Merits:
•It is simple to shift
the operating point anywhere in the active region by merely changing the base
resistor (RB).
•A very small number
of components are required.
•
Demerits:
•The collector current
does not remain constant with variation in temperature or power supply voltage.
Therefore the operating point is unstable.
•When the transistor
is replaced with another one, considerable change in the value of β can be
expected. Due to this change the operating point will shift.
•For small-signal
transistors (e.g., not power transistors) with relatively high values of β
(i.e., between 100 and 200), this configuration will be prone to thermal
runaway. In particular, the stability factor, which is a measure of the change
in collector current with changes in reverse saturation current, is
approximately β+1. To ensure absolute stability of the amplifier, a stability
factor of less than 25 is preferred, and so small-signal transistors have large
stability factors.
Usage:
•Due to the above inherent drawbacks, fixed
bias is rarely used in linear circuits (i.e., those circuits which use the
transistor as a current source). Instead, it is often used in circuits where
transistor is used as a switch. However, one application of fixed bias is to
achieve crude automatic gain control in the transistor by feeding the base
resistor from a DC signal derived from the AC output of a later stage.
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