3.2.4 Beyond Inverters | Summary and Q&A

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July 12, 2019
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3.2.4 Beyond Inverters

TL;DR

Complementary pullup and pulldown circuits in CMOS logic gates control the voltage of the output node, resulting in valid digital outputs of 0 or 1.

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Key Insights

  • 👻 Complementary pullup and pulldown circuits in CMOS logic gates control the voltage of the output node, allowing for valid digital outputs.
  • 🥺 Non-complementary circuits can lead to short circuit currents and are considered undesirable.
  • 🎨 Complementary circuits exhibit a symmetry in their design, with NFET switches in series for the pulldown circuit and PFET switches in parallel for the pullup circuit.
  • 🔬 A NAND gate is a CMOS combinational device that utilizes complementary pullup and pulldown circuits.
  • 🫵 The layout of MOSFETs in CMOS logic gates can be observed in the integrated circuit surface view, with NFETs connected in series and PFETs in parallel.

Questions & Answers

Q: What happens when the pullup and pulldown circuits in CMOS logic gates are not complementary?

When the pullup and pulldown circuits are not complementary, there can be a path between the power supply voltage and ground, leading to large amounts of short circuit current flow. This can result in damage to the circuit and is undesirable. In such cases, the output voltage is considered unknown or "X."

Q: How does a non-complementary pullup and pulldown circuit affect the output node?

In a non-complementary circuit, neither the pullup nor the pulldown circuit conducts, causing the output node to have no connection to either power supply voltage. This results in the output node being electrically floating, and any charge stored by the nodal capacitance will remain there for some time.

Q: What is the significance of complementary pullup and pulldown circuits?

Complementary pullup and pulldown circuits ensure that when one circuit is conducting, the other is not. This allows for a valid digital output of either 0 or 1. They also exhibit a symmetry in their design, with NFET switches in series for the pulldown circuit and PFET switches in parallel for the pullup circuit.

Q: What is a NAND gate?

A NAND gate is a type of CMOS combinational device that uses series NFETs in the pulldown circuit and parallel PFETs in the pullup circuit. It provides a digital output of 1 unless both inputs are 1, in which case the output becomes 0. It is considered the inverse of the AND function.

Summary & Key Takeaways

  • Complementary pullup and pulldown circuits in CMOS logic gates allow for control over the output voltage, ensuring it either rises quickly to become a digital 1 or falls quickly to become a digital 0.

  • Non-complementary circuits can lead to short circuit currents, while complementary circuits prevent this issue and provide a valid digital output.

  • Complementary circuits exhibit a symmetry in their design, such as NFET switches in series for the pulldown circuit and PFET switches in parallel for the pullup circuit.

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