XNOR and XOR

To implement XNOR and XOR logic functions first of all we will implement XNOR gate and then use inverter stage at output of XNOR gate to realize XOR gate. The truth table of XOR and XNOR gates is shown in Table below.

The Boolean expression for both the gates are given as :

For XNOR gate : F = ––A ––B + AB For XOR gate : F = ––A B + A ––B

From both the Boolean expression we can write, F = ––F = ––––––––––––––––A B + A ––B

Now we will design the PDN and PUN of F i.e. F = ––––––––––––––––A B + A ––B

Construction of PDN :

In order to get the inverted inputs i.e. ––A and ––B, the inverters are used as shown in

Figure The implementation of the PDN of function F =––––––––––––––––A B + A ––B is also shown in

Figure.

Construction of PUN :

The PUN of function F = ––––––––––––––––A B + A ––B i.e. XNOR gate is the dual network of PDN as shown in Figure. Here also for the inverted inputs i.e ––A and ––B, the inverters are used as shown in Figure.

Construction of XNOR gate :

The two input XNOR gate can be constructed by combining PDN and PUN as shown in Figure.

Construction of XOR gate :

The XOR gate can be constructed by connecting inverter at the output stage of two input XNOR gate. The complete CMOS implementation of XOR gate is as shown in

Figure below.