0195136047.pdf

430 DIGITAL CIRCUITS

The DTL NAND gate circuit shown in Figure 9.2.1 and others closely related to it belong to

the DTL logic family. Since all circuits in the same family have the same high and low ranges,

they can be interconnected to build up digital systems. Although we have described the DTL gate

as our first example for our convenience and easy understanding, the DTL technology is almost

obsolete and is replaced by TTL.

TTL NAND Gate

The TTL logic family uses only transistors instead of a combination of transistors and diodes.

TTL has the same high and low ranges as DTL. TTL is the most popular BJT logic family. A

primary reason for the popularity of TTL over DTL is its higher speed. TTL has a typical fan-out

of 10 or more, small propagation delays on the order of 2 to 10 ns, and a power consumption

of about 2 mW. By contrast, DTL has a typical fan-out of 8 to 10, a propagation delay on

the order of 30 to 90 ns, and a power consumption of around 15 mW. The noise margins and

fan-out of DTL are generally better than those of RTL (resistor–transistor logic; see Problem

9.2.8), but the switching speeds are about the same. The emitter-coupled logic (ECL) family

is also available with increased switching times, although not discussed here in any detail.

The name arises from the common attachment of the emitters of the input transistors. The

propagation delays are on the order of 1 ns, but the power consumption is quite high (on the

order of 25 mW per gate). Low power consumption combined with relatively small noise margins

(<0.3 V) has made TTL a popular choice and, in particular, the high-speed Schottky diode

TTL gates.

A typical TTL NAND gate is shown in Figure 9.2.2. TransistorT 1 is a multiple-emitter

transistor (npnBJT) which acts as an AND gate. Replacing the base–emitter and base–collector

junctions with diodes,T 1 can be represented as shown in Figure 9.2.3.

VA V

o

VCC = 5 V

X

VVB Y

C

R 2

R 1

T 1 T 2 T 3

R 3

F

Figure 9.2.2TTL NAND gate.

VA

DA

DB

DC

Y

X

Base-collector

diode D

VB Base-emitter diodes

VC

Figure 9.2.3Diode replacement of three-input transistorT 1 of

Figure 9.2.2.