In article ,
"William Sommerwerck" wrote:

Temperature compensation for the bias? (This seems unlikely, as the devices
would be silicon. But who knows?)

Not uncommon, and a B-E junction has a matching tempco, which a standard
diode does not. RCA used to make (when they made transistors) a special
"diode" which was a B-E junction in a two-wire package, for just that
purpose.

After saying all that, though, my bet is that the fifth device has
something to do with running the CD drive motor.

Isaac