Design Spotlight: JFET Current Limiters for the Masses

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.