Discussion
What do emitter ballast resistors do in a paralleled output stage?
Answer: D
A bipolar device's forward drop falls as it heats, so the hottest device would take more current and run away; a fraction of an ohm in each emitter develops an opposing voltage and equalises the sharing. They raise the output impedance slightly rather than lowering it, and provide no gain or filtering. Any device whose conduction improves as it heats needs a ballast element when paralleled, which is exactly the property MOSFETs lack and why they parallel freely. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.
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