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FacebookThe fixed bias circuit is modified by attaching an external resistor to the emitter. This resistor introduces negative feedback that stabilizes the Q-point
Merits:
• The circuit has the tendency to stabilize operating point against changes in temperature and β-value.
Demerits:
• As β-value is fixed for a given transistor, this relation can be satisfied either by keeping RE very large, or making RB very low.
Ø If RE is of large value, high VCC is necessary. This increases cost
as well as precautions necessary while handling.
Ø If RB is low, a separate low voltage supply should be used in the base circuit. Using two supplies of different voltages is impractical.
• In addition to the above, RE causes ac feedback which reduces the voltage gain of the amplifier.
Usage:
The feedback also increases the input impedance of the amplifier when seen from the base, which can be advantageous. Due to the above disadvantages, this type of biasing circuit is used only with careful consideration of the trade-offs involved.
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