Mumbai University > COMPS > Sem 3 > Digital Logic Design and Analysis

Marks: 5 M

Year: June2014

i) Fan-Out:

• It is the greatest number of inputs of gates of the same type to which the output can be safely connected. The maximum fan-out of an output measures its load driving capability.
• Fan out for CMOS family depends upon the input and output current ratings of the logic circuit.

• E.g. for an HC Series of CMOS, whose current ratings are:

  $I_{IH} = 1µA, I_{IL} = -1µA, I_{OH} = -0.02mA, I_{OL} = 0.02mA$

  So, it has LOW state as well as HIGH state fan out of 20, Therefore HC Series of CMOS family has fan out of 20.

ii) Fan In:

• Fan-in is the number of inputs a gate can handle. Physical logic gates with a large fan-in tend to be slower than those with a small fan-in.
• Fan in of CMOS family can be more than 2 by extending series-parallel design of CMOS using NAND and NOR gates.
• A gate with fan-in ‘n’ is obtained by using n series and n parallel transistors. In principle, n can have any value but there are practical limitations on the maximum value of n. So depending on various factors the value of fan-in varies in CMOS family.