EE PRO for TI - 89, 92 Plus
Equations - Linear Amplifiers
106
28.4 FET (Common Gate)
The equations in this section focus on an FET amplifier in the common gate configuration. The amplification factor
µ
is described in the first equation in terms of the transconductance gm and the drain resistance rd. In the second
equation the input resistance Rin is described as a function of load resistance Rl, rd and
µ
. The voltage gain Av is
defined by the third equation in terms of Rl, rd and
µ
. The final equation computes the output resistance Ro in
terms of rd,
µ
, and the external gate resistance RG.
µ =
⋅
gm rd
Eq. 28.4.1
Rin
Rl
rd
=
+
+
b
g
µ
1
Eq. 28.4.2
Av
Rl
rd
Rl
=
+ ⋅
+
µ
1
b
g
Eq. 28.4.3
Ro
rd
RG
=
+
+ ⋅
µ
1
b
g
Eq. 28.4.4
Example 28.4 -
An FET amplifier connected in a common gate mode has a load of 10 k
Ω
. The external gate
resistance is 1 M
Ω
, and the drain resistance is 125 k
Ω
. The transconductance is 1.6 x 10
-3
siemens. Find the mid-
band parameters.
Entered Values
Calculated Results
Solution -
All of the equations need to be used to compute the solution for this problem. Press
„
to display the
input screen, enter all the known variables and press
„
to solve the equations. The computed results are shown in
the screen displays above.
-PQYP8CTKCDNGUIO
AUKGOGPU
TF
AM
Ω
4)
A/
Ω
4N
AM
Ω
%QORWVGF4GUWNVU
#X
µµ
4KP
A
Ω
4Q
'A
Ω
28.5 FET (Common Source)
These four equations represent the key properties of an FET amplifier in the mid frequency range. The first equation
defines the amplification factor
µµ
in terms of transconductance gm and drain resistance rd. The second equation
computes input resistance Rin as a function of load resistance Rl, rd and
µµ
. The voltage gain Av is defined in the
third equation. The final equation computes the output resistance, Ro.
µ =
⋅
gm rd
Eq. 28.5.1