Active and Passive circuit elements?

Papabravo

Joined Feb 24, 2006
16,104
What are Active and Passive circuit elements?
Passive elements are incapable of producing gain, they only attenuate. Examples are resistors, capacitors, and inductors.
Active elements are capable of producing gain. Examples are transistors, operational amplifiers, and vacuum tubes.
 

Papabravo

Joined Feb 24, 2006
16,104
Diodes don't generate gain, but are still generally considered active devices.
That is true. There are some devices that don't fit neatly into one of the two categories. I don't think there is a category for devices that are neither strictly active nor strictly passive. If there is I can't remember what it was called. I was trying to keep things simple for the TS. (Disparaging remark removed by moderator.)
 

MrAl

Joined Jun 17, 2014
8,239
EDIT:
This came out a little long, perhaps the short answer is that it just doesnt matter how we classify a circuit element (active or passive or anything in between).

Hello there,

I have seen this question come up many times in the past both in forums and in regular conversation, and with that there is always some disagreement as to what exactly a passive circuit element is versus an active circuit element. After a while i start to wonder if it even makes sense to try to classify elements as either active or passive because there are variations that not only depend on the element itself but also on how it is being used. I guess what the goal is is to be able to prepare some list or set of lists where we can write down the name of a circuit element in one of the lists and then somehow feel accomplished in that we achieved something so elegant that we can always refer to when asked. But do such lists really exist? It probably depends on your personal definition of what is active and what is passive and there is probably no universal definition except in a given forum and that is probably by mutual agreement. But let's think about this a little more.

Probably most of us here believe that a resistor is passive, and a transistor is active, but what would be base that on. Well, the resistor just consumes energy, a good argument for passive, and the transistor controls signals or amplifies things, a good argument for active. But isnt it just possible that we can use a resistor to sense temperature, and therefore that resistor might be the heart of the control system for heating and cooling an entire building? Is it active then? It is surely controlling something and something rather important especially if you are on an expedition in the Arctic Circle.

Then what about a voltage reference? We can use diodes to rectify, an application where the diode has to control the flow of current, or we can use a diode as a voltage reference by biasing it with a resistor and voltage source or a current source. Is it active then? Or is it both active and passive?

Perhaps the definitions for active and passive that we chose in making these kinds of decisions were not good enough. For example, another definition might involve the determination of the power consumption or delivery of a device as compared to the power of the most power absorbing or delivering element in the system. Or how about an active element being a device that can change the current or voltage in a circuit abruptly like a diode or transistor. But we already have a classification for that we call 'nonlinear'. Is a circuit element active just because it is nonlinear?
Maybe it depends on the ratio of how one element is behaving in a circuit as compared to how most other elements are behaving. For example in the resistor temperature control system the resistor sensing the temperature is active but all the other resistors are passive. This idea would give rise to the ideas of active and passive 'modes'.

The other question is does it do us any good to classify components as active or passive. It does not seem to add any new information to any circuit analysis i dont think, because we base that not only on the component itself but also how it is being used. I never once sat down and looked at a new circuit presented to me for the first time and thought, "OK, first thing i have to do is make a list of all the active and passive devices because that will help me write the equations for this circuit". So we dont need to classify an element in that way. Maybe in some class it would make sense just as a matter of discussion, but it seems more like a topic of confusion than anything else. So what if we make a list it does not change anything about the analysis because the analysis techniques already include ways of handling different kinds of circuit elements. So really even if we *COULD* make a concise set of lists with every component on earth placed in one list or another it would not change anything about the analysis or even our understanding of any circuit.

So maybe the best answer is, "It doesnt matter". :)
 
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crutschow

Joined Mar 14, 2008
27,184
I agree that it doesn't matter much in circuit design and analysis.
We could limit passive devices to linear ohmic devices, and non-varying capacitors and inductors (at least not intentionally non-linear or non-varying).
All the rest could be then be considered active.
But that would seem to be of interest only in an academic sense.
 

LvW

Joined Jun 13, 2013
1,247
So maybe the best answer is, "It doesnt matter". :)
Yes - it does not matter at all. I think there are much more interesting and important things to discuss in analog electronics than the question of active or passive.
 

RBR1317

Joined Nov 13, 2010
631
The only time I can remember when the issue of active/passive is important was for the parameterization of network models. If you are into this (I'm not) then an important qualification is that the network transformation theorems are valid only for passive circuits, i.e. where the component models do not include dependent sources.
 
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