FIGURE 2. Useful
variations of the
gates in words.
• Buffers — A
buffer is a non-inverting amplifier that
has an output drive capacity that is far
greater than its input drive requirement, i.e., it has a high fan-out and
gives a logic 1 output for a logic 1
(EX-NOR) gate is
an EX-OR gate with
a negated output,
which goes to logic
1 if both inputs are
in the same logic
state, and goes to
logic 0 only if a
single input is at
• Inverters — An inverter (also known
as a NOT gate) is a high fan-out
amplifier that gives a logic 1 output for
a logic 0 input, and vice versa.
• AND Gates — An AND gate has an
output that is normally at logic 0 and
only goes to logic 1 when all inputs
are at logic 1, i.e., when inputs A and
B and C, etc., are high.
• NAND Gates — A NAND gate is an
AND gate with a negated (inverted)
output; the output is normally at logic
1 and only goes to logic 0 when all
inputs are at logic 1.
• OR Gates — An OR gate has an output that goes to logic 1 if any input is
at logic 1, i.e., if inputs A or B or C,
etc., are high. The output goes to logic
0 only if all inputs are at logic 0.
FIGURE 3. Symbols, truth tables, and
Boolean expressions for the eight basic
types of logic gates.
• NOR Gates — A NOR gate is an OR
gate with a negated output; it has an
output that goes to logic 0 if any input
is at logic 1, and goes to logic 1 only
when all inputs are at logic 0.
• XOR Gates — An exclusive-OR
(EX-OR) gate has two inputs, and its
output goes to logic 1 only if a single
input (A or B) is at logic 1; the output
goes to logic 0 if both inputs are in the
same logic state.
Figure 3 shows
how the functions
of the eight basic
types of gates can
also be presented
in tabular form via
truth tables (which
show the logic
state of the output
at all possible input
logic state combinations) or symbolically via Boolean
Note that, by convention, all logic
gate inputs are
notated alphabetically as A, B, C,
etc., and the output terminal is notated as Y (in counters
and flip-flops, etc., the main output is
usually notated as Q).
The actual logic states
may be represented
by 0 and 1, as shown,
or by L (= Low logic
level) and H (= High
logic level). Also note
in the Boolean expressions that a negated
output is indicated by
a negation bar drawn
above the basic
• XNOR Gates — An exclusive-NOR
FIGURE 5. Any AND or OR gate can be
used as a non-inverting buffer element.
FIGURE 4. Basic sets
of two-input positive
and negative logic
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