Common Drain Amplifier Configuration (Source Follower) Notes

Introduction

• Overview of the common drain amplifier configuration of the MOSFET, also known as the source follower.
• Previous videos covered common source and common gate amplifier configurations.

Configuration Details

• AC Ground: The drain terminal is at AC ground.
• Input/Output:
  • Input signal applied between gate and ground.
  • Output measured between source and ground.
  • Drain terminal is common between input and output.
• Voltage Gain: Less than 1 (close to 1).
• Output Impedance: Very low compared to other configurations; useful for driving low impedance loads.

Small Signal Analysis

• Typical source amplifier circuit includes biasing.
• Capacitors act as an open circuit for DC analysis and short circuit for AC analysis.
• Replace MOSFET with small signal model for analysis.

Input Impedance

• Definition: Ratio of input voltage to input current (Vin/Iin).
• Calculation:
  • Gate acts as an open circuit at low frequencies.
  • Input impedance (Z_in) = R1 || R2 (parallel combination of biasing resistors).
  • Without biasing resistors, input impedance is ideally infinite but becomes finite due to R1 and R2.
  • To achieve large input impedance, R1 and R2 values should be high (typically in mega ohms).

Voltage Gain

• KVL Application: Vgs = Vin - Vo.
• KCL Application:
  • V0/Rs = gm * Vgs.
  • Substitute Vgs to find V0/Vin.
• Expression:
  • V0/Vin = (gm * Rs) / (1 + gm * Rs)
  • Voltage gain is always less than 1, but can approximate 1 if gmRs >> 1.
  • Positive voltage gain indicates output and input signals are in phase.*

Output Impedance

• Calculation:
  • Output impedance (Z_out) = Rs || (1/gm).
  • For finite output resistance, Z_out = Rs || (1/gm) || R0, considering drain-source output resistance.
  • Low output impedance is beneficial for driving low impedance loads.

Application

• Source follower useful in multistage amplifiers to improve voltage gain when driving low impedance loads.
• Example:
  • Common source amplifier connected to a 50 Ohm load.
  • Voltage gain without source follower: 20 (significantly reduced with load).
  • Adding source follower improves voltage gain to approximately 8.88.

Conclusion

• Understanding of small signal analysis for source follower.
• Source follower as an output stage in multistage amplifiers to drive low impedance loads.
• Encouragement to ask questions or give suggestions in comments.