William Sommerwerck wrote:
An active device provides signal amplification, or it uses amplification as
the basis of its operation. That has always been the definition.

Is digital circuitry active? I'd say yes. The ability to switch a tube or
transistor "on" depends on the device's ability to amplify its input to the
point it's driven into saturation. The amplification mechanism is exactly the
same as when the device amplifies a continuously varying signal.

How about controlled rectifiers? These are four-layer (sometimes more) devices
sometimes modeled as two transistors in a sort of "soixante neuf" arrangement,
a kind of flip-flop. Again, the same transistor amplification mechanism makes
them work.

https://en.wikipedia.org/wiki/TRIAC

Magnetic amplifiers use a small current to control a larger one. They're (as
the British would say) "valves", just as tubes and transistors are.

https://en.wikipedia.org/wiki/Magnetic_amplifier


I'd say amplification is a *sufficient* condition but not a necessary
one. A diode can be used as a switch, without amplifying, which I would
argue makes it an active device.