PNP biasing:

For a PNP transistor, situation is just a mirror image of the case for an NPN device. The diodes are turned around opposite way, the arrow points inward rather than outward in transistor symbol, and all polarities are reversed. The dual diode PNP model, along with the actual bipolar transistor circuit, is shown in the figure given below.

You need not be concerned with 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 base voltage, EB, cause small changes in base current, IB. This induces large fluctuations in current IC through transistor.

In the following discussion and in most circuits that appear later in this book, you'll see NPN transistors used almost exclusively. This does not mean that NPN is better than PNP; in almost every case, you can replace each NPN transistor with the PNP, reverse polarity, and get same results. The motivation is to save the space and avoid redundancy.