An AND gate is a basic digital logic gate that implements logical conjunction. It outputs true or 1 only when all its inputs are true or 1. Here's a breakdown of how an AND gate works internally:

1. Transistor-Level Implementation

An AND gate can be implemented using transistors, such as MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors). The two most common types of transistors used in digital circuits are NMOS and PMOS transistors.

CMOS AND Gate

In CMOS (Complementary Metal-Oxide-Semiconductor) technology, an AND gate is constructed using a combination of NMOS and PMOS transistors.

  • PMOS Transistors: These transistors are used in the pull-up network. They conduct when the input is 0.
  • NMOS Transistors: These transistors are used in the pull-down network. They conduct when the input is 1.

Here’s how a two-input CMOS AND gate works:

  1. Pull-up Network (PUN):

    • Consists of two PMOS transistors in series.
    • When both inputs (A and B) are 0, the PMOS transistors conduct, pulling the output (Y) to Vcc (logic high).
  2. Pull-down Network (PDN):

    • Consists of two NMOS transistors in parallel.
    • When both inputs (A and B) are 1, the NMOS transistors conduct, pulling the output (Y) to ground (logic low).

2. Truth Table

The truth table for a two-input AND gate is:

3. Boolean Expression

The Boolean expression for an AND gate is:
Y=ABY = A \cdot B

This expression means that the output Y is true (1) only when both A and B are true (1).

4. Operation

  • Both inputs low (A = 0, B = 0):

    • PMOS transistors in the pull-up network conduct.
    • NMOS transistors in the pull-down network do not conduct.
    • Output Y is pulled high (1).
  • One input low, one input high (A = 0, B = 1 or A = 1, B = 0):

    • The pull-up network has one conducting PMOS transistor and one non-conducting PMOS transistor.
    • The pull-down network has one conducting NMOS transistor and one non-conducting NMOS transistor.
    • Output Y is pulled high (1).
  • Both inputs high (A = 1, B = 1):

    • PMOS transistors in the pull-up network do not conduct.
    • NMOS transistors in the pull-down network conduct.
    • Output Y is pulled low (0).