引用出處https://www.quora.com/Why-is-the-small-signal-model-of-an-npn-transistor-identical-to-an-pnp-transistor

As you can see, the components are the same, but the battery is now placed inversed. Since there are DC-blockers at the input and outputs, this doesnt matter for the normal functioning.

Remember that in small signal analysis all the indempendent voltage sources are shorted. It only rests the model for the transistors but if you take a look at the next image, they actually have the same behavior (only the conventions are changed). So, as you can see, both circuits have the same model.

As it says, the small signal model is a... model. Models serve to solve problems that usually cannot or are too difficult to be solved by simple mathematics, however they work for certain conditions. For the small signal model, the condition is that all the signals involved be small (hence the name os the model) in such a way that the transistor does not go out of the operation point.

So the only diference between these circuits happens when you try to amplify something that saturates the output, and thats why we have lab classes with this subject.

A small signal transistor model is a linearized model at the operation point (quiescent point) for analyzing small amplitude AC signal. Since DC supply is AC ground, this eliminates the issue with power supply and ground differences between NPN and PNP. A small amplitude AC signal is both adding and subtracting from the quiescent point; with the polarities of Ic, Ie and Ib being consistent, the ac signal cannot tell the difference between NPN and PNP. Therefore, the model can be exactly the same.

This website will help you understand all there is to know about the transistors:

Difference Between an NPN and a PNP Transistor

"An NPN transistor receives positive voltage at the collector terminal. This positive voltage to the collector allows current to flow across from the collector to emitter, given that there is a sufficient base current to turn the transistor on."

"A PNP transistor receives positive voltage at the emitter terminal. The positive voltage to the emitter allows current to flow from the emitter to the collector, given that there is no current flowing into the base."