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Facebook 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-R2network. 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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