# Inverter propagation delay using the RC delay model

• Toyona10
This is because the output is connected to VDD and the PMOS acts as a resistor in this case. In summary, the RC delay model takes into account the on-resistance of both the NMOS and PMOS when calculating delays, as they are equivalent in terms of their connection to the output.
Toyona10
Hi~

Actually I am fine with calculating the delay but I haven't got the concept clearly enough. Both nmos and pmos have their equivalent RC models. I understand how the RC model of nmos works when its on or off but I don't get how the pmos turns out this way. When its off, the upper part won't work because its open circuit but when its on, isn't the resistor supposed to be included? Why isn't it there in the inverter's equivalent RC model when the pmos is on?

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The resistor in the RC delay model is either the PMOS or the NMOS. They are both connected to "ground" because in either case the output is connected to a small signal ground (VDD in the PMOS case or VSS in the NMOS case) so as far as the delay model is concerned they are equivalent.

So to answer your question, yes the delay model does include the on-resistance of the PMOS when it is on.

## 1. What is the RC delay model and how does it relate to inverter propagation delay?

The RC delay model is a mathematical model used to estimate the delay of signals in a digital circuit, specifically for inverters. It takes into account the capacitance and resistance of the circuit components and their impact on the signal propagation time. Inverters have a delay due to the time it takes for the input signal to propagate through the inverter and produce an output signal. The RC delay model helps to calculate this delay and optimize circuit design for faster operation.

## 2. How does the size of the inverter affect its propagation delay using the RC delay model?

The size of an inverter, specifically the width of its transistors, has a direct impact on its propagation delay. A larger inverter will have a shorter delay because it has a wider channel for the signal to pass through, reducing the resistance and therefore the delay. Conversely, a smaller inverter will have a longer delay due to a narrower channel and higher resistance.

## 3. What other factors can affect inverter propagation delay besides the RC delay model?

While the RC delay model is a useful tool for estimating inverter propagation delay, there are other factors that can also impact the delay. These include the technology used for the circuit, temperature, and power supply voltage. Different technologies have different resistances and capacitances, and temperature and voltage can affect the speed of signal propagation through the circuit.

## 4. Can the RC delay model be used for other digital circuits besides inverters?

While the RC delay model is commonly used for inverters, it can also be applied to other digital circuits, such as NAND and NOR gates. However, these circuits may have additional factors to consider, such as the number of inputs and the logic function they perform. In some cases, a more complex delay model may be necessary for more accurate calculations.

## 5. How can the RC delay model be used to optimize circuit design for faster operation?

The RC delay model can be used to estimate the delay of a circuit and identify areas for improvement. By adjusting the size of transistors or using different technologies, the resistance and capacitance of the circuit can be optimized to reduce overall propagation delay. Additionally, routing techniques and placement of components can impact delay and should be considered during circuit design.

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