Common-Source JFET Amplifier: Design, Analysis, and Practical Worksheet

Common‑Source JFET Amplifier Overview

The common‑source configuration is the workhorse of JFET amplifier design. By biasing the gate at a negative voltage relative to the source, the device operates in the saturation region, providing high voltage gain while maintaining a high input impedance.

Key Electrical Characteristics

• Transconductance (gm): 2–5 mS per 1 V drain‑source voltage.
• Voltage gain (Av): 40–70 dB (≈ 50–100 V/V) when properly biased.
• Input impedance: > 10 MΩ, making it ideal for signal sources.
• Output impedance: < 1 kΩ, suitable for driving subsequent stages.

Biasing & Small‑Signal Model

To keep the JFET in saturation, a drain resistor (Rd) and source resistor (Rs) are chosen such that the drain current (Id) remains roughly constant. The small‑signal model uses the transconductance parameter gm and the output resistance ro, yielding the familiar gain equation:

Av ≈ -gm × (Rd || ro)

Typical Applications

• Audio pre‑amplifiers: low‑noise, high‑gain front‑end.
• RF front‑ends: wide bandwidth with minimal distortion.
• Instrumentation: precision signal conditioning with high input impedance.

Hands‑On Practice

Download our JFET Amplifiers Worksheet to calculate bias points, gain, and frequency response for a selected JFET. The worksheet guides you through step‑by‑step design, ensuring you grasp both theory and practical implementation.

For deeper insight, refer to the Analog Devices JFET technology overview, which provides real‑world data sheets and application notes.