While I (most of the time) know from experience what components are generally considered active or passive, I have yet to come across a satisfying definition.
Why do we divide electrical components into those two main categories at all?
Some examples:
From http://www.electricaltechnology.org/2013/06/the-main-difference-between-active-and.html
Active: Those devices or components which produce energy in the form of Voltage or Current are called as Active Components
Passive: Those devices or components which store or maintain Energy in the form of Voltage or Current are known as Passive Components
How does a Diode "produce" energy?
From http://www.differencebetween.com/difference-between-active-and-vs-passive-components/
What is the difference between active and passive components?
1. Active devices inject power to the circuit, whereas passive devices are incapable of supplying any energy
2. Active devices are capable of providing power gain, and passive devices are incapable of providing power gain.
3. Active devices can control the current (energy) flow within the circuit, whereas passive devices cannot control it.
What exactly means "can control current flow"? Isn't a (passive) capacitor able to control or at least influence current flow as well?
Some people argue, that it depends on the context in which the component is used to be able to consider it active or passive. This doesn't make things easier.
Especially for diodes there are so many conflicting/different arguments:
- "In most cases (rectifier, Zener, etc.) a diode is, no doubt, a PASSIVE device. Only in some special cases like with a tunnel diode, when its negative resistance region is used, it can be considered as an ACTIVE device."
- "it is an active device.since its impedence is positive,or v-i chara lies in 1&2 quadrants."
- "Yes it is an active device since it requires an external power source, to operate it in forward or reverse bias."
- "Diode is an active device, since it can be used as an waveform generator (half wave rectifier, for ex)."
- "If the i-v characterisitics of the diode are in region I and III, then it is a passive device (always dissipating power). I think most diodes fall into this category."
I am pretty sure that there is no "one rule" and you always have to ask several questions about a component that must be satisfied to classify it. But what are those criteria exactly?
Answer
There is a clear definition: Passive elements have no function of gain, or control over voltage or current: their controlling function is linear -> I/V = R in the case of a resistor. There are exactly four kinds of passive elements: Resistors, Capacitors, Inductors and Memristors. All other components are active. Source http://de.wikipedia.org/wiki/Elektrisches_Bauelement
Active Elements have a function of gain or control, meaning the connection of the controlling parameters nonlinear. Diodes control current, transistors amplify current, etc.
The reason for the distinction is mathematical: You can use certain mathematical approaches to solve the equations of a device that contains only passive elements, while the same approaches would not work with active elements. If you have active elements, you may have to first approximate a passive network at the working conditions before you calculate.
This does not mean you cannot build advanced devices out of passive Elements. Analog filters are often made from passive elements and can be quite complicated.
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