How it works
Fixed bias connects one resistor R_B directly from V_CC to the base. The base current I_B = (V_CC − V_BE) / R_B is set only by V_CC, V_BE, and R_B — there is no mechanism that pushes back when β changes or temperature rises. As temperature increases, V_BE decreases by about 2 mV/°C and I_C0 doubles every 10°C; both effects increase I_C. With no emitter resistor providing negative feedback, I_C can keep rising until the transistor saturates (V_CE drops below ~0.2 V), wiping out the AC signal swing. This thermal instability makes fixed bias unsuitable for analog amplifiers where the transistor can get warm.
Key points to remember
The DC load line equation is V_CE = V_CC − I_C·R_C; at cut-off I_C = 0 and V_CE = V_CC = 12 V; at saturation V_CE ≈ 0 and I_C = V_CC/R_C ≈ 5.5 mA for 2.2 kΩ. The Q-point should be placed at mid-load-line for maximum undistorted output swing. Stability factor S = dI_C/dI_CO equals (1 + β) for fixed bias — this is the worst possible value, meaning a 1 µA increase in I_CO causes I_C to increase by (1 + β) µA. For β = 200, S = 201, confirming that fixed bias is extremely sensitive to leakage current changes with temperature.
Exam tip
The examiner always asks you to calculate the Q-point (I_C, V_CE) for a fixed bias circuit and then explain why the stability factor S = 1 + β makes this configuration thermally unreliable — show the derivation of S from first principles.