FET Friday: Understanding the Common-Source JFET Amplifier

At Linear Integrated Systems, we love shining a spotlight on JFET technology and the classic analog circuits that showcase its unique advantages. This week for #FETFriday, we're highlighting one of the most fundamental JFET configurations: the Common-Source Amplifier.

What is a Common-Source JFET Amplifier?

The common-source amplifier is a go-to building block in analog design. Much like the common-emitter stage in a BJT amplifier, this configuration offers voltage gain and is known for its high input impedance, low noise characteristics, and simplicity.

It uses an N-channel JFET with the source terminal grounded (hence "common source") and the signal applied to the gate. A load resistor is placed at the drain, and biasing resistors ensure the transistor operates in its active region.

Here’s a simplified schematic from Electronics-Tutorials.ws:

Key Components:

• N-Channel JFET (such as the LSK170 or LSK389 for low-noise applications)

• Bias resistors to set gate voltage

• Source resistor for stability (can be bypassed with a capacitor for increased gain)

• Drain resistor as the load

  
  

Why Use a JFET in This Configuration?

JFETs offer an exceptionally high input impedance—ideal for buffering sensitive analog signals. When configured as a common-source amplifier, the JFET’s low gate leakage and voltage-controlled nature allow for clean signal amplification with minimal distortion.

At Linear Systems, our parts like the LSK389, LSK170, and LSJ74 are optimized for low-noise and precision audio or instrumentation designs—making them excellent choices for this topology.

Applications

You’ll find this configuration in:

• Audio preamplifiers

• Sensor front ends

• High-impedance measurement circuits

• Low-noise RF circuits

  
  

Learn More

To dive deeper into the operation and theory behind the Common-Source JFET Amplifier, check out the full article:👉 Common-Source JFET Amplifier – Electronics-Tutorials.ws

Got a favorite JFET design or a question about our parts? Drop a comment or connect with us!Stay tuned for more analog fun next #FETFriday!