# Simple circuit lets you characterize JFETs

When working with discrete JFETs, designers may need to accommodate a large variation in device parameters for a given transistor type. A square-law **equation** is usually used as an approximate model for the drain-current characteristic of the JFET: I_{D}=β(V_{GS}−V_{P})^{2}, where I_{D} is the drain current, V_{GS} is the gate-to-source voltage, β is the transconductance parameter, and V_{P} is the gate pinch-off voltage. With this approximation, the following **equation** yields the zero-bias drain current at a gate-to-source voltage of 0V: I_{DSS}=βV_{P} ^{2}, where I_{DSS} is the zero-bias drain current.

**Figure 1**is a plot of this characteristic for N-channel JFETs showing the variation possible in a collection of devices. For example, the 2N4416A’s data sheet lists a pinch-off voltage of −2.5 to −6V, and the zero-bias drain current can range from 5 to 15 mA. You can observe the correlation between these two parameters across a sample of devices. The outer curves in the plot represent these extreme cases, and the center curve represents perhaps a typical case of a pinchoff voltage of −4V and a zero-bias drain current of 8 mA.

Although you can design around a certain amount of device variation for a mass-produced circuit, you sometimes need a tool to quickly characterize an assortment of discrete devices. This tool allows you to select a device that will optimize one circuit or perhaps to find a pair of devices with parameters that match reasonably well.

**Figure 2** shows a simple test circuit for this purpose. Although the **figure** shows the JFET as an N-channel device, the JFET DUT (device under test) may be of either polarity, as selected by switch S_{1}. An external voltmeter connects to the terminals on the right. Switch S_{2} selects two distinct measurement modes—one for the pinch-off voltage and another for the zero-bias drain current. In the pinch-off-voltage mode, the external voltmeter directly reads the pinch-off voltage; in the zero-bias-drain-current mode, the measured voltage is the zero-bias drain current across an apparent resistance of 100Ω.

With S_{2} in the pinch-off-voltage mode, R_{1} allows a few microamps of drain current to flow in the JFET under test, and the source voltage is a close approximation of the negative of the pinch-off voltage. The op amp acts as a unity-gain buffer, with negative feedback through R_{3}, so you can directly read the negative of the pinch-off voltage with the external voltmeter.

In the zero-bias-drain-current mode, however, the resistance from JFET source to ground is only 10Ω, so the drain current is a close approximation of the zero-bias drain current. The op amp’s feedback also switches to a gain-of-10 configuration, with the inclusion of R_{4} and R_{5} in the feedback-voltage divider. This gain allows the voltmeter to easily read the small voltage across R_{2}, with the resulting reading being the zero-bias drain current times 100Ω. For example, if the voltmeter reads 1V, this voltage corresponds to a zero-bias drain current of 10 mA.

_{1}to suppress any tendency for high-frequency oscillation. R

_{6}isolates the op-amp feedback loop from any parasitic capacitance in the voltmeter and its leads, preserving the loop stability. R

_{7}protects against accidental shorts, and you can replace R

_{4}and R

_{5}with one 1.1-kΩ resistor. You are more likely to have on hand resistors with the values in the

**figure**, however.

By clipping in samples from a collection of JFETs and throwing a switch, you can very quickly find the two parameters that determine where each JFET’s characteristic falls in the range that **Figure 1** illustrates and select devices to optimize circuit performance.

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