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 S₁. An external voltmeter connects to the terminals on the right. Switch S₂ 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.

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