The inverter (NOT circuit) performs the operation called inversion or complementation. The inverter changes one logic level to the opposite level. In terms of bits, it changes a 1 to a 0 and a 0 to a 1.

Standard logic symbols for the inverter are shown in Figure 1. Part (a) shows the distinctive shape symbols, and part (b) shows the rectangular outline symbols. In this textbook, distinctive shape symbols are generally used; however, the rectangular outline symbols are found in many industry publications, and you should become familiar with them as well. (Logic symbols are in accordance with ANSI/IEEE Standard 91-1984 and its supplement Standard 91a-1991.)

FIGURE 1 Standard logic symbols for the inverter (ANSI/IEEE Std. 91-1984/
Std. 91a-1991).



The Negation and Polarity Indicators



The negation indicator is a “bubble” ( o ) that indicates inversion or complementation when it appears on the input or output of any logic element, as shown in Figure 1(a) for the inverter. Generally, inputs are on the left of a logic symbol and the output is on the right. When appearing on the input, the bubble means that a 0 is the active or asserted input state, and the input is called an active-LOW input. When appearing on the output, the bubble means that a 0 is the active or asserted output state, and the output is called an active LOW output. The absence of a bubble on the input or output means that a 1 is the active or asserted state, and in this case, the input or output is called active-HIGH.

Inverter Truth Table

When a HIGH level is applied to an inverter input, a LOW level will appear on its output. When a LOW level is applied to its input, a HIGH will appear on its output. This operation is summarized in Table 3–1, which shows the output for each possible input in terms of levels and corresponding bits. A table such as this is called a truth table.




EXAMPLE 1
A waveform is applied to an inverter in Figure :

Determine the output waveform corresponding to the input and show the timing diagram. According to the placement of the bubble, what is the active output state?

SOLUTION :

The output waveform is exactly opposite to the input (inverted), as shown in Figure :



Logic Expression for an Inverter

In Boolean algebra, which is the mathematics of logic circuits and will be covered thoroughly in Chapter "Boolean algebra", a variable is generally designated by one or two letters although there can be more. Letters near the beginning of the alphabet usually designate inputs, while letters near the end of the alphabet usually designate outputs. The complement of a variable is designated by a bar over the letter. A variable can take on a value of either 1 or 0. If a given variable is 1, its complement is 0 and vice versa.
The operation of an inverter (NOT circuit) can be expressed as follows: If the input variable is called A and the output variable is called X, then

This expression states that the output is the complement of the input, so if A = 0, then X = 1, and if A = 1, then X = 0. Figure 2 illustrates this. The complemented variable A can be read as “A bar” or “not A.”

FIGURE 2 The inverter
complements an input variable