
Ampliers: Basic MOSFET Congurations
473
Differentiating,
di
i
T
dT
i
v
dv
DS
DS DS
GS
GS
=
δ
δ
+
δ
δ
di
T
T
Kv VdTK
T
T
vVdv
DS
r
nGSthn
r
GS th GS
3
2
2
3
5
2
3
()
=− −+
−
di Kv V
T
T
dT
T
K
T
T
vVdv
DS nGSth
r
n
r
GS th GS
3
2
2
2
()
=− −
+
−
di i
dT
T
K
T
T
vVdv
DS DS n
r
GS th GS
3
2
2
3
=− +
−
Now, let’s consider the four-resistor biasing network, as in Figure 12.6. Since
from the input KVL we obtain
2
21
vVRi
R
RR
i
GS GSDS DD SDS
=− =
+
−
thus
dv Rdi
=−
, and the previous equation becomes
di
i
K
T
T
vVR
dT
T
DS
DS
n
r
GS th S
3
2
12
3
=
−
+
−
It is evident that the presence of the source resistor
S
improves the over-
all bias stability, so that the greater
S
is, the less current drifts with ...