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Transistor geometries in a CMOS inverter have been adjusted to meet the requirement for worst case charge and discharge times for driving a load capacitor C. This design is to be converted to that of a NOR circuit in the same technology, so that its worst case charge and discharge times while driving the same capacitor are similar. The channel lengths of all transistors are to be kept unchanged. Which one of the following statements is correct?

1. Widths of PMOS transistors should be doubled, while widths of NMOS transistors should be halved.
2. Widths of PMOS transistors should be doubled, while widths of NMOS transistors should not be changed.
3. Widths of PMOS transistors should be halved, while widths of NMOS transistors should not be changed.
4. Widths of PMOS transistors should be unchanged, while widths of NMOS transistors should be halved.

Correct answer: Widths of PMOS transistors should be doubled, while widths of NMOS transistors should not be changed.

Solution

In a NOR gate, the PMOS transistors need to provide sufficient drive strength to pull the output high, which is why their widths should be increased. Since NMOS transistors are already optimized for the original design, their widths can remain unchanged to maintain the same discharge characteristics.

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