
Ampliers: Basic BJT Congurations
391
However, as
EQ BQ CQ
and
CQ BQ
, the result is
EQ BQ
1 ; therefore,
the KVL equation can be rewritten as
VRIV
CC BQ BEQE BQ
1
1
from which
≅
++β
I
VV
BQ
CC BEQ
E
1
1
and
≅β ≅β
++β
II
CQ BQ
CC BEQ
E
1
1
()
≅+β≅
+
+β
II
R
R
EQ BQ
CC BEQ
E
1
1
1
If we meet the conditions of
E
1
1
and
1 (the latter practically
always veried) then
≅
I
R
CQ
CC BEQ
E
The result is that
CQ
is now practically independent of β. It seems a good solution
but it may not be, because
E
1
1
means that
1
is low or
E
is large, but
1
low cannot bias the base-emitter junction correctly, and a large
E
can drasti ...