Merits and demerits of fixed bias, Electrical Engineering

Merits:

1.      It is very simple to shift the operating point anywhere in the active region by just changing the base resistor (RB).

2.      A very small number of components are needed.

Demerits:

  • The collector current does not stay constant with difference in temperature or power supply voltage. Hence the operating point is unstable.
  • Variations in Vbewill change IBand so cause REto change. In turn this will alter the gain of the stage.
  • When the transistor is changed with other one, considerable change in the value of β can be supposed. Because of this change the operating point will shift.
  • For small-signal transistors (for example not power transistors) with comparatively high values of β (that is, between 100 and 200), this configuration will be prone to thermal runaway. Particularly, the stability factor which is a measure of the change in collector current with changes in reverse saturation current is almost β+1. To make sure absolute stability of the amplifier, a stability factor of less than 25 is chosen and so small-signal transistors comprises large stability factors.

 

Posted Date: 1/10/2013 7:06:30 AM | Location : United States







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