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Elegant analog design often comes down to simplicity. This 4–20 mA current-sense circuit shows how a single JFET can do the job with minimal parts. "Sometimes the simplest solution is the best. Using a single N channel JFET as a current sink, a 4-20mA instrument monitor loop can be read with a minimum of components. A 2N4392 diverts a current of 4mA until the load circuit voltage exceeds the Zener threshold where the remaining current up to 20 mA is diverted giving a linear indication from 4-20mA. Given that the two-wire polarity is known,  the Bridge rectifier can be eliminated, further simplifying the circuit. The bridge simply makes it fool proof...." - Kirkwood Rough

Many widely used ADI op-amps are excellent building blocks-but pairing them with a discrete JFET input stage can unlock lower noise, higher input impedance, and more design flexibility , especially for high-Z sensors and precision analog front ends. Where discrete JFETs shine:   • Reduce input current noise and 1/f noise • Improve performance with high-impedance sources • Allow custom gain, bandwidth, and linearity tuning • Enable better thermal tracking and matching Designers are increasingly using discrete JFET front ends ahead of precision or high-speed op-amps to outperform integrated FET-input solutions—especially in instrumentation, audio, and low-frequency sensor applications. 🔄 Op-Amp Upgrade Paths: From Monolithic to Discrete JFET Front Ends Looking to improve noise, drift, or flexibility in common Analog Devices op-amp designs? Below are suggested discrete JFET upgrade paths using Linear Systems devices — and  why designers make the switch . OP27 (Precision BJT, low drift) →  Upgrade:  LSK389 + ADA4522 →  Benefit:  Lower current noise with customizable gain OP07 (Low offset, low cost) →  Upgrade:  LSK170 + low-noise op-amp →  Benefit:  Higher input impedance and lower bias current AD8610 (FET-input, low noise) →  Upgrade:  LSK170 + OPA627 →  Benefit:  ~7× lower noise and a lower 1/f noise corner AD8620 (Dual AD8610) →  Upgrade:  LSK489 + dual precision amplifier →  Benefit:  Lower drift with increased design flexibility AD825 (High-speed instrumentation amp) →  Upgrade:  LSK170 + fast op-amp →  Benefit:  Lower noise and fully customizable gain stages AD829 (High-speed, audio) →  Upgrade:  LSK170 + audio-grade op-amp →  Benefit:  Lower distortion and improved headroom AD620 (Instrumentation amp for sensors) →  Upgrade:  LSK389 + discrete differential amplifier →  Benefit:  Better matching and lower offset drift AD8237 (Low-power INA) →  Upgrade:  LSK170 + low-bias op-amp →  Benefit:  Improved low-frequency performance ADA4627-1 (FET-input, high voltage) →  Upgrade:  LSK170 + ADA4898-1 →  Benefit:  Lower 1/f noise and more flexible input tuning ADA4898-1 (Ultra-low distortion) →  Upgrade:  LSK170 + discrete gain stage →  Benefit:  Custom linearity with reduced 1/f noise  🔊 Low-Noise Op-Amp Alternatives Using Discrete JFET Front Ends For designs where noise performance is critical, many engineers move beyond monolithic op-amps and use discrete JFET input stages for better matching, lower bias current, and improved low-frequency behavior. Below are common ADI op-amps, their noise levels, and suggested JFET-based upgrade paths: AD797 (0.9 nV/√Hz, BJT) →  Upgrade:  LSK389 + ADA4522 →  Benefit:  Matched JFET input ideal for high-impedance sources ADA4898-1 (1.0 nV/√Hz, BJT) →  Upgrade:  LSK170 + ADA4898-1 →  Benefit:  Same op-amp with improved front-end performance AD8597 (1.1 nV/√Hz, BJT) →  Upgrade:  LSK170 + audio-grade op-amp →  Benefit:  Lower bias current with flexible layout options AD8599 (Dual version) →  Upgrade:  LSK489 + precision amplifier →  Benefit:  Monolithic dual JFET input stage with improved matching AD8429 (Instrumentation amplifier) →  Upgrade:  LSK389 + discrete differential amplifier →  Benefit:  Better matching and lower drift for sensor interfaces ADA4627-1 (3.3 nV/√Hz, JFET) →  Upgrade:  LSK170 + ADA4898-1 →  Benefit:  Lower input noise and reduced 1/f noise corner AD8610 (6.0 nV/√Hz, JFET) →  Upgrade:  LSK170 →  Benefit:  ~7× lower voltage noise AD8620 (6.0 nV/√Hz, dual JFET) →  Upgrade:  LSK489 →  Benefit:  Improved noise, drift, and channel-to-channel matching AD743 (2.9 nV/√Hz, JFET) →  Upgrade:  LSK389 →  Benefit:  Monolithic dual JFET replacement with better SNR AD745 / AD746 (3.5 nV/√Hz, JFET) →  Upgrade:  LSK170 / LSK489 →  Benefit:  Higher input impedance with lower noise AD820 (~7.5 nV/√Hz, JFET) →  Upgrade:  LSK170 →  Benefit:  Significant noise reduction   Summary Recommendation: For precision designs, LSK389 offers a monolithic dual solution with unmatched thermal tracking and <1 nV/√Hz noise. For cost-sensitive upgrades, LSK170 provides low noise and high impedance in a single-channel format. For drop-in dual replacements, LSK489 offers matched performance with reduced layout complexity. Use our JFETs to build lower-noise, custom-tuned, and sensor-friendly analog front ends that outperform ADI’s integrated FET-input amplifiers. Have you tried a discrete JFET front end in your designs?

