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

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MONOLITHIC DUAL, N-CHANNEL, JFET VOLTAGE CONTROLLED RESISTOR < Back VCR11N MONOLITHIC DUAL, N-CHANNEL, JFET VOLTAGE CONTROLLED RESISTOR The VCR11N Monolithic Dual, N-Channel, JFET Voltage Controlled Resistor is a direct Siliconix VCR11 Series. It is Ideal for Variable Resistor and Grain Control Applications. This part is available in the TO-71 4L RoHS package, as well as in die form. Advanced screening options are available for our diverse product lineup, featuring JFETS, Bipolar transistors, MOSFETs, current regulators, and Diodes. Our special screening covers all the parameters listed in the standard datasheet, including comprehensive package pin-out. Connect with our experienced technical team to discuss your specific needs and tailor your requirements—email us at support@linearsystems.com or call (510) 490-9160. MOQ applies to these specialized services. Ordering Information Below are the options you have when ordering this part: VCR11N TO-71 6L VCR11N SOT-23 6L VCR11N DFN 8L VCR11N Die Datasheet Spice Model Application Notes

< Back JFETs for VC Circuits Introduction Linear Integrated Systems manufactures a variety of FETs (Field Effect Transistors). In particular they have a variety of matched dual products. There are advantages in having matched devices. For example, if you are building a twochannel stereo audio product, having two or four devices in the same package allows for the two audio channels to be more closely matched. This paper will explore using FETs in voltage controlled circuits. Several approaches will be shown: 1. Using FETs as voltage controlled resistors. 2. Using FETs as voltage controlled amplifiers and active mixers. 3. Using FETs as voltage controlled phase shifters for processing music. 4. Using FETs as voltage controlled band pass filters. We will also explore ways to reduce nonlinearities or distortions and automatically bias the FETs. Read More