
Ampliers: Variants
487
The quiescent current I
CQ
ows through the collector resistive load
R
C
and causes a large waste of DC power that does not contribute to
the useful AC output power.
P
V
i
o
AC
def
rmsR rmsR
But since
v
V
rmsR
CEQ
C
,
=
2
and
i
I
rmsR
CQ
C
,
=
then
I
o
AC
CEQCQ
=
1
2
Finally
P
P
VI
VI
2
2
0.25
o
AC
i
DC
CEQCQ
CEQCQ
η=
that is,
% 25%
This value corresponds to the maximum efciency of a class A amplier, which
is the lowest among all amplier classes. Furthermore, this is a theoretical value
but practically it is even lower, 15–20% in best cases, because actually we do not
make the signal maximally swing up to the borders of the cutoff and saturation
reg ...