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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.
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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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