Comparing JFETs in Small Signal Applications

For more than three decades, designers have turned to JFETs for their ability to deliver precision, low-noise performance in sensitive analog circuits. Unlike bipolar junction transistors (BJTs), which operate as current-controlled devices, JFETs function as voltage-controlled current sources, giving designers a unique balance of flexibility, stability, and ultra-high input impedance.

Why JFETs Matter

JFETs provide giga-ohm-level input impedances and picoamp-level input leakage currents — characteristics that make them ideal for applications demanding high sensitivity and low noise. These traits make them indispensable in instrumentation, sensor interfaces, and high-end audio designs.

Key parameters engineers evaluate include:

• IDSS (drain-source saturation current)

• VGS(OFF) (pinch-off voltage)

• Gfs (transconductance)

• en (voltage noise density)

• CISS and CRSS (input and reverse transfer capacitances)

Each of these influences circuit gain, linearity, and noise performance — meaning design engineers often balance one characteristic against another to meet specific performance goals.

Matched Dual JFETs for Precision Design

When circuit matching and temperature tracking are critical — such as in differential amplifier stages — monolithic dual JFETs are the best choice. Linear Integrated Systems’ matched pairs like the LSK389, LSK489, and LSJ689 are fabricated on the same chip, ensuring tight parameter tracking and ultra-low noise performance. These devices deliver exceptional common-mode rejection ratio (CMRR) and minimal offset voltage, making them the preferred choice for low-noise amplifiers, sensor preamps, and precision audio front ends.

Applications That Benefit from JFETs

JFETs excel in both high-performance and low-power environments, including:

• Audio: Microphone and phono preamplifiers for dynamic, ribbon, or electret transducers

• Instrumentation: Vibration and piezoelectric sensor amplifiers

• Scientific and medical: Photodiode, Geiger tube, or ECG/EKG front-end circuits

• Low-Power Designs: Battery or energy-harvesting circuits powered by solar, thermal, or motion energy

Beyond Amplification — Switching and Oscillation

JFETs aren’t limited to amplification. They can also function as voltage-controlled resistors or switches, as in the LS26VPS, or serve in ultra-low-power oscillator circuits, such as those using the LSK170A. These designs can operate with supply voltages as low as 20 mV, making JFETs viable in applications where every microamp matters.

In Summary

From high-end audio and RF front ends to precision instrumentation and energy-scarce systems, JFETs remain unmatched for small-signal design flexibility and low-noise performance. Whether configured as amplifiers, oscillators, or variable resistors, Linear Integrated Systems’ JFETs provide the control, precision, and reliability engineers demand.

🔗 Learn more: LSK389 Series Datasheet, LSK489 Series Datasheet, LSJ689 Series Datasheet, Comparing JFETs Application Note