What’s the Difference Between Passive and Active Components?
If you’re remotely interested in electrical engineering, or even consumer electronics, then you might have come across the terms ‘active’ and ‘passive’, perhaps while browsing RS Components. You might even have some intuitive grasp of what these terms imply, mostly gleaned from marketing: an active loudspeaker is one you need to plug into a power source; a passive one needs one the audio cable from a separate amplifier.
But what do the terms ‘active’ and ‘passive’ actually mean? Let’s take a look.
What are Active Components?
An active component, put simply, is any component which delivers energy into the circuit from outside of it. A transistor is an often-cited example of this, as it comes with three legs: the base, the collector and the emitter. One of these legs is designed to source power depending on the state of another. This arrangement, among other things, allows for a small signal to control a larger one. Thus transistors form the basis of many amplifiers.
The relationship between the power going into the component and the power coming out can vary enormously according to the component. This allows circuit designers to create all manner of circuits to suit the particular application. Of course, active components cannot generate power from nothing, and thus the power being sourced can be considered the upper limit. An operational amplifier is a common, flexible type of IC which makes use of active components to achieve a range of effects. Among these is a simple comparator, which quickly saturates to the positive and negative voltage inputs when one of the incoming signals is higher than the other.
What is a Passive Component?
A passive component, by contrast, is one that makes use of energy that’s already in the circuit. Resistors, capacitors and diodes all fall into this category. Signal processing invariably makes use of passive components at some stage along the path; many circuits are in fact constructed using entirely active components. A current-limiting resistor, for example, might be placed in series with an LED to provide a power indicator light.
Capacitors, in all of their various forms, are another example of a passive component. From larger electrolytic ones to smaller ceramics, capacitors are found on just about every kind of circuit there is; they eliminate ripple in power supplies, filter out noise in signals, and ensure that external clocks are able to function properly.
Diodes are passive components which limit the direction of power. When current flows the wrong way, the gate closes and the signal shrinks to zero. Diode arrangements are used in rectifier circuits, which turn AC signals into DC ones.