and tying them to one of
the IC’s supply lines. In
CMOS ICs, the shorted
inputs can be wired directly to either supply line, but
in TTL ICs the inputs must
(to give minimum quiescent current consumption
with good stability) be tied
directly to the 0V rail, as
shown in Figure 12.
Sometimes, when
using NAND gate ICs, you
may not want to use all of
a gate’s input terminals. In
this case, the unwanted
inputs can be disabled by
either tying them high
(directly in CMOS gates, or
via a 1K resistor in TTL
types) or by simply shorting them directly to a used
input. Figure 13 shows
examples of a three-input
TTL NAND gate wired for
use as a two-input type.
Note that the fan-in
of a TTL NAND gate is
an almost constant ‘1,’
irrespective of the number
of inputs used. Thus,
CMOS or TTL NAND gates
can be converted into
simple inverters by simply
shorting all of their inputs
together. Figure 14 shows examples of
TTL NAND gates used as inverters.
Also note that NAND gates are fairly
versatile elements, as demonstrated in
Figure 15, which shows ways of using
two-input elements to make a two-input or three-input AND gate or a
three-input NAND gate.
FIGURE 6. Functional diagram of the 74LS10
triple three-input NAND gate IC.
FIGURE 7. Functional diagram of the 4023B
triple three-input NAND gate IC.
FIGURE 8. Functional diagram of the 74LS20
dual four-input NAND gate IC.
FIGURE 9. Functional diagram of the 4012B
dual four-input NAND gate IC.
FIGURE 10. Functional diagram of the 74LS30
eight-input NAND gate IC.
FIGURE 11. Functional diagram of the 4068B
eight-input NAND gate IC.
resistor, as shown (for example) in the
three-input OR gate of Figure 16. The
diode OR gate is reasonably fast,
very cost-effective, and can readily be
expanded to accept any number of
inputs by simply adding one more
diode to the circuit for each new
input.
Relatively few dedicated OR gate
ICs are available. Figure 17 lists basic
FIGURE 12. Methods of disabling unwanted
TTL NAND gates.
FIGURE 13. Basic
method of disabling
unwanted TTL NAND
gate inputs.
Practical
OR Gate
IC Circuits
The output of an OR gate
goes high when any of
its inputs (A or B, etc.)
go high. The simplest
way to make an OR
gate is via a number of
diodes and a single
FIGURE 14.
Methods of
using TTL
NAND gates
as simple
inverters.
June 2007 69
