
Ampliers: Basic BJT Congurations
381
Let’s refer to the BJT’s model of Figure11.8.
For a null output voltage v
ce
, the hybrid BJT model results with output
shorted. In this model, h
fe
expresses unitless current-to-current ratio. As a
consequence h
fe
i
b
represents a parallel CCCS (Section 5.3 in Chapter 5) in
output parallel conguration, since i
c
= h
fe
i
b
+ h
oe
v
ce
is a sum of currents.
This CCCS takes into account the part of the input current that “inuences”
the output one, representing a feedforward (reverse of feedback) effect.
h
oe
since i
c
= h
fe
i
b
+ h
oe
v
ce
we can write
=
=
h
i
v
oe
c
ce
i
b
Therefore, h
oe
is the ratio of i
c
and v
ce
variations (Section ...