Voltage-divider biasing gets all the textbook attention, but sometimes you just need a stable common-emitter stage with the fewest possible parts. Collector-to-base feedback bias (also called shunt feedback bias) uses a single resistor from collector to base to establish the operating point — and that resistor gives you automatic thermal stabilization for free.

The topology is dead simple: R_C from V_CC to the collector, and R_B from collector to base. That's it. No emitter resistor required, no divider. The magic is in the negative feedback loop: if I_C tries to rise (say, from temperature increase), V_C drops, which reduces the voltage across R_B, which reduces I_B, which pulls I_C back down. Self-correcting.

How it stabilizes: In a fixed-bias design (R_B from V_CC to base), a 10°C temperature rise can shift I_C by 20-30% because β drifts and V_BE falls ~2 mV/°C. With collector-to-base feedback, the same thermal event might shift I_C by only 5-8%. Not as good as emitter degeneration with a divider, but vastly better than fixed bias — with one fewer resistor.

Design equations. Set V_C to roughly V_CC/2 for maximum symmetric swing. Then:

• R_C = (V_CC − V_C) / (I_C + I_B) ≈ (V_CC − V_C) / I_C
• R_B = (V_C − V_BE) / I_B = (V_C − 0.7) · β / I_C

Worked example: Design a 1 mA bias stage from V_CC = 9 V with a 2N3904 (β ≈ 150). Target V_C = 4.5 V. Then R_C = (9 − 4.5)/1 mA = 4.5 kΩ (use 4.7 kΩ). I_B = 1 mA / 150 = 6.67 µA. R_B = (4.5 − 0.7)/6.67 µA = 570 kΩ (use 560 kΩ). Two resistors, one transistor, done.

Real-world use: This is the classic biasing scheme for the input stage of a cheap guitar fuzz pedal (think Fuzz Face or Rangemaster). Vintage germanium designs relied on collector-feedback bias precisely because germanium's leakage current makes fixed bias unusable — the feedback loop compensates for the huge I_CBO. It's also common in RF preamps where minimizing parts count reduces parasitics.

The catch: R_B also creates Miller feedback at signal frequencies, reducing input impedance to roughly R_B/(1+A_v) and killing high-frequency gain. For a stage with A_v = 100, a 560 kΩ R_B looks like only ~5.5 kΩ at the input. Split R_B into two halves with a bypass cap to AC ground at the midpoint if you need to preserve input impedance.