Bidirectional current limiting presents an interesting analog design challenge—and the unique structure of a JFET offers some creative ways to solve it. In this Design Spotlight, three approaches are explored, from dual-device configurations to a particularly simple single-JFET solution that takes advantage of the interchangeability of the Source and Drain. The designs highlight differences in crossover behavior, current stability, and voltage compliance, including considerations that become especially important when the current source is directly in an audio signal path. Below, Kirkwood Rough walks through the design approach, tradeoffs, and practical considerations behind each configuration. “Considering the scope of analog circuit design, various concepts are used repeatedly as applied functional modules. Within the fray of modules are various forms of DC current sources, but occasionally one is needed for AC use. Since most three terminal gain parts are considered unipolar, the first order design consideration for a bidirectional current source would use at least two parts. However less obvious is the particularly unique feature of an N or P type JFET, that is, that the Source and Drain are interchangeable with respect to the gate terminal. A JFET is a gate diffusion disposed between two ends of an opposite polarity doped channel where depletion of the channel can be achieved in either direction. Some common approaches to solving a bidirectional current source are presented. Diode isolation of a standard JFET current source works well in anti-parallel but with a slight crossover distortion through Zero as seen through Rs1. Another approach sensed by Rs2 uses the gate to body of one JFET as a diode source to a standard JFET current source and as a tandem dual can set a current in each direction. This configuration is good at the full current set values but with a slightly nonlinear slope from one polarity to the other due to Gm shift with voltage. A third circuit sensed by Rs3 consists of a single JFET using both Source and Drain interchangeably and still having the ability to source current asymmetrically to a load as a function of two resistor choices. 1 Meg resistors allow the Gate to have an ohmic relation to the source end facing a negative voltage for N-channel. This example for all three configurations is for a 1mA bidirectional current source where for the devices chosen, are fully limited to a set current by the time 3V is crossed. Smaller values of voltage compliance can be achieved by selecting a smaller Vgs of operation at the current needed for an application or a higher current setpoint. For example, selecting lower value resistors in this circuit to set a higher current will set that current at a lower voltage for saturation than the 3V shown here at 1mA. The current setpoint for circuit 1 and 2 is stable over a wide range of applied voltage. Circuit 3 is positively affected a couple percent by increasing voltage but presents a linear crossover that is more useful in audio circuitry where a bidirectional current source is in the signal path. Most bidirectional applications can use all three configurations, but audio use is where the single JFET circuit was needed. Use of the J500 series parts in tandem is the same as circuit #2 and provides the simplest solution overall for this purpose. The graphs show transition through zero voltage crossover and the threshold voltage where a set current is stable from that point on. As Usual …….. Kirkwood Rough” Looking for a JFET Current Source Solution? Linear Systems offers a range of precision JFETs and J500 Series Current Regulating Diodes for current regulation, audio, instrumentation, and other analog applications. View the J500 Series and our JFET product families to find the right device for your design. Have a specific application or design question? Contact our technical team—we’re happy to help.

