

Optimizing the 2N5458 JFET Lab
Semiconductor device experiment examining the 2N5458 nchannel general purpose JFET.
Theoretical concepts discussed in the lecture course Analog and Semiconductor Devices
(EE 211) and verified using laboratory experiments, hand calculations, and computer simulations.
Objectives:
1. To measure the transconductance, and static and dynamic
drain resistances of a junction field effect transistor (JFET) at a specified Q‑point.
2. To measure the common‑source output (drain)
characteristics and the transfer characteristic of a JFET.
3. To utilize PSPICE to generate common‑source output
and transfer characteristic curves.
4. To match the PSPICE generated output and transfer
characteristics to those of a specific transistor by adding/modifying model
parameters in the PSPICE .MODEL statement.
Procedures:
1. From curve tracer measurements, accurately record the output characteristics of two 2N5458 FETs.
Sweep V_{DS} from 0 to 20V, and set the step generator for V_{GS} ≤0.5V per
step. Draw the characteristics on at least half of a sheet of graph paper, and pay particular
attention to the slope of the V_{GS} curves in the beyond pinch off region.
2. Determine the static and dynamic drain resistances (R_{D}
and r_{d}) for each of the two 2N5458 FETs at V_{GS} = 0V, V_{DS}
= 15V. Compare these values to those shown in the 2N5458 specifications.
3. From curve tracer measurements, accurately record the
transfer characteristics of the two FETs at V_{DS} = 15V. Again, use
at least half of a sheet of graph paper for each curve.
9. Modify the .MODEL statement(s) to include values for VTO,
BETA, and LAMBDA which allow the improved PSPICE generated transfer
characteristics to match those of the real devices.
10. Compare the final PSPICE generated characteristics to
those measured on the curve tracer, by superimposing one on the other.
Procedure & Data:
1. In the given experiment the output characteristics of two 2N5458 JFET
were examined using two specific techniques. First the devices were
examined with the use of a curve tracer. To measure the voltage step rather
then the current step generally associated with the Type 575 curve tracer a 1k
Ohm resister was inserted from the base to emitter post. With the device
properly connected in the curve tracer the VDS was swept from 0 to 20V and V_{GS}
≤ 0.5V per step. The output family of curves were then examined and
sketched for each device (figure 1.1 & 2.1).
Once an understanding of the output characteristics were
plotted the static R_{D}, and dynamic rd drain resistance were then
approximated from the output plots. This was approached by picking a value on
V_{GS} = 0 in the operating range and determining the I_{D}
at that point (figure 1.1 & 2.1). R_{D} and r_{d} were
then compared with the values given on the specification sheet for the
2N5458 JFET (table 1).
Table 1: Drain Resistance vs. Given Specifications for 2N5458 JFET's
Next the transfer curves were measured on the curve tracer
and plotted. This was accomplished by setting the V_{GS} = 0.5V/div.
and I_{D} = 0.5mA/div. and examining the drain current vs. gate source
voltage. Once plots were drawn for each of the 2N5458 JFET's it was required to
determine the transconductance (gm) for each of the devices. Setting V_{GS}
= 0V, V_{DS} = 15V, calculating gin (E.1), and comparing the results to
the given specifications the transconductance was determined (table 2).
gm = ∆ ∙ I / ∆ ∙ V_{GS} (E.1)
Table 2: Transconductance (gm) for 2N5458 JFET's
After completion of the above steps it was required to
repeat the procedure above at a given Q‑point where I_{D} = 1mA, V_{DS}
= 10V. Upon examining R_{D}, r_{d} for graph 2.1 it was found
that the Q‑point lied between two curves. Therefor by determining the
slope of both the above and below curves and average the two curves resulted in
the best approximation for the drain resistance for graph 2.1 (table 3). As for
determining the transconductance (gm) for the given Q‑point was similar
to the above method (E.1). The transconductance was calculated from the
transfer curves using straight line approximation and the newly found data
recorded (table 3).
Table 3: Drain Resistance & Transconductance (gm) for 2N5458 JFET's
2. In the second part of examining the output
characteristics of the 2N5458 JFET's, Pspice was used to simulate the output
response. To understand the general behavior of the family of curves in graphs
1.1 and 2.1, a Pspice generic N‑Channel NET simulation was ran (graph 3.1)
and compared to the measured output taken from the Type 575 curve tracer. To
better simulate the measured output the Model statement was used to define VTO,
BETA, and LAMBDA. VTO or the pinch off voltage was determined by examining the
measured output (graph 1.2, 2.2) from the transfer curves where the slope of
the curve approaches zero (table 4). BETA was calculated (E.2), again using the
data taken off the measured transfer curve (graph 1.2, 2.2) where I_{DSS}
was the maximum point of the curve.
BETA = I_{DSS} / (V_{P})^{2} (E.2)
In determining LAMBDA (λ), the slope of the transfer
curve needed to be approximated using straight line approximating from the
measured transfer curve (graph 2.1, 2.2) over it's complete range. Therefor
using equation (E.3) for determining the slope of the line, applying the equation
for the slope intercept (E.4) and solving for the inverse of the x intercept
(E.5) for V_{G} produced the λ result required for the Pspice
simulation (graph 3.2, 3.3).
Next transfer characteristics simulations were ran on Pspice
and compared to the measured plots. First to gain a better understanding of the
transfer characteristics a simulation was ran using only the default transfer
settings (graph 4.1). As with before once an understanding of the default
transfer curve was gained a more specific simulation using VTO, BETA, and
LAMBDA (λ) were plotted (graph 4.2, 4.3) (table 4). In determining the
specific parameters required for the plot the following equations and technique
was used as above (E.3), (E.4), (E.5). Upon completion, the plots were then
compared to graphs 1.2, 2.2 take from the Type 575 curve tracer.
Conclusion & Discoveries:
It was learned that in analysis of a NET device one must
carefully 'pick' points off the family of curves, in the operating range to
approximate the output characteristics of the device. Analysis of the device is
not mathematically derived, yet it is rather experimentally derived.
Analysis of the transfer curve was accomplished by straight
line approximation. One may ask the question if a mathematical equation can be
derived to better approximate the curve?
Variations of Ohms law allowed for one to simply calculate
the output characteristics of the devices.
In closing, the 2N5458 JFET device was examined closely with
two specific techniques applied. It was discovered that the measurements taken
off the curve tracer scope in comparison with the data simulations produced
from Pspice agreed with in acceptable error bounds (±5%). Overall the
characteristics of the 2N5458 NET devices were learned and a general
understanding of the outputs will provide one with a better understanding for
using the device in an applied application.
Electrical Engineering lab key words: JFET, junction field effect transistor, JFET Model, JFET optimizing, QPoint, common source
output characteristics, 2N5457, 2N2458, 2N5459, general purpose JFET, nchannel, transfer characteristics, drain, gate, source,
transresistance, transconductance, and differential amplifiers. Offset voltage, bias current, offset current, two port models,
frequency response, transfer characteristics and transfer functions, nonlinear distortion and nonlinear devices.
