Designing Logic Gates using CMOS Logic

Overview

• Logic gates' characteristics (fan-out, voltage level, speed, power consumption) depend on the technology used.
• Popular logic families: TTL, CMOS, ECL.
• Focus on CMOS (Complementary Metal Oxide Semiconductor) logic.

CMOS Technology

• Uses complementary NMOS and PMOS transistors.
• Transistors act as switches controlled by gate-source voltage.
  • NMOS Transistors:
    • Open circuit when gate-source voltage < threshold.
    • Closed switch when gate-source voltage > threshold.
  • PMOS Transistors:
    • Threshold voltage is negative.
    • Open circuit when gate-source voltage > threshold.
    • Closed switch when gate-source voltage < threshold.

Design Characteristics of CMOS Logic Gates

• Static Power Consumption: Low compared to NMOS or PMOS alone.
• Noise Margin: Higher than NMOS or PMOS alone.
• PMOS used as pull-up network; NMOS as pull-down network.
• Step by step explanation on why NMOS is used for pull-down and PMOS for pull-up.

NMOS and PMOS Logic Gates

• NMOS Inverter:

  • Input high: NMOS closed, output grounded.
  • Input low: NMOS open, output at supply voltage.
  • Static power dissipation due to current flow when input is high.
  • Produces weak logic 1 and strong logic 0.

• PMOS Inverter:

  • Passes strong logic 1, weak in passing logic 0.
  • Used in pull-up network.

CMOS Inverter

• Uses both NMOS and PMOS transistors.
• PMOS provides strong logic 1, NMOS provides strong logic 0.
• No direct path for current between supply and ground, reducing static power consumption.

CMOS Logic Gates Implementation

• CMOS NAND Gate:

  • NMOS transistors in series for AND operation.
  • PMOS transistors in parallel for dual network.
  • Truth table verified using transistor states.

• CMOS NOR Gate:

  • NMOS transistors in parallel for OR operation.
  • PMOS transistors in series for dual network.
  • Implementation verified with truth table.

• CMOS XOR Gate:

  • Expression: A'B + AB'
  • NMOS network: series for A'B and AB', connected in parallel.
  • PMOS network: dual of NMOS.

• CMOS XNOR Gate:

  • Expression: AB + A'B'
  • Similar implementation as XOR, with complementary logic.

Power Consumption in CMOS

• Static Power Consumption:

  • Negligible due to no direct path from Vdd to ground.
  • Slight due to leakage currents.

• Dynamic Power Consumption:

  • Occurs during charging/discharging of load capacitance.
  • Finite time when both NMOS and PMOS are ON contributes to power dissipation.

Advantages of CMOS Logic

• Low overall power consumption compared to other logic families.
• High noise margin due to full voltage swing.

Conclusion

• CMOS logic provides efficient and low-power designs for digital circuits.
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