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I know that whenever you have series transistors multiply the equivalent W/L of the inverter by the number of series transistors. In the parallel case W/L remains the same. I don't know how to apply those two rules to this problem. All of the examples I can find are far simpler than this and more symmetric.

How do I apply parallel and series rules to transistors?

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    \$\begingroup\$ Start by drawing the transistor level schematic of this function, ignore the W/L first, just draw the circuit. After that worry about W/L \$\endgroup\$ – Bimpelrekkie Jan 10 '18 at 20:47
  • \$\begingroup\$ that's not the problem i already drew it. \$\endgroup\$ – Mostafa Mekawy Jan 10 '18 at 20:52
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If you have already sketched out the transistor diagrams take the worst case N path (4 transistors if I am not mistaken) and size these to give you the required overall w/l. The other transistors on the N side can be smaller, you have a set of three one of which is already sized from the above. Size the other two to match the needed overall W/L. Finally you have one which acts on its own size that one accordingly.

Repeat the exercise for the P side transistors and you are done.

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  • \$\begingroup\$ okey, so they can be the same size as the critical path ?! \$\endgroup\$ – Mostafa Mekawy Jan 10 '18 at 21:07
  • \$\begingroup\$ Yes they can but silicon area is expensive and the extra gate capacitance reduces performance you would get extra marks from me for making them as small as possible. \$\endgroup\$ – RoyC Jan 10 '18 at 21:15

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