Nuts & Volts - May 2007

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