Page 63 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Using Transistors to Build

Primitive Logic Functions

There are several different families of transistors available to designers

and, although the actual implementations vary, each can be used to construct
primitive logic gates. This book concentrates on the metal-oxide semiconductor
field-effect trumistors (MOSFETs) introduced in Chapter 4, because their sym-
bols, construction, and operation are easier to understand than are bipohr
junction trumistors (BJTs).
Logic gates can be created using only NMOS or only PMOS transistors;
however, a popular implementation called complementary metal-oxide semi-

conductor (CMOS) makes use of both NMOS and PMOS transistors
connected in a complementary manner.
CMOS gates operate from two voltage levels, which are usually given the
labels VDD and vss. To some extent the actual values of VDD and V,, are irrele-
vant as long as VDD is sufficiently more positive than Vss. There are also two
conventions known as positive logic and negative Under the positive logic

convention used throughout this book, the more positive VDD is assigned the
value of logic I, and the more negative V,, is assigned the value of logic 0.
In Chapter 5 it was noted that truth table assignments can be specified using
the abstract values FAL5E and TRUE. However, for reasons that are more fully

examined in Appendix B, electronic designers usually represent FAL5E and
TRUE as 0 and 1, respectively.

NOT and BUF Gates
The simplest logic function to implement in CMOS is a NOT gate
(Figure 6-1). The small circle, or bobble, on the control input of transistor Tr,
indicates a PMOS transistor. The bobble is used to indicate that this transistor

1 In conversation, the term CMOS is pronounced “C-MOS”; that is, spelling the letter “C”
followed by “MOS” to rhyme with “boss”.
The positive- and negative-logic conventions are discussed in detail in Appendix B.