Understanding Digital
Buffer,, Gate, and Logic
IC Circuits
by Ray Marston
PART 2: Digital Inverter and ‘AND Gate’ Circuits
Ray Marston describes practical digital inverter and AND gate
logic ICs in this second installment.
FIGURE 1. Any NAND or
NOR gate can be used as
an inverting buffer element.
FIGURE 2.
Fourteen
popular
inverter ICs.
Last month’s
opening article
explained modern TTL and CMOS
logic gate basics and gave practical
descriptions of some of the most
popular digital buffer ICs that are
available. This month, we expand on
this basic theme and take detailed
looks at some popular digital inverter and AND gate ICs. As you read
through this, note that some of the
ICs mentioned may not be available
via your local supplier, but should still
be available from specialist dealers.
Practical Digital
Inverter Circuits
The digital inverter (or NOT gate)
is the
most
< FIGURE 3. Functional
diagram of the 7404,
74LS04, 74HC04, 74HCU04,
or 4069UB Hex inverter ICs.
basic of all digital logic elements,
and is sometimes called an inverting
buffer. If you ever need only a few
simple inverters, one cheap way to
get them is to make them from spare
TTL or CMOS NAND or NOR elements, connected in the basic way
shown in Figure 1, in which the input
terminals of a two input NAND or
NOR gate are shorted together to
accept a single input and give a
single (but inverted) output signal.
Figure 2 lists basic details of some
popular digital inverter ICs. When
using these ICs, note that all unused
inverters must be disabled by tying
their inputs to one of the IC’s low voltage supply lines. In CMOS devices,
the unused inputs can be tied directly
to either supply line, but in TTL
devices, it is best (for lowest quiescent
current consumption) to tie all unused
inputs directly to the 0V rail. If the
unused inverter is a three-state type, it
should (if it has independent controls)
be set into its normal mode via its
< FIGURE 4. Functional
diagram of the 74LS05 or
7406 Hex inverters with open
collector outputs.
> FIGURE 5. Functional
diagram of the 74LS14,
74HC14, or 40106B Hex
Schmitt inverter ICs.
68
May 2007