Question

A CMOS inverter consists of

• A. A PMOS pull-up and an NMOS pull-down network
• B. An NMOS pull-up and a PMOS pull-down network
• C. Two NMOS transistors
• D. Two PMOS transistors

Hint:

In standard CMOS digital logic design: - PMOS transistors are always placed in the upper network to pull the output up to \(V_{DD}\). - NMOS transistors are always placed in the lower network to pull the output down to Ground.

Answer 1

The Correct Option is A

Concept: CMOS (Complementary Metal-Oxide-Semiconductor) circuits combine matched pairs of PMOS and NMOS field-effect transistors to create efficient digital logic structures. The most fundamental circuit in this family is the CMOS inverter, which implements the logical NOT operation. The architecture of a CMOS inverter uses a specific layout to control the path between the supply voltage and ground:
• Pull-Up Network (PUN): Built using a PMOS transistor. Its source is connected to the positive power supply rail (\(V_{DD}\)), and its drain is connected to the output node. PMOS devices are ideal pull-up elements because they conduct strongly when their gate voltage is low (Logic 0), allowing them to pull the output node cleanly up to the full rail voltage \(V_{DD}\) without any threshold voltage loss.
• Pull-Down Network (PDN): Built using an NMOS transistor. Its drain is connected to the output node, and its source is tied to the ground rail (\(V_{SS}\)). NMOS devices are ideal pull-down elements because they conduct strongly when their gate voltage is high (Logic 1), allowing them to pull the output node cleanly down to \(0\text{ V}\). Let's look at how the circuit operates for each input logic state:
• When Input \(V_{\text{in}}\) = Logic 0 (\(0\text{ V}\)): The low voltage turns the NMOS pull-down transistor off, breaking the connection to ground. At the same time, it turns the PMOS pull-up transistor on, creating a direct path from the supply to the output. This pulls the output voltage up to \(V_{DD}\) (\(V_{\text{out}}\) = Logic 1).
• When Input \(V_{\text{in}}\) = Logic 1 (\(V_{DD}\)): The high voltage turns the PMOS pull-up transistor off, breaking the connection to the supply. Meanwhile, it turns the NMOS pull-down transistor on, creating a path from the output to ground. This pulls the output voltage down to \(0\text{ V}\) (\(V_{\text{out}}\) = Logic 0). This complementary operation confirms that a CMOS inverter requires a PMOS pull-up transistor and an NMOS pull-down transistor working together.