Active versus passive devices
An active device is any type of
circuit component with the ability to electrically control electron flow
(electricity controlling electricity). In order for a circuit to be
properly called electronic, it must contain at least one active
device. Components incapable of controlling current by means of another
electrical signal are called passive devices. Resistors,
capacitors, inductors, transformers, and even diodes are all considered
passive devices. Active devices include, but are not limited to, vacuum
tubes, transistors, silicon-controlled rectifiers (SCRs), and TRIACs. A
case might be made for the saturable reactor to be defined as an active
device, since it is able to control an AC current with a DC current, but
I've never heard it referred to as such. The operation of each of these
active devices will be explored in later chapters of this volume.
All active devices control the flow of
electrons through them. Some active devices allow a voltage to control
this current while other active devices allow another current to do the
job. Devices utilizing a static voltage as the controlling signal are,
not surprisingly, called voltage-controlled devices. Devices
working on the principle of one current controlling another current are
known as current-controlled devices. For the record, vacuum tubes
are voltage-controlled devices while transistors are made as either
voltage-controlled or current controlled types. The first type of
transistor successfully demonstrated was a current-controlled device.
|
