Common-Drain MOSFET Configuration Basics

The common-drain configuration, also known as the source follower, is one of the three basic ways to connect a Field-Effect Transistor (FET) in an amplifier circuit. In this setup, the input signal is applied to the gate, the output is taken from the source terminal, and the drain terminal is common to both input and output (usually connected to the power supply). This configuration offers unique characteristics that make it suitable for specific applications.

Key Characteristics of Common-Drain

• High Input Impedance: The gate terminal is reverse-biased, drawing almost no current. This results in a very high input impedance ($Z_{in}$), which prevents the amplifier from significantly loading the preceding signal source.
• Low Output Impedance: The output impedance ($Z_{out}$) is relatively low, making it capable of driving loads that require significant current without a substantial drop in the output voltage.
• Voltage Gain Near Unity: The voltage gain ($A_v$) is slightly less than one, typically represented as $A_v \approx 1$. It effectively transfers the input voltage to the output without significant amplification or attenuation.
• Current Gain: While the voltage gain is close to unity, the current gain is high.
• No Phase Inversion: The output signal at the source follows the input signal at the gate, meaning there is no phase inversion between the input and output voltages.

Voltage Buffer Functionality

A voltage buffer is a circuit designed to transfer a voltage level from a high-impedance source to a low-impedance load without altering the voltage level significantly. Its primary function is to isolate the source from the load.

The common-drain MOSFET configuration perfectly matches the requirements of a voltage buffer due to its characteristics:

• The high input impedance ensures that the signal source is not loaded down.
• The low output impedance allows the buffer to drive a connected load effectively, maintaining the voltage level from the input.
• The voltage gain close to unity ($A_v \approx 1$) means the output voltage closely mirrors the input voltage, fulfilling the buffering requirement.

Comparison with Other Options

• Transconductance Amplifier: This requires converting input voltage to output current. A common-source configuration is typically better suited for this role as it provides voltage amplification and can be configured for transconductance operation.
• Current Buffer: While the common-drain configuration has high current gain, its primary function isn't precise current replication or buffering in the same way it handles voltage. A current mirror or a specific common-source/common-gate topology might be more appropriate depending on the exact requirement.
• Transresistance Amplifier: This converts input current to output voltage. This function is usually achieved using common-gate or common-source configurations, not common-drain.

Therefore, the MOSFET biased in a common-drain configuration is best suited for designing a voltage buffer.