Last week, we looked at how to read a JFET datasheet and the key specifications engineers should evaluate when selecting a device. This week, we're taking a closer look at one of those parameters: transconductance (gfs). Transconductance is an important JFET characteristic because it tells us how effectively the device converts a change in gate-to-source voltage into a change in drain current. Put simply: How strongly does the JFET respond to a change at its input? What Is Transconductance? A JFET is a voltage-controlled device. Changes in gate-to-source voltage (VGS) control the current flowing from drain to source (ID). Transconductance describes the relationship between those two changes: gfs = ΔID / ΔVGS It is typically expressed in millisiemens (mS). A JFET with higher transconductance produces a greater change in drain current for a given change in gate voltage. Think of it as the device's ability to translate a small voltage signal at the gate into a larger current variation at the drain. Why Does gfs Matter? In an amplifier, transconductance is directly related to how effectively the input device can respond to a signal. Higher gfs can contribute to greater stage gain and is often desirable in low-noise analog front ends. But, as with most semiconductor specifications, one number doesn't tell the entire story. Transconductance varies with operating conditions, particularly drain current. The gfs value shown on a datasheet therefore needs to be considered alongside the conditions under which it was measured. When comparing JFETs, engineers should ask: What is the specified transconductance? At what drain current was it measured? What VDS was used? Is the datasheet showing a minimum, typical, or maximum value? How does gfs change at the actual operating point of my circuit? gfs and Gain In many JFET amplifier configurations, transconductance plays a significant role in determining voltage gain. All else being equal, greater transconductance gives the device more ability to convert a small input-voltage change into a drain-current change that can then be developed into an output voltage by the surrounding circuit. But actual circuit gain also depends on the topology, load, source resistance, bias point, feedback, and other circuit elements. So higher gfs does not automatically mean a better amplifier. It means the designer has another important parameter to balance against noise, capacitance, current consumption, linearity, and the required operating point. Transconductance and Noise Transconductance is also particularly important in low-noise design. For many JFET applications, achieving useful transconductance at the desired operating current is part of optimizing the front end for low noise. This matters when the signal entering the circuit is extremely small and the first amplification stage can determine the noise performance of the entire signal chain. Examples include: Precision instrumentation Microphone preamplifiers Hydrophones and acoustic sensors Photodetector front ends Medical electronics Scientific instrumentation High-resolution data acquisition Other low-level sensor interfaces Don't Evaluate gfs by Itself This is where reading the entire JFET datasheet becomes important. A device with attractive transconductance may not be the right choice if it also introduces too much input capacitance for a high-impedance sensor, requires more drain current than the design can support, or doesn't provide the noise performance the application requires. The better question isn't: "Which JFET has the highest gfs?" It's: "Which JFET provides the transconductance I need at the operating point my application requires?" That distinction is especially important in precision analog design, where performance is usually the result of balancing multiple device characteristics rather than maximizing a single specification. Engineering Takeaway Transconductance tells you how effectively a JFET turns a small change in gate voltage into a change in drain current. It's a key indicator of how the device will behave as an amplifier—but it should always be evaluated at the intended operating point and alongside noise, capacitance, leakage, IDSS, and the other parameters that determine real-world circuit performance. Understanding gfs doesn't just help you read the datasheet. It helps you understand what the JFET will actually do in your circuit. Linear Systems Learning Series: Designed for Precision. Built for Performance. #LinearSystems #LearningSeries #JFET #Transconductance #AnalogDesign #LowNoise #PrecisionAnalog #Semiconductors #ElectronicsEngineering #AnalogElectronics

High-voltage electron beam systems demand precision—and protecting sensitive measurement circuitry is critical to maintaining accuracy and minimizing costly downtime. In this latest FET Design Spotlight, we look at how Linear Systems PAD Diodes have provided reliable circuit protection in electron beam applications for more than 35 years. Electron Beam Circuit Protection by PAD Diodes "High-voltage applications happen to be a difficult place to involve sensitive measurement and control electronics. Often, because of the high impedances involved, certain control nodes need protection from transients, like high-voltage arcs and parasitic oscillations within the elements themselves. High-voltage sensing resistors will invariably have distributed capacitances acting as low-reactance impedances with the fast rise times of an arc. Because op-amps used in these circuits typically have FET inputs for the least loading, protection diodes are needed to prevent transient voltage surges from damaging circuitry. Diodes used here must have the same or lower leakage as the amplifier nodes they protect to maintain measurement accuracy. For these applications, LS PAD diodes have been used with consistent results on all of the Electron Beam circuits I've designed over the past 35 years. This application uses PAD-1 diodes to protect nA current sensing and high-voltage monitoring of a semiconductor wafer bias supply floating on a supply of 10–30 kV. Noise in charged beam columns is minimized when all supplies are referenced to a common point in parallel, as shown here. Without Picoamp Diode circuit protection, even 1 pF distributed circuit capacitances could, and would, render resident instrumentation inoperative after an arc and cost, sometimes, days of downtime. This circuit measures the electron current yield on the wafer, whether it was negative or positive, regardless of the wafer bias voltage value. The graph shows there to be minimal bias voltage excursion effect on the wafer current measurement by using complementary summing." — Kirkwood Rough Questions about this application? Comment here or contact our engineering team: HERE

