Figure 3.21(a) shows a
twoinput AND gate using NMOS fieldeffect transistors. This diagram is a
modification of the AND gate using switches shown in Fig. 3.21(b). The AND gate
constructed using mechanical switches was discussed in Section 24 using Fig.
2.4. Figure 3.21(b) is a reproduction of Fig. 2.4 for reference. The mechanical
switches of 3.21(b) are replaced with NMOS transistors in Fig. 3.21(a). We find
that in Fig. 3.21(a), if A = 0 and B = 0 (i.e., A and B have 0volt inputs), transistors T_{1 }and T_{2}
will remain in the OFFstate. Then, output Z
= 0 volt (logic 0). The same
situation prevails, when A = 0 and B = 1, or B = 0
and A = 1 (here logic 0 = 0
volt). Now, if A = B = 1,
then both T_{1} and T_{2} conduct and develop +V_{DD}, across the load
transistor T_{3}. This means
that the output Z = logic 1. These may be summarized as given
Table 3.2. The entries given in Table 3.2 are found to be that of the AND
operation. So, we conclude that the logic gate shown in Fig. 3.21(a) performs
the positivelogic AND operation.
Table 3.2 NMOS AND
A

B

Z

0
0
1
1

0
1
0
1

0
0
0
1

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