Electronic Circuits Lecture 32 - MOSFET Second-Order Effects

Introduction

• Lecture by Basil Razavi
• Focus on the MOSFET device, specifically second-order effects

Review of Last Lecture

• Discussed VDS (Drain-Source Voltage) exceeding VGS (Gate-Source Voltage) minus VTH (Threshold Voltage).
• Observed channel pinching near the drain when VDS becomes larger.
• Identified two regions of operation:
  • Triode Region: VDS < VGS - VTH
  • Saturation Region: VDS >= VGS - VTH
• In saturation, the drain current (ID) remains constant with changes in VDS, operating as an ideal current source.

Current Source and Voltage Relation

• In saturation, ID is a function of VGS, not VDS, and follows a quadratic relationship.
• VGS values affect electron density and current capability.

Channel Length Modulation

• Similar to the early effect in bipolar transistors.
• Real ID vs. VDS measurements show a slight increase, not constant.
• Channel length decreases as VDS increases, affecting ID.
• Introduced correction factor to model this:
  • ID = (1/2) * µn * Cox * (W/L) * (VGS - VTH)^2 * (1 + λ * VDS)
  • λ (Lambda) is the channel length modulation coefficient.

Biasing in Circuits

• Critical for MOSFET amplification.
• Without proper biasing, MOSFET cannot amplify signals effectively.
• Biasing sets operating point (VGS, ID) to ensure MOSFET is active.
• Aim to achieve a reasonable swing in output voltage with a reasonable resistance value (RL).

Transconductance (gm)

• Indicates how effectively a MOSFET converts voltage changes to current changes.
• Defined as the derivative of ID with respect to VGS.
• Higher gm indicates a stronger device.
• Calculated as:
  • gm = µn * Cox * (W/L) * (VGS - VTH)
  • gm = 2 * ID / (VGS - VTH)
  • gm = 2 * ID * µn * Cox * (W/L)

Amplifier Design Attempt

• First attempt failed due to lack of biasing.
• Second attempt added a threshold voltage, but resulted in high resistance requirements.
• Third attempt improved biasing, selecting V0 > VTH.
  • Resulted in more reasonable resistance value (25 kΩ).

Conclusion

• Proper biasing is essential for MOSFET operation in amplifier circuits.
• Understanding second-order effects like channel length modulation is crucial for accurate circuit behavior modeling.
• Transconductance is a key parameter in evaluating MOSFET performance.

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Key Takeaway: Properly biasing a MOSFET ensures its effective operation in amplifiers, and understanding its behaviors and model corrections, like channel length modulation, is vital for designing efficient circuits.