Question

How does a CMOS inverter perform logic inversion using complementary MOSFETs?

Answer 1

A CMOS inverter is a fundamental building block in digital integrated circuits and is used to perform logic inversion. It utilizes complementary metal-oxide-semiconductor (CMOS) technology, which consists of both n-type and p-type MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) in a single circuit configuration.

A CMOS inverter typically consists of two MOSFETs: one is an n-type MOSFET (NMOS) and the other is a p-type MOSFET (PMOS). These transistors are connected in a way that creates an inverter function, i.e., it converts a logic high (1) input to a logic low (0) output and vice versa. Let's see how it works:

NMOS (n-type MOSFET): The n-type MOSFET is a majority carrier device, which means it conducts when a positive voltage (logic high) is applied to its gate terminal. When the input signal (VIN) is high (1), the gate of the NMOS is positively biased, and it allows current to flow from the drain to the source terminal. This creates a low resistance path between the output (VOUT) and ground (GND), making VOUT close to GND voltage, which corresponds to logic low (0).

PMOS (p-type MOSFET): The p-type MOSFET, on the other hand, is a minority carrier device, which means it conducts when a negative voltage (logic low) is applied to its gate terminal. When the input signal (VIN) is low (0), the gate of the PMOS is negatively biased, and it allows current to flow from the source to the drain terminal. This creates a low resistance path between the output (VOUT) and the power supply voltage (VDD), making VOUT close to VDD voltage, which corresponds to logic high (1).

By combining these two complementary MOSFETs, the CMOS inverter can perform logic inversion:

When the input (VIN) is high (1), the NMOS is ON, and the PMOS is OFF, resulting in a low output (VOUT = 0).
When the input (VIN) is low (0), the NMOS is OFF, and the PMOS is ON, resulting in a high output (VOUT = 1).

In summary, the CMOS inverter's ability to perform logic inversion is achieved through the coordinated action of the n-type and p-type MOSFETs, which provide complementary switching characteristics based on the input voltage level. This configuration enables efficient and low-power operation, making CMOS technology widely used in modern digital circuits.