
Principles of Analog Electronics
406
R
oBJT
can be written in the equivalent form
R
hR Rhhhh
h
oBJT
ie s
ie s
=
+−
=
||
12
12
ooe s
RR h||
12
+∆
where Δh = h
ie
h
oe
− h
fe
h
re
.
Finally for R
o
:
R
o
v
i
ce
l
Ropen
def
" "
L
Observations
Knowing the output resistance value is fundamental for different reasons,
among which are the satisfaction of the matching conditions with the load and
the evaluation of the ltering blocking capacitor C
B2
.
When h
re
can be considered practically zero, so that its feedback effect is null
(Section 11.3.2.2), and when
, it follows that no current will ow through
the input loop i
b
= 0 and, a