Active Discussion About Passive Components

People talk about active and passive components like they are two distinct classes of electronic parts. When sourcing components on a BOM, you have the passives, which are the little things that are cheaper than a dime a dozen, and then the rest that make up the bulk of the cost. Diodes and transistors definitely fall into the cheap little things category, but aren’t necessarily passive components, so what IS the difference?

Resistors, Capacitors, Inductors, Transformers, Diodes*, and Memristors

That’s the list. Those are your passive components. Well, it’s not that easy. Also add in a bunch of types of sensors, because they are still passive. A photoresistor is a sensor but it’s still a resistor, even though its resistance changes based on an external influence. Any sensor whose measurement is a change in resistance, capacitance, or inductance still qualifies as a passive device. Also for fun let’s add a piezo buzzer.

The memristor is weird because it has only recently been proven to exist despite being theorized in the 70s, and is still not quite commercially available. There are now theories about meminductors and memcapacitors, which would also be passive devices, but they don’t exist yet.

It Depends on What Your Definition of Active Is

Part of the problem is it seems people have varying definitions of active. Rather than debunk all the wrong ones and spread bad ideas, here’s what’s correct. A device is active if any of these conditions are met:

  • It is a source of power
  • It amplifies any voltage or current
  • It acts as a switch

Applying this to the obviously active devices, like microcontrollers, it makes sense. It does all of those things on a GPIO pin. A transistor can amplify or act as a switch. A battery is a source of power.

A circuit remains passive until a single active component is added, so an RC or LC network is still passive. A piezo buzzer has an equivalent circuit of entirely passive elements, so it is also a passive device.

The equivalent circuit of a piezo buzzer is all passive elements.

As a side note, every circuit has at least one active device (a source of power). Also, an electromechanical device like a physical switch is considered passive.

The Diode

There is an exception with the diode. The vast majority of the time, it is a passive device, so it’s handy to just add it to the list of passive devices and mostly forget about it. It wouldn’t be interesting, though, unless we delve into what makes it sometimes active for that single, and rarely used exception, and to do that we have to get into quantum tunneling.

The tunnel diode is very fast (microwave frequencies), and is used in frequency converters and detectors, especially in space where its resistance to ionizing radiation, low voltage, high frequency, and longevity are desirable qualities. There is a specific condition of the tunnel diode in which it has negative resistance so that increasing voltage results in decreased current. Even the tunnel diode acts like a normal passive diode everywhere except this special region.

IV curve of a tunnel diode. The descending section is the area of negative resistance where increased voltage results in decreased current. By Mcguireatneuroticadotcom CC BY-SA 3.0

A charged particle moving across a barrier needs enough energy to get over the barrier or else it can’t cross. With a normal diode there is a PN junction that acts as the barrier. A power supply gives enough energy (called the forward voltage) for the electrons to get over that barrier, and the current flows through it. According to quantum mechanics, though, there is a non-zero probability that the electron will just jump to the other side of the barrier without going over it. This is quantum tunneling. In most diodes the barrier is high enough (controlled by the doping of the PN junction), that the tunneling is unlikely, so no current will flow until there is enough forward voltage to get the electrons over the barrier. In a tunnel diode, the PN junction has a lot more doping, increasing the likelihood of tunneling. These diodes work at much lower voltages than normal diodes because of the high doping.

At really low voltages, the electrons tunnel frequently and there is some current. As the voltage increases, tunneling increases to a peak and starts going down. It goes down because the electrons on one side of the barrier have more and more energy, but there are not the same holes on the other side of the barrier to accept them from tunneling. Once the forward voltage is high enough, the electrons have enough energy to get over the barrier without tunneling, and the tunnel diode acts like a normal diode again. This behavior allows the tunnel diode to act as an amplifier or as an oscillator, which puts it into the active category. We covered negative resistance in the tunnel diode a few months ago, and a post on diodes kicked off the active/passive debate in the comments.

Does it Matter?

Nah, not really. This is well into the realm of the esoteric, and has no practical use other than to annoy people at parties and probably below in the comments. Active and passive are generic terms for components and whether a particular component is classified as one or another doesn’t change how it is used. Quantum tunneling is neat, though, and the fact that we have harnessed it makes me wonder how close we are to warp speed and teleporters.

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