This configuration
employs negative feedback to prevent thermal runaway and stabilize the
operating point. In this form of biasing, the base resistor RF is connected to the collector instead of connecting it
to the DC source Vcc. So any thermal runaway will induce a voltage drop
across the Rc resistor that will throttle the transistor's base
current.
Merits:
• Circuit
stabilizes the operating point against variations in temperature
and β (i.e. replacement of transistor)
Demerits:
• As
β -value is fixed (and generally unknown) for a given transistor, this relation
can be satisfied either by keeping Rc fairly large or making
Rf very low.
Ø If Rc is large, a high Vcc is necessary,
which increases cost as well as
precautions necessary while handling.
ØIf Rf is low, the reverse bias of the collector–base region
is small, which limits the range of collector voltage swing that leaves the
transistor in active mode.
•The
resistor Rf causes an AC feedback, reducing the voltage gain of the
amplifier. This undesirable effect is a trade-off for greater Q-point
stability.
Usage: The feedback also decreases the input impedance of the
amplifier as seen from the base, which can be advantageous. Due to the gain
reduction from feedback, this biasing form is used only when the trade-off for
stability is warranted.
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