The fixed bias circuit is modified by attaching an external resistor to the emitter. This resistor introduces negative feedback that stabilizes the Q-point. From Kirchhoff's voltage law, the voltage across the base resistor is

From Ohm's law, the base current is

The way feedback controls the bias point is as follows. If V is held constant and temperature increases, emitter current increases. However, a larger I increases the emitter voltage V = IR, which in turn reduces the voltage V across the base resistor. A lower base-resistor voltage drop reduces the base current, which results in less collector current because I = β I. Collector current and emitter current are related by I = α I with α ≈ 1, so the increase in emitter current with temperature is opposed, and the operating point is kept stable.

Similarly, if the transistor is replaced by another, there may be a change in I (corresponding to change in β-value, for example). By similar process as above, the change is negated and operating point kept stable.

For the given circuit,

Advantages:

The circuit has the tendency to stabilize operating point against changes in temperature and β-value.

Disadvantages:


which is approximately the case if


उपयोग:

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.

Collector-Stabilized Biasing

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