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**is the gate-to-source voltage, β is the trans conductance parameter, and

_{GS}**V**is the gate pinch-off voltage. With this approximation, the following

_{P}**equation**yields the zero-bias drain current at a gate-to-source voltage of 0V:

**I**where I

_{DSS}=βV_{P}^{2},_{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 pinch off 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 micro amps 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.

**Read More: **Simple circuit lets you characterize JFETs