
Ampliers: Basic BJT Congurations
431
Observation
When we can consider
h
0 and h
, then
A
h
h
R
vBJT
fb
ib
CL
≅ .
α
v,i
vi
eb
s
v
v
=
From the input voltage divider
v
RR
RR R
v
eb
EiBJT
sE iBJT
=
+
||
||
, so
α
vi
EiBJT
RR
RR R
,
||
||
=
+
The overall voltage gain is then
A
v
A
vBJT
,
A
hR
hhhhhR
RR
RR R
v
fb CL
ib ib ob fb rb CL
EiBJT
sE iBJT
=−
+−
()
+
||
||
11.7.2.4
C.B.: Output Resistance
We want here to determine the value of the output resistance of the amplier as
already dened in Section 10.2.2 of Chapter 10. The output resistance R
is the
one that the amplier presents to its load. Therefore,
acts as the source resis-
tance for that load.
Similarly to what ...