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Use Curves to Design ClassA Amplifiers
Almost No Math! With this method, you can design amplifiers with
JFETs, depletion MOSFETs, and even triode tubes. In truth, you still need a
little math. Yet if you slogged through a semester of highschool algebra,
you'll do fine. If not, follow along. You might be in for a pleasant surprise. The
process is simple: You draw over the manufacturer's drain (or plate)
graphs. Then you apply Ohm's Law, which is a simple division, voltage divided by
current. Two of the three resistor values for your circuit pop out. The third
resistor is a given. Result: A ClassA amplifier schematic. Some electronic
engineers use a more elaborate version of this same, graphic method.
♦CAUTION, Engineering Students.
The design method on this page is just an introduction. This page
isn't precise enough for your EE projects. If in doubt, ask your prof.
Prerequisites
 Depletion Mode. The device must be a depletionmode silicon MOSFET or JFET. (No BJT
transistors. No enhancementmode MOSFETs. No GaAsFet, SiC or Ge devices.) For our
examples, we'll use a Supertex LND150 MOSFET.
 Source Bias. This technique works on sourcebiased ("selfbiased")
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.)
 PChannel FETs. This design process will work with Nchannel or Pchannel FETs. All examples
refer to Nchannel FETs, which are more common. To use a Pchannel 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.)

Supertex LND150

Study the Datasheet
Obtain the datasheet for your device from the manufacturer or catalog house Web site. For
instance, Mouser and
DigiKey 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 Xaxis (bottom) that represents drain voltage (V_{DS}). The
Yaxis (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 sourceresistor value will be a complete guess. If
you want to design circuits without using characteristic curves, click Design without curves!

Drain curves for LND150 MOSFET. Mouse over for example of homemade graph.

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 (flattop 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 negativegate area is the region of tubelike operation. Mouse over
the graph. Notice that a green shade highlights the negative curves. The name of this area is the
“depletion mode.”

Saturated vs. unsaturated regions. Mouse over for negative gatebias area.

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 Xaxis or bottom of the graph. The line will then slant left to intercept
the Yaxis. This axis represents the maximum amount of current that passes through your
drain resistor R_{D}. (Sometimes sourceresistor 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.)

Mouse over for load line.


Contents
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