The 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.
0 comments:
Post a Comment