I'm always curious, but didn't pay attention back in Electronics class to find out why.....
Why would you choose a PNP transistor for one circuit and an NPN for another?
Roger
The reasons are largely historical now.
Originally, it was impossible to make transistors transist at all unless you used germanium, and the making was hugely easier if you made them PNP. This really threw all the tube guys for a loop, because all the circuits functioned "upside down" from a schematic point, and I think this is what led to a lot of confusion on the part of early solid state electronicists.
You can make NPN germaniums, but it's much harder, and they don't work as well (power, frequency response, leakage, etc.), so the world was PNP for a while.
Then came silicon. Silicon was better than germanium in many ways, but it's easier to make NPN devices out of silicon than PNP devices, so the silicon devices were faster, lower leakage, higher gain, etc if you used NPNs. Any you could now flip your schematics and power supplies back over right side up so the tubes and transistors pointed the same way.
Better processing narrowed the gap between PNP and NPN in both silicon and germanium for discrete transistors. However, it is substantially impossible to make both good NPNs and PNPs on the same chip with the same process, so ICs all came to be made from NPNs primarily, with the poor PNPs on them only used when you HAD to have a PNP. And the gap widened - "normal" transistors are NPN.
Today you can get NPN and PNP devices of about equal goodness in silicon - similar gains, power, voltages, etc. The PNPs are usually slightly lower gain, and maybe only 350MHz instead of 600MHz, that kind of thing. There are fewer selections, but they're OK.
Generally you pick the cheapest device, or one you have in your hand (which may be the same thing), but there's little to choose from for audio use.
ICs are another matter - they're 99 44/100% NPN inside.
Sometimes you pick based on the power supply you already have and what you want to do.
But the issues don't have to do with the device itself much anymore.
excellent post, RG!
Inside one circuit, also, PNP and NPN devices may be used as a full wave, class b-style amplefier.
Generally, NPNs are nicer to used, becuase they allow negative ground, which is "conventional," and works better with power supplies....