When a single power BJT can't handle the current or dissipation you need, the obvious move is to parallel two or more devices. The obvious move is also the one that blows up your amplifier. BJTs have a negative temperature coefficient of V_BE — roughly −2 mV/°C at constant current. Tie two emitters together, share the same base drive, and whichever transistor happens to be a few degrees warmer will pull more current, heat up faster, pull even more current, and thermally run away until it fails shorted. This is current hogging, and it's why paralleled BJTs need emitter ballast resistors.

The trick: put a small resistor (typically 0.1 Ω to 1 Ω) in each emitter leg. Now if one transistor tries to hog current, the extra drop across its ballast resistor subtracts from its V_BE, throttling it back. The resistors enforce current sharing through simple local negative feedback.

Sizing rule of thumb: pick R_E so that the voltage drop at rated current is roughly 2× the worst-case V_BE mismatch you expect to see between devices at operating temperature. A common heuristic is a drop of 100–500 mV per resistor at full current. Bigger resistors give better sharing but waste more power and reduce output swing.

Worked example: A 200 W audio amplifier output stage uses four parallel NPN power BJTs (like MJL21194) delivering a peak 10 A total, so 2.5 A per device. Pick R_E = 0.22 Ω. At 2.5 A the drop is 0.55 V — comfortably larger than the ~50 mV V_BE spread you'd see across a matched quad at similar junction temperatures. Power dissipated per resistor at peak: I²R = 2.5² × 0.22 = 1.375 W, so you specify 3 W wirewound (or better, a 5 W non-inductive type to avoid ringing at audio's upper edge).

Real-world where you'll see this:

• Hi-fi power amps (Crown, Bryston, classic Threshold) universally use 0.22 Ω or 0.33 Ω emitter resistors on the output pairs.
• Linear bench supplies pass-banks stacking multiple 2N3055s use 0.1–0.5 Ω ballasts.
• Welding inverters and induction heaters paralleling IGBTs use inductive ballasting (small air-core coils) instead, because IGBTs have a positive tempco at high current and don't need resistive sharing — but they do need dynamic sharing during switching.

MOSFET aside: Vertical power MOSFETs have a positive R_DS(on) tempco, so they self-ballast for DC current sharing. You still need gate resistors (typically 4.7–22 Ω per gate) to prevent parasitic oscillation between paralleled devices — a different problem with a similar-looking fix.