Consider a 2-input NOR gate with inputs A and B. Suppose the signal driving A arrives earlier than the one driving B.

It is better to connect input B to the PMOS at the top of the stack (i.e., the PMOS with its source terminal connected to Vdd).

Question

Consider a 2-input NOR gate with inputs A and B. Suppose the signal driving A arrives earlier than the one driving B.

It is better to connect input B to the PMOS at the top of the stack (i.e., the PMOS with its source terminal connected to Vdd).

BlackTom AI · Accepted Answer

We start by restating the given statement to analyze it clearly: The scenario describes a 2-input NOR gate with inputs A and B, where A’s signal arrives earlier than B’s, and it claims that it is better to connect input B to the PMOS at the top of the stack (the PMOS whose source is connected to Vdd).

First, consider the topology of a 2-input NOR gate in standard CMOS. In a NOR gate, the pull-up network formed by the PMOS transistors is designed so that the output is high when either input is 0. In many textbook CMOS NOR implementations, the PMOS devices are not arranged in a long, multi-stage stack; instead, they are connected in a relatively simple network to Vdd, with their conduction controlled by the respective inputs. Consequently, the notion of a single PMOS transistor sitting at the “top of the stack” with a clearly defined “top” PMOS does not apply in the typical 2-input NOR pull-up arrangement as it might for multi-stage logic or certain deep-stack designs. This makes the premise about “the PMOS at the top of the stack” somewhat ambiguous or irrelevant for a basic 2-input NOR gate.

Even if one were considering a deeper stack or a nonstandard layout, the core idea behind assigning inputs to particular transistors to optimize speed is more nuanced. In dynamic or sequential scenarios, factors such as transistor sizing (W/L ratios), drive strength, and the effect of stacking (series-connected transistors causing reduced effective drive for the branch) determine which input assignment would minimize delay and improve noise margins. The claim that it is categorically better to connect B to the PMOS at the top of the stack presumes a specific stacked path and timing interaction that does not generalize reliably to a simple 2-input NOR with a straightforward pull-up network.

Now evaluating the option in light of these design considerations: the assertion that B should be connected to the PMOS at the top of the stack is not a universal rule for improving performance in a 2-input NOR gate. The conventional CMOS NOR design does not rely on a single top-of-stack PMOS as a primary determinant of timing for such a simple gate, and drive-logic optimization typically focuses on balanced pull-up and pull-down strengths, as well as avoiding deep stacking whenever possible. Therefore, the statement is not a generally valid design guideline for a standard 2-input NOR.

In summary, the claim about connecting input B to the PMOS at the top of the stack either misinterprets the typical NOR pull-up topology or relies on an overly specific and non-generalized layout rationale. This makes the statement inaccurate or at least not generally correct for a conventional 2-input NOR implementation.

FA25 ECE 555 001 Power, Combinational Circuits

Consider a 2-input NOR gate with inputs A and B. Suppose the signal driving A arrives earlier than the one driving B. It is better to connect input B to the PMOS at the top of the stack (i.e., the PMOS with its source terminal connected to Vdd).

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