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?

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    \$\begingroup\$ I think it's just a syntax thing and frankly has very little meaning/significance in the real world. So long as you know what the device is/does you could call it pactive and it wouldn't matter. The definition I was taught is that an active device needs to consume power from supply rail(s) to perform its function while a passive component does not. I personally like that definition and it works well for me. Generally the only time I use this distinction is when I refer to the "passives" on a board I'm working on. Like all definitions if you look hard enough you'll find that it's not perfect. \$\endgroup\$ – Doov Sep 19 '13 at 8:45
  • \$\begingroup\$ With just passive components you can not build a circuit that amplifies the power in a signal. (You can amplify the voltage or the current, but not the product.) \$\endgroup\$ – Wouter van Ooijen Sep 19 '13 at 10:08

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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    \$\begingroup\$ This is the best answer. It's a bit scary that this many people believe the distinction is arbitrary. For many military and industrial screening and quality purposes, the distinction is critical. You'll meet many a quality engineer who wants to know if you replaced or screened any "active" components. If you then ask what they mean by "active", you'll be in an embarrassing situation. \$\endgroup\$ – scld Sep 19 '13 at 12:24
  • \$\begingroup\$ Huh, never even heard of a memristor. @Chris L, I agree: the difference is fundamental. \$\endgroup\$ – Bob Sep 19 '13 at 16:45
  • \$\begingroup\$ Memristors are relatively new and (also relatively, but hopefully more so on this forum) exciting. \$\endgroup\$ – scld Sep 19 '13 at 16:47
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    \$\begingroup\$ I am still struggling with this. Why has a forward biased diode more "control" over voltage then a capacitor that is charging. Both can influence voltage and current in a non-linear way. Do we need to reason with the mathematical distinction in this case? What exactly means "control"? How does this apply to a diode? \$\endgroup\$ – Rev1.0 Sep 25 '13 at 7:42
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    \$\begingroup\$ Have any memristers been constructed whose characteristics are sufficiently close to ideal that within a bounded but usable set of operating conditions that they can be accurately modeled as passive devices in the same way resistors, caps, and inductors can? A cap which is more leaky at higher voltages than at lower ones can't be regarded as a "passive" component, but most caps are used in operating ranges where that's not a particular problem. \$\endgroup\$ – supercat Oct 4 '13 at 20:30

Personally, I have always gone with definition 3 "3. Active devices can control the current (energy) flow within the circuit, whereas passive devices cannot control it."

Essentially I think that a passive component can change the current flow in a constant way, whereas active components change the current flow dependant on the current flow in other parts of the component.

Hence it is of my opinion that a diode is a passive component.

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    \$\begingroup\$ I also would consider most diodes use-cases to be of passive nature, similar to what I cited in the question. In common literature however, diodes seem to be classified as active most of the time... \$\endgroup\$ – Rev1.0 Sep 19 '13 at 8:45
  • \$\begingroup\$ @Rev1.0: Diodes are active. Take two 0.7-volt-drop diodes and two resistors, tying D1 anode directly to a 1V supply positive, D1 cathode through a 100-ohm resistor to supply negative, D2 anode through the second 100-ohm resistor to supply positive, and D2 cathode directly to supply negative. If the diodes drop 0.7 volts, then supply current will be 6mA. Now connect D1 cathode to D2 anode. Adding that connection would drop the supply current to about 5mA. \$\endgroup\$ – supercat Apr 2 '20 at 15:46
  • \$\begingroup\$ @Rev1.0: The voltage across each resistor would increase from 0.3V to 0.5V, increasing the current through each resistor by 2mA, but the current through the LEDs would drop from 3mA each to zero, with all of the resistor current flowing through the series pair of resistors. Adding a wire to an AC-driven passive network could reduce current flow, but adding a wire to a DC-driven network can only reduce current if some components are active. \$\endgroup\$ – supercat Apr 2 '20 at 15:47
  • \$\begingroup\$ @supercat How did you stumble upon this 7 year old answer/question? ;) \$\endgroup\$ – Rev1.0 Apr 3 '20 at 7:23
  • \$\begingroup\$ @Rev1.0: I don't recall, but I'd just encountered the thing about diodes awhile ago and have been eager to share it ever since. Rather cool and unintuitive I think that closing a switch in a network of diodes and resistors could reduce the amount of current passed through it at a fixed voltage, or increase the voltage drop of the network if fed with a fixed current. \$\endgroup\$ – supercat Apr 3 '20 at 14:12

I think that passive components are those whose only purpose is to do some sort of energy transformation (for example electrical energy to thermal, or magnetic).

  • \$\begingroup\$ I like how this would make things simple, but consider a LED. It just transforms energy by emitting it in the form of photons, yet it is considered to be an active component. \$\endgroup\$ – Rev1.0 Sep 19 '13 at 10:45
  • \$\begingroup\$ LED is also a diode, that's why I edited to "whose only purpose". Alternatively, what you think about: A passive component is a component that doesn't need any power source to do it's work. \$\endgroup\$ – Paolo Elwood Morandini Sep 19 '13 at 12:20
  • \$\begingroup\$ But isn't a crystal a passive component despite its need for voltage to oscillate? \$\endgroup\$ – Rev1.0 Sep 19 '13 at 12:41
  • \$\begingroup\$ It is. It is. Sadly I can't think of better definitions that cover all the cases. \$\endgroup\$ – Paolo Elwood Morandini Sep 19 '13 at 14:38
  • \$\begingroup\$ Exactly my reason for this post ;) Thanks for your feedback. \$\endgroup\$ – Rev1.0 Sep 19 '13 at 17:59

<=2 Legs = Passive

>=3 Legs = Active

OK, it's stupid, but it's a start :D

Edit: OK, I really can't believe anyone is taking this post seriously enough to downvote it, but since it keeps happening I'll point out that this post was meant as a light-hearted joke and not to be taken seriously.

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    \$\begingroup\$ ... and a potentiometer is ...? :-) \$\endgroup\$ – user16324 Sep 19 '13 at 9:30
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    \$\begingroup\$ Active 'cos someone actively moves it! \$\endgroup\$ – John U Sep 19 '13 at 9:43
  • \$\begingroup\$ There are various negative-resistance devices that should be categorized as active. \$\endgroup\$ – Wouter van Ooijen Sep 19 '13 at 10:07
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    \$\begingroup\$ Negative resistance - is that like two wrongs making a right? \$\endgroup\$ – John U Sep 19 '13 at 11:58
  • \$\begingroup\$ A potentiometer is passive: it's just a resistor that you happen to be able to change. \$\endgroup\$ – Bob Sep 20 '13 at 12:24

An active component change its behaviour according to the amount of current passing through it in a non linear way. Regarding the diode; a transistor can be thought of as two diodes, this is what makes a transistor an active component, hence the diode is an active component.

The non linear behaviour if a diode is due to the potential barrier that creates the pn-junction (positive/negative). Chew on that while you remember that transistors are npn and pnp.

  • \$\begingroup\$ posipiet's answer says esentially the same thing. \$\endgroup\$ – Jasen Feb 7 '16 at 7:30

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