NOR Gate using CMOS:
CMOS NOR gate circuit uses four MOSFETs just like the NAND gate, except that its transistors are differently arranged. Instead of two paralleled sourcing (upper) transistors connected to Vdd and two series-connected sinking (lower) transistors connected to ground, the NOR gate uses two series-connected sourcing transistors and two parallel-connected sinking transistors as shown in above figure.
Fig: Truth table for a NOR circuit:
From the above both figures when both inputs, A and B, are logic 0, Q1 and Q2 are “on,” and Q3 and Q4 are “off,” and the output is logic 1. This confirms the first row of the truth table above.
With both inputs logic 1, Q3 and Q4 are “on,” and Q1 and Q2 are “off,” producing a logic 0 output that confirms the last row of the truth table.
For the rest two input combinations, either Q1 is “off” and Q3 is “on” or Q2 is “off” and is Q4 “on”. In these cases, the output is logic 0 which is consistent with the above truth table.