PNP biasing

For a PNP transistor, the situation is just a “mirror image” of the case for an NPN device. The diodes are turned around the opposite way, the arrow points inward rather than outward in the transistor symbol, and all the polarities are reversed. The dual-diode PNP model, along with the actual bipolar transistor circuit, are shown in Fig. 1. In
the discussion above, simply replace every occurrence of the word “positive” with the word “negative.”
You need not be concerned with what actually goes on inside the semiconductor materials in NPN and PNP transistors. The important thing is the fact that either type of device can serve as a sort of “current valve. “ Small changes in the base voltage, E_B, cause small changes in the base current, I_B. This induces large fluctuations in the current I_C through the transistor.
In the following discussion, and in most circuits that appear later in this book, you’ll see NPN transistors used almost exclusively. This doesn’t mean that NPN is better than PNP; in almost every case, you can replace each NPN transistor with a PNP, reverse the polarity, and get the same results. The motivation is to save space and avoid redundancy.

Fig. 1: At A, simple PNP circuit using dual-diode modeling. At B, the actual transistor circuit.