How it works
With R_B connected from collector to base, the base current I_B = (V_CE − V_BE) / R_B. Writing KVL for the collector loop: V_CE = V_CC − I_C·R_C − I_B·R_C ≈ V_CC − I_C·R_C (since I_B << I_C). Substituting: I_B = (V_CC − I_C·R_C − V_BE) / R_B. With β = 100, V_BE = 0.7 V, this gives a self-consistent system. The feedback acts because any increase in I_C lowers V_CE, directly lowering V_B, reducing I_B, and opposing the original increase — this is series-shunt negative feedback at DC.
Key points to remember
Stability factor S for collector feedback bias is S = (1+β)/(1 + β·R_C/R_B); for R_B = 330 kΩ and R_C = 3.3 kΩ with β = 100, S ≈ 10 — significantly better than fixed bias S = 101. The trade-off: R_B loading the collector reduces AC gain because R_B appears in parallel with R_C for AC signals (with a Miller effect multiplication). This configuration uses fewer components than voltage divider bias (only one extra resistor versus two) and is common in single-supply, single-transistor RF and audio stages. Maximum output voltage swing is reduced compared to voltage divider bias because V_C is not independently set.
Exam tip
The examiner always asks you to compare collector feedback bias with fixed bias by calculating stability factor S for both — show that S_fixed = 1 + β while S_collector = (1+β)/(1 + β·R_C/R_B), and explain the physical feedback mechanism in one sentence.