Understanding Noise Performance in Precision Design As precision analog systems continue to push the limits of sensitivity, stability, and signal integrity, noise performance is no longer a “nice to have” — it’s a design requirement . Engineers must prioritize noise performance to ensure that their designs meet the necessary standards for reliability and accuracy. At Linear Integrated Systems, ultra-low-noise JFETs have long played a critical role in applications where engineers can’t afford variability or guesswork. From medical sensing and instrumentation to radar, defense, and professional audio, designers increasingly need components with predictable, repeatable performance , not just typical or sampled specifications. The Importance of 100% Noise Testing That’s why we continue to spotlight our approach to 100% noise testing across our ultra-low-noise JFET portfolio, including the LSK170 single JFET and LSK389 monolithic dual JFET . Every device is verified before shipment — helping engineers reduce design risk, shorten validation cycles, and improve long-term system stability. Applications of Ultra-Low-Noise JFETs Ultra-low-noise JFETs are used in various demanding applications. These include: Medical Devices : In medical sensing, precision is crucial. Noise can significantly impact the accuracy of readings. Radar Systems : Noise performance is vital for reliable detection and tracking. Professional Audio : In audio applications, noise can degrade sound quality. Engineers must consider these factors when selecting components for their designs. Why Full Noise Verification is Critical In our latest press release, we share: Why full noise verification is becoming more critical as systems grow more sensitive. How monolithic dual JFETs improve matching and thermal stability. Where ultra-low-noise JFETs are being used across today’s most demanding applications. Enhancing Design Reliability By focusing on verified noise performance, engineers can enhance the reliability of their designs. This is especially important in fields where performance is non-negotiable. 👉 Read the full press release here: https://www.prnewswire.com/news-releases/linear-integrated-systems-spotlights-industrys-only-100-noise-tested-ultra-low-noise-jfets-302643454.html Conclusion In conclusion, verified noise performance is essential in precision design. As systems become increasingly sensitive, the need for reliable, low-noise components will only grow. By prioritizing noise testing, engineers can ensure their designs meet the highest standards of performance and reliability. ---wix---

Application Notes At Linear Systems, we provide a broad collection of application notes written by top industry professionals to guide designers in unlocking the full potential of our components. LSK389 App Note CMR Ratio in Dual JFETs Interfacing Sensors & Transducers LSJ689 App Note Consider the JFET LSK489 App Note LS844 App Note Design Specs for JFETs FETs for Sensor Applications FETching Discrete Amplifier Military Sensor Signal Chains J201 App Note DMOS App Note Low Leakage Diodes Current Regulating Diodes Solving the Noise Puzzle 3N163 App Note JFETs for VC Circuits Headphone Amp Eval Board

Bipolar Transistors - Linear Systems’ Monolithic Dual NPN and PNP transistors consist of two transistors on the same piece of silicon resulting in better matching and better performance over temperature. Bipolar Transistors Linear Systems’ Monolithic Dual NPN and PNP transistors consist of two transistors on the same piece of silicon resulting in better matching and better performance over temperature. These parts are designed for general purpose amplifier and switching applications. IT120 Series MONOLITHIC DUAL, NPN TRANSISTOR Read More LS318 LOG CONFORMANCE, MONOLITHIC DUAL, NPN TRANSISTOR Read More LS3550 Series MONOLITHIC DUAL AND SINGLE, PNP TRANSISTOR Read More LS310 Series TIGHTLY MATCHED, MONOLITHIC DUAL, NPN TRANSISTOR Read More IT124 MONOLITHIC DUAL, NPN TRANSISTOR Read More LS3250S - Single LS3250 Single NPN High-Speed Switching Transistor Read More LS358 LOG CONFORMANCE, MONOLITHIC DUAL, PNP TRANSISTOR Read More LS350 Series TIGHT MATCHING, MONOLITHIC DUAL, PNP TRANSISTOR Read More IT130 Series MONOLITHIC DUAL, PNP TRANSISTOR Read More LS3250 Series - Dual TIGHTLY MATCHED, MONOLITHIC DUAL, NPN TRANSISTOR Read More LS301 Series HIGH VOLTAGE, SUPER BETA, MONOLITHIC DUAL, NPN TRANSISTOR Read More

Linear Systems’ Voltage Controlled Resistors feature a narrow range of resistance and high input impedance resulting in better performance of the device. Voltage Controlled Resistors Linear Systems’ Voltage Controlled Resistors feature a narrow range of resistance and high input impedance resulting in better performance of the device. These parts are ideal for Variable Gain Amplifiers, Automatic Gain Controls, Voltage Controlled Oscillators, Small Signal Attenuations and Filter Range Controls. LS26VNS N-CHANNEL JFET, VOLTAGE CONTROLLED RESISTOR Read More LS26VPS_ P-CHANNEL JFET, VOLTAGE CONTROLLED RESISTOR Read More VCR11N MONOLITHIC DUAL, N-CHANNEL, JFET VOLTAGE CONTROLLED RESISTOR Read More