Transistor biasing: fixed bias vs voltage-divider bias

Objective

Set the Q-point with a voltage-divider (emitter-stabilised) bias and see why it is more stable than fixed bias.

Procedure

1. Run and read the Q-point.
2. Change the transistor's β from 100 to 300 in the Inspector: Ic barely moves.
3. Build the fixed-bias variant (Rb 560 kΩ) and repeat.

Theory

Divider bias: Vb = 12·10/57 = 2.1 V, Ve = Vb − 0.65 = 1.45 V, Ic ≈ Ie = 1.45 mA, Vce = 12 − Ic(Rc+Re) = 7.4 V. Ic hardly depends on β. Fixed bias gives Ic = β·Ib, which is proportional to β.

Questions to answer

1. Why does the emitter resistor stabilise Ic?

Components used

Battery / DC source, 4 × Resistor 220 ohm 1/4W, Transistor BC547 (NPN)

About the model

Ideal wires; component models as listed in the catalogue (no parasitics, no temperature drift). Built without a breadboard so the schematic stays readable.

More Electronic Devices Lab experiments

• BJT as a switch
• SCR (thyristor) latching
• LDR response to light
• Photodiode under illumination
• Half-wave rectifier