9.3 CMOS AND OTHER LOGIC FAMILIES 431
EXAMPLE 9.2.2
Considering the TTL NAND gate circuit of Figure 9.2.2, with one or more inputs low, show that
the output will be high.
Solution
Vo
VCC
X
Y
B
C
A
R R^2
1
iC 1
iB 1
T 1
T 3
T 2
R 3
F
Figure E9.2.2TTL NAND gate with one input
low and the other two high.
Let one of the emitters, sayA, be grounded and therefore letVAbe low whileVBandVCare high.
The situation is then as shown in Figure E9.2.2. There is a current path fromVCCdown throughR 1
and emitterAto ground. WithvX=0.7 V a base currentiB 1 ∼=(VCC− 0. 7 )/R 1 will be flowing in
transistorT 1. Note, however, thatiC 1 must be flowing out of the base of transistorT 2. That being
the wrong direction for the base current of annpntransistor, it must represent the reverse current
through one of the junctions inT 2. Reverse currents being very small,iB 1 >> iC 1 /β. HenceT 1
will be saturated, and its collector-to-emitterAvoltage will beVCEsat, which is about 0.2 V. Thus,
the voltage atYwill be about 0.2 V, which is much less than the 1.4 V needed to forward-bias the
emitter junctions ofT 2 andT 3. ThusT 3 is cut off and outputVoatFis high.
The TTL family is large and widely used. TTL circuits can switch fairly quickly allowing
data rates on the order of 10 to 40 Mbits/s (mega stands for 2^20 ). They also have good output
current capability. However, they consume too much power and space to be used in LSI. Hence,
TTL is used primarily in SSI and MSI, which are the less densely packed ICs. MOS (metal-oxide
semiconductor) technology, which has low power consumption and high packing density, but low
output current capability, is widely used in LSI.
9.3 CMOS and Other Logic Families
Both DTL and TTL are based on the saturating BJT inverter. The transistor acts as a switch
that connects or disconnects the collector and emitter. The switch is closed when sufficient base
current is applied to saturate the transistor. While the BJT technologies have dominated during
the 1970s, logic families based on MOSFET technology are now more widely used because of the
advantages of fewer fabrication steps and generally lower power consumption. After PMOS and
NMOS technologies, CMOS emerged as the dominant MOS technology and remains so today.
Let us consider the simple MOSFET inverter with resistive load shown in Figure 9.3.1(a),
quite similar in principle to the BJT inverter, although the circuit is rather impractical, as we shall
see later. The load line forRD=23 kandVDD=7 V is shown along with the transistorI–V
characteristics in Figure 9.3.1(b). On findingvout(=vDS) for different values ofvin(=vGS), the
