Common‑Drain Amplifier (IGFET): Design, Function, and Applications

What Is a Common‑Drain IGFET?

A common‑drain amplifier, also known as a source‑follower, is a simple but powerful configuration used in MOSFET and IGFET circuits. It offers a voltage‑unity gain, very high input impedance, and low output impedance, making it ideal for buffering and impedance matching.

How It Works

In the common‑drain topology, the input signal is applied to the gate, the source follows the gate voltage with a slight drop (VGS‑Vth), and the output is taken from the source. Because the source is connected to the load, the device behaves like a follower.

Key Advantages

• Voltage gain ≈ 1 (unity)
• High input impedance (kΩ to MΩ)
• Low output impedance (tens of ohms)
• Excellent linearity over the mid‑band
• Easy to bias with a single supply

Typical Applications

Common‑drain IGFETs are widely used in:

• Analog front‑ends of ADCs and DACs
• Audio pre‑amplifiers and headphone drivers
• Buffer stages between high‑impedance sensors and low‑impedance amplifiers
• Voltage‑regulated power supplies

Design Tips

• Select a transistor with a high transconductance (gm) to reduce source‑resistor loading.
• Keep the source resistor small to preserve bandwidth.
• Use a gate‑bias network that sets VGS just above threshold for linear operation.
• Add a small‑signal resistor in parallel with the load to limit output swing if necessary.

Resources

For a deeper dive, see the Wikipedia article on Common‑Drain Amplifiers and the CircuitLab simulation.