0195136047.pdf

432 DIGITAL CIRCUITS

RD

VGS = 6

VGS = 7

VGS = 5

VGS ≤ 3

VGS = 4

VDS, V

ID, mA

vout

vin

VDD = 7 V

= 23 kΩ

Pinch-off or

active region

Load line

Ohmic or

triode

region

1.0

0.8

0.6

0.4

0.2

0

(a) (b)

246810

7

vout, V

vin, V

6 5 4 3 2 1

02341

(c)

567

Figure 9.3.1MOSFET inverter with resistive

load.(a) Circuit with n-channel enhancement

MOSFET and a resistor.(b)TransistorI–Vcharac-

teristics.(c)Voltage-transfer characteristic for (a).

voltage-transfer characteristic (voutversusvin) is plotted in Figure 9.3.1(c). By choosing the low

range to be 0 to 3 V (i.e., less than the threshold voltage) and the high range to be 5 to 7 V, we can

see that any input voltage in the low range gives an output of 7 V (high) and inputs in the high

range give outputs in the low range. Thus, the circuit is seen to be an inverter.

While the circuit of Figure 9.3.1(a) functions correctly as an inverter, in order to maintain

low current consumption, large values ofRDare needed, but they are undesirable in ICs because

they occupy too much space. In order to increase the number of circuits per IC,RDis usually

replaced by a second MOS transistor, which is known as anactive load. WhenRDis replaced

by ann-channel MOSFET, it results in the logic family known as NMOS, which is widely used

in VLSI circuits such as memories and microprocessors. NMOS logic circuits are more compact

and, as such, more of them can be put on each chip. Also, MOS fabrication is simpler than

bipolar fabrication; with fewer defects, production costs are less. As mentioned before, the main

disadvantage of MOS technology, as compared to TTL, is low output current capacity.