Electronics I - Laboratory 8 Rev A
Junction Field Effect Transistor (JFET) – Drain Characteristic Curves
I. Objectives
1. Develop the drain characteristics curves of a Junction Field Effect Transistor.
2. Develop the Transconductance curve of a JFET.
II. Pre-Lab Requirements
1.
Junction Field Effect Transistor Drain Characteristic Curves
A JFET is a semiconductor device that uses voltage applied to the gate terminal to
control current flowing between the drain terminal and the source terminal. The JFET is
referred to as a voltage controlled device and is contrasted with the BJT which is a
current controlled device. In this lab the student will build a simple test circuit to
evaluate a JFET’s gate voltage, drain current, and drain-source voltage relationships.
Prior to lab, the student should review sections 4-1 and 4-2 of their text Fundamentals of
Analog Circuits. Figure 1. Shows the drain characteristic curves of a typical JFET
Figure 1. JFET Drain Characteristic Curves.
2. JFET Transconductance Curve
If on a graph, the output of a JFET (ID) is plotted versus the input voltage (VGS) the
trace that is formed is called the transconductance curve. If a JFET biasing point is
selected on the transconductance curve, that point would be defined by the
corresponding values of ID and VGS. If a straight line is constructed on the graph such
that it is tangent to the transconductance curve at the selected bias point (VGS, ID), then
the slope of the straight line represents the JFET’s transconductance at that point.
Transconductance, denoted by the variable gm, is used to determine the voltage gain of
a JFET amplifier.
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Figure 2. JFET Transconductance Curve
III. Laboratory Requirements
1. Required Parts and Equipment
A.
B.
C.
D.
E.
F.
2 - DC power supplies
1 - Bench DMM
2 - Fluke hand-held DMMs
1 - Proto-Board (PB-103)
1 – 2N5459 Transistor
Wires and leads for circuit connections.
2. Required Information
A. Transistor Data Sheets
The following is a link to the specification sheet for the 2N5459:
http://www.fairchildsemi.com/ds/2N/2N5459.pdf
2N5459 Pinout
Figure 3 shows the pin orientation for the 2N5459
Figure 3. 2N5459 Pin Orientation
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3. Laboratory Procedure
Measuring the Drain Characteristics of the 2N5459 JFET
In this experiment a test circuit will be constructed and measurements taken to
develop the data required to plot the drain characteristic curves and
transconductance curve for the for a 2N5459 JFET.
A. Construct the circuit shown in Figure 4.
Figure 4. JFET Characteristic Curve and Transconductance Test Circuit
B. After building your test circuit instrument to monitor VGS, VDS, and ID. Connect
the Keithly DMM to monitor ID, and a Fluke to monitor VDS and VGS.
C. When your circuit is built and instrumented, you are ready to take data. You will
be taking data for VGS = 0 V, -0.5V, -2V, and -3V. The methodology for
collecting the data is to set the value of VGS by adjusting the voltage of VGG and
then sweep the voltage of VDD in incremental steps from 0V to 15V recording
data for each step. Since the data and therefore the curve, exhibits the most
change in the ohmic region, take at least 8 data samples in this region. In the
constant current region, take 2 to 3 samples until a VDS value of 15 volts is
reached.
After completion of all measurements for the first VGS value, increment VGS and
repeat the procedure.
D. Tranconductance Curve Data: With your circuit still instrumented, set VDS to a
value within the constant current region, then sweep VGS in 12 steps from 0V to 5V. Measure ID for each value of VGS.
4. Data Reduction and Lab Report
This Lab submittal will be an informal report. Your report should be in Word with
graphics pasted in.
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A. Make an Excel table and graph to display your characteristic curves. Label your
graph properly and show the location of the ohmic region and the constant current
region. On this graph label the pinch-off voltage for each value of VGS.
B. Compare in a chart, compare the following values with those found in the 2N5459
data sheet:
a. Gate to Source Cutoff Voltage (VGS (off))
b. Zero-Gate Voltage Drain Current (IDSS)
C. What was the most power that you dissipated in your JFET during testing? What is
the rated total device power dissipation (PD) for the 2N5459?
D. Make and Excel table and graph of the JFET’s transconductance curve. What is the
transconductance at VGS= -1V?
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