Adding a resistance at the output of a CMOS circuit creates an RC (resistor-capacitor) circuit with the load capacitance at the output. The rate at which the output voltage changes is determined by the time constant of this RC circuit, which is the product of the resistance and capacitance.
The presence of resistance in the output path affects the charging and discharging of the load capacitance, causing additional time for the output voltage to reach its intended logic state. As a result, the propagation delay increases compared to a scenario without the added resistance.
The impact of the delay depends on the magnitude of the resistance. A higher resistance value extends the charging or discharging time, leading to an increased propagation delay. Conversely, a lower resistance value reduces the delay.
In conclusion, including a resistance at the output of a CMOS circuit increases the propagation delay due to the formation of an RC circuit with the load capacitance. Designers must carefully assess the trade-offs between delay, noise immunity, and other circuit requirements when deciding to incorporate resistance in the output path.