components (diodes, electrolytic caps).
If you need one PNP in a mostly NPN circuit, there is no general solution.
There may be solutions, depending on the configuration of the PNP stage.
For example, if the PNP transistor was being used as an emitter follower, and you have the headroom, you may be able to use an NPN in common emitter, with Rc=Re so that its gain is (approx) 1.
If the PNP transistor was in a complementary power output stage but you can only find low power PNP transistors, I remember seeing an arrangement using a PNP driver transistor and an NPN power transistor to "replace" the non-existent PNP power transistor. Peter Walker did this around 1970 for the Quad 303 power amp (I believe 3 transistors were involved) when there was no PNP version of the famous 2N3055.
And there may be other such substitutions.
If you have the exact NPN equivalent you only need to mount the circuit as it is drawn, so that the base, collector and emitter terminals of the NPN are connected where base, collector and emitter of the PNP should go, and then reverse the power supply polarity. This trick works in most cases, but it depends on the circuit we are speaking about. The exact same thing works if you want to use a PNP instead of a NPN. Remember that all the transistors in the circuit should be changed with theyr complementary, so if you have PNP's and NPN's in your circuit there's no way out, you need both types of transistors.
Electron mobility is always higher then hole mobility, and since mobility is closely related to conductivity, for the same semiconductor, NPN transistor is always faster, have better current gain and better current capabilities then his PNP counterpart. Now, it has perfect sense to turn NPN with it's current gain, speed and current capabilities into PNP transistor if you need to use PNP in your scheme. Best scheme for this purpose is Sziklai pair. Overall, pair act as one powerful PNP transistor, while most of the output current is contributed by NPN transistor.
