Fixed bias with emitter resistor:

Figure: Fixed bias with emitter resistor

The fixed bias circuit is changed through attaching an external resistor to the emitter. This resistor establishes negative feedback which stabilizes the Q-point. From the voltage law of Kirchhoff, the voltage across the base resistor is

V_Rb = V_CC - I_eR_e - V_be.

From the law of Ohm, the base current is

I_b = V_Rb / R_b.

The manner feedback controls the bias point is like this. If V_be is stay constant and temperature get increases, emitter current also increases. Though, a larger I_e increases the emitter voltage V_e = I_eR_e, in turn which decreases the voltage V_Rb across the base resistor. A lower base-resistor voltage drop decreases the base current that results in less collector current since I_c = ß I_B. Collector current and emitter current are related through the I_c = α I_e with α ≈ 1, thus an increase in emitter current with temperature is opposed, and operating point is kept stable.