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CMOS#

Complementary MOS (CMOS) refers to the use of both PMOS and NMOS to build digital circuits.

There are many reasons why CMOS dominates digital circuits in today's world.

Technical reasons:

  • Low power dissipation: It consumes very little power compared to other technologies.
  • High noise immunity: It is largely insensitive to electrical interference or disturbances. Moreover, noise is reduced as a signal propagates further through a circuit.
  • Clean logic levels: It produces distinct and reliable high/low voltage signals.
  • Single supply voltage: It only requires one power source to operate.
  • Cascadability: Components can be easily connected in series.

Economic reasons:

  • Simple design: Circuits are relatively easy to design using CMOS.
  • Mature manufacturing: The production process has been well-understood and mastered.
  • High integration: It allows for a very high density of transistors, making it suitable for complex integrated circuits (chips).

CMOS Design#

Every Boolean function \(f: \{0,1\}^n \to \{0,1\}\) can be implemented via CMOS.

Algorithm: CMOS Design from Boolean Expression

We are given an arbitrary Boolean expression of a Boolean function \(f: \{0,1\}^n \to \{0,1\}\) and want to implement \(f\) via CMOS:

  1. Create a pull-up network (PUN) from PMOS transistors by traversing the expression recursively.
  • Each conjunction (AND) in the expression is physically implemented by arranging the corresponding sub-networks in parallel.
  • Each disjunction (OR) in the expression is physically implemented by arranging the corresponding sub-networks in series.
  • The gate of each PMOS is connected to exactly one literal.
  • The sources of the top-most PMOS transistors are connected to the supply \(V_{\text{DD}}\).
  1. Create a pull-down network (PDN) from NMOS transistors by traversing the expression recursively.
  1. Connect the two networks by connecting the drains of the bottom-most PMOS transistors to the drains of the top-most NMOS transistors and drives this common connection into an inverter.