Ben79 wrote: I don't properly understand the difference between NPN and PNP, this is the problem.
Let's think of the BOG standard common emitter stage powered by a single pole power supply with earth/common connected to the negative leg of your battery/power supply. So I'm talking about *everything* in 0 volts and +9 volts.
NPN has the emitter referenced to ground (so imagine a couple hundred mV worth of positive voltage between the emitter and the emitter resistor [if any], and the base is your PN junction drop above that...let's just simplify and call that one volt on your base.
PNP is inverted, so your emitter is hanging off the positive rail and your collector points towards ground. In an identical circuit the base of your PNP stage will sit at 8 volts (+9 minus 1).
You can size up your emitter resistors to nudge the emitter (and consequently your base voltage) toward center point of the rails, which, will act in tandem with the collector resistor to provide a working environment for your transistor. So, it's possible to limit the full excursion of any given stage to inside a voltage window. Being that the NPN window is generally closer to the +9 volts, and the PNP base circuit operates in its most linear fashion in the same window you can get away with direct coupling the two stages. Mind you, I do this stuff while watching a scope, so you're either flying blind with your ears out or using math to dial in the best range of values - which hinge on the transistors used..
But ultimately, the only difference between NPN & PNP is the way voltage is referenced in the circuit.