LTSpice offers one way to generate IV curves without an
expensive curve tracer. However, how well LTSpice IV curves match the actual
device’s IV curves depends on how accurately the JFET model matches the actual
device. Not all LTSpice models are created equal. Some models are bare bones
and will only give you a a rough estimate of the actual IV curves. Most models include the pinchoff voltage and
beta parameter, which take into account the drain to source saturation current.
Additionally, most models are only specified for a given set of process
conditions. This process condition most often corresponds to the minimum data
specifications on the data sheet.
JFET IV Characteristics
(Curve Tracing)
The basic circuit for generating IV curves for a N channel is shown below. It utilizes two supplies, one for generating the gate to source
voltage (VGS) and one for generating the drain to source voltage (VDS). During the
simulation, the gate to source voltage is kept constant and the drain to source
voltage is stepped from 0 Volts to a maximum drain to source voltage.
However, a complete set of IV curves requires that the
drain-to-source current be measured at different VGS voltages. The typical LTSpice simulation command for
generating a set of JFET IV
curves is
.DC VDS 0 15 0.01 VGS -1.5 0 -0.3
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| LTSPICE Circuit Schematic for Generating IV Curves for a JFET |
Th e DC sweep command instructs the simulator to first set the gate
voltage to -1.5 volts and then sweep the VDS power supply from 0 to 15 volts in
0.01 volt steps. Once that sweep is complete, the VGS supply is incremented by -0.5 Volts to -1.5 Volts and VDS is swept
again from 0 to 15 volts This process
continues until VGS reaches 0 V. A N-Channel JFET is fully on when the gate-to-source voltage is 0 V. When VGS is at the pinchoff voltage the drain to source current is zero. For the default JFET LTSPICE model used in this example, the pinchoff parameter is at its default value of - 2 Volts.
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| IV Characteristics of Default N-Channel JFET LTSPICE Model |
The default N-Channel default model also uses LAMBA = 0. Because LAMBDA is zero, the slope of the IV curves in the saturation region is zero also. For the most part, this is not the way a real JFET operates. The IV curves in the saturation region have a small slope, which is set with the value of LAMBDA.
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