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RF Engineers know that temperature effects matter in RF measurement. This constant temperature substitution design tackles that challenge in a clever way using pico-amp diodes.   JFET Design Spotlight - A Radio Frequency RMS Metering Circuit   “When Linear Technology introduced the LT1088 RMS converter, it was packaged in a complicated iso-thermal DIP package to allow the published RMS measuring circuit application some working room. The problem with that application was temperature increasing with measured power allowing only limited precision and repeatability over environmental temperature fluctuations. However, by using the additional concentric 250Ω resistors, another control option was obviously available creating possible constant temperature operation regardless of input power measured. Here, the 250Ω resistors raise the Die temperature to about 40°-60°C where any additional thermal influence of incoming RF can be substitute balanced by complimentary reduction of the 250Ω power input. Common mode voltage of the LT1088 D1&D4 diodes sets the temperature and differentially balances out the input RF at 50Ω  with a DC equivalent power at 50Ω. The Voltage across D1&D4 are therefore constant for the input measurement range of 80mW. I talked Jim Williams out of some plastic packaged LT1088’s to prove that the expense of isothermal packaging was unnecessary. Accuracy improved over 10-fold from the original application circuit. As is the case for many of these instrument circuits, Clamp and level fixing diodes used with FET input Op-Amps by habit use pico-amp diodes like the LIS PAD-1‘s. By the way, Dual PAD pico-amp diodes have a little known feature of exhibiting significantly less nodal leakage than the Datasheet illustrates when used in symmetrical clamp bias function as shown in the earlier published 10Kv amplifier protection. These on DIE diode pairs function similarly to the gate bias cancellation of dual pico-amp JFET’s illustrated in the Accelerometer circuit presented earlier. Close proximity allows uniform doping distribution and nearly perfect matching for Dual PAD diode parts so symmetric bias currents balance to near nothing. For me, it’s a time-tested habit.” Kirkwood Rough   Why this matters:   Improved measurement accuracy through thermal stability Symmetrical diode behavior enables ultra-low leakage performance A great example of where Linear Systems PAD pico-amp diodes shine   Looking for ultra-low leakage diode solutions for your next design? Our PAD series offers exceptional matching and performance for precision analog and RF applications.   Have you used pico-amp diodes in similar circuits or solved temperature stability challenges in a different way? Share your experience in the comments.   Learn more and request samples in the comments.   #AnalogDesign #RFEngineering #JFET #ElectronicsDesign #LowNoise #Semiconductors #LinearSystems

Every engineer has faced it at some point: a critical component in your design is suddenly obsolete, unavailable, or impossible to source reliably. And unlike digital systems, analog designs don’t always give you the luxury of a quick swap. So what do you do when a key part disappears? Why Obsolescence Hits Analog Designs Harder In digital design, you can often work around a part change with firmware or minor adjustments. In analog circuits, even small parameter differences can create real problems. Things like noise performance, matching, leakage current, and biasing behavior are tightly tied to how the circuit performs. A “close enough” replacement can easily lead to increased noise, drift in operating points, degraded signal integrity, or even a full redesign. What Engineers Are Running Into Right Now From conversations with engineers and customers, a few patterns come up consistently. Legacy parts are disappearing, especially widely used JFETs and small-signal devices. Supply is often unpredictable, with long lead times or inconsistent availability. Secondary markets introduce risk, including counterfeit or non-conforming parts. And in many cases, redesign simply isn’t practical, especially for long-life or regulated systems. How Engineers Are Solving It The teams navigating this best are taking a more strategic approach. They start by focusing on the parameters that actually matter in their circuit, such as noise, gm, leakage, and matching, rather than just looking for a “similar” part on paper. They also look to modern equivalents designed to replace legacy components, often improving consistency and long-term availability. In many cases, the replacement ends up being an upgrade rather than just a workaround. Validation is critical. Simulation helps, but real confidence comes from testing in-circuit, verifying biasing, and measuring actual performance. And importantly, they’re not doing it alone. Working directly with manufacturers who understand legacy components and application behavior can significantly reduce risk and speed up the process. A Shift in Mindset The most successful teams don’t just react to obsolescence. They use it as an opportunity to improve performance, reduce future supply risk, and strengthen their designs overall. It’s not just about replacing a part. It’s about making the design more robust long term. How We’re Helping Engineers Navigate This At Linear Systems, we work with engineers every day who are dealing with obsolete JFETs, hard-to-source components, and performance-sensitive designs. We help identify strong replacement options, provide cross reference guidance, supply samples for evaluation, and support real-world validation. The goal is simple: help you keep your design performing the way it should. Need Help with an Obsolete Part? If you’re dealing with a part that’s no longer available or becoming difficult to source, we’re happy to help. Drop the part number or application in the comments or message me directly and we’ll point you in the right direction. Quick Question What’s the biggest challenge you face with obsolete components? Finding a true replacement, maintaining performance, supply chain reliability, or avoiding redesign? #AnalogDesign #ElectronicsEngineering #Semiconductors #JFET #Obsolescence #HardwareEngineering #EE #ProductDesign #SupplyChain

