N and P signify polarity of the bipolar junction transistor. NPN is "negative polarity" and PNP is "positive polarity". Basically which way the current would flow (to the emitter for NPN, from the emitter for PNP) when it was turned on. There are a lot of different types of transistors though, and they don't all have this sort of designation. I always considered NPN to be "typical" because PNP types were ass-backwards from what you normally wanted in a simple amp circuit or switch.
|