1
SMALL SIGNAL ANALYSIS
LN 2
V
in
V
BB
With the application of an input signal V
in
we
expect the BE voltage to change as a
function of time:
V
BE
(t) = V
BB
+ V
in
(t)
This will result in a change in the collector current
I
c
(t) +
I
s
e
V
BE(t)/
V
T
= I
s
e
V
BB/
V
T
e
V
in(t)/
V
T
or
i
c
(t) =
I
c
DC
e
Vin(t)/
V
T
For
V
in
(t ) < V
T
, which defines the meaning of the term
“small signal”, we obtain
.]
..........
6
1
2
1
1
[
)
(
3
2
+
+
+
+
=
T
in
T
in
T
in
DC
c
c
V
v
V
v
V
v
I
t
i
Again, for V
in
< V
T
,
higher order terms will be insignificant, resulting in
I
c
(t) =
I
c
DC +
I
c
DC
v
in
Æ
i
c
(t)
=
g
m
v
in
(t)
V
T
The factor
I
c
DC
is known as the
transconductance “g
m
”.
The transconductance
V
T
measures the change in I
c
due to a change in V
BE
~
The time varying VBE(t) also gives rise to a changing minority (as well as majority)
carrier concentration within the base. The changing storage charge gives rise to a
capacitance and the movement of the time varying excess charge gives rise to the concept
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 Spring '11
 n/a
 Volt, Input impedance, Bipolar junction transistor, small signal, CJE

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