Linear Systems is pleased to announce the release of a new SPICE model for the PAD Series low-leakage diode, developed by our engineering team. This model enables designers to simulate the SSTPAD10 in LTspice and other SPICE-compatible simulators, supporting faster circuit development and more accurate performance evaluation. The PAD Series of low-leakage diodes was designed for precision applications where minimizing leakage current and maintaining stable capacitance are critical. Its performance makes it well suited for sensitive analog circuits and high-impedance signal paths. 🔗 Explore the PAD Series Typical applications include: Precision instrumentation Low-noise analog front ends Protection circuits High-impedance signal paths Test and measurement systems The PAD Series offers controlled junction capacitance, low leakage characteristics, and stable breakdown performance, making it a reliable choice for both new designs and performance-critical applications. Providing SPICE models allows engineers to evaluate device behavior early in the design process and integrate Linear Systems components with greater confidence. We will continue expanding our SPICE model library to support additional Linear Systems devices. 📥 Download SPICE Model ( Link to SPICE model file ) 💬 Stay Connected Follow Linear Systems for new device releases, SPICE models, and application insights.

“Audio amplifiers are many and varied. Ones used for music should have a very low intermodulation distortion and when overdriven should produce a minimum of odd alien sounds. Class A amplifiers tend to be preferred over most for their “warm” and realistic sound presence. Though not efficient, The Class A and class AB1 often fit this category for the most part. Class B amplifiers are more efficient but have a nasty problem with the impedance shift at zero crossover. Here is a variation in thinking that merges both types of design. Though having a class B output with complimentary devices exclusively drawing current from their respective rails, an output stage bias is always on allowing complimentary emitters to operate with no series resistor and is thus designated as Class BA. As with most sound worthy amps, the input stage is a Low Noise differential pair of Linear Systems LSK389A JFETs. Output offset is <10mV and can be dialed to 0.0V with a small offset Pot. This circuit is inspired by an instrument buffer circuit circa: 1957 of Henry Hall and Bob Fulks at General Radio Co.” – Kirkwood Rough Interested in designing with the LSK389A ? Explore the device details and request your free samples today .

