Understanding CMOS Inverters in Circuits

Aug 14, 2024

CMOS Inverter Lecture Notes

Introduction

  • Importance of CMOS inverter in analog and digital circuits.
  • Often overlooked by students focusing solely on analog.
  • Essential for understanding both digital and analog applications.

Key Concepts

Buffer vs. Inverter

  • Buffer:
    • High input -> High output
    • Low input -> Low output
  • Inverter:
    • High input -> Low output
    • Low input -> High output

CMOS Inverter Circuit

  • Consists of PMOS and NMOS transistors.
  • Common input (V_in) for both transistors.
  • V_dd connected to the PMOS, output taken from the NMOS.

Pulse Input Analysis

  1. Input Pulse: Range of 0V to V_dd.
  2. Initial Conditions:
    • At T = 0, V_in = 0V:
      • PMOS is on (V_sg > V_tp), NMOS is off (V_gs < V_tn).
      • Output (V_out) starts charging.
  3. Charging Behavior:
    • PMOS behaves like a current source.
    • Capacitor (C) charges linearly, then exponentially.
    • Transition from saturation to linear in PMOS as V_out rises.
  4. Transition Points:
    • For V_out reaching V_tp, PMOS enters linear region.
    • NMOS turns on as V_in increases past V_tn.
  5. Discharge Phase:
    • V_in transitions from high to low, NMOS conducts, discharges capacitor.
    • Exponential discharge behavior observed.

Ramp Input Analysis

  • V_in gradually increased from 0 to V_dd.
  • Analyzing regions:
    1. 0 to V_tn:
      • PMOS on, NMOS off.
    2. V_tn to V_dd/2:
      • Both PMOS and NMOS on, but PMOS handles more current initially.
    3. V_dd/2 to V_dd:
      • NMOS conducts more heavily, PMOS switches to linear region.

Output Characteristics

  • Voltage Transfer Characteristics (VTC):
    • Indicates the relationship between V_in and V_out.
    • Switching threshold occurs when both transistors are in saturation.

CMOS Inverter Configuration Changes

  • Interchanging PMOS and NMOS leads to non-inverting behavior.
  • High input does not guarantee low output, thus acting like a buffer.

Small Signal Analysis

  • Voltage gain derived by analyzing small-signal model.
    • Gain = -G_m * (R_on1 || R_on2) for PMOS and NMOS.
  • Output resistance is simply R_on1 || R_on2.

Summary and Applications

  • CMOS inverters are crucial for both voltage level shifting and logic operations in circuits.
  • Understanding the operational principles allows for better design of analog and digital systems.