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Lab 8: Transistor Based Amplifiers
Section A1: FET Operating parameters
In section A1, a circuit was built in order to find the relationship between drain
source current and drain source voltage dependence for an N channel silicon FET. First,
V
GS
was set to 0 and R was adjusted until V
DS
was about 5V. The value of R that was
used was 1.5kOhms. This was done to find the current Id.
Id was then measured as a
function of V
DS
for a range of V
GS
values from 5V to 0V. The gate voltage was decreased
from zero in order to find g
m
. g
m
was found to be .00053 using the equation
GS
D
m
dV
dI
g
/
=
, which is the ratio of change in source current to the change in gate
voltage. Next a graph of sqrt(I
d
) vs V
GS
. The following was observed.
Sqrt(Current) vs. Gate Source Voltage
y = 0.0365x + 0.0849
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
6
5
4
3
2
1
0
Gate Source Voltage
Sqrt(Current) Amps
Series1
Series2
Linear (Series2)
Figure 1.1, Graph of sqrt(Id) verse Gate source voltage
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View Full DocumentFigure 1.2 Circuit used in A1
The graph was very useful in that we could solve for two different constants from
the following equation.
2
)
(
T
GS
DS
V
V
K
I
+
=
The constants that we desired to find out
were K and V
T
.
First K was solved for, using the relation that the square root of K is the
slope of the line. This was found to be .191. Knowing this, V
T
could next be found
knowing that square root K times V
T
is the Y intercept. Using this, we solved and
obtained a value of 2.326 for V
T
. (Note, after doing this lab, some labs were told to do
this part in EWB, however we were told to do it in Lab). Looking back at the difference
between the lab and EWB is that the line appears to be linear instead of flattening out at 0
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 Spring '08
 Staff
 Electronics, Current

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