A JFET datasheet contains dozens of electrical characteristics, graphs, and performance curves—but which specifications matter most? The answer depends on your application. A designer building a low-noise microphone preamplifier may prioritize different parameters than someone designing a current source or a high-impedance sensor interface. Understanding what each specification represents is the first step in selecting the right device. 1. Noise Voltage For precision analog front ends, this is often one of the first specifications engineers review. Input voltage noise determines how much electrical noise the JFET contributes to the signal. Lower noise is especially important in applications such as photodiode amplifiers, hydrophones, medical instrumentation, and professional audio. Engineering Tip: Don't stop at the 1 kHz specification—also consider low-frequency (1/f) noise for slow-changing signals. 2. IDSS (Drain-to-Source Saturation Current) IDSS indicates the drain current when the gate and source are connected together. This parameter affects biasing and operating point selection and often determines which device grade is most appropriate for a circuit. Engineering Tip: Choose the IDSS range that best matches your intended bias conditions. 3. Gate-to-Source Cutoff Voltage (VGS(off)) This specification indicates how much reverse gate voltage is required to reduce drain current to essentially zero. It provides insight into the device's operating range and helps establish the available gate bias window. 4. Input Capacitance Input capacitance becomes increasingly important when working with high-impedance signal sources. Higher capacitance can reduce bandwidth and place additional loading on sensitive sensors. Applications such as photodiodes and piezoelectric sensors often benefit from devices with low input capacitance. 5. Gate Leakage Current Although typically measured in picoamps, gate leakage can significantly affect circuits measuring extremely small currents. The lower the leakage current, the less the input device disturbs the signal source. 6. Transconductance (gfs) Transconductance describes how effectively changes in gate voltage control drain current. Higher transconductance generally provides greater gain, but the optimal value depends on the application and circuit topology. 7. Package Options Electrical performance is only part of the selection process. Package choice can influence board layout, parasitic effects, thermal performance, assembly requirements, and overall system integration. The Big Picture No single specification tells the whole story. Selecting the right JFET means evaluating how noise, capacitance, leakage, transconductance, operating current, and package work together to meet the requirements of your application. The best engineers don't focus on one number—they understand the complete device. Engineering Takeaway A datasheet isn't just a list of specifications—it's a guide to understanding how a device will perform in your design. Linear Systems Learning Series: Helping engineers better understand precision analog design.

Not all noise is created equal. When engineers compare low-noise devices, they often focus on the 1 kHz noise specification. While that's an important benchmark, it doesn't tell the whole story. At lower frequencies, many semiconductor devices exhibit 1/f noise, also known as flicker noise. As frequency decreases, this noise increases, making it especially important in applications that measure slow-changing or very small signals. Examples include: Precision instrumentation Medical electronics Seismic sensors Hydrophones Geophysical equipment Condenser microphone preamplifiers High-resolution data acquisition systems In these applications, low-frequency noise can limit measurement resolution long before broadband noise becomes the dominant factor. That's why engineers evaluate both broadband noise and 1/f noise when selecting input devices for precision analog front ends. Devices such as the LSK389 are designed with exceptionally low broadband and low-frequency noise, making them well suited for demanding analog applications where preserving weak signals is critical. The LSK389 is specifically designed for ultra-low-noise applications and is 100% tested to meet its 1/f and broadband noise specifications. Engineering Tip If your circuit measures slow-changing signals, don't stop at the 1 kHz noise specification. Evaluate low-frequency (1/f) noise as well—it can have a significant impact on real-world performance. Linear Systems Learning Series: Helping engineers better understand precision analog design.

Another glimpse into the creative mind of analog designer Kirkwood Rough. "‘No batteries allowed on this instrument,’ was the statement from the luthier. So, a phantom-powered preamp needed to be configured using a standard guitar cable. 'Nothing special.' First, the piezo loading had to be minimal so its charge capacity would not run out and sound 'buzzy' on the low end. Second, the piezo needed to be sensed differentially and therefore shielded. Third, no batteries could be attached to the instrument, so an active preamp had to be powered through a single- or two-wire guitar cable, preferably a single cable so that any standard guitar cable could be used. Fourth, the frequency response could not be affected by the length or loading of the guitar cable. Piezo loading is critical where charge capacity is limited, and too low a resistance can deplete this charge at low frequencies. A distorted, chirpy sound can evolve from the E and A strings, as well as from transient loading during power chords. This particular piezo required a 20 MΩ or greater load. Therefore, a low-noise JFET front end was powered by a voltage from a low fixed-load impedance to minimize cable effects. One JFET drain is AC-decoupled so that the other can current-modulate the cabled voltage source. This current modulates the Q1 emitter's Z < 50 Ω, and its drain current then flows through R2. That voltage, buffered by Q2, becomes a low-impedance signal output. Overall, not much to it. In the end, the gain needed to be reduced another 10 dB with an improved sensor through a couple of resistor changes in the resolver circuit. As a matter of habit, Linear Systems parts proved to be an effective drop-in solution requiring very little board space in surface-mount fabrication. The -3 dB bandwidth was nearly 70 kHz. Note: High-value resistors perform best when they are no smaller than a 1206 package and micro-pen formed for the lowest excess noise. As usual... Kirkwood" Discover why engineers continue to choose Linear Systems for the world's most demanding analog designs. Our ultra-low-noise JFETs, precision bipolar transistors, MOSFETs, and custom screening services help designers achieve exceptional performance in audio, instrumentation, medical, aerospace, defense, and scientific applications. Want to learn more? Contact one of our engineers by clicking HERE.

