Active versus passive devices & Amplifiers

Chapter 1.2 Active versus passive devices

Lessons In Electric Circuits Volume III – Semiconductors Book
Pages 528
Views 5,195
Downloads : 7 times
PDF Size : 3.5 MiB

Summary of Contents

Lessons In Electric Circuits Volume III – Semiconductors Book

  • 1.2. ACTIVE VERSUS PASSIVE DEVICES3trons through solid semiconductorsubstances rather than through a vacuum, and so transistortechnology is often referred to as solid-stateelectronics.1.2Active versus passive devicesAn activedevice is any type of circuit component with the ability to electrically control electronflow (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 bymeans of another electrical signal are called passivedevices. Resistors, capacitors, inductors,transformers, and even diodes are all considered passive devices. Active devices include, butare 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 ableto control an AC current with a DC current, but I’ve never heard it referred to as such. Theoperation 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 avoltage 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-controlleddevices. Devices working on the principle of one current controlling another currentare known as current-controlleddevices. For the record, vacuum tubes are voltage-controlleddevices while transistors are made as either voltage-controlled or current controlled types. Thefirst type of transistor successfully demonstrated was a current-controlled device.1.3AmplifiersThe practical benefit of active devices is their amplifyingability. Whether the device in ques-tion be voltage-controlled or current-controlled, the amount of power required of the control-ling signal is typically far less than the amount of power available in the controlled current.In other words, an active device doesn’t just allow electricity to control electricity; it allows asmallamount of electricity to control a largeamount of electricity.Because of this disparity between controllingand controlledpowers, active devices may beemployed to govern a large amount of power (controlled) by the application of a small amountof power (controlling). This behavior is known as amplification.It is a fundamental rule of physics that energy can neither be created nor destroyed. Statedformally, this rule is known as the Law of Conservation of Energy, and no exceptions to it havebeen discovered to date. If this Law is true – and an overwhelming mass of experimental datasuggests that it is – then it is impossible to build a device capable of taking a small amount ofenergy and magically transforming it into a large amount of energy. All machines, electric andelectronic circuits included, have an upper efficiency limit of 100 percent. At best, power outequals power in as in Figure 13,1.2.Usually, machines fail even to meet this limit, losing some of their input energy in the formof heat which is radiated into surrounding space and therefore not part of the output energystream. (Figure 13,1.3)Many people have attempted, without success, to design and build machines that outputmore power than they take in. Not only would such a perpetual motionmachine prove that the