Linear Systems is pleased to announce the release of the first SPICE model for the VCR11N, developed by our engineer Allan Cabiluna.   This new model allows designers to simulate the VCR11N voltage-controlled resistor in LTspice and other SPICE-compatible simulators, enabling faster circuit development and performance evaluation.   The VCR11N is an N-channel JFET designed for use as a voltage-controlled resistor, allowing the drain-source resistance to be adjusted using gate voltage. This makes it ideal for applications requiring precise signal attenuation and gain control.   Typical applications include:   Amplifier gain control Oscillator amplitude stabilization Programmable attenuators Analog signal processing Filter tuning   The device is designed for low-level AC signals with no DC component, providing predictable RDS variation with VGS control and allowing it to operate as a symmetrical resistor with no DC bias in the signal loop.   The VCR11N is also a pin-for-pin replacement for the Siliconix VCR11N, supporting both legacy designs and new circuit development.   Providing SPICE models helps engineers evaluate performance early in the design cycle and integrate Linear Systems devices with greater confidence.   We plan to expand our SPICE model library to support additional Linear Systems devices in the future.   spice .MODEL VCR11N_UPDATED NJF( + VTO=-9.29  BETA=3.36E-4  VTOTC=-2.2m        + BETATCE=-0.5 LAMBDA=1E-4  RD=46.2  RS=46.2            + IS=1E-14 ISR=1E-13 N=1.0 NR=2.0  XTI=3.0 BVK=80.0  IBV=10U            + ALPHA=1E-3 VK=0.7 CGS=7.5P CGD=7.5P           + PB=1.0 M=0.5 MJ=0.5 FC=0.5             + KF=1E-17 AF=1 GDSNOI=1.0 NLEV=2             + TNOM=27            + MFG=Linear_Systems Learn more about the VCR11N and access the datasheet and product details here: https://www.linearsystems.com/voltagecontrolledresistors/vcr11n

A Design Guide to the LSK389 Dual and LSK170 Single JFETs When designing ultra-low-noise analog front ends, device selection matters. Whether you're building instrumentation, audio, or sensing systems, the transistor you choose can directly impact signal integrity, stability, and overall system performance. Two of the most trusted solutions for engineers worldwide are the LSK389 dual JFET  and LSK170 single JFET  — precision components engineered for applications where noise, matching, and reliability cannot be compromised. Why Engineers Still Choose JFET Front Ends JFETs continue to dominate critical signal paths because they offer: extremely low noise floors ultra-high input impedance predictable biasing behavior excellent linearity For sensitive measurement or audio circuits, these characteristics are often essential — not optional. LSK389 — Matched Dual JFET on a Single Die The LSK389  eliminates one of analog design’s biggest challenges: matching discrete transistors. Because both JFETs are fabricated on the same piece of silicon, designers get: tight electrical matching excellent thermal tracking long-term stability Key Performance Highlights 1.6 nV/√Hz noise (typical @ 1 kHz) 20 mS transconductance (typical) 15 mV max VGS offset matching 40 V breakdown voltage This monolithic architecture provides tighter performance than hand-matching discrete devices while reducing design time and cost. LSK170 — Benchmark Single Ultra-Low-Noise JFET The LSK170  is designed for applications that require the lowest possible noise from a single device. Key Performance Highlights 0.9 nV/√Hz noise (typical @ 1 kHz) ~20 GΩ input impedance 22 mS transconductance (typical) ≤22 pF capacitance It’s also a direct replacement for the classic 2SK170 , making it ideal for legacy upgrades or modern redesigns. When to Use Each Device Choose LSK389 if your design needs: differential input stages matched pairs precision instrumentation Choose LSK170 if your design needs: lowest single-device noise ultra-high impedance inputs compact layouts Typical Applications These devices are widely used in: microphone preamps sensor interfaces instrumentation amplifiers hydrophones and acoustic detection medical electronics test and measurement systems Design Insight Engineers Appreciate Narrow IDSS grading means less biasing guesswork and more predictable circuit performance, especially in low-voltage or battery-powered systems. That translates directly into: ✔ faster prototypes✔ fewer revisions✔ more consistent production results Need Help Choosing the Right Grade? Linear Integrated Systems offers custom screening options and technical support to help you select the optimal device for your design. 📩 Contact our engineering team: support@linearsystems.com

