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To the best of my knowledge, a device is considered active if:
1. its a power source.
2. it amplifies current or voltage signals.
3. it behaves like a switch, meaning it has states that are equivalent to ON and OFF.
I'd like to hear too about that issue (also about passive components).
In the most basic terms, active components change their behavior based on the signals they interact with (diodes, transistors, etc.) Passive components behavior is not changed by the signals going to them (resistors, capacitors, etc).
The most exact definition says that an active device is one that generates
energy in the circuit, but then we can ask the question of where that energy
came from in the first place, and then several other devices dont actually
generate anything but they do regulate energy so it seems like they should
be called active too. Still yet other devices convert energy...should they
be called active too?
So dont feel too bad, because lots of people and web sites are also confused.
The reason why is because how you classify a given element depends partly on
how you are analyzing it. Most classify resistors as passive, but that's about the
end of it.
One saving grace is to not worry so much about what is active and what is
passive. Just forget about this for now. Instead, concentrate on each
element you know about, taking one at a time, until you know a lot about
that one, then move on to the next. Of course start with the resistor and
Once you get a feel for how several devices work you can make your own
judgment about what is active and what is passive. You will find that the
definition varies a bit.
Passive parts are considered to be capacitors, resistors, and inductors, and their value, to a first approximation, is constant. They do not require any added power to perform their function.
Active devices usually contain semiconductors, (tubes in the olden days) which modify the signal in some manner (switch on/off, gain, non-linear wave shaping, etc.) that requires some form of added power.