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) * 碌n * Cox * (W/L) * (VGS - VTH)^2 * (1 + 位 * VDS)
- 位 (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 = 碌n * Cox * (W/L) * (VGS - VTH)
- gm = 2 * ID / (VGS - VTH)
- gm = 2 * ID * 碌n * 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 k惟).
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.
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.