The previous two chapters showed how transistors — bipolar and FET — work as amplifying devices, using a small current or voltage to control a much larger current. The application to a simple machine like a record-player (yes, yes, I know — but CDs are much more complicated and we’ll have to leave them until later) is an obvious one. The small electric signal produced by the player’s pick-up cartridge must be amplified to a sufficiently large extent to drive a speaker.
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