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 VGS
≤ 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 RD, and dynamic rd drain resistance were then
approximated from the output plots. This was approached by picking a value on
VGS = 0 in the operating range and determining the ID
at that point (figure 1.1 & 2.1). RD and rd 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 VGS = 0.5V/div.
and ID = 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 VGS
= 0V, VDS = 15V, calculating gin (E.1), and comparing the results to
the given specifications the transconductance was determined (table 2).
gm = ∆ ∙ I / ∆ ∙ VGS (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 ID = 1mA, VDS
= 10V. Upon examining RD, rd 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 IDSS
was the maximum point of the curve.
BETA = IDSS / (VP)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 VG produced the λ result required for the Pspice
simulation (graph 3.2, 3.3).
