When we started talking about “passives” in previous articles, we stirred up a hornet’s nest.1 Several engineers2 inside Maxim Integrated and in the larger engineering community immediately challenged the definition of “passive.” We are still trying to find a short, accurate definition that is universally accepted. The most common definition is simply “not active.” Thus, a typical active device uses power to do something like create gain. But there are always exceptions.
- Passive Components Aren't Really So Passive (Part 1): Capacitors
- Passive Components Aren't Really So Passive (Part 2):
- ASIC Fixes For Noisy Analog "Oops" Moments
For example, an emitter follower is active, uses power, converts impedance, and has a gain just less than unity. The goal of these articles on passives, therefore, has been to warn people that what we think is a passive can and does cause nonlinear responses that can change the signal. So, resistor voltage dependence or capacitive absorption (soakage) can cause harmonic distortion. Hydroscopic printed-circuit boards (PCBs) can change offset.
How does one define a passive component? It is a tough question. Engineers in a chat room had some good suggestions. The IEEE dictionary3 defines:
“Passive device, A device that does not require power and contains no active components.”
“Passive Electric Network, An electric network containing no source of energy.”
Davor Vujatovic in the Encyclopedia of Life Support Systems (EOLSS) suggests a passive definition:4
“A passive component denotes a component that is unable to deliver more energy to an external circuit than it initially stores. To determine whether the component is passive, the total energy absorbed by it must be greater or equal to zero. In other words, a component that absorbs more energy than it delivers is passive. If the total energy delivered by the component is greater than the total absorbed energy, the component is active, i.e. the active component is capable of delivering energy to the outside world.”
The chat room engineers also suggested the Wikipedia entry under “Passivity (engineering).”5 It has an interesting perspective in the first two paragraphs:
“Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems. A passive component, depending on field, may be either a component that consumes (but does not produce) energy (thermodynamic passivity), or a component that is incapable of power gain (incremental passivity).
“A component that is not passive is called an active component. An electronic circuit consisting entirely of passive components is called a passive circuit (and has the same properties as a passive component). Used without a qualifier, the term passive is ambiguous. Typically, analog designers use this term to refer to incrementally passive components and systems, while control systems engineers will use this to refer to thermodynamically passive ones.”
Then depending on one’s engineering discipline, Wikipedia says:
“Thermodynamic passivity: In control systems and circuit network theory, a passive component or circuit is one that consumes energy, but does not produce energy. Under this methodology, voltage and current sources are considered active, while resistors, capacitors, inductors, transistors, tunnel diodes, glow tubes, metamaterials and other dissipative and energy-neutral components are considered passive.”
“Incremental passivity: In circuit design, informally, passive components refer to ones that are not capable of power gain; this means they cannot amplify signals. Under this definition, passive components include capacitors, inductors, resistors, diodes, transformers, voltage sources, and current sources. They exclude devices like transistors, vacuum tubes, relays, tunnel diodes, and glow tubes.”
The Wikipedia article really sums it up in the second paragraph: “Used without a qualifier, the term passive is ambiguous.”
We included the wording “seems to be passive” in our article definition in an effort to “weasel word” the definition to allow nonlinear distortion from something that we expect to be “inert” or “benign.” “Seems” as used above drew lightning for the engineers, so now adding “inert” or “benign” will probably add more fuel to the fire. We are still trying to find a short, accurate definition of “passive” that is universally accepted. The most common definition is “not active,” and it is not sounding so bad after all.
1. “Passive Components Aren’t Really So Passive (Part 1): Capacitors,” Bill Laumeister, http://electronicdesign.com/power/passive-components-aren-t-really-so-passive-part-1-capacitors
3. The IEEE Standard Dictionary of Electrical and Electronics Terms, sixth edition, IEEE Std. 100-1996; terms are “Passive device” and “passive electric network.”
4. Vujatovic, Davor, Electronic Engineering Vol I –Active Networks, ©2009 Encyclopedia of Life Support Systems, EOLSS Publishers Co Ltd, ISBN-13: 978-1848269774).
5. Wikipedia: http://en.wikipedia.org/wiki/Passivity_(engineering)
Bill Laumeister is an engineer in strategic applications at Maxim Integrated Products. He works with customers who use Maxim’s analog and digital integrated circuits. He has more than 30 years of experience and holds several patents.