CMOS Interview Question

Occasionally you will come across a CMOS circuit where the complementary nature of the n-channel pull-downs and p-channel pull-ups are not obvious, as in the circuit shown below:

1. Construct a table that gives the on-off status of each transistor in the circuit above for all combinations of inputs A and B.
2. Compute the output, Y, for each input combination and describe the function of the above circuit.

Solution:

1. 

2. The output Y is connected to four pairs of transistors in series, so each of these pairs can affect the output.

when A=0 and B=0, transistors T4 and T5 are on, so Y=0 when A=0 and B=1, transistors T6 and T7 are on, so Y=1 when A=1 and B=0, transistors T2 and T3 are on, so Y=1 when A=1 and B=1, transistors T8 and T9 are on, so Y=0

Putting this together, we conclude that Y = XOR(A,B).