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3.1 Variations of the Early Voltage with the Operating Bias Points
The variations of the drain current and output resistance with the drain bias of
an n-channel MOS transistor operating in the weak inversion region is shown in
Fig. 3. It is observed that as the drain bias is increased beyond the saturation
value, the drain current increases with drain bias. Therefore, the output resis-
tance values reduce with drain bias. The physical causes are the channel length
modulation effect and the DIBL phenomenon [5,6]. For scaled MOS transistor,
the threshold voltage of a MOS transistor reduces as the drain bias is increased.
This is referred to as the DIBL phenomenon. However, as the gate bias is in-
creases the fall of output resistance is somewhat less. This is because with the
increase of gate bias, the gate achieves better control and due to carrier mobility
degradation effect, the magnitude of the drain current is reduced. This physics
of the scaled MOS transistor has significant impact on the Early voltage of the
MOS transistor which is defined as
I D
∂I D
V A =
∂V DS
(3)
The variations of the Early voltage with drain bias for different V GS are shown
in Fig. 4. It is observed that for scaled MOS transistor, the Early voltage does
not remain constant with the operating bias points. Selection of suitable drain
voltage is therefore, extremely important. In addition, the magnitude of the Early
voltage also depends upon the gate bias to some extent. This characteristics of
the Early voltage needs to be incorporated within the design procedure. This is
discussed in the next section.
Fig. 5. Two-stage Miller OTA Circuit
 
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