
Ampliers: Basic BJT Congurations
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11.10.3.2 Stability versus V
BEQ
The other stability factor S
V
BEQ
, according to its denition, can be then calculatedas
VR R
V
CQ
BEQ
def
∆
=−
++
β1
In addition, since, typically, β ≫ 1 we can approximate
S
RR
R
R
R
R
K
V
TH E
E
E
TH
E
BEQ
≅−
+
≅− =−
+
β
β
β
1
1
In this case, it is useful to have a high value of R
E
to guarantee stability versus
V
BEQ
variations.
Observation
Note the similarity of the expressions for S
I
CBO
and S
V
BEQ
with the ones already
found for the collector feedback bias network, now with R
TH
replacing R
F
.
11.10.3.3
Stability versus β
Let us determine now the stability factor S
β
.
We know that (Section 8.6.3 in ...