When designing a precision differential amplifier, low noise is only part of the equation. The two input transistors must behave as similarly as possible. Even small differences in electrical characteristics can introduce input offset voltage, reduce common-mode rejection (CMRR), increase temperature drift, and ultimately limit the accuracy of the entire circuit. That's why device matching is so important. Rather than pairing two individual transistors, many engineers choose a monolithic dual JFET, where both devices are fabricated on the same piece of silicon. Sharing the same die allows the transistors to track each other much more closely—electrically and thermally—resulting in more stable, predictable performance over temperature and time. The LSK389 was developed for exactly these types of applications. Its monolithic dual construction combines ultra-low noise with tightly matched electrical characteristics, making it well suited for differential input stages used in instrumentation, medical electronics, scientific measurement equipment, sensor interfaces, and professional audio. Key Benefits of the LSK389 Monolithic dual JFET construction Ultra-low noise (1.6 nV/√Hz typical @ 1 kHz) Tight gate-to-source voltage matching (15 mV max) Excellent thermal tracking Optimized for precision differential amplifier input stages In precision analog design, matching isn't simply another specification—it directly affects stability, accuracy, and long-term performance. Linear Systems Learning Series: Helping engineers better understand precision analog design.

Last week we discussed why engineers continue to choose discrete JFETs for precision analog designs. But selecting the right JFET involves more than comparing noise specifications. For high-impedance applications such as photodiodes, hydrophones, piezoelectric sensors, condenser microphones, and precision instrumentation, input capacitance can have a significant impact on circuit performance. Higher input capacitance can reduce bandwidth, increase signal loading, and make compensation more challenging. That's why many engineers evaluate both ultra-low noise and low input capacitance when selecting an input device. The LSK489 was designed with this balance in mind, combining: Ultra-low noise (1.8 nV/√Hz typical) Typical 4 pF input capacitance Excellent monolithic matching Multiple package options, including bare die The result is a device well suited for demanding analog front ends where preserving weak signals is critical. Over the coming months, we'll continue sharing practical engineering insights like this as part of the new Linear Systems Learning Series, exploring the concepts behind precision analog design—not just the products. What analog design topic would you like us to cover next? #JFET #AnalogDesign #Semiconductors #LowNoise #Instrumentation #Photodiodes #Hydrophones #ElectronicsEngineering #LinearSystems

We're always fascinated by the way Kirkwood Rough approaches analog circuit design. Here's another one of his design stories. "Sometimes a sound gets a grip on your senses and that happened to be the Bass lines from Moulin Rouge. They were Fat, rich, percussive and Smooth with a high octave layered presence. That started a design process that ended up being the LadyMarmalade™ effect. It had to enhance even ordered harmonics without a sharp clipping effect, even when overdriven, while accentuating the rise and fall times at high drive. The core of this was an op-amp with relatively low loop gain and could be non-uniformly loaded by some nonlinear impedance. The typical distortion pedal, a Proco Rat for example, uniformly limits a gain adjusted/high loop gain/low output Z op-amp with two silicon diodes back-to-back. Uniform clipping results in high odd harmonic content; shred –metal sound with a fatiguing long-term listening effect. Adding a 390kΩ resistor between the diode node and the +9V trace will make a ProcoRat sound significantly better by asymmetry. However, non-linear circuit loading by Germanium diode biased silicon and germanium transistors creates a very soft clipped differential impedance load to the op-amp. The combined loading effect produces a wide even ordered harmonic content increasing with gain while maintaining, by soft clipping, a reduced upper frequency harmonic spread. The nonlinear loading of these two clamping circuit elements are sensed with a non-loading high impedance buffer using a GM multiplied JFET/Bipolar pair. Using two 9V batteries in a split supply gives plenty of headroom for popped-string transient response without rail limiting. Since most all pedals are turned on by the shorting of an input ring contact to ground, a single MOSFET is used to turn on both batteries at the same time. Combined with a FatBottomGirl™ front end, endless forms of Bass conditioning can be achieved. Here, the acoustic subtlety of discrete semiconductor elements in an analog convolution is hard to beat. Linear Systems parts, reliably, are time tested answers for complex analog function applications well beyond these circuits. As usual …… Kirkwood" Discover the analog advantage. Whether you're designing professional audio equipment, precision instrumentation, sensor interfaces, or the next generation of analog circuits, Linear Integrated Systems offers industry-leading ultra-low-noise JFETs, bipolars, MOSFETs, and precision discrete semiconductors trusted by engineers worldwide. 📩 Request free samples: HERE

