A Class-AB output stage needs a small, temperature-tracking bias voltage between the bases of its complementary output transistors — typically 1.2 V to 1.4 V — to keep quiescent current in the 20-100 mA range without letting it thermally run away. You could stack two forward-biased diodes there, but you're stuck with whatever voltage two junctions give you. The V_BE multiplier, sometimes called a "rubber diode" because you can stretch its voltage to any value you want, solves this beautifully with one transistor and two resistors.

The circuit: a BJT with its collector and emitter forming the two terminals, and a resistor divider (R1 top, R2 bottom) from collector to emitter with the tap driving the base. Because negligible base current flows (assuming high β), the divider forces V_BE across R2, which sets the current through both resistors. The collector-to-emitter voltage becomes:

V_CE = V_BE × (1 + R1/R2)

That's the whole trick — the transistor multiplies its own V_BE by the divider ratio.

Concrete example: Say you're building a 20 W audio amp with TIP41/TIP42 outputs and want V_bias = 1.3 V across the base-to-base gap. Assume V_BE = 0.65 V for a 2N3904 multiplier transistor at room temp. Then R1/R2 = (1.3/0.65) − 1 = 1.0, so pick R1 = R2 = 1 kΩ. Better yet, replace R1 with a 500 Ω resistor in series with a 1 kΩ trimmer so you can dial in quiescent current on the bench.

Why it beats diode stacks: the real magic is thermal tracking. Bolt the V_BE multiplier transistor to the same heatsink as the output devices. As the outputs heat up and their V_BE drops (roughly −2 mV/°C per junction), the multiplier sees the same temperature rise and its V_CE shrinks by (1 + R1/R2) × 2 mV/°C — exactly canceling the output V_BE drift. Without this tracking, quiescent current climbs with temperature, dissipation rises, temperature climbs further, and you get thermal runaway: melted output transistors and a smoking amplifier.

Rule of thumb: keep the bias current through the multiplier at 5–10× the base current it needs to supply — usually a few mA works. Too little current makes it noisy and slow to respond; too much wastes power and heats the multiplier transistor independently of the outputs, breaking the tracking.

Add a small capacitor (0.1 µF) across the multiplier to lower its AC impedance, since any impedance here shows up as crossover distortion.