JFET Design Spotlight 🎸 “JFET Guitar Wah Pedal — Look Ma, No Inductors!”   The variable band pass filter for musical voicing effects most notably, the Cry Baby from VOX, became famously known as a WAH Pedal. The original WAH used a 500mH inductor in a feedback loop to produce a variable filter with foot-pedal potentiometer activation. Issues with this pedal were, few were much alike. This design variation of the WAH effect is to eliminate the inductor having varied Q’s making most all Wah Pedals different. By substituting a bandpass filter designed at General Radio by Henry Hall in 1952 for Null Meters, a more uniform distributed gain over frequency could be realized. Incorporation of this technique makes for an unusually clean sweep and low noise effect for Lead Guitar, Bass, with 3 capacitor swaps, and any other Funkadelic need worth messing with. The Linear Systems LSK389s make implementation of this low loop gain JFET OP-Amp remarkably linear and highly musical. Loop gain may be cheap but doesn’t necessarily sound good. For Audio, Linearity certainly does!” Kirkwood Rough JFET Guitar Wah Pedal JFET Guitar Wah Plot What’s your preference for tone shaping — classic inductor wah or active filter designs? How can we help you? If you have questions, need technical information, samples, etc.: Contact Us !

“An accelerometer is typically a capacitive transducer and varies Cacc with acceleration rate. A common problem with resident current fed accelerometer preamps is excessive DC offset drift when used up to 100 C. The object here was to keep the output bias voltage within +/- 25% of the 10-volt setting. The LS5906 Low bias Dual JFET addresses this temperature dependency by a unique internal DIE coupling in its construction. In this circuit, compensation of J1’s gate current over temperature happens by an inter-DIE Gate/Substrate coupling current where J2’s gate/drain junction is negative relative to J1’s gate where J1’s Gate current relates to the positive end of the drain channel. The JFETs close proximity on a common DIE assures uniform doping distribution and results in equal junction characteristics. By setting both FETs to have approximately the same Vgd, Gate currents are nearly the same and with opposed junctions, will null out an operating bias current of the active FET over a wide range of temperature. Normal doubling of leakage in silicon every 7 C is then reduced to less than a few percent. Though there is a lot of latitude to modify the dual JFETs Vgd balance for even better performance, this circuit satisfied an immediate requirement and was refined no further. Tests were successful up to 100 C.  However, the principle of balanced opposing gate currents for temperature compensation here should be noted for other possible single ended designs where gate current temperature drift is a critical issue.” - Kirkwood Rough     What are your thoughts on using balanced opposing JFET gate currents to mitigate temperature drift? Interested in samples of the LS5906 or another Linear Systems part?   🔗 Request samples here: https://www.linearsystems.com/about-2

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---

Community Design Spotlight 🔍 WSF Amplifier “The Watt Sucking Fireball  amplifier illustrates a wide use range of Linear Systems  JFETs and matched bipolar transistors. It has an all JFET front end using a (Differential-Cascode-Transimpedance)™ amp driving a resistor loaded differential current to voltage gate driver; a lot to say in one sentence. Though simple in overall design concept, the goal of this design was to prove out an all N-channel JFET amplifier having almost no intermodulation distortion using a linear low loop gain signal path and minimal feedback. The output JFETs are vertical Power JFETs made by Sony in the 1970s. The vertical C7 driver FETs were my own fabrication in 1994. But that’s a whole other story. Though the debate rages on, audio is always AC so capacitors selected well are valid, and Nelson Pass said so. This input diff stage has capacitor coupled sources, as does the output stage driver. Independent current sources set preamp quiescent voltage matching. Independent current sources make output zeroing simple via a differential current steering integrator to balance gate drive current sources. Output JFETs are negatively biased gate to source and thus balanced to zero the output DC voltage. Linear Systems  single & dual JFETs and Matched bipolars are effectively used in the entire bias management system.  The DCT  (Differential Cascode Transimpedance)™ preamp was invented for the Linear Systems  2009 Burning-Amp Festival Demo. Following founder John Hall, I have been designing discrete transistor and JFET circuitry with Linear Systems parts since their first year of founding in 1987, and before that, Micropower Systems , much of it in Semiconductor fabrication machine instrumentation. Discrete Music amplifier design is now a 60-year continued preoccupation.” - J Kirkwood H Rough