2:4 decoder solution confusion

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anhnha

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Hi.
I am confused a bit about the solution for 2:4 decoder in the picture below.
Please help me with the question in the figure.
Thanks.

 

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    anhnha

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For the same drive strength, the NAND-NOT combination needs less silicon space than the NOR solution.
Vice versa, by using your NOR solution on the same silicon area as the NAND-NOT combination, the NOR output has less drive strength.

However, if you don't mind about drive strength (or silicon area consumption), your NOR solution is probably faster than the NAND-NOT combination, because it needs just 1 gate delay to achieve the result, instead of 2 propagation delays for the NAND-NOT combination.
 
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    anhnha

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Thank you.
Could you explain why the NOR output has less drive strength?
 

anhnha said:
Could you explain why the NOR output has less drive strength?
Click to expand...

Because the 2 PMOS in series - at corresponding W/L ratio - have much less drive strength for the logical 1 than the NAND's 2 NMOS in series for a logical 0. In CMOS, the NMOSFETs are much stronger than the PMOSFETs, for the same W/L ratio - about the factor µn/µp .

See explanations on Logical Effort, e.g. from here:
 

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