Prerequisites

• Depletion Mode. The device must be a depletion-mode silicon MOSFET or JFET. (No BJT transistors. No enhancement-mode MOSFETs. No GaAsFet, SiC or Ge devices.) For our examples, we'll use a Supertex LND150 MOSFET.

• Source Bias. This technique works on source-biased ("self-biased") preamplifiers.

• Minimum I_DSS. Keep the quiescent current above the minimum I_DSS. Otherwise, the device drops out of saturation and operates erratically or not at all. (You can test that as I did.)

• P-Channel FETs. This design process will work with N-channel or P-channel FETs. All examples refer to N-channel FETs, which are more common. To use a P-channel FET, reverse all power polarities. (For example, transpose the drain and source leads.) Also reverse all polar devices that connect to the FET. (For example, electrolytic capacitors and diodes.)

Study the Datasheet

Obtain the datasheet for your device from the manufacturer or catalog house Web site. For instance, Mouser and Digi-Key offer manufacturer datasheets.

Examine the datasheet. Pay particular attention to the device's drain curve graph (“characteristic curves”). Here's how you'll recognize a drain curve graph: This graph has an X-axis (bottom) that represents drain voltage (V_DS). The Y-axis (left) represents drain current (I_D). Both axes present maximum and not quiescent figures. (See the example curve set at right.)

If you can't obtain the datasheet, draw a rough graph similar to the figure nearby. Omit the horizontal lines, because they only apply to the LND150 device. Your drawing will help you to learn the process. Yet don't expect to design a working circuit: For example, your source-resistor value will be a complete guess. If you want to design circuits without using characteristic curves, click Design without curves!

What are curves?

Bias voltage curves. The curves appear as several horizontal lines. These curves represent device performance at different gate bias voltages. On FET performance graphs, bias voltage curves are nearly horizontal. Toward the left side of the graph, some of the curves become more vertical. This vertical area indicates performance when the device falls out of saturation (ohmic mode). The saturated region (flat-top curves) is the area that linear amplifiers use. Our design process only considers the saturated mode.

Use the Negative Curves

Negative gate bias area. See the bias curve graph at right. Notice that only some of the curves refer to device operation with a negative gate voltage. (If this were a JFET or tube graph, all curves would represent negative voltages.) With some MOSFETs including the LND150 (right), even a positive gate can control the device.

Only use the negative curves. Our design method depends on keeping the gate negative with respect to the source. The negative-gate area is the region of tube-like operation. Mouse over the graph. Notice that a green shade highlights the negative curves. The name of this area is the “depletion mode.”

Introducing Load Lines

Load lines. During design, you'll draw a load line. Your load line portrays a resistor. Resistances appear as slanted lines. The line for your drain resistor will pass through the horizontal point that represents your power voltage. This point is on the X-axis or bottom of the graph. The line will then slant left to intercept the Y-axis. This axis represents the maximum amount of current that passes through your drain resistor R_D. (Sometimes source-resistor value R_S exceeds 10 percent of the R_D value. In that case, the load line equals R_D plus R_S. For now, we'll ignore this possibility.)