CMOS Logic Gates Explained | Logic Gate Implementation using CMOS logic

Summary

This video explains the CMOS logic gate implementations for XOR and XNOR gates. It covers the fundamental principles behind these gates, demonstrating how NMOS and PMOS networks are used to construct them. The video also explores how to verify the logic of these implementations using truth tables. Additionally, it addresses power consumption in CMOS circuits, highlighting the differences between static and dynamic power consumption. While static power is minimal, dynamic power is more significant, but overall, CMOS circuits are known for their efficiency in terms of power consumption, making them ideal for low-power applications.

Takeaways

• 😀 CMOS logic gates use both NMOS and PMOS transistors to implement Boolean functions with low static power consumption.
• 😀 The XOR gate can be implemented in CMOS using the expression A'B + AB', or equivalently (AB + A'B')'.
• 😀 In CMOS, the NMOS network for XOR connects A and B in series, while A' and B' are also in series, and these two series combinations are then connected in parallel.
• 😀 The PMOS network for XOR is the dual of the NMOS network, with A and B connected in parallel, and A' and B' in parallel, followed by a series connection of these parallel combinations.
• 😀 To verify the XOR gate logic, the truth table can be used, confirming the correct outputs for all input combinations.
• 😀 The XNOR gate is the complement of the XOR gate, with its output expressed as AB + A'B'.
• 😀 In CMOS, the NMOS network for XNOR involves connecting A'B and AB' in series, and these two series connections are then combined in parallel.
• 😀 The PMOS network for XNOR is the dual of the NMOS network, where A'B and AB' are connected in parallel and then connected in series.
• 😀 Static power consumption in CMOS logic gates is negligible, primarily due to the use of complementary transistors that do not conduct current when in their 'off' states.
• 😀 Dynamic power consumption in CMOS gates occurs due to the charging and discharging of capacitance during logic state transitions (0 to 1 or 1 to 0), and during the brief period when both NMOS and PMOS transistors are on.
• 😀 Despite dynamic power consumption, CMOS logic circuits consume less overall power than other logic families, making them more efficient in terms of energy use.

Q & A

What is the Boolean expression for a two-input XOR gate?

The Boolean expression for a two-input XOR gate is A bar B + A dot B bar, or equivalently, A B + A bar B bar whole bar.

How is the NMOS network for the XOR gate designed?

The NMOS network for the XOR gate involves connecting A and B in series, and A bar and B bar in series as well. These two series combinations are then connected in parallel.

How is the PMOS network for the XOR gate designed?

In the PMOS network for the XOR gate, A and B are connected in parallel, and A bar and B bar are also connected in parallel. These two parallel networks are then connected in series.

What is the Boolean expression for a two-input XNOR gate?

The Boolean expression for a two-input XNOR gate is A dot B + A bar dot B bar, or equivalently, the complement of the XOR gate (A bar B + A dot B bar whole bar).

How is the NMOS network for the XNOR gate implemented?

The NMOS network for the XNOR gate involves connecting A bar and B in series, and A and B bar in series as well. These two series connections are then connected in parallel.

How is the PMOS network for the XNOR gate designed?

In the PMOS network for the XNOR gate, A bar and B are connected in parallel, and A and B bar are also connected in parallel. These two parallel networks are then connected in series.

What role do inverters play in the CMOS implementation of XOR and XNOR gates?

Inverters are used to generate the complements (A bar, B bar) of the input signals when these complements are not already available. This is necessary for both XOR and XNOR gate implementations.

What is the static power consumption in CMOS logic gates?

Static power consumption in CMOS logic gates is typically negligible because there is no current flowing through the transistors when they are in the off state. However, there may be some leakage current that causes minimal static power consumption.

What is dynamic power consumption in CMOS logic gates?

Dynamic power consumption in CMOS logic gates occurs when there is a transition in the output from logic 0 to logic 1, or from logic 1 to logic 0. During this transition, the load capacitance at the output of the gate is charged or discharged, consuming power.

What are the factors contributing to dynamic power consumption in CMOS circuits?

Dynamic power consumption in CMOS circuits is caused by the charging and discharging of the load capacitance at the gate's output, which includes the capacitance of the next gate’s input and the interconnects between gates. Additionally, finite rise and fall times of inputs cause temporary current flow between supply and ground, leading to power dissipation.