Common Transistors

CHAPTER 3: Basic Hacks 47

Figure 3-24 Using a PNP bipolar transistor transistor. If for some reason (and this does happen) we wanted to switch the positive side, then we would need to use a PNP equivalent of the NPN 2N3904, such as the 2N3906. NPN stands for Negative-Positive-Negative, and yes, you can guess what PNP stands for. That is because transistors are kind of semiconductor sandwiches, with material of either N or P type as the bread. If the bread is N type (the most common), then the base voltage needs to be higher than the emitter voltage (by about 0.5V) before the transistor starts to turn on. On the other hand, a PNP transistor turns on when the base voltage is more than 0.5V lower than the emitter voltage. If we wanted to switch the positive side, we could use a PNP transistor (as shown in the PNP alternative to Figure 3-21) displayed in Figure 3-24.

MOSFETs also have their own equivalent of PNP transistors called P-channel, their version of the more common NPN being called N-channel.

Common Transistors

The transistors in Table 3-3 will cover a wide range of transistor applications. There are thousands and thousands of other transistors, but in this book we only really use them for switching, so these will cover most “bases”!

Name Appendix Code Type Max Switching Current Notes

Low/medium-current switching 2N3904 S1 NPN bipolar 200mA Current gain about 100 2N3906 S4 PNP bipolar 200mA Current gain about 100 2N7000 S3 N-channel MOSFET 200mA 2.1V gate-source threshold voltage; turns on when gate is 2.1V higher than source

High-current switching FQP30N06 S6 N-channel MOSFET 30A 2.0V gate-source threshold voltage; turns on when gate is 2.0V higher than source

TABLE 3-3 Really Useful Transistors