When switching speed and signal integrity matter, every nanosecond counts. The SST211/SD211DE Series of N-channel lateral DMOS switches is designed for high-speed, low-glitch analog switching in demanding applications where low capacitance and fast response are critical. Key Features ⚡ 1 ns maximum turn-on time ⚡ Ultra-low 0.2 pF reverse transfer capacitance (Crss) ⚡ Low rDS(on) ⚡ Low turn-on threshold voltage ⚡ Integrated Zener ESD protection ⚡ Available in TO-72 4-Lead (SD211DE) and SOT-143 4-Lead (SST211) packages. Typical applications include: High-speed analog switching Sample-and-hold circuits Pixel-rate switching DAC deglitching High-speed drivers If your design requires fast switching with excellent signal integrity, the SST211/SD211DE Series is built to deliver. 📄 Learn more or request samples HERE

Artificial intelligence is transforming nearly every industry, and the spotlight is often on powerful GPUs, advanced processors, and sophisticated software. But behind every AI server is an equally important layer of technology that rarely gets the attention it deserves: precision analog electronics. From routing high-speed signals to monitoring critical system functions, analog components help ensure AI servers operate with the speed, reliability, and accuracy today's data centers demand. AI Infrastructure Depends on More Than Processing Power Modern AI servers are incredibly complex systems. In addition to massive computational workloads, they must continuously manage power distribution, thermal performance, high-speed communications, timing, and precision signal routing. Many of these functions rely on analog circuits that operate at extremely high speeds while preserving signal integrity in electrically noisy environments. As AI infrastructure continues to scale, the performance of these supporting analog devices becomes increasingly important to overall system reliability. Why High-Speed Analog Switching Matters High-speed analog switches play a critical role in moving and controlling signals throughout server hardware. As data rates increase, engineers face growing challenges with propagation delay, insertion loss, capacitance, and signal integrity. Applications commonly include: High-speed analog switching Sample-and-hold circuits DAC deglitching High-speed drivers Precision signal routing These applications require components capable of switching rapidly while minimizing signal loss and maintaining exceptional electrical performance. The SST211 Series was designed specifically for these demanding applications, offering ultra-high-speed switching, low reverse capacitance, and low on-resistance. Designed for High-Speed Performance Linear Integrated Systems' SST211 Series of N-channel lateral DMOS switches is engineered for high-speed analog switching applications where performance and signal integrity are critical. Key features include: Ultra-high-speed switching (1 ns maximum turn-on) Ultra-low reverse transfer capacitance (0.2 pF maximum) Low on-resistance Low turn-on threshold voltage High-speed lateral DMOS architecture Integrated ESD protection These characteristics make the SST211 Series well suited for high-frequency analog signal paths where minimizing capacitance and switching delay is essential. Supporting the Next Generation of AI Servers As AI server manufacturers continue pushing the limits of performance, demand for ultra-fast analog components continues to grow. High-speed analog switches may represent only a small part of a server's overall bill of materials, but they play an essential role in enabling reliable, high-performance system operation. At Linear Integrated Systems, we've seen increasing interest in the SST211 Series for AI server infrastructure, where engineers require analog devices that combine speed, low capacitance, and dependable performance. While processors perform the computation, precision analog components help ensure the signals that support those processors move accurately, efficiently, and reliably. The Future of AI Requires Precision Analog Electronics The future of artificial intelligence isn't built on digital technology alone. Behind every AI server is a foundation of analog electronics responsible for switching, sensing, monitoring, and controlling the signals that keep complex systems operating around the clock. As AI data centers continue to expand, high-performance analog semiconductors will remain an essential part of the infrastructure powering tomorrow's computing platforms. Learn More Interested in learning more about the SST211 Series? Explore the datasheet or contact the Linear Integrated Systems engineering team to discuss how our high-speed lateral DMOS switches can support your next design.

Integrated circuits continue to advance, but many of the highest-performance analog front ends still begin with a discrete JFET. Why? Because when you're trying to capture extremely weak signals, the input stage often determines the performance of the entire system. Engineers continue to choose discrete JFETs for applications requiring: Ultra-low noise High input impedance Low input capacitance Excellent linearity Whether designing photodiode amplifiers, hydrophone preamplifiers, precision sensor interfaces, or instrumentation front ends, these characteristics can make the difference between detecting a signal—or missing it. That's why devices like the LSBF862, LSK170, LSK389, and LSK489 continue to be designed into new systems where signal integrity starts at the input stage. 👉 For free samples or to discuss your application with our engineering team, complete the form HERE!