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"An active device or component is something which can provide an infinite energy or non-zero power for an infinite interval"

I learned this definition for an active device in a book, which is valid for capacitor, inductor and batteries (I have evaluated). I was wondering what is a "PN junction", "junctions of a transistor", "OPAMP", "JFET", "MOSFET"? I mean what are the above mentioned devices inside the quotation marks? Are they active devices or passive? If either one then please provide explanation for my understanding.

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  • \$\begingroup\$ Where does the quote come from and what relevance does it have to your question? \$\endgroup\$
    – Andy aka
    Commented Sep 23, 2014 at 18:15

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First, capacitors, resistors and inductors, all they are passive devices. An active device is one that is able to impose the voltage or the current in a given circuit.
That is, an active device (ideal) may be a voltage source which delivers a constant voltage, current whenever required. It can also be a source of current (ideal) that provides a permanent electric current to any power requirement.

OPAMP (operational amplifier), BJT, JFET (transistors), it is called active devices, because of its ability to manipulate and maintain voltage or constant current (within specified operating ranges, unlike the ideal elements).

Here, you can find a more detailed explanation.

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  • \$\begingroup\$ How PN JUNCTION is considered as active? \$\endgroup\$
    – Pankaj Kumar
    Commented Sep 23, 2014 at 17:56
  • \$\begingroup\$ @PankajKumar PN junction is the "basic module" to make BJT's, OPAMP, etc. The most simple device with a PN junction is the diode. \$\endgroup\$
    – Martin Petrei
    Commented Sep 23, 2014 at 17:58
  • \$\begingroup\$ With all due respect I consider your statement as understandable and true but see if i have to literally explain the concept that how PN junction is active so what would be the explanation that is what I am expecting to seek. \$\endgroup\$
    – Pankaj Kumar
    Commented Sep 23, 2014 at 18:24
  • \$\begingroup\$ @PankajKumar Maybe you can think about the model that describes a PN junction in the context of a circuit, no the physics model. Usually, a PN junction is modeled with a resistor and a battery in series, so the model is made with active components (the battery). Here: en.wikipedia.org/wiki/Diode_modelling you can find a explanation about the three commonly used diode (PN junction) models. See at "Piecewise linear model" \$\endgroup\$
    – Martin Petrei
    Commented Sep 23, 2014 at 18:29
  • \$\begingroup\$ THANX A LOT @Martin Petrei \$\endgroup\$
    – Pankaj Kumar
    Commented Sep 23, 2014 at 18:31
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I agree that this is somewhat of a confusing topic. Even Wikipedia doesn't seem to have a clear definition of what an active device is.

I think it's best to think of an passive device as one which linearly affects the circuit. By this I mean, if you create a circuit out of passives and send a frequency through it. There will be no other frequencies that come out. Resistors, capacitors and inductors all fall into this category.

An active element affects the input in a non-linear way. If you send a frequency in, you'll very likely get a multiple frequencies out.

From this definition you can see that a PN junction would definitely alter the frequency characteristics of an input.

This describes linear circuits in more detail:
http://en.wikipedia.org/wiki/Linear_circuit

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  • \$\begingroup\$ I never would mix linear/non-linear characteristics with the question active/passive. Both categories are completely different. \$\endgroup\$
    – LvW
    Commented Sep 23, 2014 at 19:59
  • \$\begingroup\$ @LvW Not according to the accepted answer that this question is now duped over to. electronics.stackexchange.com/questions/82787/… A quote from there: "their controlling function is linear" \$\endgroup\$
    – horta
    Commented Sep 23, 2014 at 20:05
  • \$\begingroup\$ What means "accepted"? Here is another quote from this "accepted answer": "Diodes control current". Control current??? And what does a resistor ? Is a resitor linear? I continue to state: Linearity has nothing to do with the question active/passive (BTW: I do not blindly rely on wikipedia). \$\endgroup\$
    – LvW
    Commented Sep 25, 2014 at 7:56
  • \$\begingroup\$ @LvW You make good points. Accepted with 20 votes means that 20 of their peers agree that it's the most correct answer. If you believe you know a better answer, please post it in the duped question area and add an (at)horta to it so I get a notice please. Even Andy Aka with years of experience and plenty of reputation here wasn't sure there was a clear cut definition. At any rate, most of us are here to learn, so if you have some knowledge that the rest of us don't, please stop debating and instead start teaching. Thanks in advance for your help! \$\endgroup\$
    – horta
    Commented Sep 25, 2014 at 14:27

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