How do passive circuits work with no battery/power source?
In: 173
Passive circuits are generally components in a larger system, they have power flowing into them from somewhere else.
For example, in a passive low pass filter the power is coming from the input signal, and some of it is dissipated by the filter. The power of the output signal decreases as the frequency increases, but is never more than the power of the input signal.
Some radio receivers can operate without a power source since the radio waves hitting the antenna can induce some, albeit very little, current (E.g. [the foxhole radio](https://en.wikipedia.org/wiki/Foxhole_radio?wprov=sfla1)). I would imagine that the same concept might be applied in some modern devices.
All circuits have a power source of some kind. Passive means it does not input any power into the circuit but can still modify things with the power that was provided.
Active circuits can provide power to the circuit.
The volume example is great. It provides resistance and changes the output but adds nothing.
An active circuit needs power outside of it’s input signal to perform. Passive just needs the input signal.
Another definition is that all passive circuits are made of integral, linear, or derivitive transforms of their inputs, while active has non-linearity.
One example might be the volume and tone controls on an electric guitar.
You don’t turn up the volume or the tone (treble frequencies), you turn down the amount which is lost to ground.