When referring to the "source" of an active or passive device, it's often misunderstood as a power supply. In reality, the term "source" can mean either a driving source or simply a reference point, but in electronics, this term is frequently misapplied when talking about power supplies.
The main difference between active and passive devices lies in whether their port characteristics depend on an external source. To make this distinction clear, we usually consider ideal components, which help eliminate confusion caused by real-world manufacturing variations. If a composite component contains at least one active element, the entire system is classified as active.
Take an ideal resistor or resistor network, for example. Its voltage-current (V-I) characteristics remain constant regardless of external conditions. Even without any external influence, the behavior of a resistor is independent, making it a passive device.
On the other hand, an ideal bipolar junction transistor (BJT) has a characteristic relationship such as Ic = βIb, which only holds true under specific biasing conditions—like a reverse-biased CE junction. Without these external conditions, the relationship cannot be established. Therefore, the transistor’s behavior depends on an external source, classifying it as an active device.
Now consider an ideal diode. From a basic perspective, its V-I characteristics appear independent of external factors, so it is considered a passive device. However, in practice, different types of diodes have additional properties. For instance, LEDs exhibit electro-optical characteristics, and photodiodes respond to light intensity. These behaviors go beyond the simple ideal model and require an external stimulus, such as light or current. Because of these complexities, many practical diodes are often categorized as active devices. Still, in theory, the ideal diode remains a passive component.
In summary, while the classification of active and passive devices is based on dependency on external sources, real-world applications often complicate this distinction. Understanding the ideal behavior helps clarify the fundamental nature of each component, even if practical implementations introduce additional factors.
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