This is the most commonly used arrangement for
biasing as it provide good bias stability. In this arrangement the emitter
resistance ‘RE’ provides stabilization. The resistance ‘RE’ cause a voltage drop in a direction so as to
reverse bias the emitter junction. Since the emitter-base junction is to be
forward biased, the base voltage is obtained from R1-R2
network. The net forward bias across the emitter base junction is equal to VB- dc
voltage drop across ‘RE’. The base voltage is set by Vcc and R1 and R2. The dc
bias circuit is independent of transistor current gain. In case of amplifier,
to avoid the loss of ac signal, a capacitor of large capacitance is connected
across RE. The
capacitor offers a very small reactance to ac signal and so it passes through
the condensor.
Merits:
• Operating point is almost independent of β
variation.
• Operating point stabilized against shift in
temperature.
Demerits:
• As β-value is fixed for a given transistor, this
relation can be satisfied either
by keeping RE fairly large, or
making R1||R2 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 R1 || R2 is low, either R1 is low, or R2 is low, or
both are low. A low R1 raises VB closer to VC, reducing the
available swing in collector voltage, and limiting how large RC can be made without driving the transistor out of
active mode. A low R2 lowers Vbe, reducing the allowed collector current. Lowering both
resistor values draws more current from the power supply and lowers the input
resistance of the amplifier as seen from the base.
Ø AC as well as DC feedback is caused by RE, which
reduces the AC voltage gain of the amplifier. A method to avoid AC feedback
while retaining DC feedback is discussed below.
Usage:
The circuit's
stability and merits as above make it widely used for linear circuits.
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